48 THE ORIGIN OF SPECIES n 



between the same two species. It is not always equal in degree 

 in a first cross, and in the hybrid produced from this cross. 



' ' In the same manner as in grafting trees, the capacity 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 breeding 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 repro- 

 ductive 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 con- 

 ditions 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 the offspring ; and that slight changes in the con- 

 ditions 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 hybrids produced from it, and the capacity of 

 being grafted together though this latter capacity evidently 

 depends on widely different circumstances should all run to a 

 certain extent parallel with the systematic affinity of the forms 

 which are subjected to experiment; for systematic affinity, 



