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188 HTBEIDISM. CHAp<XIX> 



with terrestrial animals, that this is the case ; as most males and 

 females pair for each birth, and some few for life. 



On the whole we may conclude that with animals the sterility 

 of crossed species has not been slowly augmented through 



natural selection ; and as this sterility follows the same general 



laws in the vegetable as in the animal kingdom, it is improbabl 

 though apparently possible, that with plants crossed species should 

 have been rendered sterile by a different process. From this 

 consideration, and remembering that species which have never 

 co- existed in the same country, and which therefore could not 

 have received any advantage from having been rendered mu- 

 tually infertile, yet are generally sterile when crossed; aud 

 bearing in mind that in reciprocal crosses between the same 

 two species there is sometimes the widest difference in their 

 sterility, we must give up the belief that natural selection has 

 come into play. 



As species have not been rendered mutually infertile through 

 the accumulative action of natural selection, and as we may 

 safely conclude, from the previous as well as from other and 

 more general considerations, that they have not been endowed 

 through an act of creation with this quality, we must infer 

 that it has arisen incidentally during their slow formation in 

 connection with other and unknown changes in their organisa- 

 tion. By a quality arising incidentally, I refer to such cases 

 as different species of animals and plants being differently 



affected by poisons to which they are not naturally exposed, 

 and this difference in susceptibility is clearly incidental on 

 other and unknown differences in their organisation. So again 

 the capacity in different kinds of trees to be grafted on each 

 other, or on a third species, differs much, and is of no advantage 

 to these trees, but is incidental on structural or functional dif- 

 ferences in their woody tissues. We need not feel surprise at 

 sterility incidentally resulting from crosses between distinct 

 species,— the modified descendants of a common proge] 

 when we bear in mind how easily the reproductive system is 

 affected by various causes— often by extremely slight" changes 

 in the conditions of life, by too close interbreeding, and by 

 other agencies. It is well to bear in mind such cases, as that 

 of the Passiflora alata, which recovered its self-fertility from 



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