268 THE ORIGIN OF SPECIES 



tinct species, the sexual elements should so generally have become 

 more or less modified, leading to their mutual infertility, we do not 

 know; but it seems to stand in some close relation to species having 

 been exposed for long periods of time to nearly uniform conditions 

 of life. 



It is not surprising that the difficulty in crossing any two spe- 

 cies, and the sterility of their hybrid offspring, should in most 

 cases correspond, even if due to distinct causes; for both depend 

 on the amount of difference between the species which are 

 crossed. Nor is it surprising that the facility of effecting a first 

 cross, and the fertility of the hybrids thus produced, and the ca- 

 pacity of being grafted together — though this latter capacity evi- 

 dently depends on widely different circumstances — should all 

 run, to a certain extent, parallel with the systematic affinity of 

 the forms subjected to experiment; for systematic affinity in- 

 cludes resemblances of all kinds. 



First crosses between forms known to be varieties, or suffi- 

 ciently alike to be considered as varieties, and their mongrel off- 

 spring, are very generally — ^but not, as is so often stated, invar- 

 iably — fertile. Nor is this almost universal and perfect fertility 

 surprising, when it is remembered how liable we are to argue in 

 a circle with respect to varieties in a state of nature; and when 

 we remember that the greater number of varieties have been pro- 

 duced under domestication by the selection of mere external dif- 

 ferences, and that they have not been long exposed to uniform 

 conditions of life. It should also be especially kept in mind, that 

 long-continued domestication tends to eliminate sterility, and is 

 therefore little likely to induce this same quality. Independently 

 of the question of fertility, in all other respects there is the 

 closest general resemblance between hybrids and mongrels, in 

 their variability, in their power of absorbing each other by re- 

 peated crosses, and in their inheritance of characters from both 

 parent-forms. Finally, then, although we are as ignorant of the 

 precise cause of the sterility of first crosses and of hybrids as we 

 are why animals and plants removed from their natural condi- 

 tions become sterile, yet the facts given in this chapter do not 

 seem to me opposed to the belief that species aboriginally existed 

 as varieties. 



