ANIMAL PSYCHOLOGY. 255 



to their bodies the lightness necessary to flight; whence 

 they have not only a double circulation of the blood, 

 and an aerial respiration, but they also respire by other 

 cavities besides the lungs. In most animals the quan- 

 tity of respiration is moderate, because they are formed 

 to walk rather than to run; in reptiles, which are 

 formed to creep or hop, it is lower still; while in fishes 

 it is least of all, since they are suspended in a medium 

 of nearly their own specific gravity, and require but 

 little muscular strength for motion. These differ- 

 ences are chiefly produced by variations of the same 

 organs. From the fact that the vertebrate animals 

 share a common typical structure, a strong presump- 

 tion arises that they also share a common principle of 

 intelligence. 



This presumption is materially strengthened by 

 other considerations. The structure of the higher 

 animals leads directly to the inference that each of 

 their organic forms was designed to be actuated and 

 governed by a thinking principle; a principle not 

 only capable of receiving impressions conveyed by 

 the organs of sense, but also of making a rational 

 use of the perceptions which these organs were 

 designed to throw perpetually under its cognizance. 

 To deny the existence of the principle, or its power 

 to act, is a denial of the obvious purpose of the elab- 

 orate mechanism of the animal frame. 



From every point in which the structural relations 

 of the vertebrate animals are considered, a common 

 plan of creation is not only seen, but this, in turn, 

 becomes deeply significant upon the question of sim- 

 ilar mental endowments. These intimacies of struct- 

 ure are the foundation of corresponding intimacies in 



