248 ON THE ORIGIN OF SPECIES 



of one species or variety to take on another, is incidental 

 on generally unknown differences in their vegetative 

 systems, so in crossing, the greater or less facility of one 

 species to unite with another, is incidental on unknown 

 differences in their reproductive systems. There is no 

 more reason to think that species have been specially 

 endowed with various degrees of sterility to prevent 

 them crossing and blending in nature, than to think 

 that trees have been specially endowed with various and 

 somewhat analogous degrees of difficulty in being 

 grafted together in order to prevent them becoming 

 inarched in our forests. 



The sterility of first crosses between pure species, 

 which have their reproductive systems perfect, seems 

 to depend on several circumstances ; in some cases 

 largely on the early death of the embryo. The sterility 

 of hybrids, which have their reproductive systems 

 imperfect, and which have had this system and their 

 whole organisation disturbed by being compounded of 

 two distinct species, seems closely allied to that sterility 

 which so frequently affects pure species, when their 

 natural conditions of life have been disturbed. This 

 view is supported by a parallelism of another kind ; — 

 namely, that the crossing of forms only slightly different 

 is favourable to the vigour and fertility of their offspring ; 

 and that slight changes in the conditions of life are 

 apparently favourable to the vigour and fertility of all 

 organic beings. It is not surprising that the degree of 

 difficulty in uniting two species, and the degree of 

 sterility of their hybrid - offspring should generally 

 correspond, though due to distinct causes ; for both 

 depend on the amount of difference of some kind 

 between the species which are crossed. Nor is it 

 surprising that the facility of effecting a first cross, 

 the fertility of the hybrids produced from it, and the 

 capacity of being grafted together — though this latter 

 capacity evidently depends on widely different circum- 

 stances — should all run, to a certain extent, parallel 

 with the systematic affinity of the forms which are 

 subjected to experiment ; for systematic affinity 



