is present in all cases. Its conspicuous presence in the four hermaphrodites would seem, therefore, to indicate that the birds started as females (ZW), which is inconsistent with the dislocation hypothesis. The alternate would be that in all these cases the Z part always included the region of the oviduct, which seems improbable. There is another possibility, viz, that in birds a sex-factor is carried by the W chromosome, and ZW is a female not because of one Z, but due to the presence of W. If so, then one Z or two Z’s might give the same result, viz, female. If a bird started as female, (ZW) and chromosomal dislocation occurred, then the Z parts would be female and the male part W. Until we get evidence on this point it is not worth elaborating. Without genetic evidence from hybrids, the interpretation of hermaphrodites in birds can have at present only a speculative interest. We may hope some day to get the same kind of evidence as in the case of _Drosophila_. Hermaphrodite hybrid pheasants that have been often described might seem to furnish a hopeful field, for they appear to be quite common and to show characteristics of both races. As yet, however, no one has, I think, succeeded in finding a simple interpretation of the results. It is also not unlikely that many of the pheasant cases are not true hermaphrodites, but due to failure of normal development of the reproductive gland, which gives an intermediate or mixed type of secondary sexual characters. PART II. DARWIN’S THEORY OF SEXUAL SELECTION. Darwin seems to have felt the necessity of giving some other explanation for the secondary sexual differences between the male and female than that such differences were only a by-product or concomitant of sex itself. His reason for searching further was probably a part of the general point of view he had reached in regard to the utility of special structures of animals, namely, that their presence finds its explanation on the basis of utility. Believing as he did that most of the adaptations of plants and animals have been built up by the accumulation of small steps, it must have appeared to Darwin inconceivable that the highly developed ornamentation exhibited in the secondary sexual characters could have been simply the by-product of sex itself, especially when the ornamentation may have been entirely absent in males of closely related species. To-day we are not, I think, so oppressed with the difficulties of the situation, for we have become familiar with the fact that very slight genetic differences may cause very great differences in the end-product. In a word, the problem seems less formidable to us than it did to Darwin. Darwin appealed to three processes to account for the facts: (1) to natural selection between the members of the same sex; (2) to choice on the part of the “other” sex; (3) to the “inheritance of use.” Since each of these appeals to a different procedure, let us take them up separately.