430 The Bird 
reptilian horizon, and greatly altered all the conditions 
of their life. The history of the egg of a bird, from the 
time it is laid until it hatches, has an all-important effect 
on its form, colour, and even upon the number of eggs 
laid. This is not strange when we consider that every 
minute of the bird’s life is open to many dangers, and 
that the egg stage—that bridging over of generations— 
is a most precarious period. 
That which adds the greatest interest to anything is 
the why of it, and a vast collection of eggs, beautiful 
though they are, yet, if ignorantly looked at, is worse than 
useless. Why one bird lays twenty eggs and another 
but two; why one bird’s eggs are white, another’s of varied 
colours, we will never learn from blown museum speci- 
mens. Not until we have the patience and skill to watch 
and to find the most deadly enemies which threaten the 
nests and eggs of birds, their number and modes of attack, 
can we hope for successful soiutions to the thousand and 
one problems which offer themselves. What we know in 
respect to eggs is fragmentary and rests on so slight a 
degree of proof that every theory is attacked and re- 
attacked in turn. 
Supposing that the eggs of the early forms of birds 
were round,—that being the most typical form of a single 
cell,—we find many variations in shape among the eggs 
of living species. Many of the eggs which are laid in 
hollow trees still retain the primitive spherical form, per- 
haps an advantage in keeping the eggs in a close group 
in the centre of the floor of the cavity. 
So characteristic of the eggs of birds is the pear-shape 
