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Chap. 8. Principles of Nat. Selection

sexes will be brought about. But our supposed species will 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 can exist, depends, not on destruction by enemies, but on the amount of food, increased fertility will lead to severe competition and to the survivors being badly fed. In this case, if the sexes were equalized 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 presently to be described.

An excess of either sex, we will again say of the males, could however, apparently be eliminated through natural selection in an indirect manner by an actual diminution of the males, without any increase of the females, and consequently without any increase in the productiveness of the species. From all characters being variable, we may feel assured that of the pairs inhabiting any locality, some would produce a somewhat larger and others a somewhat smaller excess of superfluous males. The latter pairs would produce a less actual number of

[Descent 1: 316-17: "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.

An excess of either sex, we will again say of the males, could, however, apparently be eliminated through natural selection in another and indirect manner, namely by an actual diminution of the males, without any increase of the females, and consequently without any increase in the productiveness of the species. From the variability of all characters, we may feel assured that some pairs, inhabiting any locality, would produce a rather smaller excess of superfluous males, but an equal number of productive females."]

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Chap. 8

Amazons, says that the males are much more numerous than the females, even in the proportion of a hundred to one. In N. America, Edwards who has had great experience, estimates the males to the females in the genus Papilio as four to one; and Mr. Walsh, who informs me of this fact, says that with P. Turnus this is certainly the case. In S. Africa Mr R. Trimen found that the males were in excess in 19 species 58* (U); & in one of these, which swarms in open places, he estimated the number of males as fifty to one female. with In another species, the males of which are numerous in certain localities, he collected during seven years only five females. I the island of Bourbon, M. Maillard says states that the males of one species of Papilio are twenty times as numerous as the females 58*(V). Mr Trimen informs me that as far as he has himself seen, or heard from others, it is rare for the females of any butterfly to exceed in number the males, but this is perhaps the case with three S. African species.

Mr. Wallace 59*(W) states that the females of Ornithoptera crœsus of the Malay Archipelago were are more plentiful common and more easily caught than the males; but this is a rare butterfly. I may here add that in [text excised]thra, a genus of moths, Guenée says that from

[Descent 1: 309-10: "In regard to Butterflies in a state of nature, several observers have been much struck by the apparently enormous preponderance of the males.56 Thus Mr. Bates,57 in speaking of the species, no less than about a hundred in number, which inhabit the Upper Amazons, says that the males are much more numerous than the females, even in the proportion of a hundred to one. In North America, Edwards, who had great experience, estimates in the genus Papilio the males to the females as four to one; and Mr.
56 Leuckart quotes Meinecke (Wagner, 'Handwörterbuch der Phys.' B. iv. 1853, s. 775) that with Butterflies the males are three or four times as numerous as the females.
57 'The Naturalist on the Amazons,' vol. ii. 1863, p. 228, 347.
Walsh, who informed me of this statement, says that with P. turnus this is certainly the case. In South Africa, Mr. R. Trimen found the males in excess in 19 species;58 and in one of these, which swarms in open places, he estimated the number of males as fifty to one female. With another species, in which the males are numerous in certain localities, he collected during seven years only five females. In the island of Bourbon, M. Maillard states that the males of one species of Papilio are twenty times as numerous as the females.59 Mr. Trimen informs me that as far as he has himself seen, or heard from others, it is rare for the females of any butterfly to exceed in number the males; but this is perhaps the case with three South African species. Mr. Wallace60 states that the females of Ornithoptera crœsus, in the Malay archipelago, are more common and more easily caught than the males; but this is a rare butterfly. I may here add, that in Hyperythra, a genus of moths, Guenée says, that from four to five females are sent in collections from India for one male.
59 Quoted by Trimen, 'Transact. Ent. Soc.' vol. v. part iv. 1866, p. 330.
60 'Transact. Linn. Soc.' vol. xxv. p. 37."]