CLAIMS OF OOLOGY TO BE AN EXACT SCIENCE. 237 
that at least seven well-marked substances exist in the 
colouring matter of eggs, to the admixture of which, in 
certain proportions, all their tints are due (Newton, 
Dictionary of Birds). But absolutely nothing is known 
about these substances, their investigation being attended 
with extreme difficulty. It is highly interesting, however, 
to note that Professor Newton believes that one of the seven 
may possibly be due to the growth of minute fungi. The 
relative strength of the shell also demands the serious con- 
sideration of the oologist. Dr Rey has invented a remark- 
ably clever and reliable means of testing the strength of the 
shell; and he has also elaborated a very scientific process 
by which the combined values of the size and weight of the 
egg may be ascertained; but in such a paper as this, it 
would be out of place to describe them. As a rule, those 
eggs which are laid in exposed situations, as, for example, 
on the bare ground, have strong shells; while, conversely, 
such as are laid in holes or crevices have thin shells. The 
thickest egg-shell known to Dr Rey is that of Francolinus 
granti, Hartl., whose nest is a mere hollow scraped in the 
ground. The learned Doctor tells us that an egg of this 
species is nearly six times the weight of a pigeon’s egg of 
exactly the same dimensions. But there are many excep- 
tions to the rule. The egg of the Albatross is laid on the 
ground, yet its shell is relatively extremely fragile; while 
the ege of the Cuckoo is notably remarkable for its strength 
of shell, and is so irrespective of the circumstance whether 
it is placed in a nest made in a hole or not. The egg of 
the Woodcock, though placed on the ground, is, as every- 
one who has blown one can testify, possessed of a very 
thin shell, and so affords a marked contrast to that of the 
Pheasant, though the nesting sites of the two birds are 
practically identical. 
2. THE NUMBER OF THE CLUTCH. 
This is a matter which, though of considerable import- 
ance to the oologist, intimately concerns the ornithologist. 
It is interesting, however, to note that the number of eggs 
VOL. I. 18 
