Chapter VI
The Distribution and the Ætiology of the CrayfishesSO far as I have been able to discover, all the crayfishes which inhabit the British islands agree in every point with the full description given above, at p. 230. They are abundant in some of our rivers, such as the Isis, and other affluents of the Thames; and they have been observed in those of Devon; [note 1] but they appear to be absent from many others. I cannot hear of any, for example, in the Cam or the Ouse, on the east, or in the rivers of Lancashire and Cheshire, on the west. It is still more remarkable that, according to the best information I can obtain, they are absent in the Severn, though they are plentiful in the Thames and Severn canal. Dr. M'Intosh, who has paid particular attention to the fauna of Scotland, assures me that crayfish are unknown north of the Tweed. In Ireland, on the other hand, they occur in many localities; [note 2] but the question whether their diffusion, and even their introduction into this
island, has or has not been effected by artificial means, is involved in some obscurity. English zoologists have always termed our crayfish Astacus fluviatilis; and, up to a recent period, the majority of Continental naturalists have included a corresponding form of Astacus under that specific name.
Thus M. Milne Edwards, in his classical work on the Crustacea, [note 3] published in 1837, observes under the head of "Écrevisse commune. Astacus fluviatilis:" "There are two varieties of this crayfish; in the one, the rostrum gradually becomes narrower from its base onwards, and the lateral spines are situated close to its extremity; in the other, the lateral edges of the rostrum are parallel in their posterior half and the lateral spines are stronger and more remote from the end."
The "first variety," here mentioned, is known under the name of "Écrevisse à pieds blancs" [note 4] in France, by way of distinction from the "second variety," which is termed "Écrevisse à pieds rouges," on account of the more or less extensive red coloration of the forceps and ambulatory limbs. This second variety is the larger, commonly attaining five inches in length, and sometimes reaching much larger dimensions; and it is more highly esteemed for the market, on account of its better flavour.
In Germany, the two forms have long been popularly distinguished, the former by the name of "Steinkrebs,"
or "stone crayfish," and the latter by that of "Edelkrebs," or "noble crayfish." Milne Edwards, it will be observed, speaks of these two forms of crayfish as "varieties " of the species Astacus fluviatilis; but, even as far back as the year 1803 some zoologists began to regard the "stone crayfish" as a distinct species, to which Schrank applied the name of Astacus torrentium, while the "noble crayfish" remained in possession of the old denomination, Astacus fluviatilis; and, subsequently, various forms of "stonecrayfishes" have been further distinguished as the species Astacus saratilis, A. tristis, A. pallipes, A. fontinalis, &c. On the other hand, Dr. Gerstfeldt, [note 5] who has devoted especial attention to the question, denies that these are anything more than varieties of one species; but he holds this and Milne Edwards's "second variety" to be specifically distinct from one another.
We thus find ourselves in the presence of three views respecting the English and French crayfishes.
1. They are all varieties of one species--A. fluviatilis.
2. There are two species--A. fluviatilis, and A. torrentium, of which last there are several varieties.
3. There are, at fewest, five or six distinct species.
Before adopting the one or the other of these views, it is necessary to form a definite conception of the meaning of the terms "species" and "variety."
The word "species" in Biology has two significations; the one based upon morphological, the other upon physiological considerations.
A species, in the strictly morphological sense, is simply an assemblage of individuals which agree with one another, and differ from the rest of the living world in the sum of their morphological characters; that is to say, in the structure and in the development of both sexes. If the sum of these characters in one group is represented by A, and that in another by A + n; the two are morphological species, whether n represents an important or an unimportant difference.
The great majority of species described in works on Systematic Zoology are merely morphological species. That is to say, one or more specimens of a kind of animal having been obtained, these specimens have been found to differ from any previously known by the character or characters n; and this difference constitutes the definition of the new species, and is all we really know about its distinctness.
But, in practice, the formation of specific groups is more or less qualified by considerations based upon what is known respecting variation. It is a matter of observation that progeny are never exactly like their parents, but present
small and inconstant differences from them. Hence, when specific identity is predicated of a group of individuals, the meaning conveyed is not that they are all exactly alike, but only that their differences are so small, and so inconstant, that they lie within the probable limits of individual variation. Observation further acquaints us with the fact, that, sometimes, an individual member of a species may exhibit a more or less marked variation, which is propagated through all the offspring of that individual, and may even become intensified in them. And, in this manner, a variety, or race, is generated within the species; which variety, or race, if nothing were known respecting its origin, might have every claim to be regarded as a separate morphological species. The distinctive characters, of a race, however, are rarely equally well marked in all the members of the race. Thus suppose the species A to develope the race A + r; then the difference x is apt to be much less in some individuals than in others; so that, in a large suite of specimens, the interval between A + r and A will be filled up by a series of forms in which r gradually diminishes.
Finally, it is a matter of observation that modification of the physical conditions under which a species lives favours the development of varieties and races.
Hence, in the case of two specimens having respectively the characters A and A + U, although, prima facie, they are of distinct species ; yet if a large collection shows us that the interval between A and A + U is filled up by forms of A having traces of U, and forms of A + n in which n becomes less and less, then it will be
concluded that A and A + U are races of one species and not separate species. And this conclusion will be fortified if A and A + U occupy different stations in the same geographical area. Even when no transitional forms between A and A + U are discoverable, if U is a small and unimportant difference, such as of average size, colour, or ornamentation, it may be fairly held that A and A + U are mere varieties; inasmuch as experience proves that such variations may take place comparatively suddenly; or the intermediate forms may have died out and thus the evidence of variation may have been effaced.
From what has been said it follows that the groups termed morphological species are provisional arrangements, expressive simply of the present state of our knowledge.
We call two groups species, if we know of no transitional forms between them, and if there is no reason to believe that the differences which they present are such as may arise in the ordinary course of variation. But it is impossible to say whether the progress of inquiry into the characters of any group of individuals may prove that what have hitherto been taken for mere varieties are distinct morphological species; or whether, on the contrary, it may prove that what have hitherto been regarded as distinct morphological species are mere varieties.
What has happened in the ease of the crayfish is this:
the older observers lumped all the Western European forms which came under their notice under one species, Astacus fluviatilis; noting, more or less distinctly, the stone crayfish and the noble crayfish as races or varieties of that species. Later zoologists, comparing crayfishes together more critically, and finding that the stone crayfish is ordinarily markedly different from the noble crayfish, concluded that there were no transitional forms, and made the former into a distinct species, tacitly assuming that the differential characters are not such as could be produced by variation. It is at present an open question whether further investigation will or will not bear out either of these assumptions. If large series of specimens of both stone crayfishes and noble crayfishes from different localities are carefully examined, they will be found to present great variations in size and colour, in the tuberculation of the carapace and limbs, and in the absolute and relative sizes of the forceps.
The most constant characters of the stone crayfish are:--
1. The tapering form of the rostrum and the approximation of the lateral spines to its point; the distance between these spines being about equal to their distance from the apex of the rostrum (fig. 61, A).
2. The development of one or two spines from the ventral margin of the rostrum.
3. The gradual subsidence of the posterior part of
the post-orbital ridge, and the absence of spines on its surface. 4. The large relative size of the posterior division of the telson (G).
On the contrary, in the noble crayfish
1. The sides of the posterior two-thirds of the rostrum are nearly parallel, and the lateral spines are fully a third of the length of the rostrum from its point; the distance between them being much less than their distance from the apex of the rostrum (B).
2. No spine is developed from the ventral margin of the rostrum.
3. The posterior part of the post-orbital ridge is a more or less distinct, sometimes spinous elevation.
4. The posterior division of the telson is smaller relatively to the anterior division (H).
I may add that I have found three rudimentary pleurobranchiæ in the noble crayfish, and never more than two in the stone crayfish.
