ORGANIC EVOLUTION 



263 



IV. Organs developed in both, but better developed in 

 adult. 



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

 al 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- 

 ner of life of the organ- 

 ism to comprehend this. 

 Even the most abject 

 parasite, losing all or- 

 gans for independent 

 existence, is advancing 

 in its own peculiar way 

 of getting on in the 

 world. 



There is also a sense 



FIG. 161. Leaves of the rag weed (Ambrosia ,.,| 1 : r ,U rpcrrpcci-i'o 



aitemissfolia). a, cotyledons; b to e, m WniCH regreSSl\6 



leaves successively formed in youth, m, rlpAT-plnnmp-nt ic tn KP> 



the mature leaf form ; M to 5, the dimin- development IS tO DC 



ishing series of leaves successively formed r-nncirlprprl 3 n p r t of 



during the period of seed production; a' Considered apart 



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 



