186 EGGS 
that when the bird has attained her full vigour improvement stops, 
and a few years later the intensity of hue begins to decline. It 
would be weil 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 chrysaetus, 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 generally a pretty constant quantity— 
allowance being made for individual constitution; but it often 
happens—especially in 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 specimen 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 generally 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 most BuNTINGS, Hmberizide. Such markings 
must not only have been deposited while the egg was at rest, but 
it must have remained motionless until the pigment was completely 
set, or blurred instead of sharp edges would have been the result. 
1 The principal oological works with coloured figures are the following: 
Thienemann, Fortpflanzungsgeschichte der gesammten Vogel (4to, Leipzig: 1845) ; 
Lefévre, Atlas des wujs des oiseaux d’ Europe (8vo, Paris: 1845); Hewitson, 
Coloured Illustrations of the Eggs of British Birds (8vo, Ed. 3, London: 1856) ; 
Brewer, North American Oology (4to, Washington: 1859) ; Taczanowski, Oologia 
Ptakéw Polskich (8vo, Warszawa: 1862); Baideker, Die Hier der Huwropiischen 
Vogel (fol. Leipzig: 1863); Wolley, Ootheca Wolleyana (8vo, London: 1864)— 
some of which have never been completed. The above is not, and does not 
profess to be, an exhaustive list, and perhaps some others deserve inclusion in 
it; but there are works, chiefly on British oology, which have unfortunately 
attained considerable notoriety, though really unworthy of serious notice, either 
from the recklessly inaccurate statements to be found in the text which accom- 
panies the plates. or the misleading tendencies of the plates. I prefer passing 
