vii ON THE INFERTILITY OF CROSSES 175 



Now. let us suppose that a partial sterility of the hybrids 

 between the two forms arises, in correlation with the different 

 modes of life and the slight external or internal peculiarities 

 that exist between them, both of which we have seen to be 

 real causes of infertility. The result will be that, even if the 

 hybrids between the two forms are still freely produced, these 

 hybrids will not themselves increase so rapidly as the two 

 pure forms : and as these latter are, by the terms of the 

 problem, better suited to their conditions of life than are 

 the hybrids between them, they will not only increase more 

 rapidly, but will also tend to supplant the hybrids altogether 

 whenever the struggle for existence becomes exceptionally 

 severe. Thus, the more complete the sterility of the hybrids 

 the more rapidly will they die out and leave the two parent 

 forms pure. Hence it will follow that, if there is greater 

 infertility between the two forms in one part of the area than 

 the other, these forms will be kept more pure wherever 

 this greater infertility prevails, will therefore have an 

 advan: g : each recurring period of severe struggle for 

 nee, and will thus ultimately supplant the less infertile 

 or completely fertile forms that may exist in other portions 

 of the area. It thus appears that, in such a case as here 

 supposed, natural selection would preserve those portions of 

 the two breeds which were most infertile with each other, or 

 whose hybrid offspring were most infertile : and would, 

 therefore, if variations in fertility continued to arise, tend to 

 increase that infertility. It must particularly be noted that 

 this effect would result, not by the preservation of the 

 infertile variations on account of their infertility, but by the 

 inferiority of the hybrid offspring, both ; - " sing fewer in 

 numbers, less able to continue their race, and less adapted to 

 the conditions of existence than either of the pure fornix It 

 is this inferiority of the hybrid offspring that is the essential 

 point ; and as the number of these hybrids will be per- 

 manently less where the infertility is greatest, therefore those 

 portions of the two forms in which infertility is greatest will 

 have the advantage, and will ultimately survive in the struggle 

 for existence. 



The differentiation of the two forms into distinct species, 

 with the increase of infertility between them, would be 



