EVOLUTION 



13 



do not appear to be again productive of more generalized 

 groups. In all lines of descent specialized forms do not 

 appear to regain by regression or degeneration the potential 

 characters of their ancestral condition. A generalized form 

 is like potter's clay, plastic in the hands of nature, readily 

 to be converted into a needed kind of cup or vase ; but 

 when thus specialized may never resume unaltered its 

 ancestral condition : the clay survives ; the cup perishes. 



