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THE AMERICAN NATURALIST [Vol. LITI 



in the sense of a description) of how evolution has come 

 to pasg. For many years past, I have been endeavoring 

 to weigh the evidence for and against both of these hy- 

 potheses and I have reached the same verdict with re- 

 spect to the two : each is both proved and disproved. It 

 is not that adequate evidence is lacking, as some assume. 

 Rather, in each case, is the evidence well-nigh over- 

 whelming— oh both sides. 



Now, obviously, no single proposition can be both true 

 and untrue at the same time. What is meant here is 

 this. I believe the selection of virtually continuous vari- 

 ations and the inheritance of functional and environ- 

 mental modifications to have both played some part in 

 evolution. And I do not hesitate to say that the evi- 

 dence in favor of such a view is of the same general char- 

 acter as the evidence for the evolution theory itself, and 

 nearly as convincing. 



On the other hand, it seems no less probable that the 

 operation of each of these factors is strictly limited. In- 

 deed, it would appear likely that much of the adaptive- 

 ness in nature is not adequately accounted for by either 

 process or by both taken together. There may well be 

 other factors the existence of which is as little suspected 

 to-day as was that of natural selection before the time of 

 Darwin and Wallace. 



But will our explanations remain purely naturalistic, 

 or will they find room for extra-natural directive agents, 

 by whatever name called? Will they, like the two chief 

 historic theories, base themselves on tlie contingency of 

 every adaptive variation in structure or function, ante- 

 , cedent to the test of experience, or will they be forced to 

 concede a primary adaptiveness iMlicrcnt in living 

 matter. 



Many of those who admit the widcsjin ad occuni'nco 

 of natural selection as a process, are wont to deny to it 

 any explanatory value. To quote a now familiar saying, 

 it is said that the survival of the fittest does not account 

 for the origin of fitness. The real cause of modification, 

 these writers insist, is to be sought in the process by 



