74 STRUCTURE AND COMPARISON. 



where they sweep past each other alternately in long straight 

 strokes, giving the greatest possible force and efficiency. 



Everything that could give speed in the water has been 

 adopted in the grebe and the loon, and with what wonderful 

 success! In all kinds of aquatic feats they lead all other 

 birds. Yet at what a cost do they hold this supremacy in one 

 particular ! When we see the grebe's foot, put on at the very 

 hinder end of the body, flattened as if crushed by a boot heel, 

 with its toe's set in the same straight line as its shank, and its 

 flexed heels nipping close together so that the toes turn out- 

 \vard, we see at once that this bird cannot walk. 



A perfect swimmer, fitted with all appliances for speed and 

 endurance in swimming, he has been over-developed in one 

 direction, and is good for nothing but swimming. He is put 

 at the very foot of the list as the lowest organization of all, 

 while our little bluebird and robin, that seem to have no 

 special accomplishments but are good " all-round " birds, stand 

 at the very top. It is a harm to a bird as well as to a man 

 to be so much developed along one line that he is weak in 

 other directions. So in science we say that " the most special- 

 ized" forms are the lower, and the "most generalized" forms 

 — that is, the "good all-rouud" forms — are the higher 

 structures. 



