THE MORPHOLOGICAL UNITY OF LIVING THINGS. 287 
among crayfishes; nevertheless the churches have cer- 
tainly 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. 