In order to ascertain whether no crayfish exist in which the characters of the parts here referred to are intermediate between those defined, it would be necessary to examine numerous examples of each kind of crayfish from all parts of the areas which they respectively inhabit. This has been done to some extent, but by no means thoroughly; and I think that all that can be safely said, at present, is that the existence of intermediate forms is not proven. But, whatever the constancy of the
differences between the two kinds of crayfishes, there can surely be no doubt as to their insignificance; and no question that they are no more than such as, judging by analogy, might be produced by variation. From a morphological point of view, then, it is really impossible to decide the question whether the stone crayfish and the noble crayfish should be regarded as species or as varieties. But, since it will, hereafter, be convenient to have distinct names for the two kinds, I shall speak of them as Astacus torrentium and Astacus nobilis. [note 6]
In the physiological sense, a species means, firstly, a group of animals the members of which are capable of completely fertile union with one another, but not with the members of any other group; and, secondly, it means all the descendants of a primitive ancestor or ancestors, supposed to have originated otherwise than by ordinary generation.
It is clear that, even if crayfishes had an unbegotten ancestor, there is no means of knowing. whether the stone crayfish and the noble crayfish are descendants of the same, or of different ancestors, so that the second sense of species hardly concerns us. As to the first sense, there is no evidence to show whether the two
kinds of crayfish under consideration are capable of fertile union or whether they are sterile. It is said, however, that hybrids or mongrels are not met with in the waters which are inhabited by both kinds, and that the breeding season of the stone crayfish begins earlier than that of the noble crayfish. M. Carbonnier, who practises crayfish culture on a large scale, gives some interesting facts bearing on this question in the work already cited. He says that, in the streams of France, there are two very distinct kinds of crayfishes-the red-clawed crayfish (L'Écrevisse à pieds rouges), and the white-clawed crayfish (L'Écrevisse à pieds blancs), and that the latter inhabit the swifter streams. In a piece of land converted into a crayfish farm, in which the white-clawed crayfish existed naturally in great abundance, 300,000 red-clawed crayfish were introduced in the course of five years; nevertheless, at the end of this time, no intermediate forms were to be seen, and the "pieds rouges" exhibited a marked superiority in size over the "pieds blancs." M. Carbonnier, in fact, says that they were nearly twice as big.
On the whole, the facts as at present known, seem to incline rather in favour of the conclusion that A. torrentium and A. nobilis are distinct species; in the sense that transitional forms have not been clearly made out, and that, possibly, they do not interbreed. [see End note 17]
As I have already remarked, the very numerous
specimens of English and Irish crayfishes which have passed through my hands, have all presented the character of Astacus torrentium, with which also the description given in works of recognised authority coincides as far as it goes. [note 7] The same form is found in many parts of France, as far south as the Pyrenees, and it is met with as far east as Alsace and Switzerland. I have recently [note 8] been enabled, by the kindness of Dr. Bolivar, of Madrid, who sent me a number of crayfishes from the neighbourhood of that city, to satisfy myself that the Spanish peninsula contains crayfishes altogether similar to those of Britain, except that the subrostral spine is less developed. Further, I have no doubt that Dr. Heller [note 9] is right in his identification of the English crayfish with a form which he describes under the name of A. saxatilis. He says that it is especially abundant in Southern Europe, and that it occurs in Greece, in Dalmatia, in the islands of Cherso and Veglia, at Trieste, in the Lago di Garda, and at Genoa. Further, Astacus torrentium appears to be widely distributed in North Germany. The eastern limit of this crayfish is uncertain; but, according to Kessler, [note 10] it does not occur within the limits of the Russian empire. Astacus torrentium appears to be particularly addicted to rapid highland streams and the turbid pools which they feed.
Astacus nobilis is indigenous to France, Germany, and the Italian peninsula. It is said to be found at Nice and at Barcelona, though I cannot hear of it elsewhere in Spain. Its south-eastern limit appears to be the Lake of Zirknitz, in Carniola, not far from the famous caves of Adelsberg. It is not known in Dalmatia, in Turkey, nor in Greece. In the Russian empire, according to Kessler, this crayfish chiefly inhabits the watershed of the Baltic. The northern limit of its distribution lies between Christianstad, in the Gulf of Bothnia (62 degrees 16' N), and Serdobol, at the northern end of Lake Ladoga. "Eastward of Lake Ladoga it is found in the Uslanka, a tributary of the Swir. It appears to be the only crayfish which exists in the waters which flow from the south into the Gulf of Finland and into the Baltic; except in those streams and lakes which have been artificially connected with the Volga, and in which it is partially replaced by A. leptodactylus." It still inhabits the Lakes of Beresai and Bologoe, as well as the affluents of the Msta and the Wolchow; and it is met with in affluents of the Dnieper, as far as Mohilew. Astacus nobilis is also found in Denmark and Southern Sweden; but, in the latter country, its introduction appears to have been artificial. This crayfish is said occasionally to be met with on the Livonian coast in the waters of the Baltic, which, however, it must
be remembered, are much less salt than ordinary sea water. It will be observed that while the two forms, A. torrentium and A. nobilis, are intermixed over a large part of Central Europe, A. torrentium has a wider north-westward, south-westward, and south-eastward extension, being the sole occupant of Britain, and apparently of the greater part of Spain and of Greece. On the other hand, in the northern and eastern parts of Central Europe, A. nobilis appears to exist alone.
[Figure 75: Astacus leptodactylus]
Further to the east, a new form, Astacus leptodactylus (fig. 75), makes its appearance. Whether A. leptodactylus exists in the upper waters of the Danube, does not appear, but in the lower Danube and in the Theiss it is the dominant, if not the exclusive, crayfish. From hence it extends through all the rivers which flow into the Black, Azov, and Caspian Seas, from Bessarabia and Podolia on the west, to the Ural mountains on the east. In fact, the natural habitat of this crayfish appears to be the watershed of the Pontocaspian area, excluding that part of the Black Sea which lies southward of the Caucasus on the one hand, and of the mouths of the Danube on the other. [note 11]
It is a remarkable circumstance that this crayfish not only thrives in the brackish waters of the estuaries of the rivers which debouche into the Black Sea and the Sea of Azov, but that it is found even in the salter
southern parts of the Caspian, in which it lives at considerable depths. In the north, Astacus leptodactylus is met with in the rivers which flow into the White Sea, as well as in many streams and lakes about the Gulf of Finland. But it has probably been introduced into these streams by the canals which have been constructed to connect the basin of the Volga with the rivers which flow into the Baltic and into the White Sea. In the latter, the invading A. leptodactylus is everywhere overcoming and driving out A. nobilis in the struggle for existence, apparently in virtue of its more rapid multiplication. [note 12]
In the Caspian and in the brackish waters of the estuaries of the Dniester and the Bug, a somewhat different crayfish, which has been called Astacus pachypus, occurs; another closely allied form (A. angulosus) is met with in the mountain streams of the Crimea and of the northern face of the Caucasus; and a third, A. colchicus, has recently been discovered in the Rion, or Phasis of the ancients, which flows into the eastern extremity of the Black Sea.
With respect to the question whether these Pontocaspian crayfishes are specifically distinct from one another, and whether the most widely distributed kind, A. leptodactylus, is distinct from A. nobilis, exactly the same difficulties arise as in the case of the west European
crayfishes. Gerstfeldt, who has had the opportunity of examining large series of specimens, concludes that the Pontocaspian crayfishes and A. nobilis are all varieties of one species. Kessler, on the contrary, while he admits that A. angulosus is, and A. pachypus may be, a variety of A. leptodactylus, affirms that the latter is specifically distinct from A. nobilis. Undoubtedly, well marked examples of A. leptodactylus are very different from A. nobilis.
1. The edges of the rostrum are produced into five or six sharp spines, instead of being smooth or slightly serrated as in A. nobilis.
2. The fore part of the rostrum has no serrated spinous median keel, such as commonly, though not universally, exists in A. nobilis.
3. The posterior end of the post-orbital ridge is still more distinct and spiniform than in A. nobilis.
4. The abdominal pleura of A. leptodactylus are narrower, more equal sided, and triangular in shape.
5. The chelæ of the forceps, especially in the males, are more elongated; and the moveable and fixed claws are slenderer and have their opposed edges straighter and less tuberculated.
