74 THE FOUR SEGREGATIVE PRINCIPLES. 
they appear. The American cowbird and the English cuckoo seem 
to have entirely lost the instinct. Again, the physiological power of 
the mammalian mother to provide milk for her young seems to be 
gradually 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.—lt 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 survival of species in one period, it must have significance in the 
origin of species in the periods that follow. The sinistral mutations 
