Darwin s Artificial and Natural Selection 147 



Sterility between Species 



The care with which Darwin examined every bearing of 

 his theory is nowhere better exemplified than in his treat- 

 ment of the question of sterility between the individuals of 

 different species. It would be so obviously to the advantage 

 of the selection theory if it were true that sterility between 

 species had been acquired by selection in order to prevent 

 intercrossing, that it would have been easy for a less cautious 

 thinker to have fallen into the error of supposing that sterility 

 might have been acquired in this way. Tempting as such a 

 view appears, Darwin was not caught by the specious argu- 

 ment, as the opening sentence in the chapter of hybridism 

 shows : — 



" The view commonly entertained by naturalists is that 

 species, when intercrossed, have been specially endowed with 

 sterility, in order to prevent their confusion. This view 

 certainly seems at first highly probable, for species living 

 together could hardly have been kept distinct had they been 

 capable of freely crossing. The subject is in many ways 

 important for us, more especially as the sterility of species 

 when first crossed, and that of their hybrid offspring, cannot 

 have been acquired, as I shall show, by the preservation of 

 successive profitable degrees of sterility. It is an incidental 

 result of differences in the reproductive systems of the 

 parent species." 



In dealing with this subject Darwin points out that we must 

 be careful to distinguish between "the sterility of species 

 when first crossed, and the sterility of hybrids produced from 

 them." In the former case, the reproductive organs of each 

 individual are in a perfectly normal condition, while hybrids 

 appear to be generally impotent owing to some imperfection 

 in the reproductive organs themselves. They are not perfectly 

 fertile, as a rule, either with each other, or with either of the 

 parent forms. 