But, in all these respects, individual specimens of A. nobilis vary in the direction of A. leptodactylus and vice versâ; and if A. angulosus and A. pachypus are varieties of A. leptodactylus, I cannot see why Gerstfeldt's conclusion that A. nobilis is another variety of
the same form need be questioned on morphological grounds. However, Kessler asserts that, in those localities in which A. leptodactylus and A. nobilis live together, no intermediate forms occur, which is presumptive evidence that they do not intermix by breeding. No crayfishes are known to inhabit the rivers of the northern Asiatic watershed, such as the Obi, Yenisei, and Lena. None are known [note 13] in the sea of Aral, or the great rivers Oxus and Jaxartes, which feed that vast lake; nor any in the lakes of Balkash and Baikal. If further exploration verifies this negative fact, it will be not a little remarkable; inasmuch as two [note 14], if not more, kinds of crayfishes are found in the basin of the great river Amur, which drains a large area of north-eastern Asia, and debouches into the Gulf of Tartary, in about the latitude of York.
Japan has one species (A. japonicus), perhaps more; but no crayfish has as yet been made known in any part of eastern Asia, south of Amurland. There are certainly none in Hindostan; none are known in Persia, Arabia, or Syria. In Asia Minor the only recorded locality is the Rion. No crayfish has yet been discovered in the whole continent of Africa. [note 15],
Thus, on the continent of the old world, the crayfishes are restricted to a zone, the southern limit of which coincides with certain great geographical features; on the west, the Mediterranean, with its continuation, the Black Sea; then the range of the Caucasus, followed by the great Asiatic highlands, as far as the Corea on the east. On the north, though there is no such physical boundary, the crayfishes appear to be entirely excluded from the Siberian river basins; while east and west, though a sea-barrier exists, the crayfishes extend beyond it, to reach the British islands and those of Japan.
Crossing the Pacific, we meet with some half-a-dozen kinds of crayfishes, [note 16], different from those of the old world, but still belonging to the genus Astacus, in British Columbia, Oregon, and California. Beyond the Rocky Mountains, from the Great Lakes to Guatemala, crayfishes abound, as many as thirty-two different species having been described, but they all belong to the genus Cambarus (fig. 63, p. 248). Species of this genus also occur in Cuba, [note 17], but, so far as is at present known, not in any of the other West Indian islands. The occurrence of a curious dimorphism among the male Cambari has been described by Dr. Hagen; and a blind Cambarus
is found, along with other blind animals, in the subterranean caves of Kentucky. [Figure 76:--Australian Crayfish] [note 19]
All the crayfishes of the northern hemisphere belong to the Potamobiidæ, and no members of this family are known to exist south of the equator. The crayfishes of the southern hemisphere, in fact, all belong to the division of the Parastacidæ, and in respect of the number and variety of forms and the size which they reach, the head-quarters of the Parastacidæ is the continent of Australia. Some of the Australian crayfishes (fig. 76) attain a foot or more in length, and are as large as full-sized lobsters. The genus Engæus of Tasmania comprises small crayfish which, like some of the Cambari, live habitually on land, in burrows which they excavate in the soil.
New Zealand has a peculiar genus of crayfishes, Paranephrops, a species of which is found in the Fiji Islands, but none are known to occur elsewhere in Polynesia.
Two kinds of crayfish have been obtained in southern Brazil, and have been described by Dr. v. Martens, [note 18], as A. pilimanus and A. brasiliensis. I have shown that they belong to a peculiar genus, Parastacus. The former was procured at Porto Alegre, which is situated in 30 degrees S. Latitude, close to the mouth of the Jacuhy, at the north end of the great Laguna do Patos, which
communicates by a narrow passage with the sea; and also at Sta. Cruz in the upper basin of the Rio Pardo, an affluent of the Jacuhy, "by digging it out of holes in the ground." The latter (P. brasiliensis, fig. 64) was obtained at Porto Alegre, and further inland, in the region of the primitive forest at Rodersburg, in shallow streams. In addition to these, no crayfish have as yet been found in any of the great rivers, such as the Orinoko; the Amazon, in which they were specially sought for by Agassiz; or in the La Plata, on the eastern side of the Andes. But, on the west, an "Astacus" chilensis is described in the "Histoire Naturelle des Crustacées," (vol. ii. p. 333). It is here stated that this crayfish "habite les côtes du Chili," but the freshwaters of the Chilian coast are doubtless to be understood.
Finally, Madagascar has a genus and species of crayfish (Astacoides madagascariensis, fig. 65) peculiar to itself.
On comparing the results obtained by the study of the geographical distribution of the crayfishes with those brought to light by the examination of their morphological characters, the important fact that there is a broad and general correspondence between the two becomes apparent. The wide equatorial belt of the earth's surface which separates the crayfishes of the northern from those of the southern hemisphere, is a sort of geographical
representation of the broad morphological differences which mark off the Potamobiidæ from the Parastacidæ. Each group occupies a definite area of the earth's surface, and the two are separated by an extensive border-land untenanted by crayfishes. [Figure 77:--Map of the World, showing the geographical distribution of the Crayfishes.]
A similar correspondence is exhibited, though less distinctly, when we consider the distribution of the genera and species of each group. Thus, among the Potamobiidæ, Astacus torrentium and nobilis belong essentially to the northern, western, and southern watersheds of the central European highlands, the streams of which flow respectively into the Baltic and the North Seas, the Atlantic and the Mediterranean (fig. 77, I.); A. leptodactylus, pachypus, angulosus, and colchicus, appertain to the Pontocaspian watershed, the rivers of which drain into the Black Sea and the Caspian (I.); while Astacus dauricus and A. Schrenckii are restricted to the widely separated basin of the Amur, which sheds its waters into the Pacific (II.). The Astaci of the rivers of western North America, which flow into the Pacific (IV.), and the Cambari of the Eastern or Atlantic water-shed (V.) are separated by the great physical barrier of the Rocky Mountain ranges. Finally, with regard to the Parastacidæ, the widely separated geographical regions of New Zealand (VIII.), Australia (IX.), Madagascar (XII.), and South America (VI. and VII.), are inhabited by generically distinct groups.
But when we look more closely into the matter, it will
be found that the parallel between the geographical and the morphological facts cannot be quite strictly carried out. Astacus torrentium, as we have seen, inhabits both the British Islands and the continent of Europe; nevertheless, there is every reason to believe that twenty miles of sea water is an insuperable barrier to the passage of crayfishes from one land to the other. For though some crayfishes live in brackish water, there is no evidence that any existing species can maintain themselves in the sea. A fact of the same character meets us at the other side of the Eurasiatic continent, the Japanese and the Amurland crayfishes being closely allied; although it is not clear that there are any identical species on the two sides of the Sea of Japan.
Another circumstance is still more remarkable. The West American crayfishes are but little more different from the Pontocaspian crayfishes, than these are from Astacus torrentium. On the face of the matter, one might therefore expect the Amurland and Japanese crayfishes, which are intermediate in geographical position, to be also intermediate, morphologically, between the Pontocaspian and the West American forms. But this is not the case. The branchial system of the Amurland Astaci appears to be the same as that of the rest of the genus; but, in the males, the third joint (ischiopodite) of the second and third pair of ambulatory limbs is provided with a conical, recurved, hook-like process; while, in the females, the hinder edge of the penultimate thoracic
sternum is elevated into a transverse prominence, on the posterior face of which there is a pit or depression. [note 20], In both these characters, but more especially in the former, the Amurland and Japanese Astaci depart from both the Pontocaspian and the West American Astaci, and approach the Cambari of Eastern North America.
In these crayfishes, in fact, one or both of the same pairs of legs in the male are provided with similar hook-like processes; while, in the females, the modification of the penultimate thoracic sternum is carried still further and gives rise to the curious structure described by Dr. Hagen as the "annulus ventralis."
