since commingled by intercrossing. On this view, the aboriginal species
must either at first have produced quite fertile hybrids, or the
hybrids must have become in subsequent generations quite fertile under
domestication. This latter alternative seems to me the most probable,
and I am inclined to believe in its truth, although it rests on no
direct evidence. I believe, for instance, that our dogs have descended
from several wild stocks; yet, with perhaps the exception of certain
indigenous domestic dogs of South America, all are quite fertile
together; and analogy makes me greatly doubt, whether the several
aboriginal species would at first have freely bred together and have
produced quite fertile hybrids. So again there is reason to believe
that our European and the humped Indian cattle are quite fertile
together; but from facts communicated to me by Mr. Blyth, I think they
must be considered as distinct species. On this view of the origin of
many of our domestic animals, we must either give up the belief of the
almost universal sterility of distinct species of animals when crossed;
or we must look at sterility, not as an indelible characteristic, but
as one capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids,is an extremely general result;
but that it cannot, under our present state of knowledge, be considered
as absolutely universal.
_Laws governing the Sterility of first Crosses and of Hybrids_. We will
now consider a little more in detail the
circumstances and rules governing the sterility of first crosses and of
hybrids. Our chief object will be to see whether or not the rules
indicate that species have specially been endowed with this quality, in
order to prevent their crossing and blending together in utter
confusion. The following rules and conclusions are chiefly drawn up
from Gartner s admirable work on the hybridisation of plants. I have
taken much pains to ascertain how far the rules apply to animals, and
considering how scanty our knowledge is in regard to hybrid animals, I
have been surprised to find how generally the same rules apply to both
kingdoms.
It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
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