74 THE POUR SEGREGATIVE PRINCIPLES. 



they appeal-. The American cowbird and the English cuckoo seem 

 to have entirely losl the instinct. Again, the physiological powerof 

 the mammalian mother to provide milk for her young seems to be 

 idually declining in the human species, through the survival and 

 propagation of the children of mothers presenting individual varia- 

 tions below the standard that for countless generations was necessary 

 in order to leave descendants. If statistical investigation should 

 show that lack of sufficient milk for their young is most common in 

 mothers belonging to communities that have for the longest time, and 

 most successfully, met every deficiency of this kind, it would be a 

 strong indication that the accumulation of individual variations can 

 not be overlooked in a complete theory of the factors of evolution. 



19. Degeneration in Eyesight and its Lessons. 



Another example of a similar kind is found in the power of sight in 

 mankind. I believe it is fully recognized that in civilized races the 

 proportion of individuals with defective sight is much greater than in 

 savage races; and the best explanation that has been given is found 

 in the equally certain fact that with civilized man the standard of 

 sight necessary for individual survival has been reduced to zero, and 

 the standard necessary for attaining the highest prosperity and the 

 fullest share in the propagation of the race is far below that which is 

 necessary among savages. Is there any reason to doubt that the dif- 

 ference in the average inherited power of vision in the two cases is 

 due to the fact that for many generations individual savages with 

 deficient sight have had less opportunity for leaving descendants 

 than have individuals with the same deficiency belonging to civilized 

 races? Without the selection of individual variations in the primitive 

 races of man, the power of these races would have fallen so low that 

 the species would have been exterminated in its conflict with other 

 species. But the survival of man, due to this selection of individual 

 variations, is in no small degree determining what other species shall 

 survive; and the determination of the species that survive controls 

 the types of the mutations that from time to time appear, and would, 

 therefore, control the types of future species, even if every such new 

 form must find its origin in a mutation. 



First Lesson. — It therefore seems to be shown that the accumula- 

 tion of individual variations by selective generation has had, and 

 must continue to have, a profound influence on the course of evolu- 

 tion; for, if the selection of individual variations has significance in 

 the sia rival of species in one period, it must have significance in the 

 origin of species in the periods that follow. The sinistral mutations 



