12 BRITISH BIRDS, THEIR EGGS AND NESTS. 
in connection with the very commonest birds. Even eggs from 
the same nest may continually be met with, presenting great 
disparity in bulk; one in the number being frequently so small 
in comparison with the others as to set one invariably thinking 
it must have been the last laid, and that a partial failure of 
ége-producing power in the mother-bird must be the explanation 
of the phenomenon. 
It seems scarcely open to question that the physical condition 
of the parent-bird must exercise a great influence over its egg- 
producing capacity. Its powers may be impaired by age, by the 
past effects of injury or sickness, by a partial failure of some 
necessary element of food, by undue pressure on the egg-pro- 
ducing organs, such as must occur by the loss of one or 
more early layings. Indeed all these causes are well-known to 
interfere with the reproductive energies of animals at large, and 
it is a thoroughly ascertained fact that both the first and the last 
act most strongly in the case of many Birds. The comparative 
size of Birds’ eggs, therefore, seems to me a matter to which 
it is unnecessary, if not inexpedient, to direct the young collector’s 
attention; in any other way, at least, than as to a matter of 
curious observation and contrast. As ameans of identification it 
fails completely, and is only adverted to here for the purpose of 
obviating a portion of the perplexity which may often occur in 
practice to the youthful egg-fancier from the difference in size 
between different specimens of what are in reality eggs of the 
same species of birds, but seem to him, from their discrepancy 
of dimensions, not possibly so. 
Again, the colour and markings of many different species of 
egos are found to admit of great variation. The most familiar 
and striking instance is in the case of the Guillemot: but one 
more within the reach of every nest-hunter is presented by the 
7h, 
