STRUCTURAL ADAPTATIONS 389 



Plovers, however, have the second and third toes half-webbed ; 

 in the Avocet, and some other Plovers, the third and fourth 

 are also connected in this way. Among the Duck tribe the 

 Screamers may be cited as forms with free toes, yet they swim 

 much, while the intensely specialised Grebes have lobed feet 

 like the Coots, and Phalaropes among the Plovers. To add 

 to the complexity of this problem the Divers, which are the 

 marine counterparts of the fluviatile Grebes, have webbed feet. 

 Further, the Steganopodous birds are distinguished by the fact 

 that all the toes are enclosed in a web ; yet, save the diving 

 Cormorants and Darters, none of these birds pass more time 

 on the water than, say, the Gulls. In the Frigate-bird indeed, 

 in which the legs are much reduced, the web of the toes has 

 undergone considerable reduction. Such are the facts; the 

 interpretation thereof is by no means so simple as some seem 

 to imagine. 



In seeking for evidence as to the effects of a cursorial habit 

 we turn naturally to the pelvic girdle and limb as being the 

 parts most likely to be affected by any adaptation tending to 

 increase pedestrian locomotion. The conflicting character of 

 the results obtained by a careful comparison of these parts in 

 the most typical cursorial birds is most puzzling. 



In all cases cursorial locomotion is developed at the expense 

 of flight ; and this fact is brought out with unmistakable em- 

 phasis in the case of the Ostrich tribe which are now — save only 

 the Tinamous — absolutely flightless. But there are many 

 species among the carinate types which, though still retaining 

 the power of flight, pass most of their time upon the ground. 

 This is especially true of the granivorous types — the Bus- 

 tards, Game-birds, Pigeons and Sand-grouse. A very casual 

 survey of the forms belonging to these two groups — Neognath 

 and Palaeognath — will show that, in so far as living species are 

 concerned, the former have an unusually broad pelvis, while in 

 the latter it is surprisingly long and narrow. The one indeed is 

 the very antithesis of the other. 



The broad type of pelvis, however, it is to be noted, is by 

 no means rare amongithe Carinatse, and it is especially common 

 in the smaller Coraciiformes, such as the Kingfishers, for ex- 

 ample. It occurs, too, among the smaller Cuckoos. Indeed it 

 would seem almost as though this were a primitive character 