[Figure 78: Cambarus (Guatemala) penultimate leg]
In all the Cambari, the pleurobranchiæ appear to be entirely suppressed, and the hindermost podobranchia has no lamina; while the areola is usually extremely narrow. The proportional size of the areola in the Amurland
crayfishes is not recorded; in the Japanese crayfish, judging by the figure given by De Haan, it is about the same as in the western Astaci. On the other hand, in the West American crayfishes it is distinctly smaller; so that, in this respect, they perhaps more nearly approach the Cambari. Unfortunately, nothing is known as to the branchiæ of the Amurland crayfishes. According to De Haan, those of the Japanese species resemble those of the western Astaci as those of the West American Astaci certainly do. With respect to the Parastacidæ; in the remarkable length and flatness of the epistoma, the crayfishes of Australia, Madagascar, and South America, resemble one another. But in its peculiar truncated rostrum (see fig. 65) and in the extreme modification of its branchial system, which I have described elsewhere, the Madagascar genus stands alone.
The Paranephrops of New Zealand and the Fijis, with its wide and short epistoma, long rostrum, and large antennary squames, is much more unlike the Australian forms than might be expected from its geographical position. On the other hand, considering their wide separation by sea, the amount of resemblance between the New Zealand and the Fiji species is very remarkable.
If the distribution of the crayfishes is compared with that of terrestrial animals in general, the points of
difference are at least as remarkable as the resemblances. With respect to the latter, the area occupied by the Potamobiidæ, corresponds roughly with the Palæarctic and Nearctic divisions of the great Arctogæal provinces of distribution indicated by mammals and birds; while distinct groups of crayfishes occupy a larger or smaller part of the other, namely, the Austro-Columbian, Australian, and Novozelanian primary distributional provinces of mammals and birds. Again, the peculiar crayfishes of Madagascar answer to the special features of the rest of the fauna of that island.
But the North American crayfishes extend much further South than the limits of the Nearctic fauna in general; while the absence of any group of crayfishes m Africa, or in the rest of the old world, south of the great Asiatic table-land, forms a strong contrast to the general resemblance of the North African and Indian fauna to that of the rest of Arctogæa. Again, there is no such vast difference between the crayfishes of New Zealand, Australia, and South America, as there is between the mammals and the birds of those regions.
It may be concluded, therefore, that the conditions which have determined the distribution of crayfishes have been very different from those which have governed the distribution of mammals and birds. But if we compare with the distribution of the crayfishes, not that of terrestrial animals in general, but only that of freshwater
fishes, some very curious points of approximation become manifest. The Salmonidæ, or fishes of the salmon and trout kind, a few of which are exclusively marine, many both marine and freshwater, while others are confined to fresh water, are distributed over the northern hemisphere, in a manner which recalls the distribution of the Potamobine crayfishes, [note 21], though they do not extend so far to the South in the new world, while they go a little further, namely, as far as Algeria, Northern Asia Minor, and Armenia, in the old world. With the exception of the single genus Retropinna, which inhabits New Zealand, no true salmonoid fish occurs south of the equator; but, as Dr. Günther has pointed out, two groups of freshwater fishes, the Haplochitonidæ and the Galaxidæ, which stand in somewhat the same relation to the Salmonidæ as the Parastacidæ do to the Potamobiidæ, take the place of the Salmonidæ in the fresh waters of New Zealand, Australia, and South America. There are two species of Haplochiton in Tierra del Fuego; and of the closely allied genus Prototroctes, one species is found in South Australia, and one in New Zealand; of the Galaxidæ, the same species, Galaxias attennuatus, occurs in the streams of New Zealand, Tasmania, the Falkland Islands, and Peru. Thus, these fish avoid South Africa, as the crayfishes
do; but I am not aware that any member of the group is found in Madagascar, and thus completes the analogy. The preservation of the soft parts of animals in the fossil state depends upon favourable conditions of rare occurrence; and, in the case of the Crustacea, it is not often that one can hope to meet with such small hard parts as the abdominal members, in a good state of preservation. But without recourse to the branchial apparatus, and to the abdominal appendages, it might be very difficult to say whether a given crustacean belonged to the Astacine, or to the closely allied Homarine group. Of course, if the accompanying fossils indicated that the deposit in which the remains occur, was of freshwater origin, the presumption in favour of their Astacine nature would be very strong; but if they were inhabitants of the sea, the problem whether the crustacean in question was a marine Astacine, or a true Homarine, might be very hard to solve.
Undoubted remains of crayfishes have hitherto been discovered only in freshwater strata of late tertiary age. In Idaho, North America, Professor Cope [note 22], found, in association with Mastodon mirificus, and Equus excelsus, several species, which he considers to be distinct from
the existing American crayfishes; whether they are Cambari or Astaci does not appear. But, in the lower chalk of Ochtrup, in Westphalia, and therefore in a marine deposit, Von der Marck and Schlüter [note 23], have obtained a single, somewhat imperfect, specimen of a crustacean, which they term Astacus politus, and which, singularly enough, has the divided telson found only in the genus Astacus. It would be very desirable to know more about this interesting fossil. For the present it affords a strong presumption that a marine Potamobine existed as far back as the earlier part of the cretaceous epoch. Such are the more important facts of Morphology, Physiology, and Distribution, which make up the sum of our present knowledge of the Biology of Crayfishes. The imperfection of that knowledge, especially as regards the relations between Morphology and Distribution, becomes a serious drawback when we attack the final problem of Biology, which is to find out why animals of such structure and active powers, and so localized, exist?
It would appear difficult to frame more than two fundamental hypotheses in attempting to solve this problem. Either we must seek the origin of crayfishes in conditions extraneous to the ordinary course of natural
operations, by what is commonly termed Creation; or we must seek for it in conditions afforded by the usual course of nature, when the hypothesis assumes some shape of the doctrine of Evolution. And there are two forms of the latter hypothesis; for, it may be assumed, on the one hand, that crayfishes have come into existence, independently of any other form of living matter, which is the hypothesis of spontaneous or equivocal generation, or abiogenesis; or, on the other hand, we may suppose that crayfishes have resulted from the modification of some other form of living matter; and this is what, to borrow a useful word from the French language, is known as transformism. I do not think that any hypothesis respecting the origin of crayfishes can be suggested, which is not referable to one or other of these, or to a combination of them.
As regards the hypothesis of creation, little need be said. From a scientific point of view, the adoption of this speculation is the same thing as an admission that the problem is not susceptible of solution. Moreover, the proposition that a given thing has been created, whether true or false, is not capable of proof. By the nature of the case direct evidence of the fact is not obtainable. The only indirect evidence is such as amounts to proof that natural agencies are incompetent to cause the existence of the thing in question. But such evidence is out of our reach. The most that
can be proved, in any case, is that no known natural cause is competent to produce a given effect; and it is an obvious blunder to confound the demonstration of our own ignorance with a proof of the impotence of natural causes. However, apart from the philosophical worthlessness of the hypothesis of creation, it would be a waste of time to discuss a view which no one upholds. And, unless I am greatly mistaken, at the present day, no one possessed of knowledge sufficient to give his opinion importance is prepared to maintain that the ancestors of the various species of crayfish were fabricated out of inorganic matter, or brought from nothingness into being, by a creative fiat. Our only refuge, therefore, appears to be the hypothesis of evolution. And, with respect to the doctrine of abiogenesis, we may also, in view of a proper economy of labour, postpone its discussion until such time as the smallest fragment of evidence that a crayfish can be evolved by natural agencies from not living matter, is brought forward.
In the meanwhile, the hypothesis of transformism remains in possession of the field; and the only profitable inquiry is, how far are the facts susceptible of interpretation, on the hypothesis that all the existing kinds of crayfish are the product of the metamorphosis of other forms of living beings; and that the biological phenomena which they exhibit are the results of the interaction, through past time, of two series of
factors: the one, a process of morphological and concomitant physiological modification; the other, a process of change in the condition of the earth's surface. If we set aside, as not worth serious consideration, the assumption that the Astacus torrentium of Britain was originally created apart from the Astacus torrentium of the Continent; it follows, either that this crayfish has passed across the sea by voluntary or involuntary migration; or that the Astacus torrentium existed before the English Channel, and spread into England while these islands were still continuous with the European mainland; and that the present isolation of the English cray fishes from the members of the same species on the Continent is to be accounted for by those changes in the physical geography of western Europe which, as there is abundant evidence to prove, have separated the British Islands from the mainland.
