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of the model and presumed ancestor of the mimic is very great, the female is often alone mimetic; when the difference is comparatively small, both sexes are commonly mimetic. The Nymphaline genus Hypolimnas is a good example. In Hypolimnas itself the females mimic Danainae with patterns very different from those preserved by the non-mimetic males: in the sub-genus Euralia, both sexes resemble the black and white Ethiopian Danaines with patterns not very dissimilar from that which we infer to have existed in the non-mimetic ancestor.
(7) Although a melanic form or other large variation may be of the utmost importance in facilitating the start of a mimetic likeness, it is impossible to explain the evolution of any detailed resemblance in this manner. And even the large colour variation itself may well be the expression of a minute and "continuous" change in the chemical and physical constitution of pigments.
SEXUAL SELECTION (EPIGAMIC CHARACTERS).
We do not know the date at which the idea of Sexual Selection arose in Darwin's mind, but it was probably not many years after the sudden flash of insight which, in October 1838, gave to him the theory of Natural Selection. An excellent account of Sexual Selection occupies the concluding paragraph of Part I. of Darwin's Section of the Joint Essay on Natural Selection, read July 1st, 1858, before the Linnean Society. ("Journ. Proc. Linn. Soc." Vol. III. 1859, page 50.) The principles are so clearly and sufficiently stated in these brief sentences that it is appropriate to quote the whole: "Besides this natural means of selection, by which those individuals are preserved, whether in their egg, or larval, or mature state, which are best adapted to the place they fill in nature, there is a second agency at work in most unisexual animals, tending to produce the same effect, namely, the struggle of the males for the females. These struggles are generally decided by the law of battle, but in the case of birds, apparently, by the charms of their song, by their beauty or their power of courtship, as in the dancing rock-thrush of Guiana. The most vigorous and healthy males, implying perfect adaptation, must generally gain the victory in their contests. This kind of selection, however, is less rigorous than the other; it does not require the death of the less successful, but gives to them fewer descendants. The struggle falls, moreover, at a time of year when food is generally abundant, and perhaps the effect chiefly produced would be the modification of the secondary sexual characters, which are not related to the power of obtaining food, or to defence from enemies, but to fighting with or rivalling other males. The result of this struggle amongst the males may be compared in some respects to that produced by those agriculturists who pay less attention to the careful selection of all their young animals, and more to the occasional use of a choice mate."
A full exposition of Sexual Selection appeared in the "The Descent of Man" in 1871, and in the greatly augmented second edition, in 1874. It has been remarked that the two subjects, "The Descent of Man and Selection in Relation to Sex", seem to fuse somewhat imperfectly into the single work of which they form the title. The reason for their association is clearly shown in a letter to Wallace, dated May 28, 1864: "...I suspect that a sort of sexual selection has been the most powerful means of changing the races of man." ("More Letters", II. page 33.)
Darwin, as we know from his Autobiography ("Life and Letters", I. page 94.), was always greatly interested in this hypothesis, and it has been shown in the preceding pages that he was inclined to look favourably upon it as an interpretation of many appearances usually explained by Natural Selection. Hence Sexual Selection, incidentally discussed in other sections of the present essay, need not be considered at any length, in the section specially allotted to it.
Although so interested in the subject and notwithstanding his conviction that the hypothesis was
(7) Although a melanic form or other large variation may be of the utmost importance in facilitating the start of a mimetic likeness, it is impossible to explain the evolution of any detailed resemblance in this manner. And even the large colour variation itself may well be the expression of a minute and "continuous" change in the chemical and physical constitution of pigments.
SEXUAL SELECTION (EPIGAMIC CHARACTERS).
We do not know the date at which the idea of Sexual Selection arose in Darwin's mind, but it was probably not many years after the sudden flash of insight which, in October 1838, gave to him the theory of Natural Selection. An excellent account of Sexual Selection occupies the concluding paragraph of Part I. of Darwin's Section of the Joint Essay on Natural Selection, read July 1st, 1858, before the Linnean Society. ("Journ. Proc. Linn. Soc." Vol. III. 1859, page 50.) The principles are so clearly and sufficiently stated in these brief sentences that it is appropriate to quote the whole: "Besides this natural means of selection, by which those individuals are preserved, whether in their egg, or larval, or mature state, which are best adapted to the place they fill in nature, there is a second agency at work in most unisexual animals, tending to produce the same effect, namely, the struggle of the males for the females. These struggles are generally decided by the law of battle, but in the case of birds, apparently, by the charms of their song, by their beauty or their power of courtship, as in the dancing rock-thrush of Guiana. The most vigorous and healthy males, implying perfect adaptation, must generally gain the victory in their contests. This kind of selection, however, is less rigorous than the other; it does not require the death of the less successful, but gives to them fewer descendants. The struggle falls, moreover, at a time of year when food is generally abundant, and perhaps the effect chiefly produced would be the modification of the secondary sexual characters, which are not related to the power of obtaining food, or to defence from enemies, but to fighting with or rivalling other males. The result of this struggle amongst the males may be compared in some respects to that produced by those agriculturists who pay less attention to the careful selection of all their young animals, and more to the occasional use of a choice mate."
A full exposition of Sexual Selection appeared in the "The Descent of Man" in 1871, and in the greatly augmented second edition, in 1874. It has been remarked that the two subjects, "The Descent of Man and Selection in Relation to Sex", seem to fuse somewhat imperfectly into the single work of which they form the title. The reason for their association is clearly shown in a letter to Wallace, dated May 28, 1864: "...I suspect that a sort of sexual selection has been the most powerful means of changing the races of man." ("More Letters", II. page 33.)
Darwin, as we know from his Autobiography ("Life and Letters", I. page 94.), was always greatly interested in this hypothesis, and it has been shown in the preceding pages that he was inclined to look favourably upon it as an interpretation of many appearances usually explained by Natural Selection. Hence Sexual Selection, incidentally discussed in other sections of the present essay, need not be considered at any length, in the section specially allotted to it.
Although so interested in the subject and notwithstanding his conviction that the hypothesis was