Thus, the problem of human sexual selection is in the highest degree complicated. When we gather together such scanty data of precise nature as are at present available, we realize that, while generally according with the results which the evidence not of a quantitative nature would lead us to accept, their precise significance is not at present altogether clear. It would appear on the whole that in choosing a mate we tend to seek parity of racial and individual characters together with disparity of secondary sexual characters. But we need a much larger number of groups of evidence of varying character and obtained under varying conditions. Such evidence will doubtless accumulate now that its nature is becoming defined and the need for it recognized. In the meanwhile we are, at all events, in a position to assert, even with the evidence before us, that now that the real meaning of sexual selection is becoming clear its efficacy in human evolution can no longer be questioned. APPENDICES APPENDIX A. THE ORIGINS OF THE KISS. Manifestations resembling the kiss, whether with the object of expressing affection or sexual emotion, are found among various animals much lower than man. The caressing of the antennæ practiced by snails and various insects during sexual intercourse is of the nature of a kiss. Birds use their bills for a kind of caress. Thus, referring to guillemots and their practice of nibbling each other's feet, and the interest the mate always takes in this proceeding, which probably relieves irritation caused by insects, Edmund Selous remarks: "When they nibble and preen each other they may, I think, be rightly said to cosset and caress, the expression and pose of the bird receiving the benefit being often beatific."[196] Among mammals, such as the dog, we have what closely resembles a kiss, and the dog who smells, licks, and gently bites his master or a bitch, combines most of the sensory activities involved in the various forms of the human kiss. As practiced by man, the kiss involves mainly either the sense of touch or that of smell. Occasionally it involves to some extent both sensory elements.[197]