See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1866
1869
1872

Comparison with 1869

ought not to expect it also to produce sterility.
The sterility of hybrids is a .. different case from that of first crosses, for the reproductive organs of hybrids are more or less functionally impotent; whereas in first crosses the organs on both sides are of course in a perfect condition. As we continually see that organisms of all kinds are rendered in some degree sterile from being exposed to slightly changed conditions, we need not feel surprise at hybrids being in some degree sterile, for their constitutions can hardly fail to be disturbed from being compounded of two distinct organisations; but whether this is the true cause of their sterility I will not pretend to decide. The above 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 when crossed acquire increased vigour and fertility. So that, on the one hand, a considerable change 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 may now be found, they must in the
ought not to expect it also to produce sterility.
The sterility of hybrids is a very different case from that of a first cross, for their reproductive organs are more or less functionally impotent; whereas in first crosses, the organs of both species are in a perfect condition. As we continually see that organisms of all kinds are rendered in some degree sterile from their constitutions having been disturbed by slightly different 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 disturbed from being compounded of two distinct organisations. 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 acquire 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 serious 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