346 DISTRIBUTION AND ATIOLOGY OF THE CRAYFISHES. 
characters, and finally ended in the existing Potamobiide 
and Homarina, the fossil forms left in the track of this 
process of evolution would be very much what they 
actually are. Up to the end of the Mesozoic epoch 
the only known Potamobiide are marine animals. And 
we have already seen that the facts of distribution 
suggest the hypothesis that they must have been so, 
at least up to this time. 
Thus, with respect to the Attiology of the crayfishes, 
all the known facts are in harmony with the requirements 
of the hypothesis that they have been gradually evolved 
in the course of the Mesozoic and subsequent epochs 
of the world’s history from a primitive Astacomorphous 
form. 
And it is well to reflect that the only alternative sup- 
position is, that these numerous successive and coexistent 
forms of insignificant animals, the differences of which 
require careful study for their discrimination, have been 
separately and independently fabricated, and put into the 
localities in which we find them. By whatever verbal fog 
the question at issue may be hidden, ihis is the real 
nature of the dilemma presented to us not only by the 
crayfish, but by every animal and by every plant; from 
man to the humblest animalcule; from the spreading 
beech and towering pine to the Micrococci which lie at 
the limit of mieroscopic visibility. 
