August 15, 187S] 



NATURE 



427 



colouring matters in the shells of birds' eggs. Permit me to 

 call attention, through your journal, to a long and detailed 

 paper on this subject published by me, about four years ago, in 

 the Proceedings of the Zoological Society of London. Appa- 

 rently the authors who have lately treated on this subject have 

 not been aware of the existence of this paper. As far as they 

 go, the facts they have described fully confirm my conclusions ; 

 but, as I have shown, different birds' eggs contain at least five 

 perfectly distinct coloured substances, and not, as Liebermann 

 says, only two. One of these is closely related to a product of 

 the decomposition of hremoglobin and another to the bile pig- 

 ments, it and the bile of birds yielding the same well-marked 

 product on oxidisation. H. C. SORBY 



Kingstown, Dublin, August 6 



The Limbs 



In his interesting papers on "The Genesis of the Limbs," 

 which have recently appeared in Nature, Mr. Mivart mentions 

 that I have represented the limbs as modified portions of a 

 primitively continuous inferior azygos fin. 



That view was stated by me at length in a paper in the 

 journal of Anatomy and Physiology', 1871, vol. v., p. 

 59. It was formed from the following considerations among 

 others — First, that the mesial, or azygos fins, are essentially 

 double organs, being formed from the coalesced elements 

 of the dorsal and ventral plates of the two sides, and being 

 furnished with muscles, nerves, and blood-vessels, from the 

 two sides. They may therefore be the representatives of 

 organs which remain double, that is, of organs in which 

 the lateral elements do not coalesce. Secondly, the limbs 

 and the azygos fins do not co-exist in the same region. The 

 limbs are formed where the ventral plates are kept apart and 

 expanded by the presence of the visceral cavity, so that the 

 elements which in the dorsal and postanal regions meet and 

 unite into azygos fins, are here separated and grow out as lateral 

 limbs. The ventral plates are, moreover, continued onwards 

 beyond the outgrowing line of the limbs, and form the median 

 portion of the visceral wall which lies beneath and between the 

 limbs on the two sides. Thirdly, there is such a marked resem- 

 blance between the ventral fins in some fishes and the anal fins, 

 that the transition from the one to the other is easy ; the dagger- 

 shaped pelvic bones being the representatives of two or more 

 coalesced intraspinous bones and the ray bones of the one set 

 being, in like manner, the representatives of the ray bones of 

 the other. 



Mr. Balfour in his admirable papers on " The Development of 

 Elasmobranch Fishes," in the journal 0/ Anatomy and Physio- 

 logy shows (vol. xi. p. 133), that the limbs are the remnants of 

 continuous lateral fins, and that the ridges from which they are 

 developed are in eveiy way like the folds from which the un- 

 paired fins are formed, but the development and growth are 

 confined to two special points on each side instead of being 

 continued, as in the case of the dorsal and anal fins, along a 

 greater length of the fold. He further remarks that, externally, 

 they closely resemble the unpaired fins, and both their position 

 and nervous supply indicate that they do not belong to one 

 special segment of the body. The lateral ridges, from which 

 they are developed, I conceive to be the continuations of the 

 diverging lateral halves of the essentially double ridge of the 

 caudal fin kept asunder by the presence of the visceral cavity. 

 These are but little separated in the position of the ventral fins, 

 and are more so in the position of the pectoral fins. If this be 

 so, the limbs are specialised differentiations of primitively 

 continuous lateral folds — of portions, that is, of the diverging 

 plates of the median fold from which the caudal fin is developed. 