There is no evidence that our crayfish has been purposely introduced by human agency into Great Britain; and from the mode of life of crayfish and the manner in which the eggs are carried about by the parent during their development, transport by birds or floating timber would seem to be out of the question. Again, although Astacus nobilis is said to venture into the brackish waters of the Gulf of Finland, and A. leptodactylus, as we have seen, makes itself at home in the more or less salt Caspian, there is no reason to believe that Astacus torrentium is capable of existing in
sea-water, still less of crossing the many miles of sea which separate England from even the nearest point of the Continent. In fact, the existence of the same kind of crayfish on both sides of the Channel appears to be only a case of the general truth, that the Fauna of the British Islands is identical with a part of that of the Continent; and as our foxes, badgers, and moles certainly have neither swum across, nor been transported by man, but existed in Britain while it was still continuous with western Europe, and have been isolated by the subsequent intervention of the sea, so we may confidently explain the presence of Astacus torrentium by reference to the same operation. If we take into account the occurrence of Astacus nobilis over so large a part of the area occupied by Astacus torrentium; its absence in the British Islands, and in Greece; and the closer affinity which exists between A. nobilis and A. leptodactylus, than between A. nobilis and A. torrentium; it seems not improbable that Astacus torrentium was the original tenant of the whole western European area outside the Ponto-Caspian watershed; and that A. nobilis is an invading offshoot of the Ponto-Caspian or leptodactylus form which has made its way into the western rivers in the course of the many changes of level which central Europe has undergone; in the same way as A. leptodactylus is now passing into the rivers of the Baltic provinces of Russia.
The study of the glacial phenomena of central Europe
has led Sartorius von Waltershausen [note 24], to the conclusion that at the time when the glaciers of the Alps had a much greater extension than at present, a vast mass of freshwater extended from the valley of the Danube to that of the Rhone, around the northern escarpment of the Alpine chain, and connected the head-waters of the Danube with those of the Rhine, the Rhone, and the northern Italian rivers. As the Danube debouches into the Black Sea, and this was formerly connected with the Aralo-Caspian Sea, an easy passage would thus be opened up by which crayfishes might pass from the Aralo-Caspian area to western Europe. If they spread by this road, the Astacus torrentium may represent the first wave of migration westward, while A. nobilis answers to a second, and A. leptodactylus, with its varieties, remains as the representative of the old Aralo-Caspian crayfishes. And thus the crayfishes would present a curious parallel with the Iberian, Aryan, and Mongoloid streams of westward movement among mankind. If we thus suppose the western Eurasiatic crayfishes to be simply varieties of a primitive Aralo-Caspian stock, their limitation to the south by the Mediterranean and by the great Asiatic highlands becomes easily intelligible.
The extremely severe climatal conditions which obtain in northern Siberia may sufficiently account for the
absence of crayfishes (if they are really absent) in the rivers Obi, Yenisei, and Lena, and in the great lake Baikal, which lies more than 1,300 feet above the sea, and is frozen over from November to May. Moreover, there can be no doubt that, at a comparatively recent period, the whole of this region, from the Baltic to the mouth of the Lena, was submerged beneath a southward extension of the waters of the Arctic ocean to the Aralo-Caspian Sea and Lake Baikal, and a westward extension to the Gulf of Finland. The great lakes and inland seas which stretch, at intervals, from Baikal, on the east, to Wenner in Sweden, on the west, are simply pools, isolated partly by the rising of the ancient sea-bottom and partly by evaporation; and often completely converted into fresh water by the inflow of the surrounding land-drainage. But the population of these pools was originally the same as that of the Northern Ocean, and a few species of marine crustaceans, mollusks, and fish, besides seals, remain in them as living evidences of the great change which has taken place. The same process which, as we shall see, has isolated the Mysis of the Arctic seas in the lakes of Sweden and Finland, has. shut up with it other arctic marine crustacea, such as species of Gammarus and Idothea. And the very same species of Gammarus is imprisoned, along with arctic seals, in the waters of Lake Baikal.
The distribution of the American crayfishes agrees equally well with the hypothesis of the northern origin of
the stock from which they have been evolved. Even under existing geographical conditions, an affluent of the Mississippi, the St. Peter's river, communicates directly, in rainy weather, with the Red river, which flows into Lake Winnipeg, the southernmost of the long series of intercommunicating lakes and streams, which occupy the low and flat water-parting between the southern and the northern watersheds of the North American Continent. But the northernmost of these, the Great Slave Lake, empties itself by the Mackenzie river into the Arctic Ocean, and thus provides a route by which crayfishes might spread from the north over all parts of North America east of the Rocky Mountains. The so-called Rocky Mountain range is, in reality, an immense table-land, the edges of which are fringed by two principal lines of mountainous elevations. The tableland itself occupies the place of a great north and south depression which, in the cretaceous epoch, was occupied by the sea and probably communicated with the ocean at its northern, as well as at its southern end. During and since this epoch it became gradually filled up, and it now contains an immense thickness of deposits of all ages from the cretaceous to the pliocene--the earlier marine, the later more and more completely freshwater. During the tertiary epoch, various portions of this area have been occupied by vast lakes, the more northern of which doubtless had outlets into the Northern sea. That crayfish existed in the vicinity of the Rocky Mountains
in the latter part of the tertiary epoch is testified by the Idaho fossils. And there is thus no difficulty in understanding their presence in the rivers which have now cut their way to the Pacific coast. The similarity of the crayfish of the Amurland and of Japan is a fact of the same order as the identity of the English crayfish with the Astacus torrentium of the European Continent, and is to be explained in an analogous fashion. For there can be no doubt that the Asiatic continent formerly extended much further to the eastward than it does at present, and included what are now the islands of Japan. Even with this alteration of the geographical conditions, however, it is not easy to see how crayfishes can have got into the Amur-Japanese fresh waters. For a north-eastern prolongation of the Asiatic highlands, which ends to the north in the Stanovoi range, shuts in the Amur basin on the west; while the Amur debouches into the sea of Okhotsk, and the Pacific ocean washes the shores of the Japanese islands.
But there are many grounds for the conclusion that, in the latter half of the tertiary epoch, eastern Asia and North America were connected, and that the chain of the Kurile and Aleutian islands may indicate the position of a great extent of submerged land. In that case, the sea of Okhotsk and Behring's sea may occupy the site of inland waters which formerly placed the mouth of the Amur in direct communication with the Northern Ocean, just as the Black Sea, at present, brings the basin of the
Danube into connection, first with the Mediterranean and then with the western Atlantic; and, as in former times, it gave access from the south to the vast area now drained by the Volga. When the Black Sea communicated with the Aralo-Caspian sea, and this opened to the north into the Arctic sea, a chain of great inland waters must have skirted the eastern frontier of Europe, just such as would now lie on the eastern frontier of Asia if the present coast underwent elevation. Supposing, however, that the ancestral forms of the Potamobiidæ obtained access to the river basins in which they are now found, from the north, the hypothesis that a mass of fresh water once occupied a great part of the region which is now Siberia and the Arctic Ocean, would be hardly tenable, and it is, in fact, wholly unnecessary for our present purpose.
The vast majority of the stalk-eyed crustaceans are, and always have been, exclusively marine animals; the cray-fishes, the Atyidæ, and the fluviatile crabs (Thelphusidæ) being the only considerable groups among them which habitually confine themselves to fresh waters. But even in such a genus as Penæus most of the species of which are exclusively marine, some, such as Penæus brasiliensis, ascend rivers for long distances. Moreover, there are cases in which it cannot be doubted that the descendants of marine Crustacea have gradually accustomed themselves to fresh water conditions, and have, at the same time, become more or less modified,
so that they are no longer absolutely identical with those descendants of their ancestors which have continued to live in the sea. [note 25], In several of the lakes of Norway, Sweden and Finland, and in Lake Ladoga, in Northern Europe; in Lake Superior and Lake Michigan, in North America; a small crustacean, Mysis relicta, occurs in such abundance as to furnish a great part of the supply of food to the fresh water fishes which inhabit these lakes. Now, this Mysis relicta is hardly distinguishable from the Mysis oculata which inhabits the Arctic seas, and is certainly nothing but a slight variety of that species.
