THE EVOLUTION OF THE BIRDS 449 



the " merrythought " or furcula. The carpals, metacarpals, 

 and phalanges are much reduced and fused together, but in 

 the young Hoatzin (South American) among others, the hand 

 is well developed and represented by three digits ending in 

 claws. With the help of these claws the young Hoatzin 

 clambers about on trees in a manner suggestive of the presumed 

 habits of its ancestors. The barbs of the flight-feathers are 

 connected to one another by the hooks on the barbules, and 

 most of them are active flyers. All make nests of one kind 

 or another and incubate the eggs, except the cuckoo which is 

 parasitic in that it lays its eggs in the nests of other birds for 

 them to incubate, and the Megapodes which lay their eggs in 

 decaying vegetable matter, relying on the heat engendered by 

 the latter to incubate them. 



A point of no small importance in these birds is the high 

 development of courtship-activities and structures, which play 

 a large part in knitting parents together into a family. Normally 

 it is the male which is the active partner in courtship and 

 possesses well-developed " secondary sexual " or courtship 

 characters. In some, such as the Great Crested Grebe, males 

 and females are similar in appearance and in the ardour of 

 their behaviour. In others, of which the Phalarope is an 

 example, the female is the more brilliant and active, and the 

 male incubates the eggs, and in fact does almost everything 

 except lay them. 



As regards their anatomy, these birds are very much alike ; 

 so much so that it is a matter of extreme difficulty to arrive 

 at a satisfactory scheme of classification for them. This is 

 partly because the birds are a comparatively recent group, and 

 because they have been so successful in the walk of life to 

 which they have become adapted that little extinction has 

 taken place. Most of them are strictly aerial animals. Others 

 such as the common fowl and the extinct Dodo of Mauritius 

 have secondarily become terrestrial as a result of reduction of 

 the powers of flight. Many have become adapted to swim- 

 ming on water, and have developed webs of skin between the 

 toes of the feet, which are then described as webbed feet. 

 This has taken place independently in a great many groups, 



2 G 



