THE MORPHOLOGICAL UNITY OF LIVING THINGS. 287 



among crayfishes ; nevertheless the churches have cer- 

 tainty not been developed from a common ancestor, but 

 have been built separately. Whether the different kinds 

 of crayfishes have been built separately, is a problem we 

 shall not be in a position to grapple with, until we have 

 considered a series of facts connected with them, which 

 have not yet been touched upon. 



