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.