Chap. XIV. 
EECAPITULATIOm 
461 
produced under domestication; and as domestication 
(I do not mean mere confinement) apparently tends to 
eliminate sterility, we ought not to expect it also to 
produce sterility. 
The sterility of hybrids is a very different case from 
that of first crosses, for their reproductive organs are 
more or less functionally impotent; whereas in first 
crosses the organs on both sides are in a perfect con¬ 
dition. As we continually see that organisms of all 
kinds are rendered in some degree sterile from their 
constitutions having been disturbed by slightly dif¬ 
ferent and new conditions of life, we need not feel 
surprise at hybrids being in some degree sterile, for 
their constitutions can hardly fail to have been dis¬ 
turbed from being compounded of two distinct organisa¬ 
tions. This parallelism is supported by another parallel, 
but directly opposite, class of facts; namely, that the 
vigour and fertility of all organic beings are increased 
by slight changes in their conditions of life, and that 
the offspring of slightly modified forms or varieties ac¬ 
quire from being crossed increased vigour and fertility. 
So that, on the one hand, considerable changes in the 
conditions of life and crosses between greatly modified 
forms, lessen fertility; and on the other hand, lesser 
changes in the conditions of life and crosses between 
less modified forms, increase fertility. 
Turning to geographical distribution, the difficulties 
encountered on the theory of descent with modification 
are grave enough. All the individuals of the same 
species, and all the species of the same genus, or even 
higher group, must have descended from common 
parents ; and therefore, in however distant and isolated 
parts of the world they are now found, they must in the 
course of successive generations have passed from some 
one part to the others. We are often wholly unable 
