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CHAPTER VII.

COLOUR OF EGGS.

THOUGH We may safely lay it down as an invariable principle of nature that nothing is made in vain, and that every circumstance connected with organic life is designed to fulfil some purpose; yet we are frequently at a loss in our researches to discover the designs of the Creator in particular instances. In reference to this consideration, the subject of colours is one of considerable interest, but of no less difficulty. It has given origin to many ingenious theories founded upon a few facts partially selected; while little has hitherto been done in grouping ascertained facts into a general view. It has been maintained, for example, that the varied colours of flowers were intended to please the eye of man, an assumption to which, as well as to many others of a similar kind, the lines of Pope may perhaps be thought a sufficient answer:

"While man exclaims, 'see all things for my use,'

'See man for mine!' replies a pamper'd goose."

Again, if we suppose that the colour of each particular flower is peculiarly adapted to its economy by the refraction or reflection of various-coloured rays of light, an opinion, at first sight, extremely plausible, we are immediately met by the fact of the varying colour in the same species, when equally healthy, and, so far as we can perceive, efficient in performing the functions of growth and seed-bearing. We have, for example, at present a collection of primroses and another of heart's-ease (Viola tricolor), of almost every

shade of colour from nearly pure white to dark purplish black, and yet all these varieties seem to be equally thriving and untainted with disease, or what the Linnæan botanists are accustomed to designate monstrosity. The case is the same with animals, as may be shown, among numerous other instances, in the banded snail-shell (Helix nemoralis). Within the space of half a mile we have collected not less than a dozen varieties of this species, having from one band to seven, and the bands as diversified in the shades of their colour as in their breadth and arrangement, some being very pale and others of a dark blackish brown. In the garden spider (Epeira diadema) there is an equal diversity of markings and shades of colour, some being bright orange, others dark brown, and others greenish grey, while the spots are sometimes large and conspicuous, and sometimes small and indistinct. The causes of this diversity we have been unable to trace in a satisfactory manner, though we have for many years paid considerable attention to the subject; we have mentioned it here merely to introduce what we are about to say on the subject of the diversity of colour in the eggs of birds, by reminding the reader that the phenomenon, however obscure as to its final cause, is not unexemplified in other natural productions.

If we advert to the manner in which the shell of an egg is formed, we may discover some of the circumstances attendant on the colouring and the markings. The shell has been ascertained to be a secretion whose basis is lime, derived from the glands of the egg tube after the nucleus, consisting of the yolk and white, has advanced from the egg-bag (ovarium). The white colour of the eggs of the barn-door fowl, the dull cream colour of those of the pheasant, the greenish brown of those of the nightingale, and the * J, R..

bright pale blue of those of the redstart and the hedge-sparrow (Accentor modularis), must be produced, like the black bones in the fowls of Malabar, by a uniform colouring matter in the calcareous secretion; and we accordingly find, that when subjected to the action of dilute hydrochloric acid, the whole shell of an egg, uniformly coloured, becomes dissolved, the dissolution being as perfect in the instance of the blue egg of the hedge-sparrow and the green egg of the nightingale as in the ivory-white egg of the woodpecker. A similar experiment upon any uniformlycoloured shell, such as the common pale yellow snailshell (Helix hortensis) is attended with nearly the same result, a thin membranous pellicle only remaining undissolved by the acid. But when the banded snail-shell (H. nemoralis) is subjected to the acid, the coloured bands having less calcareous and more animal matter, remain in a loosened and somewhat flocculent form, but considerably thicker than the portion where the ground colour prevails, proving that the bands consist chiefly of animal matter. On examining the mantle of the snail from which the shell is secreted, we find that it is marked with dark translucent bands, exactly correspondent to the bands on the shell; and it may be that these bands, being less brittle than the other portions of the shell, are intended to strengthen its texture.

The various markings on the eggs of many species of birds are, in all probability, formed in a similar manner to the bands on snail-shells, namely, by glands secreting colouring matter, distributed amongst those which secrete the general ground colour or the uncoloured portions. From the formation of the eggshell, however, taking place within the egg-tube, it is out of the reach of observation, while the process of forming the snail-shell can be seen at every step. This probability is strengthened by the similar effect

of the dilute hydrochloric acid on the markings of the eggs of birds; for, if we apply this acid to the egg of a song-thrush, which has a ground colour of a bright blue, with irregular spots and blotches of black, these blotches remain while the blue portion is dissolved and disappears. The same takes place with respect to the markings on the eggs of the lapwing, the chaffinch, the yellow-hammer, the butcherbird, the magpie, and the house-sparrow.

From the various examinations which we have made of the coloured markings of these and other eggs, they appear to consist of an animal oil, partly soluble in spirits of wine, similar to that which, as M. Odier has shown, forms the colouring matter of the wingcases (elytra) of beetles*; but there seems to be some mucilage also present. This view of the matter is further confirmed by the observations of Mr. Knapp, who tells us that the calcareous matter is partly taken up during incubation, the markings upon the eggs remaining little injured, and even to the last being almost as strongly defined as when the eggs are first laid t.

From these markings being for the most part either exclusively or more numerous at the larger end of the egg. it would appear that the glands that secrete the colouring matter require the stimulus caused by the pressure of the egg to bring them into activity; and hence, also, we may account for the zone of markings frequently conspicuous on the eggs of the white-throat, the flusher (Lanius Collurio), and many others. We have just witnessed a fact corroborative of this explanation. A hen canary which we paired with an aberdevine (Carduelis Spinus) in her second laying, (the first having proved abortive,) had two eggs of different sizes, one of the regular size, * Mem. Soc. d'Hist. Nat. de Paris, tome i. Journal of a Naturalist, p. 223, 3d edit.

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like an encampment along the banks. They stood fifteen or twenty yards distant from the water, on a high marsh about four feet perpendicular above the water. I knew them to be the nests of the crocodile, having had a description of them before, and now expected a furious and general attack, as I saw several large crocodiles swimming abreast of the buildings. These nests being so great a curiosity to me, I was determined at all events immediately to land and examine them. Accordingly, I ran my bark on shore at one of their landing places, which was a sort of nick or little dock from which ascended a sloping path or road up to the edge of the meadow where their nests were; most of them were deserted, and the great whitish egg-shells lay broken and scattered upon the ground round about them. The nests or hillocks are of the form of an obtuse cone, four feet high and four or five feet in diameter at their bases; they are constructed with mud, grass, and herbage. At first they lay a floor of this kind of tempered mortar on the ground, upon which they deposit a layer of eggs, and upon this a stratum of mortar seven or eight inches in thickness, and then another layer of eggs; and in this manner, one stratum upon another, nearly to the top. I believe they commonly lay from one to two hundred eggs in a nest. These are hatched, I suppose, by the heat of the sun; and perhaps the vegetable substances mixed with the earth, being acted upon by the sun, may cause a small degree of fermentation, and so increase the heat in those hillocks *.'

A later writer, M. Descourtilz, while he denies the employment of vegetable substances in the nest, and proves the number of eggs to be exactly twenty-eight, adds the interesting fact that the mother alligator slopes the top of the nest in order to let the rain run *Bartram's Travels, p, 125, edit. London, 1794.

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