[152]

Oct 22 (one tied)

Genista fragrans

[data not transcribed]

(Used)

[152v]

84 96

R Malvern

Chap. 8. Principles of Nat. Selection.

On the power of Natural Selection to regulate the proportional numbers of the sexes. ─ A few remarks may here be added on how far natural selection, or the survival of the fittest, has the power, as far as we can judge, of regulating the proportional numbers of the sexes. In some peculiar cases, which need not here be considered, an excess of one sex over the other may be a great advantage to the species, as in the case of sterile of females, though sterile, with social insects, or of males when more than one male is requisite to fertilise the female, as with certain cirripedes, and perhaps with certain fishes. In other cases an excess of females may would be no injury to a species, as with highly polygamous animals. An excess in number in one sex would indeed in all cases be so far advantageous that the more vigorous individuals of the sex would alone leave offspring; but the offspring from the parents which produced the sexes in unequal numbers would have so little advantage in the general battle for life over those offspring from the parents which produced the sexes in equal numbers, that the former would hardly tend to increase;─ more specifically as the actual number of their offspring capable of reproduction as would be diminished by the excess of one sex. Hence with species

[Descent 1: 315-17: "On the Power of Natural Selection to regulate the proportional Numbers of the Sexes, and General Fertility.—In some peculiar cases, an excess in the number of one sex over the other might be a great advantage to a species, as with the sterile females of social insects, or with those animals in which more than one male is requisite to fertilise the female, as with certain cirripedes and perhaps certain fishes. An inequality between the sexes in these cases might have been acquired through natural selection, but from their rarity they need not here be further considered. In all ordinary cases an inequality would be no advantage or disadvantage to certain individuals more than to others; and therefore it could hardly have resulted from natural selection. We must attribute the inequality to the direct action of those unknown conditions, which with mankind lead to the males being born in a somewhat larger excess in certain countries than in others, or which cause the proportion between the sexes to differ slightly in legitimate and illegitimate births.
Let us now take the case of a species producing from the unknown causes just alluded to, an excess of one sex—we will say of males—these being superfluous and useless, or nearly useless. Could the sexes be equalised through natural selection? We may feel sure, from all characters being variable, that certain pairs would produce a somewhat less excess of males over females than other pairs. The former, supposing the actual number of the offspring to remain constant, would necessarily produce more females, and would therefore be more productive. On the doctrine of chances a greater number of the offspring of the more productive pairs would survive; and these would inherit a tendency to procreate fewer males and more females. Thus a tendency towards the equalisation of the sexes would be brought about. But our supposed species would by this process be rendered, as just remarked, more productive; and this would in many cases be far from an advantage; for whenever the limit to the numbers which exist, depends, not on destruction by enemies, but on the amount of food, increased fertility will lead to severer competition and to most of the survivors being badly fed. In this case, if the sexes were equalised by an increase in the number of the females, a simultaneous decrease in the total number of the offspring would be beneficial, or even necessary, for the existence of the species; and this, I believe, could be effected through natural selection in the manner hereafter to be described. The same train of reasoning is applicable in the above, as well as in the following case, if we assume that females instead of males are produced in excess, for such females from not uniting with males would be superfluous and useless. So it would be with polygamous species, if we assume the excess of females to be inordinately great."]