630 AVES [CH. 



With regard to the care of the young, the Mound-builders of 

 Australia (Megapodidae) exhibit the most primitive conditions, for 

 they do not even sit on their eggs, but bury them in a mass of 

 decaying vegetation, so that they may be hatched by the heat of 

 fermentation just as Reptiles bury their eggs where they may be 

 hatched by the heat of the sun. All other birds sit on their eggs. 

 But in the case of some birds, such as the common hen, when the 

 chicks are hatched they are covered with down and are able to run 

 about and feed themselves : such chicks are said to be nidifugous. 

 In the case of other Birds, such as the Swallow, the chicks emerge 

 from the egg in a blind, naked condition and have to be fed with 

 unremitting care by the parents : such chicks are said to be 

 nidicolous: and this is certainly a more modified condition of 

 affairs than the nidifugous condition. 



If we now, starting from Archaeopteryx, ask ourselves what the 

 most primitive form of bird was like, we may arrive at certain 

 probable conclusions. Birds, we have already concluded are derived 

 from Reptiles, and they must have originated from active tree- 

 climbing Reptiles of moderate size, which jumped from branch to 

 branch, for in this way only can we understand how the power of 

 flying was evolved. 



The wings in the first birds must have been actively functional, 

 for only by their functional importance can we account for their 

 evolution ; but we need not credit the first birds with great powers 

 of flight any more than the first aviators, whose short flights pale 

 into insignificance in comparison with the flying feats of to-day. 

 Now there is one group of birds which to a large extent retain these 

 primitive characters to-day : these are the Game-Birds, the so-called 

 Cock-like Birds (Galliformes). They roost in trees for the most part 

 and make short flights only when they are disturbed or alarmed. 

 Their young are nidifugous. 



Included in this group are two remarkable genera. In one of 

 these, the South American Tinamou, the palate is of the dromaeo- 

 gnathous type, and there is a penis in the dorsal wall of the cloaca. 

 In the other (OpistJwcomus) the chick, when it emerges from the 

 egg, has claws on the thumb and index finger and the wing feathers 

 are not developed, and for the first few days of its life it runs about 

 over the branches like the arboreal reptile from which the group of 

 Birds is descended. It is in fact a bird larva. In all other Galli- 

 formes, except the Tinamou, the palate is schizognathous and there is 

 no penis. It is customary to separate the Tinamou on this account 



