Problems and Concepts of Evolution 13 



individuals are the fortunate possessors of these advantageous modifications. 

 Because individuals do survive, it is the custom to assume on the basis of 

 Darwin's hypothesis that they are better able to meet the conditions of life than 

 those which were eliminated, but an accurate analysis and measurement of this 

 problem is utterly lacking, nor can it be attacked, excepting through a long- 

 continued, painstaking analytical investigation upon favorable materials in the 

 laboratory and in nature. 



If it proved to be true on adequate investigation that the elimination in 

 natural populations is entirely a matter, or even largely a matter, of the chance 

 position of the individuals when the accidents of life occur, and that the sur- 

 vivors are on the average no better provided with equipment for life than those 

 which are eliminated, then Darwin's hypothesis as a means of transmutation 

 and species formation in nature must be abandoned, or at the least assigned a 

 minor position as a productive cause of diversity in nature. 



It is not worth while at this place to enter into a lengthy discussion of Dar- 

 win's hypothesis. There has been too much discussion and too little investiga- 

 tion ; but one aspect needs attention at this place, namely, the idea advanced by 

 many, perhaps most vividly insisted upon by De Vries, that " natural selection " 

 acts solely as a sieve which allows certain individuals to persist, while others are 

 eliminated thereby. This " sieve-like action," however, seems to me to be one 

 quite different from that which Darwin conceived of in his hypothesis. There 

 is unquestionably in nature an action of the conditions of the environment which 

 in the main eliminates extremes in any population, and further, under adverse 

 conditions it is the mediocre in the race which has the best chance of survival. 

 This seems to be a fairly well substantiated generalization, as far as evidence 

 at the present moment goes, and in my o\vn work I have frequently found that 

 this principle was actively operating when populations were subjected to hostile 

 conditions. This action would have a tendency of conservation, to hold the 

 population in a stable condition, and distinctly would not have the action which 

 natural selection was supposed to possess by Darwin — one of producing diver- 

 gence. It is possible that two operations have been confused, and that there 

 should be recognized the possibility of there being in nature a " natural selective 

 process," which might be productive of the results which Darwin supposed to 

 be the basis of the origin of species; on the other hand, there are clearly in 

 nature operations which do exactly the things which De Vries attributes to 

 natural selection. These latter I have often encountered in my work, and in 

 this report I have adopted the name " factors of conservation," meaning thereby 

 an association of agencies within and without the organisms, which, so long as 

 they remain constant, maintain the race in uniformity. These are agencies 

 acting for the retention of specific form, habitat, and distribution, but they are 

 distinctly not formative forces in the sense that they produce anything new. 

 Their operation is the reverse, and hence their name. In portions of these 

 reports I shall show how experiments with these two aspects of the problem 

 operate in some instances, and especially in relation to the two questions which 

 have been raised with regard to Darwin's hypothesis — whether it is chance 

 favorable constitution or chance favorahle position that determines the survival 

 of the individual. 



Through inability to make progress with the neo-Darwinian concepts, many 

 earnest students of the evolution problems, in the latter part of the nineteenth 



