CHAP. XL EVOLUTION. L. 



(See above, the remarks on the males of the Amphi- 

 poda.) Some variations, indeed, from their very nature, 

 can only occur when the young animal has attained the 

 adult stage of development. Thus the Sea Caterpillars 

 (Polynoe) at first possess only a few body-segments, 

 which, during development, gradually increase to a 

 number which is different in different species, but con- 

 stant in the same species ; now before a young animal 

 could exceed the number of segments of its parents, it 

 must of course have attained that number. We may 

 assume a similar supplementary progress wherever the 

 deviation of the descendants consists in an addition of 

 new segments and limbs. 



Descendants therefore reach a new goal, either ~by de- 

 viating sooner or later whilst still on the way towards the 

 form of their parents, or ~by passing along this course 

 without deviation, hut then, instead of standing still, ad-\ 

 vance still farther. 



The former mode will have had a predominant action 

 where the posterity of common ancestors constitutes a 

 group of forms standing upon the same level in es- 

 sential features, as the whole of the Amphipoda, Crabs, 

 or Birds. On the other hand we are led to the as- 

 sumption of the second mode of progress, when we seek 

 to deduce from a common original form, animals some 

 of which agree with young states of others. 



In the former case the developmental history of the 

 descendants can only agree with that of their ancestors 

 up to a certain point at which their courses separate, 

 as to their structure in the adult state it will teach us 



