ile 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 {255} 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 Gaertner'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. It is
surprising in how many curious ways this gradation can be shown to exist;
but only the barest outline of the facts can here be given. When pollen
from a plant of one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species of the same
genus applied to the stigma of some one species, yields a perfect gradation
in the number of seeds produced, up to nearly complete or even quite
complete fertility; and, as we have seen, in certain abnor