In the case of the lakes of Norway and Sweden, there is independent evidence that they formerly communicated with the Baltic, and were, in fact, fiords or arms of the sea. The communication of these fiords with the sea having been gradually cut off, the marine animals they contained have been imprisoned; and as the water has been slowly changed from salt to fresh by the drainage of the surrounding land, only those which were able to withstand the altered conditions have survived. Among these is the Mysis oculata, which has in the meanwhile undergone the slight variation which has converted it into Mysis relicta. Whether the same explanation
applies to Lakes Superior and Michigan, or whether the My8ia oculata has not passed into these masses of fresh water by channels of communication with the Arctic Ocean which no longer exist, is a secondary question. The fact remains that Mysis relicta is a primitively marine animal which has become completely adapted to fresh-water life. [Figure 79: Palæmon jamaicensis]
Several species of prawns (Palæmon) abound in our own seas. Other marine prawns are found on the coasts of North America, in the Mediterranean, in the South Atlantic and Indian Oceans, and in the Pacific as far south as New Zealand. But species of the same genus (Palæmon) are met with, living altogether in fresh water, in Lake Erie, in the rivers of Florida, in the Ohio, in the rivers of the Gulf of Mexico, of the West India Islands and of eastern South America, as far as southern Brazil, if not further; in those of Chili and those of Costa Rica in western South America; in the Upper Nile, in West Africa, in Natal, in the Islands of Johanna, Mauritins, and Bourbon, in the Ganges, in the Molucca and Philippine Islands, and probably elsewhere.
Many of these fluviatile prawns differ from the marine species not only in their great size (some attaining a foot or more in length), but still more remarkably in the vast development of the fifth pair of thoracic appendages. These are always larger than the slender fourth pair (which answer to the forceps of the crayfishes) ; and, in the males especially, they are very long and strong, and
are terminated by great chelæ, not unlike those of the crayfishes. Hence these fluviatile prawns (known in many places by the name of "Cammarons") are not unfrequently confounded with true crayfishes; though the fact that there are only three pair of ordinary legs behind the largest, forceps-like pair, is sufficient at once to distinguish them from any of the Astacidæ. Species of these large-clawed prawns live in the
brackish water lagoons of the Gulf of Mexico, but I am not aware that any of them have yet been met with in the sea itself. The Palæmon lacustris (Anchistia migratoria, Heller) abounds in fresh-water ditches and canals between Padua and Venice, and in the Lago di Garda, as well as in the brooks of Dalmatia; but its occurrence in the Adriatic or the Mediterranean, which has been asserted, appears to be doubtful. So the Nile prawn, though very similar to some Mediterranean prawns, does not seem to be identical with any at present known. [note 26], In all these cases, it appears reasonable to apply the analogy of the Mysis relicta, and to suppose that the fluviatile prawns are simply the result of the adaptive modification of species which, like their congeners, were primitively marine.
But if the existing sea prawns were to die out, or to be beaten in the struggle for existence, we should have, scattered over the world in isolated river basins, more or less distinct species of freshwater prawns, [note 27], the areas inhabited by which might hereafter be indefinitely enlarged or diminished, by alteration in the elevation of the
land and by other changes in physical geography. And, indeed, under these circumstances, the freshwater prawns themselves might become so much modified, that, even if the descendants of their ancestors remained unchanged in structure and habits in the sea, the relationship of the two might no longer be obvious. These considerations appear to me to indicate the direction in which we must look for a rational explanation of the origin of crayfishes and their present distribution.
I have no doubt that they are derived from ancestors which lived altogether in the sea, as the great majority of the Mysidæ and many of, the prawns do now; and that, of these ancestral crayfishes, there were some which, like Mysis oculata or Penæus brasiliensis, readily adapted themselves to fresh water conditions, ascended rivers, and took possession of lakes. These, more or less modified, have given rise to the existing crayfishes, while the primitive stock would seem to have vanished. At any rate, at the present time, no marine crustacean with the characters of the Astacidæ is known.
As crayfishes have been found in the later tertiaries of North America, we shall hardly err in dating the existence of these marine crayfishes at least as far back as the miocene epoch; and I am disposed to think that, during the earlier tertiary and later mesozoic periods, these Crustacea not only had as wide a distribution as the Prawns and Penæi have now, but were differentiated into two groups, one with the general characters of the
Potamobiidæ in the northern hemisphere, and another, with those of the Parastacidæ, in the southern hemisphere. The ancestral Potamobine form probably presented the peculiarities of the Potamobiidæ in a less marked degree than any existing species does. Probably the four pleurobranchiæ were all equally well developed; the laminæ of the podobranchiæ smaller and less distinct from the stem; the first and second abdominal appendages less specialised; and the telson less distinctly divided. So far as the type was less specially Potamobine, it must have approached the common form in which Homarus and Nephrops originated. And it is to be remarked that these also are exclusively confined to the northern hemisphere.
The wide range and close affinity of the genera Astacus and Cambarus appear to me to necessitate the supposition that they are derived from some one already specialised Potamobine form; and I have already mentioned the grounds upon which I am disposed to believe that this ancestral Potamobine existed in the sea which lay north of the miocene continent in the northern hemisphere.
In the marine primitive crayfishes south of the equator, the branchial apparatus appears to have suffered less modification, while the suppression of the first abdominal appendages, in both sexes, has its analogue among the Palinuridæ, the headquarters of which are in the southern hemisphere. That they should have ascended
the rivers of New Zealand, Australia, Madagascar, and South America, and become fresh water Parastacidæ is an assumption which is justified by the analogy of the fresh-water prawns. It remains to be seen whether marine Parastacidæ still remain in the South Pacific and Atlantic Oceans, or whether they have become extinct. In speculating upon the causes of an effect which is the product of several co-operating factors, the nature of each of which has to be divined by reasoning backwards from its effects, the probability of falling into error is very great. And this probability is enhanced when, as in the present case, the effect in question consists of a multitude of phenomena of structure and distribution about which much is yet imperfectly known. Hence the preceding discussion must rather be regarded as an illustration of the sort of argumentation by which a completely satisfactory theory of the ætiology of the crayfish will some day be established, than as sufficing to construct such a theory. It must be admitted that it does not account for the whole of the positive facts which have been ascertained; and that it requires supplementing, in order to furnish even a plausible explanation of various negative facts.
The positive fact which presents a difficulty is the closer resemblance between the Amur-Japanese crayfish and the East American Cambari, than between the
latter and the West American Astaci; and the closer resemblance between the latter and the Pontocaspian crayfish, than either bear to the Amur-Japanese form. if the facts had been the other way, and the West American and Amur-Japanese crayfish had changed places, the case would have been intelligible enough. The primitive Potamobine stock might then have been supposed to have differentiated itself into a western astacoid, and an eastern cambaroid form; [note 28], the latter would have ascended the American, and the former the Asiatic rivers. As the matter stands, I do not see that any plausible explanation can be offered without recourse to suppositions respecting a former more direct communication between the mouth of the Amur, and that of the North American rivers, in favour of which no definite evidence can be offered at present. The most important negative fact which remains to be accounted for is the absence of crayfishes in the rivers of a large moiety of the continental lands, and in numerous islands. Differences of climatal conditions are obviously inadequate to account for the absence of cray-fishes in Jamaica, when they are present in Cuba; for their absence in Mozambique, and the islands of Johanna and Mauritius, when they are present in Madagascar; and for their absence in the Nile, when they exist in Guatemala.
At present, I confess that I do not see my way to a perfectly satisfactory explanation of the absence of crayfishes in so many parts of the world in which they might, a priori, be expected to exist; and I can only suggest the directions in which an explanation may be sought.
