HYBRIDISM 51 



being grafted together in order to prevent their inarch- 

 ing 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 instances 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 explain the sterility of hybrids. 

 This view is strongly supported by a parallelism of aaother 

 kind; namely, that, first, 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 op 

 which have varied, 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 primary cause of the sterility of 

 crossed species is confined to differences in their sexual 

 elements. But why, in the case of distinct species, the 

 sexual elements should so generally have become more or 

 less modified, leading to their mutual infertility, we do 

 not know; but it seems to stand in some close relation 



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