We shall deal with the phenomenon of correlation at some length in the next chapter. It is a subject to which sufficient attention has not been paid. Even as certain characters are correlated in certain species, so in some cases are certain characters correlated with sex.
Why this should be so we are not in a position to say; this, however, does not affect the indisputable fact that such correlation does exist.
Physicians in the course of their practice sometimes come across very curious cases of correlation in human beings.
Unilateral Transmission
“It is,” writes Thomson (Heredity, p. 290), “an interesting fact that an abnormal element in the inheritance may find expression in the males only or in the females only. If we could understand this we should be nearer understanding what sex really means.
“Hæmophilia, or a tendency to bleeding, is a heritable abnormality, partly associated with weakness in the blood-vessels, which do not contract as they should and are apt to break, and partly connected with a lack of coagulating power in the blood. It is usually confined to males. But as it passes from a father through a daughter to a grandson, and so on, it must be a latent part of the germinal inheritance of the females, though for some obscure physiological reason it fails to find expression in them, or has its expression quite disguised. Colour-blindness or Daltonism has been recorded (Horner) through the males only of seven generations. Dejerine cites another case (fide Appenzeller) in which all the males in a family history had cataract through four generations. There are other instances of what is sometimes awkwardly called the unilateral transmission of abnormal qualities. Edward Lambert, born in 1717, is said to have been covered with ‘spines.’ His children showed the same peculiarity, which began to be manifest from the sixth to the ninth month after birth. One of his children grew up and handed on the peculiarity to another generation. Indeed, it is said to have persisted for five generations, and in the males only—unilateral transmission.”
In our view, these abnormalities are of such a kind that they are only possible in connection with the male organ; in other words, they are mutations of the first of the four kinds cited above—those which appear only in connection with the male organ.
It is a curious fact that the general rule in nature seems to be that the male is ahead of the female in the course of evolution. The sexes may be alike at a given period in the life-history of the species. Presently a mutation appears which is confined to the male alone; thus arises the phenomenon of sexual dimorphism. The next step in the evolution of the species is frequently a mutation on the part of the female which brings her once again into line with the male, and so the sexual dimorphism disappears, for a time at any rate. A good example of this is furnished by the sparrows; in the common sparrow of a large part of Africa (Passer swainsoni) both sexes are very plain, like the hen of the house-sparrow; in this species (P. domesticus) as every one knows, the cock, though by no means brilliant, is noticeably handsomer than his mate; while in the Tree-sparrow (P. montanus) both sexes have a plumage of masculine type, much like that of the cock house-sparrow.
If we consider in conjunction with one another the various facts we have cited above, we begin to grasp the nature of the phenomena of sexual dimorphism.
Let us consider an imaginary case of a defenceless little bird which builds an open nest. Let us suppose that it is inconspicuously plumaged. Now suppose that a mutation of the first kind shows itself, a mutation which affects the cock only and makes him more conspicuous. Let us further suppose that the cock does not share in the duties of incubation. It is quite possible that, in spite of this apparently unfavourable mutation, the species may survive, for, as we have seen, it does not affect the hen, and she, since she alone incubates, stands the most in need of protective colouring. Moreover, as Stolzmann has suggested, the species can possibly afford to lose a few males. But suppose that both cock and hen share in the duties of incubation, it is then quite likely that the mutation will cause the species to become extinct, by the elimination of all the males. Or, let us suppose that the mutation in the direction of showy plumage affects both sexes, then in such a case the species will almost certainly become extinct. If, however, the hypothetical species nested in holes in trees, it is quite possible that it might survive notwithstanding its showy plumage.