278 THE STRUCTURE AND LIFE OF BIRDS chap. 



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 to a 

 point at which they meet, and that if we proceed 

 further, we shall find the line joining another, wh'ere 

 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 



