The third and weightiest consideration, however, is this, that the fact of the formation of hybrids lies neither above nor below the scale, but altogether outside of it; following our metaphor, to apply the Darwinian idea to them would mean to seek somehow to apply the heat-scale to electric or magnetic phenomena. So soon as the evolution theory attempts to bring the fact of the formation of hybrids within its sphere of operation, it annihilates the possibility of its own existence. Natural selection is only possible in self-copulation. A self-copulation to the point of sterility is a contradiction in itself; hence Darwin himself is here obliged to have recourse to unknown impulsions. “The general sterility of crossed species may safely be looked at, not as a special acquirement or endowment, but as incidental on changes of an unknown nature in their sexual elements.”[28] Again, “The extinction of species has been involved in the most gratuitous mystery.... We need not wonder at extinction; if we must marvel, let it be at our own presumption in imagining for a moment that we understand the many complex contingencies on which the existence of each species depends.”[29] This, however, means nothing but putting the question, “Who says we have a right to inquire into everything?” And that, again, means nothing but to be a good Christian. That, of course, is not the slightest disparagement to the teaching, so long as one takes it for what it really is--a standard of measurement for the facts, a formula by means of which one may more easily express them. It would be passing sentence of death upon it, as also upon the law of the conservation of energy, if, apprehending it in childish wise, one interpreted it as a genuine world-conception, as a law that should not merely supply a _reading_ of the facts, but account for these facts themselves. When modern biology inclines to set aside the Darwinian teaching in favour of the more novel theories of mutation, it is acting like that countryman who bought himself a pair of spectacles, expecting them not only to make print clear to his eyes but also teach him how to read, and who then made complaint that the glasses did not do their duty. The theory of natural selection, as well as every other theory, may be likened to reading-glasses. It reveals the facts in such a way as to lighten the labour for weakly eyes, but it does not teach one to understand the facts themselves. And as with glasses, so with theories; one has to change them, on an average, every five years. But let us return to our subject proper.