774 B I 11 D S [EGGS. captivity having arrested the natural secretions. In like manner over excitement or debility of the organs, the con sequence of ill health, give rise to much and often very of curious abnormality. It is commonly believed that the i older a bird is the more intensely coloured will be its eggs, and to some extent this belief appears to be true. Certain Falconidce, which ordinarily lay very brilliantly-tinted eggs, and are therefore good tests, seem when young unable to sscrete so much colouring-matter as they do when older, and season after season the dyes become deeper, but there is reason to think that when the bird has attained its full vigour improvement stops, and a few years later the inten sity of hue begins to decline. It would be well if we had more evidence, however, in support of this opinion, which is chiefly based on a series of eggs of one species the Golden Eagle (Aquila ckrysaetus), in the writer s possession, among which are some believed on good grounds to have been the produce in the course of about twelve years of one and the same female. The amount of colouring-matter secreted and deposited seems notwithstanding to be gene rally a pretty constant quantity allowance being made for ties individual constitution ; but it often happens especially 5 same j n birds that lay only two eggs that nearly all the dye will be deposited on one of these, leaving the other colourless ; it seems, however, to be a matter of inconstancy which of the two is first developed. Thus of two pairs of Golden Eagles eggs also in the possession of the writer, one speci men of each pair is nearly white while the other is deeply coloured, and it is known that in one case the white egg was laid first and in the other the coloured one. When birds lay many mottled, and a fortiori plain, eggs, there is gene rally less difference in their colouring, and though no two can hardly ever be said to be really alike, yet the family resemblance between them all is obvious to the practised eye. It would seem however to be a peculiarity with some species and the Tree-Sparrow (Passer montanus) which lays five or six eggs may be taken as a striking example that one egg should always differ remarkably from the rest of the clutch. In addition to what has been said above as to the deposition of colour in circular spots indicating a pause in the progress of the egg through one part of the oviduct, it may be observed that the cessation of motion at that time is equally shewn by the clearly defined hair-lines or vermiculations seen in many eggs, and in none more commonly met with than in those of the Buntings (Ember- izidce). Such markings must not only have been deposited while the egg was at rest, but it must have remained mo tionless until the pigment was completely set, or blurred instead of sharp edges would have been the result. 1 ire of The composition of this pigment has long excited much ient curiosity, and it has been commonly and rather crudely ascribed to secretions of the blood or bile, 2 but very recently unexpected light has been shed upon the subject by the researches of Mr Sorby (Proc. Zool. Soc. 1875, p. 351), who, using the method of spectrum-analysis, has now ascer tained the existence of seven well-marked substances in the colouring-matter of eggs, to the admixture of which in cer tain proportions all their tints are due. These he names 1 The principal oological works with coloured figures are the follow ing: Thienemann, Fortpflanzungsgeschichte der gesammten Vogel (4to, Leipzig: 1845) ; Lefevre, Atlas des ceufs dcs oiseanx d Europe (Svo, Paris: 1845) ; Hewitson, Coloured Illustrations of the Erjgs of British Birds (Svo, Ed. 3, London: 1856); Brewer, North American Oology (4to, Washington: 1859); Taczanowski, Oologia Ptakow Pclskich (Svo, "Warszawa: 1862); Badeker, Die Eier d er Europdischcn Vogel (fol. Leip zig: 1863) ; Wolley, Ootheca Wolleyana (Svo, London: 1864) some of which have never been completed ; but a great number of rare eggs are also figured in various journals, as the Proceedings of the Zoologi cal Society, Naumannia, the Journal ftir OrnitJwlogie, and The Ibis. % Cf. Wilke, Naumannia, 1858, pp. 393-397, and C. Leconte, Revue et Magasin de Zooloyie, I860, pp. 199-205. Oorhodeine, Oocyan, Banded Oocyan, Yellow Ooxanthine, Rufous Ooxanthine, a substance, giving narrow absorption- bands in the red, the true colour of which he has not yet been able to decide, and lastly Lichenoxanthine. It would be out of place here to particularize their chemical proper ties, and it is enough to say that they are closely connected either with haemoglobin or bile-pigments, and in many respects resemble the latter more than do any other group of colouring-matters, but do not actually agree with them. The first is perhaps the most important of all the seven, because it occurs more or less in the shells of so great a number of eggs that its entire absence is exceptional, and it is of a very permanent character, its general colour being of a peculiar brown-red. The second and third seem when pure to be of a very fine blue, but the spectrum of the former shows no detached bands, while that of the latter has a well-marked detached absorbent-band near the red end, though the two are closely related since they yield the same product when oxidized. The fourth and fifth sub stances supply a bright yellow or reddish-yellow hue, and the former is particularly characteristic of eggs of the Emeus (Dromccus), giving rise when mixed with oocyan to the fine malachite-green which they possess, while the latter has only been met with in those of the Tinamous (Tinamidce), in which it should be mentioned that oorho- deine has not been found, or perhaps in those of a Casso wary (Casuarius), and when mixed with oocyan produces a peculiar lead-colour. The sixth substance, as before stated, has not yet been sufficiently determined, but it would seem in combination with others to give them an abnormally browner tint ; and the seventh appears to be identical with one which occurs in greater or less amount in almost all classes of plants, but is more especially abundant in and characteristic of lichens and fungi. There is a possibility however of this last being in part if not wholly due to the growth of minute fungi, though Mr Sorby believes that some such substance really is a normal constituent of the shell of eggs having a peculiar brick-red colour. That gentleman is further inclined to think that oorhodeinc is in some way or other closely related to cruentine, being probably derived from the red colouring-matter of the blood by some unknown process of secretion, and likewise that there is some chemical relation between the oocyans and the bile. The grain of the egg-shell offers characters that deserve Grain < far more consideration than they have received iintil lately, tlie s ie when the attention of Herr von Nathusius having been directed to the subject by some investigations carried on by Dr Landois 3 and Herr Rudolf Blasius, 4 he has brought out a series of remarkable papers 5 in which he has arrived at the conclusion that a well-defined type of shell-structure belongs to certain families of birds, and is easily recognized under the microscope. In some cases, as in the eggs of certain Swans and Geese (Cygnus olor and C. musicus, Anser cinereus and A. segetum) even specific differences aro apparent. The bearing of these researches on classifica tion generally is of considerable importance and must be taken into account by all future taxonomers. Here we cannot enter into details, it must suffice to remark that the grain of the shell is sometimes so fine that the surface is glossy, and this is th& case with a large number of Pi- carice, where it is also quite colourless and the contents of their eggs seen through the semi-transparent shell give an 3 Zeitschr. fur wissensch. Zooloyie, xv. pp. 1-31, 4 Op. cit. xvii. pp. 480-524. 5 Op. cit. xviii. pp. 19-21, pp. 225-270, xix. pp. 322-348, xx. pp. 106-130, xxi. pp. 330-335. A summary of these will be found in Journ. fiir Ornith. 1871, pp. 241-260, and the subject has been con tinued in the same periodical for 1872, pp. 321-332, and 1874, pp.
I 1-26.