Chapter VI (1)

THE DISTRIBUTION AND THE ÆTIOLOGY OF THE CRAYFISHES.

So 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;[15] 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^cIntosh, 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;[16] but the question whether their diffusion, and even their introduction into this {289} island, has or has not been effected by artificial means, is involved in some obscurity.

[15] Moore. Magazine of Natural History. New Series, III., 1839.

[16] Thompson. Annals and Magazine of Natural History, XI., 1843.

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_,[17] 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”[18] 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.

[17] “Histoire Naturelle des Crustacés.”

[18] Carbonnier. “L’Écrevisse,” p. 8.

In Germany, the two forms have long been popularly distinguished, the former by the name of “Steinkrebs,” {290} 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 “stone-crayfishes” have been further distinguished as the species _Astacus saxatilis_, _A. tristis_, _A. pallipes_, _A. fontinalis_, &c. On the other hand, Dr. Gerstfeldt,[19] 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.” {291}

[19] “Ueber die Flusskrebse Europas.” Mém. de l’Acad. de St.
Petersburg, 1859.

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 {292} 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 develop the race A + _x_; 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 + _x_ and A will be filled up by a series of forms in which _x_ 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 + _n_, although, _primâ facie_, they are of distinct species; yet if a large collection shows us that the interval between A and A + _n_ is filled up by forms of A having traces of _n_, and forms of A + _n_ in which _n_ becomes less and less, then it will be {293} concluded that A and A + _n_ are races of one species and not separate species. And this conclusion will be fortified if A and A + _n_ occupy different stations in the same geographical area.

Even when no transitional forms between A and A + _n_ are discoverable, if _n_ is a small and unimportant difference, such as of average size, colour, or ornamentation, it may be fairly held that A and A + _n_ 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 case of the crayfish is this: {294} 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 {295} 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 {296} 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_.[20]

[20] According to strict zoological usage the names should
be 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.

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 {297} 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.

* * * * *

As I have already remarked, the very numerous {298} 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.[21] 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[22] 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[23] 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,[24] it does not occur within the limits of the Russian empire. {299}

[21] See Bell. “British Stalk-eyed Crustacea,” p. 237.

[22] Since the statement respecting the occurrence of crayfishes
in Spain on p. 44 was printed.

[23] “Die Crustaceen des Südlichen Europas,” 1863.

[24] “Die Russischen Flusskrebse.” Bulletin de la Société
Impériale des Naturalistes de Moscow, 1874.

_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° 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 {300} 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.

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.[25]

[25] These statements rest on the authority of Kessler and
Gerstfeldt, in their memoirs already cited.

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 {302} 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.[26]

[26] Kessler (Die Russischen Flusskrebse, l. c. p. 369–70), has
an interesting discussion of this question.

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 {303} 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 {304} 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[27] 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[28], 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.[29] {305}

[27] It would be hazardous, however, to assume that none exist,
especially in the Oxus, which formerly flowed into the Caspian.

[28] _A. dauricus_ and _A. Schrenckii_.

[29] Whatever the so-called _Astacus capensis_ of the Cape
Colony may be, it is certainly not a crayfish.

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,[30] 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,[31] 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_ {306} is found, along with other blind animals, in the subterranean caves of Kentucky.

[30] 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_.

[31] Von Martens. _Cambarus cubensis._ Archiv. für
Naturgeschichte, xxxviii.

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,[32] 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° S. Latitude, close to the mouth of the Jacuhy, at the north end of the great Laguna do Patos, which {308} 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.

[32] Südbrasilische Süss- und Brackwasser Crustaceen, nach den
Sammlungen des Dr. Reinh. Hensel. Archiv. für Naturgeschichte,
XXXV. 1869.

]

[33] 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.

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 {310} 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.

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 {311} 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 {312} sternum is elevated into a transverse prominence, on the posterior face of which there is a pit or depression.[34]

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.”

[34] Kessler, l. c.

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 {313} 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 _Parastacidcæ_; 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 {314} 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 in 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 {315} 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,[35] 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.

[35] 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.

Thus, these fish avoid South Africa, as the crayfishes {316} 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[36] found, in association with _Mastodon mirificus_, and _Equus excelsus_, several species, which he considers to be distinct from {317} 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[37] 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.

[36] On three extinct _Astaci_ from the freshwater Tertiary
of Idaho. Proceedings of the American Philosophical Society,
1869–70.

[37] Neue Fische und Krebse aus der Kreide von Westphalen.
Palæontographica, Bd. XV., p. 302; tab. XLIV., figs. 4 and 5.

* * * * *

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 {318} 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 {319} 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 {320} 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 crayfishes 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 {321} 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 {322} has led Sartorius von Waltershausen[38] 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.

[38] “Untersuchungen ueber die Klimate der Gegenwart und der
Vorwelt.” Natuurkundige Verhandelingen van de Hollandsche
Maatschappij der Wetenschappen te Haarlem, 1865.

The extremely severe climatal conditions which obtain in northern Siberia may sufficiently account for the {323} 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 {324} 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 table-land 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 {325} 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 {326} 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 crayfishes, 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, {327} so that they are no longer absolutely identical with those descendants of their ancestors which have continued to live in the sea.[39]

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.