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. 
