CLAIMS OF OOLOGY TO BE AN EXACT SCIENCE. 243 
is, as the genus is at present constituted, an unfortunate one. 
Where, on the other hand, we meet with a genus in which 
there is a marked difference in colour in the eggs of one or 
more species, we are justified in assuming that that genus 
has not been properly diagnosed. It is, for example, impos- 
sible for an oologist to believe that the Song Thrush, Redwing, 
and Fieldfare belong to one and the same genus, or that the 
most recent revision of such genera as Caprimulgus or Cryp- 
turus is satisfactory. The internal colouring of birds’ eggs 
—that is, the ground colour of the inner shell substance— 
is often of great importance as a guide to the oologist. For 
example, the eggs of the White and Black Stork are both 
white and of about the same size, yet they can easily be dis- 
tinguished by the innermost pigments, which in the former is 
yellow and in the latter very dark green. Examples of the 
Kestrel’s egg sometimes very closely resemble those of the 
Sparrow Hawk, yet they can at once be detected, for the inner 
membrane of the egg of the Falcon always shows yellow, and 
that of the Hawk green. The egg of Hieracidea, thoroughly 
falcon-like in its appearance, is hawk-like in the colour of 
the inner shell-substance. The eggs of the Griffon and Black 
Vultures, and those of many Owls, may also be determined 
by the same means. This is not, in coloured eggs, due to 
the external coloration, for the inner membrane reveals its 
normal pigments after the outer one has been removed by 
acid. While the eggs of birds exhibit almost every colour 
known, white and blue, as may be supposed, are by far the 
most common, and on the other hand, black and yellow are 
extremely rare. Eggs of one and the same species of bird 
not unfrequently differ greatly, while others again are remark- 
ably constant in their coloration. Every novice knows that 
there is seemingly no limit to the variation in the colouring 
of the eggs of such birds as the Guillemot and the Tree Pipit. 
But there are far more striking cases than these: for example, 
that of the Fantail Warbler, whose eggs are well known for 
the extent to which they differ in colour. According to the 
Swiss naturalist M. Lunel, the extraordinary variation is due 
to the temperature at the time when the eggs were laid. In 
other words, he adopts, or rather he anticipates, M‘Aldowie’s 
theory that coloration is in due ratio to the amount of light 
