94 STRUCTURE AND COMPARISON. 



and that a web-footed bird seeks its living in water too deep 

 for wading. 



When we compare all the different birds, we see that there 

 are no great jumps from one extreme to another, from very 

 short legs to those ridiculously long, from tiny bills 'to those 

 enormously long or thick or wide. Somewhere in nature we 

 may expect to find a bird which just fills in the gap and 

 makes a graded series. 



From the man-of-war bird with his abbreviated legs, for 

 which, short as they are, he seems to have almost no use at 

 all, to the stilt perched up on his absurd, artificial-looking 

 shanks, extends all the long procession of birds the terns 

 and gulls and the whole race of sandpipers and others that 

 take their food less and less by pursuing it 

 on the wing, and more and more by running 

 or wading after it. The changes, after all, 

 are gradual. 



Watch the growth of the idea of a swim- 

 ming foot and see how the need of more 



or less surface to oppose the water is met 

 FIG. 20. SEMIPAL- ^ different wayg The firgt hint we t of 



MATE FOOT OF . . , 



SANDPIPER (LIFE a wg b-foot is in the slight semipalmation of 

 SIZE). ' some of the sandpipers. We need not 



suppose necessarily that this is a sign that 



the sandpiper swims much, for we find 



semipalmation in some land-birds, even in 



the hen, and we know that this is to bear 



them up in walking over snow. Perhaps in 



the sandpipers the principal use of this slight 



webbing is to help them in walking over soft FlG ' 2L LoBATE 



FOOT OF PHALA- 

 mud. But soon it becomes evident that it R0pr ( SLIGHTLY 



.aids in swimming, and the little phalaropes, REDUCED). 



