VARIATION. 29 



given species in nature is found to be' pure, must be materially 

 useful. The theory does not show the way, in which a species 

 may become pure for any quality which confers no advantage 

 to the organisms which carry it. Even if we limit ourselves to 

 domestic species, where selection has certainly been one of the 

 main factors in the reduction of variability, we can explain 

 how the white Wyandotte became pure for its white colour 

 and yellow legs, and how the silver Wyandotte became pure 

 for its peculiar feather-marking, but the explanation fails us if 

 we enquire into the reasons which have made the white Wyan- 

 dotte pure for a brown colour of its eggs, and why the sUver 

 Wyandotte always lays a pinkish egg with minute white dots, 

 and another Wyandotte a white egg. 



In the second place, the theory of natural selection, as pro- 

 posed, by Darwin and elaborated by Weismann, does explain 

 the way in which species may be thought to change if they 

 vary, but it gives no explanation of the causes underlying this 

 variation. 



If we start from the assumption that a number of animals 

 vary in height between four and ten, it is obvious enough that, 

 if the tallest animals have a decided advantage, in a short time 

 only animals of grade ten may survive. But in the first place, 

 this does not explain why the animals did vary between four 

 and ten; and further, it does not make it probable that the 

 survival of animals of grade ten texclusively implies a variation 

 of their descendants between, say, eight and twelve. 



It can be seen how, given a certain small variation, this can 

 be reduced by a suitable selection, and how ultimately the 

 group can come to consist of extremes only. And this does not 

 pre-suppose any knowledge about the causes of variation. But 

 it remains a mystery why variation should continue, and how, 

 as the result of the selection, it can now exceed the former 

 limits. 



No matter from which angle we look at the problem, we 

 must see that the first question as to evolution has to be an 

 inquiry into the causes of hereditable variation, and only if we 



