ORGANIC EVOLUTION 



263 





IV. Organs developed in both, but better developed in 



adult. 



V. Organs rudimentary or absent in young, and func- 

 tional only in adult. 



Progress in regres- 

 sion. — There is an im- 

 portant sense in which 

 all regressive develop- 

 ment spells progress. 

 One must take into ac- 

 count the whole man- 



%kl^ ^ ^ ^^^ ^^^ ^^ ^^^^ ^^ ^^^ organ- 



3^1 jj^ iJjiM^ ism to comprehend this. 



^S^jT ■\nS?" Even the most abject 



^ X \^ ^ ^ parasite, losing all or- 



_^ k k k^lK gans for independent 



^7^"*^ V ^NraQ^^ existence, is advancing 



in its own peculiar way 

 of getting on in the 

 world. 



There is also a sense 

 in which regressive 

 development is to be 

 considered a part of 

 the normal life of 

 an individual. As nutritive and reproductive functions 

 come successively into dominence in the lifetime of every 

 organism, so a retrograde development of nutritive organs 

 may begin with the taking up of the labor of reproduc- 

 tion. This is well illustrated by the common rag weed. 

 The leaves shown in figure 161 were developed at differ- 

 ent periods of the life of a single plant. They are divid- 

 ed into two series, which parallel the wax and wane of 

 vegetative vigor in the plant. The second series is the one 



b 



Fig. 161. Leaves of the ragweed {Ambrosia 

 artemiscrfolia) . a, cotyledons; b to e, 

 leaves successively formed in youth, ni, 

 the mature leaf form; n to 5, the dimin- 

 ishing series of leaves successively formed 

 during the period of seed production; z, 

 a fruiting tip. 



