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pecies of Papilio have non…mimetic males with beautifully mimetic females。 The explanation is probably to be sought in the fact that the females of Papilio are more variable and more often tend to become dimorphic than those of Cosmodesmus； while the latter group has more often happened to possess a sufficient foundation for the origin of the resemblance in patterns which； from the start； were common to male and female。
（5） In very variable species with sexes alike； mimicry can be rapidly evolved in both sexes out of very small beginnings。 Thus the reddish marks which are common in many individuals of Limenitis arthemis were almost certainly the starting…point for the evolution of the beautifully mimetic L。 archippus。 Nevertheless in such cases； although there is no reason to suspect any greater variability； the female is commonly a somewhat better mimic than the male and often a very much better mimic。 Wallace's principle seems here to supply the obvious interpretation。
（6） When the difference between the patterns 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 sound； Darwin was quite aware that it was probably the most vulnerable part of the 〃Origin〃。 Thus he wrote to H。W。 Bates； April 4； 1861： 〃If I had to cut up myself in a review I would have （worried？） and quizzed sexual selection； therefore； though I am fully convinced that it is largely true； you may imagine how pleased I am at what you say on your belief。〃 （〃More Letters〃； I。 page 183。）
The existence of sound…producing organs in the males of insects was； Darwin considered； the strongest evidence in favour of the operation of sexual selection in this group。 （〃Life and Letters〃； III。 pages 94； 138。） Such a conclusion has received strong support in recent years by the numerous careful observations of Dr F。A。 Dixey （〃Proc。 Ent。 Soc。 Lond。〃 1904； page lvi； 1905； pages xxxvii； liv； 1906； page ii。） and Dr G。B。 Longstaff （〃Proc。 Ent。 Soc。 Lond。〃 1905； page xxxv； 〃Trans。 Ent。 Soc。 Lond。〃 1905； page 136； 1908； page 607。） on the scents of male butterflies。 The experience of these naturalists abundantly confirms and extends the account given by Fritz Muller （〃Jen。 Zeit。〃 Vol。 XI。 1877； page 99； 〃Trans。 Ent。 Soc。 Lond。〃 1878； page 211。） of the scents of certain Brazilian butterflies。 It is a remarkable fact that the apparently epigamic scents of male butterflies should be pleasing to man while the apparently aposematic scents in both sexes of species with warning colours should be displeasing to him。 But the former is far more surprising than the latter。 It is not perhaps astonishing that a scent which is ex hypothesi unpleasant to an insect… eating Vertebrate should be displeasing to the human sense； but it is certainly wonderful that an odour which is ex hypothesi agreeable to a female butterfly should also be agreeable to man。
Entirely new light upon the seasonal appearance of epigamic characters is shed by the recent researches of C。W。 Beebe （〃The American Naturalist〃； Vol。 XLII。 No。 493； Jan。 1908； page 34。）； who caused the scarlet tanager （Piranga erythromelas） and the bobolink （Dolichonyx oryzivorus） to retain their breeding plumage through the whole year by means of fattening food； dim illumination； and reduced activity。 Gradual restoration to the light and the addition of meal…worms to the diet invariably brought back the spring song； even in the middle of winter。 A sudden alteration of temperature； either higher or lower； caused the birds nearly to stop feeding； and one tanager lost weight rapidly and in two weeks moulted into the olive…green winter plumage。 After a year； and at the beginning of the normal breeding season； 〃individual tanagers and bobolinks were gradually brought under normal conditions and activities；〃 and in every case moulted from nuptial plumage to nuptial plumage。 〃The dull colours of the winter season had been skipped。〃 The author justly claims to have established 〃that the sequence of plumage in these birds is not in any way predestined through inheritance。。。； but that it may be interrupted by certain factors in the environmental complex。〃
XVI。 GEOGRAPHICAL DISTRIBUTION OF PLANTS。
By SIR WILLIAM THISELTON…DYER； K。C。M。G。； C。I。E。 Sc。D。； F。R。S。
The publication of 〃The Origin of Species〃 placed the study of Botanical Geography on an entirely new basis。 It is only necessary to study the monumental 〃Geographie Botanique raisonnee〃 of Alphonse De Candolle； published four years earlier （1855）； to realise how profound and far… reaching was the change。 After a masterly and exhaustive discussion of all available data De Candolle in his final conclusions could only arrive at a deadlock。 It is sufficient to quote a few sentences：
〃L'opinion de Lamarck est aujourd'hui abandonee par tous les naturalistes qui ont etudie sagement les modifications possibles des etres organises。。。
〃Et si l'on s'ecarte des exagerations de Lamarck； si l'on suppose un premier type de chaque genre； de chaque famille tout au moins； on se trouve encore a l'egard de l'origine de ces types en presence de la grande question de la creation。
〃Le seul parti a prendre est donc d'envisager les etres organises comme existant depuis certaines epoques； avec leurs qualites particulieres。〃