158 THE ORIGIN OF SPECIES 



" First crosses between forms sufficiently distinct to be ranked 

 as species, and their hybrids, are very generally, but not universally, 

 sterile. The sterility is of all degrees, and is often so slight that 

 the two most careful experimentalists who have ever lived have 

 come to diametrically opposite conclusions in ranking forms by this 

 test. The sterility is innately variable in individuals of the same 

 species, and is eminently susceptible of favourable and unfavourable 

 conditions. The degree of sterility does not strictly follow syste- 

 matic affinity, but is governed by several curious and complex laws. 

 It is generally different, and sometimes widely different, in reciprocal 

 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. 



'* 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 in- 

 arched 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 

 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 favourable 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 syste- 

 matic affinity attempts to express all kinds of resemblance between 

 all species. 



" First crosses between forms known to be varieties, or sufficiently 

 alike to be considered as varieties, and their mongrel offspring, are 



