318 DISTRIBUTION AND TIOLOGY OF THE CRAYFISHES, 
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 cf Evolution. And there are two 
forms of the latter hypothesis; for, it may be assumed, 
on the one hand, that crayfishes have come into exist- 
ence, 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 incom- 
petent to cause the existence of the thing in question. 
But such evidence is out of our reach. The most that 
