278 THE STRUCTURE AND LIFE OF BIRDS cuHap. 
is put under a hen, and gradually increases in size as 
the white shrinks by evaporation. 
To follow in detail the kaleidoscopic development 
of the embryo from day to day, till at last by the 
help of the “egg-tooth” at the end of his beak he 
pecks his way out, is quite beyond the scope of 
this chapter. I merely wish to make clear one 
or two of the most interesting points. To the 
right understanding of these, some preliminary re- 
marks are necessary. It is generally held that the 
embryo goes through in a short space of time many 
of the various stages by which in the course of 
ages the species has become what it is. Thus the 
progenitors of the butterfly were wingless crawling 
creatures, and the change from the caterpillar to the 
perfect insect is a brief abstract of the history of long 
ages of slow development. In the same way when 
we find in the embryos of reptiles, birds, and mam- 
mals organs that are the same in nature and origin as 
the gills of fishes, we infer that these three classes 
of animals were once water-breathers. The heart 
and the blood-vessels, as they advance step by step to 
the perfected form that we find in the mature bird, 
show us clearly that if we trace upward the avian 
and reptilian pedigrees we shall come at length toa 
point at which they meet, and that if we proceed 
further, we shall find the line joining another, where 
the common ancestors of birds and reptiles drew 
apart from the more primitive types that continued 
to make their home in the water, the progenitors of 
the fish of the present geological period. 
This being so, it is well beforehand to get some 
