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.