In the second part Darwin brings forward a large body of evidence in favour of his hypothesis of sexual selection--viz. the view that, in the higher animals, some alteration, especially of the secondary sexual characters, is produced by the preferences and rejections of the sex which is sought by the other. Such results are commonly found in the males as a result of the preferences of the females accumulated through countless generations; but in some species the females court the males, and are themselves subject to the same process of improvement by selection. Opinion is still divided on this most interesting question. Wallace, more convinced than ever as to the efficiency and scope of natural selection, after first doubting, has finally come to reject sexual selection altogether. Probably the majority of naturalists are convinced by Darwin’s arguments and his great array of facts that the principle of sexual selection is real, and accounts for certain relatively unimportant features in the higher animals, and they further accept Darwin’s opinion that its action has always been entirely subordinate to natural selection. A brief third part considers sexual selection in relation to man. Darwin says, in his “Autobiography,” that sexual selection and “the variation of our domestic productions, together with the causes and laws of variation, inheritance, and the intercrossing of plants, are the sole subjects which I have been able to write about in full, so as to use all the materials which I have collected.” [Sidenote: EXPRESSION OF EMOTIONS.] “The Expression of the Emotions,” at first intended as a chapter of the “Descent,” was begun, only two days after the proofs of the latter had been corrected, on January 15th, 1871. The book was published in the autumn of the following year; the edition consisted of 7,000 copies, and 2,000 were printed at the end of the year; and this, we are told, was a mistake, as it prevented the appearance of a second edition, with notes and corrections, during the author’s lifetime. Darwin had begun to take notes on this subject when his first child was born, December 27th, 1839, for he tells us that, even then, he felt convinced “that the most complex and fine shades of expression must all have had a gradual and natural origin.”--(“Autobiography.”) In this work Darwin argues with great wealth of illustration and the record of numberless interesting observations, that the movements of expression are to be explained by three principles. The first of these is that movements made in gratifying some desire become by repetition so habitual that the slightest feeling of desire leads to their performance, however useless they may then be. The second principle is that of antithesis--“the habit of voluntarily performing opposite movements under opposite impulses.” The third principle is “the direct action of the excited nervous system on the body, independently of the will, and independently, in large part, of habit.”