WHY DID BIRDS BEGIN TO INCUBATE? 97 
toward incubation, an interesting question may arise 
as to the origin of this habit among the birds. 
Since the lowest birds, or those showing nearer 
relationship to the reptiles in structure, practice incu- 
bation less closely than do the higher groups (some 
even ignoring it altogether), we feel justified in in- 
ferring that the early birds left their eggs to be 
hatched by natural heat, and that incubation, as we 
now know it, came in after feathers. 
While the coiling of the python and other similar 
snakes around their eggs may seem like incubation, it 
may be really more a matter of maternal watefal- 
ness. The temperature of the eggs are said to be 
elevated by the act, however, but this may be due in 
part to their own heat incidental to the mere act of 
hatching, for the development of life means the de- 
velopment of some heat. Since all reptiles are cold- 
blooded, they have little heat to impart to their eggs, 
and incubation can have little significance with them. 
In fact, it has been suggested that the parent has 
coiled about the eggs to warm itself from them, and 
in this way, at least among reptiles, the habit of in- 
cubating may have had its origin. 
But, strange to say, the opposite idea has been 
advanced in the case of birds—that is, that, being in- 
tensely hot-blooded, they began first to sit upon their 
egos in order to cool their breasts. In keeping with 
this view it may be noted that one thing is fairly es- 
tablished now—that at the incubating period the 
breast and abdomen of the sitting bird are in a state 
of congestion, and, of course, the circulation and heat 
