THE DIGESTIVE AND RESPIRATORY SYSTEM 301 



probably the largest among mammals, is reported to be that of 

 Pontoporia, a South American dolphin, while the shortest 

 mammalian intestine is that found in certain insectivorous 

 bats, the proportion of which to the body length is 2:1. A 

 change in the length or volume of an organ is, however, so 

 easily effected even during an animal's lifetime, that it is prob- 

 able that members of the same species may show considerable 

 difference in length of intestine, especially if a comparison be 

 made between specimens from quite different localities where 

 the diet is different. Thus in man, the intestinal canal of the 

 Japanese, whose food is largely vegetable, exceeds in average 

 length by one-fifth that of Europeans; and in whites of me- 

 dium stature the length of the intestine proper, from pylorus 

 to anus, averages 960 cm., while the average length of the 

 same in nine negroes was but 866.7 cm ' 



This whole subject, therefore, gives but little indication of 

 phylogeny and is valuable in the present inquiry mainly as an 

 example of the complete correlation between environment and 

 structure. In the examples given mammals have been pur- 

 posely emphasized and instances of adaptations in the other 

 groups of vertebrates have been omitted as far as possible, 

 since their inclusion would convey the subject far beyond the 

 proper limits of this work. 



The function of respiration is the simplest of the major 

 functions, since it consists primarily of an interchange of gases 

 through osmosis, and involves in itself nothing save a moist 

 membrane, with air or aerated water, on one side and blood 

 on the other. The blood must be constantly renewed through 

 some form of circulation, and there is usually some auxiliary 

 mechanism to create a current in the respiratory medium also. 

 It is also imperative that the osmotic membrane be kept moist, 

 a matter of no difficulty in an aquatic animal, but one involving 

 some little additional apparatus, usually an interior chamber 

 with a regulated outlet, in terrestrial forms. Thus in aquatic 

 invertebrates the respiratory membrane is usually external, 

 often a modified portion of the integument. In many minute 

 forms in which the integument is thin, respiration takes place 