I cannot, however, assent to Mr. Mivart's view that the limbs are 

 mere appendages to the axial system, or admit that either they or 

 the limb girdles are the result of centripetal growth, " due to the 

 in growth of originally superficial structures — exoskeletal harden- 

 ings which have grown inwards and become endoskeletal." The 

 limb girdles are found in the same plane of the mesoblast as the 

 ribs, and are, as I have shown in the '* Anatomy of the Crypto- 

 branch " [Journal of Anatomy, vol. vi. p. 9, and Observations 

 on Myology), though not necessarily the serial homologues of the 

 ribs, yet like them the result of ossification in the ventral trans- 

 verse intermuscular septa. As they grow out ( hey carry before 

 them envelopes not only of skin, but of muscle, derived from 

 the body-wall, which become differentiated according to the 



requirements. Moreover, although -'the ridges from which they 

 are primarily developed may appear at first as epiblastic pro- 

 jections, these are soon supplemented by accumulations of 

 mesoblastic tissue in which the components of the limbs are 

 chiefly formed. G. M. Humphry 



Cambridge, August i 



The Darkness of Caverns 



The impenetrable darkness of caverns has been for a very 

 long time a recognised fact, without its cause having been satis- 

 factorily explained. This darkness vanishes but partially before 

 torch-light, and that only in a very limited radius. I, in my 

 explorations in the caverns of Spain, had also noticed this cir- 

 cumstance, and now that I have verified it in others in Switzer- 

 land, I venture to think that I have found the explanation of 

 this phenomenon. 



The walls as well as the roof and floor of caverns are con- 

 tinually covered with moisture, which works without interrup- 

 tion in condensing the corpuscles that float in that circumscribed 

 space. It thus performs the same function that the glycerine 

 does which varnishes the sides of the crystal box by means of 

 which Prof. TjTidall obtains an optical vacuum, the light dif- 

 fusing itself imperfectly from want of those atoms ■ which act as 

 reflecting bodies. I have had occasion to verify my supposition 

 by scattering around the torch very fine dust of different sub- 

 stances. The brightness diffused itself regularly all the time 

 that the dust maintained itself in the required state of closeness 

 and fluctuation, and vanished again slowly as the dust spread or 

 deposited itself. The earth or common dust is the one which, 

 in my experience, has produced the best effect. 



Salvador Calderon 



Scent and Colour in Flowers 



The extension of our perceptive faculties of sight and hearing 

 by various optical and acoustical instruments may enable us to 

 comprehend the possibility of these faculties existing in other 

 creatures to a degree so far surpassing ours as to seem a differ- 

 ence almost of kind. So the sight of the vulture would seem to 

 be paralleled by the faculty of smell in moths, as evidenced by 

 the detection of distant females by males. It would seem pro- 

 bable that the sense of smell may guide insects at a far greater 

 distance than that of vision ; for a consideration of the structure 

 of the eyes of insects leads to the belief that they are not capable 

 of forming clear images of distant objects. While, then, the 

 scent of its blossoms may attract insects to a plant, their colour 

 will act as a subsequent guide to the individual flowers, just as 

 variegations undoubtedly act as honey-guides when the insect 

 reaches the flower. This view is borne out, firstly, by the un- 

 doubted connection between perfume and pollination, shown by 

 Morren in the case of the orchid Alaxillaria, whose aromatic 

 perfume lasts till pollination ; and, secondly, by the well known 

 connection of odovu: both with colour and with natural groups, 

 white flowers being mostly sweet-scented, brown and orange 

 ones most fetid. The insect could thus identify species before 

 seeing them. Mr. Wallace has been, perhaps justly, blamed 

 by a writer in the Gardener's Chronicle for saying that brightly- 

 coloured flowers are seldom scented, and Dr. Taylor by "J. S. G." 

 (Nature, vol. xviii. p. 277), for saying that white flowers 

 open mostly at night. It would, I think, be truer to say that 

 few flowers are both variegated and scented, i.e., that scented 

 flowers are mostly monochromcus, and that the majority of night- 

 blowing flowers are white. The latter is a very different matter 

 from saying that the majority of white flowers are night-blowing. 

 We can perceive with difficulty that one part of a flower is more 

 scented than another, yet scent may replace the dots and point - 

 indicating lines of variegation to the senses of an insect. Natiire 

 not only often effects one purpose by divers means, but also uses 

 one means for divers ends ; so just as colour exists in plants, not 

 only to attract insects, we can understand it being absent in 

 some white flowers simply as a phenomenon of degradation and 

 not as one of specialisation. The dog-rose, white convolvulus, 

 and daisy, mentioned by Mr. Gardner as closing at eventide, are 

 all scentless. The first, according to Dr. Hermann Miiller, is 

 visited by six hymenoptera, two diptera, and twelve coleoptera. 

 The convolvulus does not close till between eight and ten p.m., 

 and re-opens by moonlight. It is visited by two diptera, Podura, 

 Thrips, one coleopteron, two hymenoptera, and the Sphinx con- 



