If we should ever attempt to pair together animals much more unlike each other than the horse and the ass, we should simply fail. They will not come together, and we cannot tell whether, if they did, they would be capable of producing progeny. We may therefore conclude that, as matter of fact, there are certain well-marked physiological species that will not breed with each other at all, while there are other species also physiologically distinct, but not so markedly separated from each other, that may be brought to breed together, their offspring being infertile.
167. The most apparent conclusion to be deduced from these facts would be that of the invariability of species, and of the impossibility of its transmutation—the infertility of hybrids being the law which prevents any such transmutation from taking place. And as the physiological species cannot be made different the apparent conclusion is that in times past they have been always the same as they are now. If this be allowed, it follows that inasmuch as they took their origin in time, they must have originally been produced very much as they are at the present moment,—a separate act of production being required for each species, or rather two separate acts for each species. This position has always been regarded as a stronghold by a certain class of theological thinkers, and they have resented the attempts of men of science to obtain any other explanation of the origin of species.
Men of science have, on the other hand, asserted their right to discuss this question with the same freedom as any other. Our point of view is somewhat different from that of either of these two parties. We think it is not so much the right or privilege as the bounden duty of the man of science to put back the direct interference of the Great First Cause—the unconditioned—as far as he possibly can in time. This is the intellectual or rather theoretical work which he is called upon to do—the post that has been assigned to him in the economy of the universe.
If, then, two possible theories of the production of any phenomenon are presented to the man of science, one of these implying the immediate operation of the unconditioned, and the other the operation of some cause existing in the universe, we conceive that he is called upon by the most profound obligations of his nature and work to choose the second in preference to the first. But we have already sufficiently discussed this question in a previous part of this book ([Art. 85]).
168. When we examine closely into the phenomena of life we find that side by side with the general law, that like produces like, there is a tendency to minor variations.
Thus we have already agreed to consider dray-horses and Arabs as varieties of the species horse; and in like manner pouters, carriers, fan-tails, and tumblers are all varieties of the species rock-pigeon. We are therefore led to ask how such varieties were originally produced, and how they become perpetuated after their production.
Now it is well known that there occurs occasionally an accountable variation, so marked in its nature as to be worthy of historical record. Two very interesting and instructive instances of this are given by Professor Huxley, and we take the liberty of quoting these in the Professor’s own words:—
‘The first of them is that of the “Ancon,” or “Otter” sheep, of which a careful account is given by Colonel David Humphreys, F.R.S., in a letter to Sir Joseph Banks, published in the Philosophical Transactions for 1813. It appears that one Seth Wright, the proprietor of a farm on the banks of the Charles River in Massachusetts, possessed a flock of fifteen ewes and a ram of the ordinary kind. In the year 1791, one of the ewes presented her owner with a male lamb differing, for no assignable reason, from its parents by a proportionally long body and short bandy legs, whence it was unable to emulate its relatives in those sportive leaps over the neighbours’ fences, in which they were in the habit of indulging, much to the good farmer’s vexation.
‘With the ’cuteness characteristic of their nation, the neighbours of the Massachusetts farmer imagined it would be an excellent thing if all his sheep were imbued with the stay-at-home tendencies enforced by nature upon the newly arrived ram, and they advised Wright to kill the old patriarch of his fold, and install the Ancon ram in his place. The result justified their sagacious anticipations.... The young lambs were almost always either pure Ancons or pure ordinary sheep. But when sufficient Ancon sheep were obtained to interbreed with one another it was found that the offspring was always pure Ancon.’