68 ZOOLOGY 



with short stamens and a tall pistil. If pollen be taken from 

 the short stamens and placed on the short pistil, many 

 vigorous seeds will be produced; and the same result will 

 follow if pollen from long stamens be placed on a long pistil. 

 But if pollen from short stamens be placed on a long pistil, 

 or if pollen from long stamens be applied to a short pistil, 

 then few and feeble seeds are produced : the result is therefore 

 exactly the same as if two distinct species had been crossed. 

 Darwin also showed that whereas in the majority of cases 

 two distinct species were absolutely sterile when crossed with 

 one another, yet this is by no means always the case ; that 

 in fact sterility is a question of degree, and that it is to be 

 regarded as a concomitant of divergence of structure. 



Darwin's teaching evoked the most violent opposition 

 because it was in sharp contrast with previous views on the 

 Universe, but amongst all the opponents of his views no 

 one could gainsay the fact of the tremendous slaughter which 

 goes on amongst wild animals ; this fact was previously not 

 denied, but conveniently ignored, and no doubt much irritation 

 was caused by the manner in which this neglected fact was 

 forced into the foreground of the controversy. The sole 

 question at issue, then, is whether this slaughter subserves 

 the welfare of the species or not, and most people will on 

 reflection be inclined to welcome Darwin's view that it sub- 

 serves a useful end. 



It is obvious that Darwin's theory leaves entirely open the 

 question of what causes those differences between individuals 

 out of which natural selection gradually piles up the divergences 

 of structure which separate species from species and genus 

 from genus. These differences are summed up under the 

 general title of Variation. In a later but less well-known 

 work, entitled The .Variation of Animals and Plants under 

 Domestication, Darwin collected together all the evidence he 

 could muster which could throw light on the origin of varia- 

 tions. This work is the grandest which he produced ; to 

 compile it was really the goal of his life, and the Origin of 

 Species was intended by him to be merely a preliminary 

 account of his work which the urgency of his friends forced 

 him into writing, when it appeared probable that his ideas 

 would be anticipated by another naturalist. Unfortunately 



