124 THE STORY OF THE BIRDS: 
condition has come out of the precocial—a sort of de- 
generate process consequent perhaps upon changed 
conditions. ‘True, in certain reptiles there is a prema- 
ture hatching of the eggs within the body, but the 
young comes forth as perfect as by the slower process 
of external heat. 
In the mammals generally a premature condition 
of the young at birth is strikingly within the lower 
forms—as in the marsupials—and its perfection be- 
fore birth is evidently a later or higher development. 
Still, since some (such as kittens and puppies) in the 
higher mammals are much more helpless and blind 
than others (as pigs and calves), it is evident that the 
condition of babyhood is even here quite variable, 
depending upon some unexplained influence. It is 
probable therefore that some change of habit or envi- 
ronment, by selective agency or otherwise, has effected 
this change in birds. While we can never know what 
it was, we can glance tentatively at the problem and 
note some interesting possible factors of the change. 
First, the condition of the nestling seems to de- 
pend largely upon the relative size of the egg, both to 
that of the parent and to that of the young at hatch- 
ing. Precocial birds generally lay much larger eggs 
in proportion to the parent’s size than do the altricials. 
But there are striking variations among the precocials 
for reasons noted later in connection with the number 
of eggs. Second, in relation of egg size to that of 
nestling, altricial eggs appear as if they were too small 
for the further development or perfection of the em- 
bryo. This development may depend upon absolute 
