XIL] THE ORIGIN OF SPECIES. 239 



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

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



&quot; 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. 



&quot; 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 organization 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 favour 

 able to the vigour and fertility of the 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 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 attempts to express all kinds of resemblance between 

 all species. 



&quot; First crosses between forms known to be varieties, or sufficiently alike 

 to be considered as varieties, and their mongrel offspring, are very gener 

 ally, but not quite universally, fertile. Nor is this nearly general and 

 perfect fertility surprising, when we remember 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 produced under 

 domestication by the selection of mere external differences, and not of 

 differences in the reproductive system. In all other respects, excluding 

 fertility, there is a close general resemblance between hybrids and 

 mongrels.&quot; Pp. 2768. 



We fully agree with the general tenor of this weighty passage ; 

 but forcible as are these arguments, and little as the value of 

 fertility or infertility as a test of species may be, it must not be 

 forgotten that the really important fact, so far as the inquiry into 

 the origin of species goes, is, that there are such things in Nature 



