CHIEF CHARACTERISTICS 9 



wider uses of the voice in parental warning and instruction 

 and in the sociable gossip of parrots. The expanding 

 utilisation of the voice and the socialisation that it made 

 possible, so familiar in the case of rooks, which have quite 

 a vocabulary, would react on the evolution of the brain, 

 would, at any rate, supply the sieve by which germinal 

 variations in the direction of better brains would be 

 approved of. 



§ 8. Nesting 



Accepting the hypothesis that birds evolved from a 

 bipedal stock of terrestrial reptiles, we get a little light on 

 the custom of nesting. Leaving the water was no doubt a 

 step of progress, but it brought with it certain penalties. 

 The terrestrial creature is confined to movement in one 

 plane, the surface of the earth, and it cannot, like the fish or 

 the frog, simply shed its eggs into the cradle of the water. 

 And it must be remembered that the conquest of the land 

 was associated with the establishment of /w^erna/ fertilisation. 

 Some reptiles transcend the first limitation referred to by 

 becoming burrowers, like the limbless lizards and a few 

 snakes ; others by becoming arboreal, like chamaeleons 

 and the little dragons {Draco volans) the turtles and sea- 

 snakes have returned to the sea. Similarly, as regards the 

 disposal of the eggs, they may be buried deeply as in the 

 Crocodile, they may be laid in a hole in a tree, or they may 

 be incubated as in Pythons. 



If, then, we think back to the incipient non-flying 

 birds, without much in the way of armour or weapons, we 

 can imagine the advantage of being able to take a running 

 leap on to the branch of a tree and the advantage of having 

 a nest in a position of relative safety. Later on, it need 

 hardly be said, secondary advantages became manifest, 

 e.g. making the nest such that the developing eggs were not 

 liable to chill and that the nestlings were not liable to fall 

 out. It should also be borne in mind that the nest may 

 have had primitively a function to serve as a relatively safe 

 retreat for pairing. 



