THE VERTEBRATES AND THEIR ENVIRONMENT 31 



the reptiles, birds, and mammals, which are more complex, are 

 adapted for hfe on the land and in the atmosphere ; the Amphibia 

 represent an intermediate state in both structure and habits. 



Feeding and Food Supply. — Like other large groups of animals, 

 the vertebrates present examples of varied types of diet. Some 

 vertebrates are plant-feeding, or herbivorous; others, flesh-feeding, 

 or carnivorous; while still others are omnivorous, thriving upon a 

 diversified food supply. The differing foods and the differences in 



Fig. 17. — Timber wolves. Tj'pical of the adaptations that enable such 

 manimals to range from the high mountains to the plains and even from tb.e 



Arctic circle to the tropics. 



(Photo, by courtesy of the AmerioMn Museum of Natural History.) 



feeding habits which they imply are, of course, related to the habits 

 and habitat of each species. 



The vertebrates, taken as a whole, show a wide range in feeding 

 habits and in adaptations related to feeding. The sources of food 

 supply may be considered in this connection, with reference not 

 only to vertebrates but to animal life in general. Animals take 

 into their bodies masses of material, the food, which is digested and 

 becomes a part of their protoplasm {cf. p. 81). Green plants, 

 on the other hand, take in simple compounds, such as water, carbon 

 dioxide, and mineral salts; with these alone they are able, in the 

 presence of sunlight, to maintain and increase the bulk of their 

 bodies. Closer analysis shows that, strictly speaking, the differ- 

 ence between plant and animal nutrition is not so great as might be 

 supposed; but for the present we are considering only the fact 



