346 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



characters, and finally ended in the existing Potamobiida 

 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 Potamobiidae 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 ^Etiology 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, this 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 microscopic visibility. 