The first of these is the existence of physical obstacles to the spread of crayfishes, at the time at which the Potamobine and the Parastacine stocks respectively began to take possession of the rivers, some of which have now ceased to exist; and the second is the probability that, in many rivers which have been accessible to cray-fishes, the ground was already held by more powerful competitors.
If the ancestors of the Potamobine crayfishes originated only among those primitive crayfishes which inhabited the seas north of the miocene continent, their present limitation to the south, in the old world, is as easily intelligible as is their extension southward, in the course of the river basins of Northern America as far as Guatemala, but no further. For the elevation of the Eurasiatic highlands had commenced in the miocene epoch, while the isthmus of Panama was interrupted by the sea.
With respect to the Southern hemisphere, the absence of crayfishes in Mauritius and in the islands of the Indian Ocean, though they occur in Madagascar, may be due to the fact that the former islands are of comparatively late volcanic origin; while Madagascar is the remnant of
a very ancient continental area, the oldest indigenous population of which, in all probability, is directly descended from that which occupied it at the beginning of the tertiary epoch. If Parastacine Crustacea inhabited the southern hemisphere at this period, and subsequently became extinct as marine animals, their preservation in the freshwaters of Australia, New Zealand, and the older portions of South America may be understood. The difficulty of the absence of crayfishes in South Africa [note 29], remains; and all that can be said is, that it is a difficulty of the same nature as that which confronts us when we compare the fauna of South Africa in general with that of Madagascar. The population of the latter region has a more ancient aspect than that of the former; and it may be that South Africa, in its present shape, is of very much later date than Madagascar. With respect to the second point for consideration, it is to be remarked that, in the temperate regions of the world, the crayfishes are by far the largest and strongest of any of the inhabitants of freshwater, except the Vertebrata; and that while frogs and the like fall an easy prey to them, they must be formidable enemies and competitors even to fishes, aquatic reptiles, and the smaller aquatic mammals. In warm climates, however, not only the large prawns which have been mentioned, but Atyæ
and fluviatile crabs (Thelphusa) compete for the possession of the freshwaters; and it is not improbable that under some circumstances, they may be more than a match for crayfishes; so that the latter might either be driven out of territory they already occupied, as Astacus leptodactylus is driving out A. nobilis in the Russian rivers; or might be prevented from entering rivers already tenanted by their rivals. In connection with this speculation, it is worthy of remark that the area occupied by the fluviatile crabs is very nearly the same as that zone of the earth's surface from which crayfish are excluded, or in which they are scanty. That is to say, they are found in the hotter parts of the eastern side of the two Americas, the West Indies, Africa, Madagascar, Southern Italy, Turkey and Greece, Hindostan, Burmah, China, Japan, and the Sandwich Islands. The large-clawed fluviatile prawns are found in the same regions of America, on both east and west coasts, in Africa, Southern Asia, the Moluccas, and the Philippine Islands; while the Atyidæ not only cover the same area, but reach Japan, extend over Polynesia, to the Sandwich Islands, on the north, and New Zealand, on the south, and are found on both shores of the Mediterranean; a blind form (Troglocaris Schmidtii), in the Adelsberg caves, representing the blind Cambarus of the caves of Kentucky.
The hypothesis respecting the origin of crayfishes
which has been tentatively put forward in the preceding pages, involves the assumption that marine Crustacea of the astacine type were in existence during the deposition of the middle tertiary formations, when the great continents began to assume their present shape. That such was the case there can be no doubt, inasmuch as abundant remains of Crustacea of that type occur still earlier in the mesozoic rocks. They prove the existence of ancient crustaceans, from which the crayfishes may have been derived, at that period of the earth's history when the conformation of the land and sea were such as to admit of their entering the regions in which we now find them. The materials which have, up to the present, time been collected are too scanty to permit of the tracing out of all the details of the genealogy of the crayfish. Nevertheless, the evidence which exists is perfectly clear, as far as it goes, and is in complete accordance with the requirements of the doctrine of evolution.
Mention has been made of the close affinity between the crayfishes and the lobsters--the Astacina and the Homarina; and it fortunately happens that these two groups, which may be included under the common name of the Astacomorpha, are readily distinguishable from all the other Podophthalmia by peculiarities of their exoskeleton which are readily seen in all well-preserved fossils. In all, as in the crayfish, there are large forceps, followed by two pairs of chelate ambulatory limbs, while
the succeeding two pairs of legs are terminated by simple claws. The exopodite of the last abdominal appendage is divided into two parts by a transverse suture. The pleura of the second abdominal somite are larger than the others, and overlap those of the first somite, which are very small. Any fossil crustacean which presents all these characters, is certainly one of the Astacomorpha. The Astacina, again, are distinguished from the Homarina by the mobility of the last thoracic somite, and the characters of the first and second abdominal appendages, when they are present; or by their entire absence. But it is so difficult to make out anything about either of these characters in fossils, that, so far as I am aware, we know nothing about them in any fossil Astacomorph. And hence, it may be impossible to say to which division any given form belongs, unless its resemblances to known types are so minute and so close as to remove doubt.
For the present purpose, the series of the fossiliferous rocks may be grouped as follows:--1. Recent and Quaternary. 2. Newer Tertiary (Pliocene and Miocene). 3. Older Tertiary (Eocene). 4. Cretaceous (Chalk, Greensand and Gault). 5. Wealden. 6. Jurassic (Purbeck to Inferior Oolite). 7. Liassic. 8. Triassic. 9. Permian. 10. Carboniferous. 11. Devonian. 12. Silurian. 1e. Cambrian.
Now the oldest known member of the group of the
decapod Podophthalmia to which the Astacomorpha belong occurs in the Carboniferous formation. It is the genus Anthrapalæmon--a small and very curious crustacean, about which nothing more need be said at present, as it does not appear to have special affinities with the Astacomorpha. In the later formations, up to the top of the Trias, podophthalmatous Crustacea are very rare; and, unless the Triassic genus Pemphix is an exception, no Astacomorphs are known to occur in them. The specimens of Pemphix which I have examined are not sufficiently complete to enable me to express any opinion about them. [Figure 80: Pseudastacus pustulosus]
The case is altered when we reach the Middle Lias. In fact this yields several forms of a genus, Eryma (fig. 80, B), which also occurs in the overlying strata almost up to the top of the Jurassic series, and presents so many variations that nearly forty different species have been recognised. Eryma is, in all respects, an Astacomorph, and so far as can be seen, it differs from the existing genera only in such respects as those in which they differ from one another. Thus it is quite certain that Astacomorphous Crustacea have existed since a period so remote as the older part of the Mesozoic period; and any hesitation in admitting this singular persistency of type on the part of the crayfishes, is at once removed by the consideration of the fact that, along with Eryma, in the Middle Lias, prawn-like Crustacea, generically identical with the existing Penæus, flourished in the sea
and left their remains in the mud of the ancient sea bottom. [Figure 81: Hoploparia longimana]
Eryma is the only crustacean, which can be certainly ascribed to the Astacomorpha, that has hitherto been found in the strata from the Middle Lias to the lithographic slates; which last lie in the upper part of the Jurassic series. In the freshwater beds of the Wealden, no Astacomorpha are known, and although no very great weight is to be attached to a negative fact of this kind, it is, so far, evidence that the Astacomorpha had not yet taken to freshwater life. In the marine deposits of the Cretaceous epoch, however, astacomorphous forms, which
are known by the generic names of Hoploparia and Enoploclytia, are abundant. The differences between these two genera, and between both and Eryma, are altogether insignificant from a broad morphological point of view. They appear to me to be of less importance than those which obtain between the different existing genera of crayfishes.
Hoploparia is found in the London clay. It therefore extends beyond the bounds of the Mesozoic epoch into the older Tertiary. But when this genus is compared with the existing Homarus and Nephrops, it is found partly to resemble the one and partly the other. Thus, on one line, the actual series of forms which have succeeded one another from the Liassic epoch to the present day, is such as must have existed if the common lobster and the Norway lobster are the descendants of Erymoid crustaceans which inhabited the seas of the Liassic epoch.
