Descent of Man, and Selection in Relation to Sex, v.1

Page 151

286 THE PRINCIPLES OF PAUT II. that variations which first appear in either sex at a late period of life, tend to be developed in the sam.e s~x alone; whilst variations which first appear early m hfe in either sex tend to be developed in both sexes. I am, however, far from supposing that this is the ~ole de­termining cause. As I have not elsewhere d1~cussed this subject, and as it has an important bearmg on sexual selection, I must here enter into lengthy and somewhat intricate details. It is in itself probable that any character appearing at an early age would tend to be inherited. equally. by both sexes, for the sexes do not differ much m constitu­tion, before the power of reproduction is gained. On the other hand, after this power has been gained and the sexes have come to differ in constitution, the gemmules (if I may again use the language of pangenesis) which are cast off from each varying part in the one sex would be much more likely to possess the proper affinities for uniting with the tissues of the same sex, and thus becoming developed, than with those of the opposite sex. I was first led to infer that a relation of this kind exists, from the fact that whenever and in whatever manner the adult male has come to differ from the adult female, he differs in the same manner from the young of both sexes. The generality of this fact is quite remarkable : it holds good with almost all mammals, birds, amphibians, and fishes; also with many crus­taceans, spiders and some few insects, namely certain orthoptera and libellulre. In all these cases the varia­tions, through the accumulation of which the male ac­quired his proper masculine characters, must have oc­curred at a somewhat late period of life; otherwise the young males would have been similarly characterised; and conformably with our rule, they are transmitted to CHAP. VIII. SEXUAL SELECTION. 287 and developed in the adult males alone. When, on the other hand, the adult male closely resembles the young of both sexes (these, with rare exceptions, being alike), he generally resembles the adult female; and in most of these cases the variations through which the young and old acquired their present characters, probably occurred in conformity with our rule during youth. But there is here room for doubt, as characters are sometimes trans­ferred to the off8pring at an earlier age than that at which they first appeared in the parents, so that the parents may have varied when adult, and have trans­ferred their characters to their offspring whilst young. There are, moreover, many animals, in which the two sexes closely resemble each other, and yet both differ from their young; and here the characters of the adults must have been acquired late in life ; nevertheless, these characters in apparent contradiction to our rule, are transferred to both sexes. We must not, however, overlook· the possibility or even probabHity of succes­sive variations of the same nature sometimes occurring, under exposure to similar conditions, simultaneously in both sexes at a rather late period of life ; and in this case the variations would be transferred to the offspring of both sexes at a corresponding late age; and there would be no real contradiction to our rule of the variations which occur late in life being transferred exclusively to the sex in which they first appeared. This latter rule seems to hold true more generally than the second rule, namely, that variations which occur in either sex early in life tend to be transferred to both sexes. As it was obviously impossible even to estimate in how large a number of cases throughout the animal kingdom these two propositions hold good, it occurred to me to inves­tigate some striking or crucial instances, and to rely on the result.