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of their mongrel offspring cannot be considered as universal; nor is their very general fertility surprising when we remember that it is not likely that either their constitutions or their reproductive systems should have been profoundly modified. Nor is the very general fertility of varieties, when crossed, surprising, when we remember that it is not likely that their reproductive systems should have been profoundly modified. Moreover, most of the varieties which have been experimentised on have been produced under domestication; and as domestication 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 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 acquire from being crossed 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
of their mongrel offspring has been asserted by so many authors to be universal, this cannot be considered correct after the facts given on the authority of Gärtner and Kölreuter. Nor is the very general fertility of varieties, when crossed, surprising, when we remember that it is not likely that their reproductive systems should have been profoundly modified. Moreover, most of the varieties which have been experimented on have been produced under domestication; and as domestication (I do not mean mere confinement) almost certainly tends to eliminate sterility, we ought not to expect it also to produce sterility.
The sterility of hybrids is a .. different case from that of a first cross, for the reproductive organs of hybrids are more or less functionally impotent; whereas in first crosses, the organs of both species are of course 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 when crossed acquire 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