Side by side with Eryma, in the lithographic slates, there is a genus, Pseudastacus (fig. 80, A), which, as its name implies, has an extraordinarily close resemblance to the crayfishes of the present day. Indeed there is no point of any importance in which (in the absence of any knowledge of the abdominal appendages in the males) it differs from them. On the other hand, in some features, as in the structure of the carapace, it differs from Eryrna, much as the existing crayfishes differ from Nephrops. Thus, in the latter part of the Jurassic epoch, the Astacine
type was already distinct from the Homarine type, though both were marine; and, since Eryma begins at least as early as the Middle Lias, it is possible that Pseudastacus goes back as far, and that the common protastacine form is to be sought in the Trias. Pseudastacus is found in the marine cretaceous rocks of the Lebanon, but has not yet been traced into the Tertiary formations. I am disposed to think that Pseudastacus is comparable to such a form as Astacus nigrescens rather than to any of the Parastacidæ, as I doubt the existence of the latter group at any time in northern latitudes.
In the chalk of Westphalia (also a marine deposit) a single specimen of another Astacomorph has been discovered, which possesses an especial interest as it is a true Astacus (A. politus, Von der Marck and Schlüter), provided with the characteristic transversely divided telson which is found in the majority of the Potamobiidæ.
If we arrange the results of palæontological inquiry which have now been stated in the form of a table such as that which is given on the following page, the significance of the succession of astacomorphous forms, in time, becomes apparent.
SUCCESSIVE FORMS OF THE ASTACOMORPHOUS TYPE.
I. Recent. Potamobiidae. Homarina. Penæus
| | |
II. Later Tertiary Astacus | | |
(Idaho).| | |
| | | |
III. Earlier Tertiary. | | Hoploparia. |
| | |
IV. Cretaceous. Astacus. Pseudastacus. Enoploclytia. Hoploparia. |
\ / |
V. Wealden \ / |
(Fresh water). \ / |
\ / |
VI. Jurassic. Pseudastacus Eryma. Penæus.
| | |
VII. Liassic. | Eryma. Penæus.
VIII. Triassic.
IX. Permian
X. Carboniferous Anthrapalæmon
XI. Devonian.
XII. Silurian.
XIII. Cambrian.
If an Astacomorphous crustacean, having characters intermediate between those of Eryma and those of Pseudastacus, existed in the Triassic epoch or earlier; if it gradually diverged into Pseudastacine and Erymoid forms; if these again took on Astacine and Homarine
characters, and finally ended in the existing Potamobiidæ and Homarina, the fossil forms left in the track of this process of evolution would be very much what they actually are. Up to the end of the Mesozoic epoch the only known Potamobiidæ are marine animals. And we have already seen that the facts of distribution suggest the hypothesis that they must have been so, at least up to this time. Thus, with respect to the Ætiology of the crayfishes, all the known facts are in harmony with the requirements of the hypothesis that they have been gradually evolved in the course of the Mesozoic and subsequent epochs of the world's history from a primitive Astacomorphous form.
And it is well to reflect that the only alternative supposition is, that these numerous successive and coexistent forms of insignificant animals, the differences of which require careful study for their discrimination, have been separately and independently fabricated, and put into the localities in which we find them. By whatever verbal fog the question at issue may be hidden, this is the real nature of the dilemma presented to us not only by the crayfish, but by every animal and by every plant; from man to the humblest animalcule; from the spreading beech and towering pine to the Micrococci which lie at the limit of microscopic visibility.
______[Author's Notes to Chapter 6]
[Note 1]: Moore. Magazine of Natural History. New Series, III., 1839.
[Note 2]: Thompson. Annals and Magazine of Natural History, XI., 1843.
[Note 3]: "Histoire Naturelle des Crustacés."
[Note 4]: Carbonnier. "L'Écrevisse," p. 8.
[Note 5]: "Ueber die Flusskrebse Europas." Mém. de 1'Acad. de St. Petersburg, 1859.
[Note 6]: According to strict zoological usage the names should he written A. fluviatilis: (var. torrentium) and A. fluviatilis (var. nobilis) on the hypothesis that the stone crayfish and the noble crayfish are varieties; and A. torrentium and A. fluviatilis on the hypothesis that they are species; but as I neither wish to prejudge the species question, nor to employ cumbrously long names, I take a third course.
[Note 7]: See Bell. "British Stalk-eyed Crustacea," p. 237.
[Note 8]: Since the statement respecting the occurrence of crayfishes in Spain on p. 44 was printed.
[Note 9]: "Die Crustaceen des Südlichen Europas," 1863.
[Note 10]: "Die Russischen Flusskrebse." Bulletin de la Société Impériale des Naturalistes de Moscow, 1874.
[Note 11]: These statements rest on the authority of Kessler and Gerstfeldt, in their memoirs already cited.
[Note 12]: Kessler (Die Russischen Flusskrebse, l. c. p. 369-70), has an interesting discussion of this question.
[Note 13]: It would be hazardous, however, to assume that none exist, especially in the Oxus, which formerly flowed into the Caspian.
[Note 14]: A. dauricus and A. Schrenckii.
[Note 15]: Whatever the so-called Astacus capensis of the Cape Colony may be, it is certainly not a crayfish.
[Note 16]: Dr. Hagen in his "Monograph of the North American Astacidæ," enumerates six species; A. Gambelii, A. klamathensis, A. leenisculus, A. nigrescens, A. oreganus, and A. Trowbridgii.
[Note 17]: Von Martens. Cambarus cubensis. Archiv. für Naturgeschichte, xxxviii.
[Note 18]: Sübrasilische Süss- und Brackwasser Crustaceen, nach den Sammlungen des Dr. Reinh. Hensel. Ariv. für Naturgeschichte, xxxv. 1869.
[Note 19]: The nomenclature of the Australian crayfishes requires thorough revision. I therefore, for the present, assign no name to this crayfish. It is probably identical with the A. nobilis of Dana and the A. armatus of Von Martens.
[Note 20]: Kessler, l. c.
[Note 21]: According to Dr. Günther their southern range is similarly limited by the Asiatic Highlands. But they abound in the rivers both of the old and new worlds which flow into the Arctic sea; and though those on the western side of the Rocky Mountains are different from the Eastern American forms, yet there are species common to both the Asiatic and the American coasts of the North Pacific.
[Note 22]: On three extinct Astaci from the freshwater Tertiary of Idaho. Proceedings of the American Philosophical Society, 1869-70.
[Note 23]: Neue Fische und Krebse aus der Kreide von Westphalen. Palæontographica, Bd. XV., p.302; tab. XLIV., figs; 4 and 5.
[Note 24]: "Untersuchungen ueber die Klimate der Gegenwart und der Vorwelt." Natuurkundige Verhandelingen van de Hollandsche Maatschappij der Wetenschappen te Haarlem, 1865.
[Note 25]: See on this interesting subject: Martens, "On the occurrence of marine animal forms in fresh water." Annals of Natural History, 1858; Lovèn. "Ueber enige im Wetter und Wener See gefundene Crustaceen." Halle Zeitschrift für die Gesammten Wissenschaften, xix., 1862; G. O. Sars, "Histoire Naturelle des Crustacés d'eau douce de Norvège," 1867.
[Note 26]: Heller, "Die Crustaceen des südlichen Europas," p. 259. Klunzinger, "Ueber eine Süsswasser-crusttacee im Nil," with the notes by von Martens und von Siebold: Zeitschrift für Wissenschaftliche Zoologie, 1866.
[Note 27]: This seems actually to have happened in the case of the widely-spread allies and companions of the fluviatile prawns, Atya and Caridina. I am not aware that truly marine species of these genera are known.
[Note 28]: Just as there is an American form of Idothea and an Asiatic form in the Arctic ocean at the present day.
[Note 29]: But it must be remembered that we have as yet everything to learn respecting the fauna of the great inland lakes and river systems of South Africa.
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