Crosses between the two species of zebra, or even between the zebra and the quagga, or the quagga and the ass, might have led to a very different result.

Again, in pre-Darwinian times it was so universally the practice to argue in a circle, and declare that the fertility of the offspring of a cross proved the identity of species of the parents, that experiments in hybridity were usually made between very remote species and even between species of different genera, to avoid the possibility of the reply: "They are both really the same species;" and the sterility of the hybrid offspring of such remote crosses of course served to strengthen the popular belief.
Now that we have arrived at a different standpoint, and look upon a species, not as a distinct entity due to special creation, but as an assemblage of individuals which have become somewhat modified in structure, form, and constitution so as to adapt them to slightly different conditions of life; which can be differentiated from other allied assemblages; which reproduce their like, and which usually breed together--we require a fresh set of experiments calculated to determine the matter of fact,--whether such species crossed with their near allies do always produce offspring which are more or less sterile _inter se_.
Ample materials for such experiments exist, in the numerous "representative species" inhabiting distinct areas on a continent or different islands of a group; or even in those found in the same area but frequenting somewhat different stations.
To carry out these experiments with any satisfactory result, it will be necessary to avoid the evil effects of confinement and of too close interbreeding.

If birds are experimented with, they should be allowed as much liberty as possible, a plot of ground with trees and bushes being enclosed with wire netting overhead so as to form a large open aviary.
The species experimented with should be obtained in considerable numbers, and by two separate persons, each making the opposite reciprocal cross, as explained at p.155.In the second generation these two stocks might be themselves crossed to prevent the evil effects of too close interbreeding.

By such experiments, carefully carried out with different groups of animals and plants, we should obtain a body of facts of a character now sadly wanting, and without which it is hopeless to expect to arrive at a complete solution of this difficult problem.

There are, however, some other aspects of the question that need to be considered, and some theoretical views which require to be carefully examined, having done which we shall be in a condition to state the general conclusions to which the facts and reasonings at our command seem to point.
_Sterility due to changed Conditions and usually correlated with other Characters, especially with Colour._ The evidence already adduced as to the extreme susceptibility of the reproductive system, and the curious irregularity with which infertility or sterility appears in the crosses between some varieties or species while quite absent in those between others, seem to indicate that sterility is a characteristic which has a constant tendency to appear, either by itself or in correlation with other characters.