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THE AMERICAN NATURALIST [Vol. XL VII 



action of these two great factors — progressive fixation, 

 which is ever tending to make characters constant and 

 to decrease variabilih ; and natural selection, which 

 operates in eliminating individuals which have become 

 too rigid in their vitally essential features, and thus in 

 encouraging those which display superior adaptability — 

 is at least helpful in presenting a clear picture of the 

 process of evolution. 



The marked conservatism which we have noticed in 

 particular structures or organs may perhaps be explained 

 in a similar way as due to their comparative unimportance 

 in the economy of the individual. The fact that the repro- 

 ductive organs in all plants and in many animals are 

 especially conservative may possibly be taken to indicate 

 that the particular method of reproduction is of less vital 

 concern to the race than are its other activities. The 

 conservatism of other structures, such as the root, is evi- 

 dently due to the comparative constancy of their sur- 

 roundings. Internal structures in general are apt to be 

 more conservative than external ones because of their 

 exposure to a less varied environment. 



Various attempts have been made to explain those phe- 

 nomena of conservatism which have been grouped under 

 the head of recapitulation. De Vries has maintained that 

 the seedling characters of plants are just as dependent on 

 the action of natural selection as are those of the adult 

 and that ancient features persist in youth only when they 

 happen to be of survival value for the early stages of the 

 plant. The same position has sometimes been maintained 

 on the zoological side. To attribute functional importance 

 •to all embryological characters, however, and to explain 

 the numberless cases where there is close correspondence 

 between ontogeny and ancestry as due simply to the opera- 

 tion of natural selection, is to burden that hypothesis 

 beyond all necessity. 



The theory of formative stimuli, which explains the 

 persistence of structures in the embryo of animals on the 

 assumption that their presence is absolutely necessary as 



