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. 
