The Conquest of the Land 



digits, was arrived at except by accident. Once arrived at, 

 however, the arrangement was adhered to with considerable 

 strictness. For although the number of digits is in some groups 

 in the birds especially reduced, the primary design is almost 

 always readily recognisable. The second function of the limb, 



FIG. 85. Development of the Frog. 



that of supporting the body, was developed very slowly. In the 

 amphibians and reptiles, and even in the lower mammals, the 

 legs are comparatively weak and sprawling, and the creature 

 crawls on the belly. 



The second great change which required to be made was of 

 course in the method of breathing. An ordinary fish, when taken 

 out of the water, dies of suffocation, because its gills become 



107 



