NATURAL SELECTION 243 
vigor and fertility of all organic beings; and secondly, that the cross- 
ing of forms, which have been exposed to slightly different conditions 
of life or 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 con- 
nects 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, lead- 
ing to their mutual infertility, we do not know; but it seems to stand in 
some Close relation to species having been exposed for long periods of 
time to nearly uniform conditions of life. 
It is not surprising that the difficulty in crossing any two species, 
and the sterility of their hybrid offspring, should in most cases corre- 
spond, even if due to distinct causes: for both depend on the amount 
of difference between the species which are crossed. Nor is it sur- 
prising that the facility of effecting a first cross, and the fertility of the 
hybrids thus produced, and the capacity of being grafted together— 
though this latter capacity evidently depends on widely different cir- 
cumstances—should all run, to a certain extent, parallel with the 
systematic affinity of the forms subjected to experiment; for system- 
atic affinity includes resemblances of all kinds. 
First crosses between forms known to be varieties, or sufficiently 
alike to be considered as varieties, and their mongrel offspring, are 
very generally, but not, as is so often stated, invariably fertile. Nor 
is this almost universal and perfect fertility surprising, when it is 
remembered 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 that they have not been 
long exposed to uniform conditions of life. It should also be espe- 
cially kept in mind that long-continued domestication tends to elimi- 
nate sterility, and is therefore little likely to induce this same quality. 
Independently of the question of fertility, in all other respects there 
is the closest general resemblance between hybrids and mongrels, 
in their variability, in their power of absorbing each other by repeated 
crosses, and in their inheritance of characters from both parent-forms. 
