2G4 6UMMAEY. Chap. VIH. 



of sterility to prevent their crossing and blending in nature, 

 than to think that trees have been specially endowed with vari- 

 ous and somewhat analogous degrees of ditliculty in being 

 grafted together in order to prevent their inarching in our 

 forests. 



Tlie sterility of first crosses and of their hylirid progeny lias 

 not, as far as we can judge, been acquired through natural se- 

 lection. 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 liybrids, it perhajis 

 depends on tlieir whole organization having been disturbed by 

 lieing compounded from two distinct forms ; tlie sterility being 

 closely allied to that Avhich so frequently aii'ects pure species, 

 when exposed to unnatural conditions of life. This view is 

 supported by a parallelism of another kind : namely, that, first, 

 the crossing of forms only slightly diiferentiated favors the 

 vigor and fertility of their offspring, while close interbreeding 

 is injurious ; and secondly, that slight changes in the conditions 

 of life apparently add to the vigor and fertility of all organic 

 beings, while greater changes are often injurious. But the 

 facts given on the sterility of the illegitimate unions of dimor- 

 phic and trimorphic plants and of their illegitimate progeny, 

 render it probable that some unknown bond in all cases con- 

 nects the degree of fertility of first unions with that of their 

 offspring. The consideration of these facts on dimorphism, as 

 well as the results of reciprocal crosses, clearly leads to the 

 conclusion that the primary cause of tlie sterility is confined to 

 differences in the sexual elements. But why, in the case of 

 species, the sexual elements should so generally liave become 

 more or less modified, leading to their mutual infertility, we do 

 not know% 



It is not surprising that the diniculty in crossing any two 

 species, and the sterility of their hybrid-offspring, should in 

 most cases correspond, even if due to distinct causes ; for both 

 depend on the amount of difference between the species which 

 are crossed. Nor is it surprising that the facility of effecting 

 a first cross, and the fertility of tlie hybrids thus produced, 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 system- 

 atic afiinity of the forms subjected to experiment ; for system- 

 atic airiiiity includes resemblances of all kinds. 



First crosses between forms known to be varieties, or snffi- 



