346 DISTEIBUTION AND ^ETIOLOGY OF THE CRAYFISHES. 



characters, and finally ended in the existing PotamohiidcB 

 and Homarina, the fossil forms left in the track of this 

 process of evolution -would be very much what they 

 actuall}' are. Up to the end of the Mesozoic epoch 

 the only known Potamohiida are marine animals. And 

 we have ah-eady seen that the facts of distribution 

 suggest the hj^Dothesis that they must have been so, 

 at least up to this time. 



Thus, with resj)ect to the ^tiologj^ of the crayfishes, 

 all the known facts are in harmonj' with the requirements 

 of the hj'pothesis that they have been gradually evolved 

 in the course of the Mesozoic and subsequent epochs 

 of the world's histor}'^ from a primitive Astacomoi-phous 

 form. 



And it is well to reflect that the onh^ alternative sup- 

 position is, that these numerous successive and coexistent 

 forms of insignificant animals, the differences of which 

 require carefid study for their discrimination, have been 

 separately and independently fabricated, and put into tffe 

 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 sjireading 

 beech and towering pine to the Micrococci which lie at 

 the limit of microscopic visibility. 



