184 DARWINISM chap. 



chance of survival in the battle of life. It is, therefore, 

 directly under the control of natural selection, which acts 

 both by the self-preservation of fertile and the self-destruction 

 of infertile stocks — except always where -correlated as above, 

 when they become useful, and therefore subject to be increased 

 by natural selection. 



Sumwiary and Concluding Remarks on Hyhridity. 



The facts which are of the greatest importance to a com- 

 prehension of this very difficult subject are those which show 

 the extreme susceptibility of the reproductive system both in 

 plants and animals. We have seen how both these classes of 

 organisms may be rendered infertile, by a change of conditions 

 which does not affect their general health, by captivity, or 

 by too close interbreeding. AVe have seen, also, that infertility 

 is frequently correlated with a difference of colour, or with other 

 characters ; that it is not proportionate to divergence of 

 structure ; that it varies in reciprocal crosses between pairs of 

 the same species ; while in the cases of dimorphic and tri- 

 morphic plants the different crosses between the same pair 

 of individuals may be fertile or sterile at the same time. It 

 appears as if fertility depended on such a delicate adjustment 

 of the male and female elements to each other, that, unless 

 constantly kejit up by the preservation of the most fertile 

 individuals, sterility is always liable to arise. This preservation 

 always occurs within the limits of each species, both because 

 fertility is of the highest importance to the continuance of the 

 race, and also because sterility (and to a less extent infertility) 

 is self-destructive as well as injurious to the species. 



So long therefore as a species remains undivided, and in 

 occupation of a continuous area, its fertility is kept up by 

 natural selection ; but the moment it becomes separated, 

 either by geographical or selective isolation, or by diversity 

 of station or of habits, then, while each portion must be kept 

 fertile inter se, there is nothing to prevent infertility arising 

 between the two separated portions. As the two |)ortions 

 will necessarily exist under somcAvhat different conditions of 

 life, and will usually have acquired some diversity of form and 

 colour --both which circumstances we know to be either the 

 cause of infertility or to be correlated with it, — the fact of 



