HYBRIDISM 267 



rically opposite conclusions in ranking forms by this test. The 

 sterility is innately variable in individuals of the same species, 

 and is eminently susceptible to action of favorable and unfavor- 

 able conditions. The degree of sterility does not strictly follow 

 systematic affinity, but is governed by several curious and com- 

 plex laws. It is generally different, and sometimes widely differ- 

 ent, in reciprocal crosses between the same two species. It is not 

 always equal in degree in a first cross and in the hybrids pro- 

 duced from this cross. 



In the same manner as in grafting trees, the capacity in one 

 species or variety to take on another, is incidental on differences, 

 generally of an unknown nature, 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 repro- 

 ductive systems. There is no more reason to think that species 

 have been specially endowed with various degrees of sterility to 

 prevent their 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 their inarching in our forests. 



The sterility of first crosses and of their hybrid progeny has 

 not been acquired through natural selection. In the case of first 

 crosses it seems to depend on several circumstances; in some in- 

 stances, in chief part on the early death of the embryo. In the 

 case of hybrids, it apparently depends on their whole organization 

 having been disturbed by being compounded from two distinct 

 forms, the sterility being closely allied to that which so frequently 

 affects pure species, when exposed to new and unnatural conditions 

 of life. He who will explain these latter cases will be able to ex- 

 plain the sterility of hybrids. This view is strongly supported by a 

 parallelism of another kind; namely, that, firstly, slight changes 

 in the conditions of life add to the vigor and fertility of all organic 

 beings; and secondly, that the crossing of forms which have been 

 exposed to slightly different conditions of life, or which have var- 

 ied, favors the size, vigor, and fertility of their offspring. The 

 facts given on the sterility of the illegitimate unions of dimorphic 

 and trimorphic plants and of their illegitimate progeny, perhaps 

 render it probable that some unknown bond in all cases connects 

 the degree of fertility of first unions with that of their offspring. 

 The consideration of these facts on dimorphism, as well as of the 

 results of reciprocal crosses, clearly leads to the conclusion that the 

 primal cause of the sterility of crossed species is confined to 

 differences in their sexual elements. But why, in the case of dis- 



