136 How Animals Changed 



structural alterations would produce the graded series of local races 

 often exhibited in the latitudinal variation of fishes." 



Food 



As has been pointed out (p. 91), animals that live in water are 

 surrounded by food in the form of small plants and animals and as 

 dissolved substances. In the ocean algae of some size and a few 

 higher plants are present along shore, but these constitute a very 

 small part of available food resources. In fresh water a considerable 

 body of submerged and emergent vascular vegetation is available. 

 On land the bulk of the plant food of animals is made up of sper- 

 matophytes. Putter's idea (1909) that aquatic animals absorb con- 

 siderable amounts of organic food through external bodily mem- 

 branes seems to have been pretty will disproved (Dakin, 1925; 

 Krogh, 1931), but some aquatic animals undoubtedly do utilize 

 such nourishment as colloidal material which is taken from water 

 passed through the enteron (Hinman, 1932; Smith, 1933). 



In any habitat animals tend to be specialists in their food rela- 

 tions, although there are always omnivorous animals of both spe- 

 cialized and generalized types. Animals commonly avoid competi- 

 tion by using diiferent foods (Pearse, 1930, 1934) . Along the 

 shores of aquatic habitats the majority of animals are vegetarians, 

 mud eaters, and scavengers (Pearse, 1929, 1932a) . Carnivores must 

 always occur in smaller numbers than the animals upon which they 

 prey. Animals which have attained terrestrial life have been obliged 

 to restrict their digestive activities to the interior of their bodies. 

 ( Organs of taste, instead of being scattered over the outside sur- 

 ^ faces, are limited to the region about the mouth (insects) or to the 

 ( buccal cavity itself (land vertebrates) . In finding foods, more de- 

 indence is usually placed on organs (olfactory, visual, auditory) 

 which receive stimuli from a distance., Glands along the digestive 

 canal are more sharply defined. Water is added to food by salivary 

 glands near the anterior end of the alimentary canal and removed 



