THE DIGESTIVE AND RESPIRATORY SYSTEM 299 



these particulars implies, not genetic relationship necessarily, 

 but a similar demand responded to in a similar way. The 

 main object to be achieved in all cases is to regulate the amount 

 of digestive surface to the demands offered by the various 

 kinds of food, and as there is but a limited number of me- 

 chanical or architectural devices possible, the same ones are 

 employed in unrelated groups of animals, having arisen in- 

 dependently in response to a similar physiological need. This 

 phenomenon of parallel development (or " analogical resem- 



\ 



FIG. 83. Colon labyrinth of Ceruus canadensis. [After WEBER.] 



blance," as Darwin calls it), may appear in any system or part 

 and has been a frequent source of error in the estimation of 

 the inter-relationship of animals. 



The relation of the total length of the intestine to the kind 

 of food has been frequently emphasized, the idea prevailing 

 that it is short in carnivores and long in herbivorous forms, in 

 accordance with the difference in nutrient qualities and the 

 ease of digestion in the two sorts of food, but this statement 

 is to be accepted only in a general way, as it is subject to 



