482 Mr. E. W. H. Holdswortli on the 



some of powerful flighty some whicli spend most of their time 

 in the water, and others, such as the Gulls, which move 

 with almost equal facility on the water, in the air, and on the 

 land. There is also some variety in the proportionate length of 

 the several parts of the leg and of the toes ; but they have the 

 one common character of webbed feet. This appears to me 

 to bear on what I regard as the main object of the backward 

 position of the feet^the lessening of resistance to the air. 



If a close view be obtained of a Duck or other web-footed 

 bird when swimming, it will be observed that although the 

 toes partially close when they are brought forward, they are 

 only slightly curved, but are bent back from the metatarsal 

 joint, and that at the backw^ard stroke they are straightened 

 and separated by the action of the extensor muscles, aided by 

 the pressure against the water. The toes of a webbed foot 

 do not in fact appear to contract naturally so much as those 

 of a true perching-bird, and the interdigital web, although 

 elastic, would to some extent interfere with both the free 

 closure and the contraction of the toes. Web-footed birds are 

 mostly of rapid flight, the feathers lying very close, and every- 

 thing being arranged to lessen resistance. If, however, the 

 partially contracted toes were held in front, the webbed foot 

 could not be so disposed of as not to offer some appreciable 

 resistance to the passage of the bird through the air, and there 

 would be some strain on the flexor muscles of the leg and 

 foot to keep them doubled up close to the body. It may be 

 said that Ducks and many other web-footed birds some- 

 times sleep whilst standing on one leg, the other being 

 doubled up under the flank-feathers ; but in those cases the 

 feathers are lying loosely, there is no resistance to the air to 

 be avoided, and some evidence of the position being one of 

 partial strain is shown by the fact that the first thing the 

 bird generally does after putting down its foot is to stretch 

 the whole limb to its fullest extent. The backward position 

 of the leg, with the tibia, tarsus, and toes extended in a 

 straight line, appears to fulfil the desired conditions of mus- 

 cular equilibrium and minimum of resistance during sustained 

 flight. 



