The second interpretation suggested above is one that has been advanced and established by genetic evidence in Drosophila, viz., dislocation of the sex chromosomes. In the case of birds the male is supposed to be duplex for the sex factors (ZZ), the female simplex (ZW), and consequently the chromosome-dislocation hypothesis must be worked out contrawise in birds and insects. We should have to suppose that such birds start as males (ZZ), and that at some division of the cells of the embryo one of the Z’s became lost (left at the cell-wall for example). All the cells that got ZZ would be male; all that got Z would be female. If the reproductive region included cells of these two kinds, an ovotestis would result. The rest of the body should be the same, or nearly so, since the soma of male and female birds is alike whether ZZ or Z, except in so far as it is affected by the secretions from the ovaries (in most races of poultry), or from the testes if the race be Sebright, Campines, or Hamburgs. Birds with ovotestis might, nevertheless, be expected, on this view, to show at times an intermediate condition of the secondary sexual characters, according to how much internal secretion is produced in the ovotestis. In other words, the chromosome loss might involve much more extensive regions than the reproduction organs, but show its effects first in that organ and then indirectly other parts of the body be affected by the luteal cells of the testis. There is one rather good piece of evidence that seems opposed to this interpretation. In the hermaphrodites the oviduct 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.
Competition of the males with each other for the female would, Darwin said, lead to the survival of those males best endowed with organs of offense and defense. The spurs of the cock are weapons dangerous for other birds; the horns of the bull and those of deer are used for offense and defense; the mane of the lion is a protection against the teeth of other lions. It is true that these same weapons and shields serve for attack and defense outside the species; but since the female lacks them or has them less developed, they would not seem necessary for survival of the individual against aggression from without. They have developed, then, through competition within the species.
Several objections of greater or less weight have been urged against Darwin’s interpretation. It has been pointed out that the combats within the species are seldom fatal and that the defeated rival finds another mate. If, as a rule, there are as many females as males within the species and monogamy is the rule, all males will find partners sooner or later, all may have offspring, and the offspring have equally good chances of survival. Under these circumstances it is not to be expected that the combat would be likely to lead to the production of males with longer spurs or larger horns.
Darwin realized this difficulty and tried to meet it by another assumption, viz, that the better endowed males would also be more likely to have more offspring. How could this be made probable? Darwin suggested that the strongest males would be in position to mate with the first females to reach maturity, and if these were more likely to have offspring, either because of maternal endowments that made them also more prolific or because the earlier broods would have a better chance of getting food, etc., then the successful competitor would sooner or later impress his advantages on the race.
At other times Darwin suggested that the exceptional vigor that led to the greater development of the character in question would itself be of value and through transmission to the offspring lead to advance in the development of the other character in question. But here the argument shifts to another field of inquiry and survival is ascribed to greater vigor, while the secondary sexual character is carried along in its wake as a sort of correlated effect.