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THE AMERICAN NATURALIST [Vol. XLVII 



There are two sets of factors in producing and deter- 

 mining the direction of evolution, those within the organ- 

 ism, and those external to it. In earlier studies of evo- 

 lution the external factors received by far the most atten- 

 tion, and only recently have the internal factors begun to 

 receive the thought they deserve. Of the external factors, 

 natural selection is the one for which the largest claims 

 have been made and the one whose claims have been most 

 cordially allowed. I think, however, there are few, if any, 

 of us who do not feel that some of the advocates of nat- 

 ural selection have gone too far. Wallace, for example, 

 would not allow that there are any characters of animals 

 or plants which are not useful to their possessors and have 

 not been perpetuated and emphasized because of this use- 

 fulness. Admitting, as we must, that we know but little 

 of the intimate life of organisms, and that the use of 

 many really most useful characters may fail to impress 

 us only because of our ignorance of real conditions, still 

 I think most of us feel not only that the claim that all 

 structures and qualities of organisms are useful is an 

 exaggerated claim, but that very many characteristics 

 are either of indifferent quality, or are so slightly useful 

 as not to be of selection value, or even are slightly dis- 

 advantageous. The more closely one studies any organ- 

 ism the more will he become impressed with the number 

 of these non-useful or doubtfully useful qualities, and 

 close and careful students are likely to find their vision 

 of natural selection grow dim, like the pilgrim who, in the 

 midst of the woods, could not see the forest for the trees. 



But as we view organic nature in its wider aspects is 

 there any other feature so prominent as the adaptation 

 of organisms to their environment and to the lives they 

 must live in the midst of this environment? However 

 many details of structure or behavior may fail to show 

 their utility, still it remains true that there is no phenom- 

 enon of organic nature more impressive than adaptation. 



In our study of evolution we have, then, this thing to 

 explain— namely, the universal prevalence of a high 

 degree of adaptation of organisms in habit, function and 

 structure, to their environment, and yet the presence of 



