Acclimatization 137 



from it near the posterior end. A snail's radula, which is an old 

 organ developed in the ocean, is an admirable mechanism for rasp- 

 ing off bits of land plants for foods. A carp has no stomach and no 

 peptic digestion (Kenyon, 1925) ; there is much enteric variation 

 among fishes according to food habits. Among mammals there are 

 many differences in food habits, but there is comparatively slight 

 variation in organs of digestion. There is no mammal without a 

 stomach. A primary distinction between reptiles, as terrestrial types, 

 and amphibians, as aquatic types, is in the teeth, which in the for- 

 mer are more firmly fastened into the jaws. Mammals make further 

 improvements in teeth. They usually have two successive sets and 

 have enamel-covered surfaces which effectively grind up hard ter- 

 restrial plants. Birds in modern times all have horny beaks which 

 tear flesh, strain small organisms from water, or crack hard seeds 

 of vascular plants. Their gizzards grind up resistant foods. 



The animals which have taken advantage of the food resources 

 on land, though consuming a great variety of foods, are on the 

 whole representatives of a few specialized groups which by particu- 

 lar adaptations have taken advantage of the great stores of organic 

 foods in land vegetation. 



Acclimatization 



Animals which change from one habitat to another must acquire 

 new ranges of toleration for environmental factors. The degree of 

 acclimatization that an animal undergoes may be judged by changes 

 in its ability to survive extreme conditions, by its success in popu- 

 lating new habitats, or its behavior after being subjected to changed 

 conditions (Shelf ord, 1929) . The reason why the majority of ter- 

 restrial animals have reached land through the fresh waters rather 

 than directly from the sea is in part due to the favorable series of 

 intermediate environments (Carter, 1931). The adaptations are 

 acquired serially, and thus more easily. The mechanisms for ac- 

 climatization appear to be concerned with modifications in the 



