Mr. Lonsdale, informs me that he placed a pair of land-snails, (Helix pomatia), one of which was weakly, into a small and illprovided garden. After a short time the strong and healthy individual disappeared, and was traced by its track of slime over a wall into an adjoining well-stocked garden. Mr. Lonsdale concluded that it had deserted its sickly mate; but after an absence of twenty-four hours it returned, and apparently communicated the result of its successful exploration, for both then started along the same track and disappeared over the wall. Even in the highest class of the Mollusca, the Cephalopoda or cuttlefishes, in which the sexes are separate, secondary sexual characters of the present kind do not, as far as I can discover, occur. This is a surprising circumstance, as these animals possess highly-developed sense-organs and have considerable mental powers, as will be admitted by every one who has watched their artful endeavours to escape from an enemy.3 Certain Cephalopoda, however, are characterised by one extraordinary sexual character, namely, that the male element collects within one of the arms or tentacles, which is then cast off, and clinging by its sucking-discs to the female, lives for a time an independent life. So completely does the cast-off arm resemble a separate animal, that it was described by Cuvier as a parasitic worm under the name of Hectocotyle. But this marvellous structure may be classed as a primary rather than as a secondary sexual character. Although with the Mollusca sexual selection does not seem to have come into play; yet many univalve and bivalve shells, such as volutes, cones, scallops, &c., are beautifully coloured and shaped. The colours do not appear in most cases to be of any use as a protection; they are probably the direct result, as in the lowest classes, of the nature of the tissues; the patterns and the sculpture of the shell depending on its manner of growth. The amount of light seems to be influential to a certain extent; for although, as repeatedly stated by Mr. Gwyn Jeffreys, the shells of some species living at a profound depth are brightly coloured, yet we generally see the lower surfaces, as well as the parts covered by the mantle, less highly-coloured than the upper and exposed surfaces. In some cases, as with shells 4 3 See, for instance, the account which I have given in my 'Journal of Researches,' 1845, p. 7. I have given (Geolog. Observations on Volcanic Islands,' 1844, p. 53) a curious instance of the influence of light on the colours of a fror descent incrustation, deposited by the surf on the coastrocks of Ascension, and formed by the solution of triturated sea-shells 5 living amongst corals or brightly-tinted sea-weeds, the bright colours may serve as a protection. But that many of the nudibranch mollusca, or sea-slugs, are as beautifully coloured as any shells, may be seen in Messrs. Alder and Hancock's magnificent work; and from information kindly given me by Mr. Hancock, it seems extremely doubtful whether these colours usually serve as a protection. With some species this may be the case, as with one kind which lives on the green leaves of algæ, and is itself bright-green. But many brightly-coloured, white or otherwise conspicuous species, do not seek concealment; whilst again some equally conspicuous species, as well as other dull-coloured kinds, live under stones and in dark recesses. So that with these nudibranch molluscs, colour apparently does not stand in any close relation to the nature of the places which they inhabit. These naked sea-slugs are hermaphrodites, yet they pair together, as do land-snails, many of which have extremely pretty shells. It is conceivable that two hermaphrodites, attracted by each other's greater beauty, might unite and leave offspring which would inherit their parents' greater beauty. But with such lowly-organised creatures this is extremely improbable. Nor is it at all obvious how the offspring from the more beautiful pairs of hermaphrodites would have any advantage over the offspring of the less beautiful, so as to increase in number, unless indeed vigour and beauty generally coincided. We have not here the case of a number of males becoming mature before the females, with the more beautiful males selected by the more vigorous females. If, indeed, brilliant colours were beneficial to a hermaphrodite animal in relation to its general habits of life, the more brightly-tinted individuals would succeed best and would increase in number; but this would be a case of natural and not of sexual selection. Sub-kingdom of the Vermes: Class, Annelida (or Sea-worms).— In this class, although the sexes, when separate, sometimes differ from each other in characters of such importance that they have been placed under distinct genera or even families, yet the differences do not seem of the kind which can be safely attributed to sexual selection. These animals are often beautifully coloured, but as the sexes do not differ in this respect, we are but little concerned with them. Even the Nemertians, though so lowly organised, "vie in beauty and variety of "colouring with any other group in the invertebrate series;" yet 5 Dr. Morse has lately discussed this subject in his paper on the Adaptive Coloration of Mollusca, 'Proc. Boston Soc. of Nat. Hist. vol. xiv., April, 1871. Dr. McIntosh cannot discover that these colours are of any service. The sedentary annelids become duller-coloured, according to M. Quatrefuges,' after the period of reproduction; and this I presume may be attributed to their less vigorous condition at that time. All these worm-like animals apparently stand too low in the scale for the individuals of either sex to exert an} choice in selecting a partner, or for the individuals of the san.e sex to struggle together in rivalry. Sub-kingdom of the Arthropoda: Class, Crustacea.