That which is conceivable, however, is by no means necessarily true; and no amount of purely morphological evidence can suffice to prove that the forms of life have come into existence in one way rather than another.
There is a common plan among churches, no less than {287} among crayfishes; nevertheless the churches have certainly not been developed from a common ancestor, but have been built separately. Whether the different kinds of crayfishes have been built separately, is a problem we shall not be in a position to grapple with, until we have considered a series of facts connected with them, which have not yet been touched upon.
CHAPTER VI. 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. McIntosh, 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.”