5. To this must be added the circumstance that no authenticated instance of variation by natural selection can be brought forward. It is true that this is not a very important argument, because our knowledge of those classes of animals in which natural selection could act is even now very incomplete; and our knowledge of their past history is still more limited, so that we are not in a condition to prove a negative. But in such a case as this the onus of proof should surely lie on the other side. It is for those who would assert the theory to bring forward positive proof of it. There is, however, one point in Mr. Darwin's view of domesticated animals which tells against his theory. The cat remains unchanged, because from its vagrant habits man has no control over its pairing [Footnote: Darwin's "Animals and Plants," vol. ii. p. 236.]. Now considering the variety of conditions under which cats exist, here is surely a great opening for natural selection. But it has produced no results. We come now to the theory of Sexual Selection, which is to account for those peculiarities and distinctions which can have no beneficial effect in the struggle of life, and which are accounted for on the supposition that they render their possessors more agreeable to the opposite sex, and so facilitate pairing, so that those animals which possess them in a remarkable degree would have the greatest chance of continuing their race. The case on which Mr. Darwin mainly rests his argument is that of birds, in which the males are frequently distinguished by exquisite colours and very graceful markings, and in which also the proceedings of the sexes can, in many cases, be more easily watched.