- In this great class we first meet with undoubted secondary sexual characters, often developed in a remarkable manner. Unfortunately the habits of crustaceans are very imperfectly known, and we cannot explain the uses of many structures peculiar to one sex. With the lower parasitic species the males are of small size, and they alone are furnished with perfect swimming-legs, antennæ and sense-organs; the females being destitute of these organs, with their bodies often consisting of a mere distorted mass. But these extraordinary differences between the two sexes are no doubt related to their widely different habits of life, and consequently do not concern us. In various crustaceans, belonging to distinct families, the anterior antennæ are furnished with peculiar thread-like bodies, which are believed to act as smelling-organs, and these are much more numerous in the males than in the females. As the males, without any unusual development of their olfactory organs, would almost certainly be able sooner or later to find the females, the increased number of the smellingthreads has probably been acquired through sexual selection, by the better provided males having been the more successful in finding partners and in producing offspring. Fritz Müller has described a remarkable dimorphic species of Tanais, in which the male is represented by two distinct forms, which never graduate into each other. In the one form the male is furnished with more numerous smelling-threads, and in the other form with more powerful and more elongated chelæ or pincers, which serve to hold the female. Fritz Müller suggests that these differences between the two male forms of the same species may have originated in certain individuals having varied in the number of the smelling-threads, whilst other individuals varied in the chape and size of their chelæ; so that of the former, those which were best able to find the female, and of the latter, those which See his beautiful monograph on 'British Annelids,' part i. 1873, P. 3. 7 See M. Perrier, l'Origine de l'Homme d'après Darwin,'' Revue Scientifique,' Feb. 1873, p. 866. were best able to hold her, have left the greatest number of progeny to inherit their respective advantages. b с In some of the lower crustaceans, the right anterior antenna of the male differs greatly in structure from the left, the latter resembling in its simple tapering joints the antennæ of the female. In the male the modified antenna is either swollen in the middle or angularly bent, or converted (fig. 4) into an elegant, and sometimes wonderfully complex, prehensile organ. It serves, as I hear from Sir J. Lubbock, to hold the female, and for this same purpose one of the two posterior legs (b) on the same side of the body is converted into a forceps. In another family the inferior or posterior antennæ are curiously zigzagged" in the males alone. Fig. 4. a. Part of right anterior an- b. Posterior pair of thoracic legs of male. c. Ditto of iemale. In the higher crustaceans the anterior legs are developed into chelæ or pincers; and these are generally larger in the male than in the female, -so much so that the market value of the male edible crab (Cancer pagurus), Labidocera Darwinii according to Mr. C. Spence Bate, is five times as great as that of the female. In many species the chelæ are of unequal size on the opposite side of the body, the right-hand one being, as I am informed by Mr. Bate, generally, though not invariably, the largest. This inequality is also often much greater in the male than in the female. The two chela of the male often differ in structure (figs. 5, 6, and 7), the smaller one resembling that of the female. What advantage is gained by their inequality in size on the opposite sides of the 8 Facts and Arguments for Darwin,' English translat. 1869, p. 20. See the previous discussion on the olfactory threads. Sars has described a somewhat analogous case (as quoted in Nature,' 1870, p. 455) in a Norwegian crustacean, the Pontoporcia affinis. 9 See Sir J. Lubbock in 'Annals and Mag. of Nat. Hist.' vol. xi. 1853, pl. i. and x.; and vol. xii. (1853) pl. vii. See also Lubbock in Transact. Ent. Soc.' vol. iv. new series, 1856-1858, p. 8. With respect to the zig-zagged antennæ mentioned below, see Fritz Muller, Facts and Arguments for Darwin,' 1869, p. 40, foot-note. body, and by the inequality being much greater in the male than in the female; and why, when they are of equal size, both are Fig. 5. Anterior part of body of Callianassa (from Milne-Edwards), showing the unequal and differently-constructed right and left-hand chelæ of the male. N.B-The artist by mistake has reversed the drawing, and made the left-hand chela the largest. Fig. 6. Second leg of male Orchestia Tucuratinga (from Fritz Müller). often much larger in the male than in the female, is not known. As I hear from Mr. Bate, the chelæ are sometimes of such length and size that they cannot possibly be used for carrying food to the mouth. In the males of certain fresh-water prawns (Palæmon) the right leg is actually longer than the whole body.10 The great size of the one leg with its chelæ may aid the male in fighting with his rivals; but this will not account for their |