626 COMPARATIVE PHYSIOLOGY. 



while the form of the animal generally agrees with what would 

 have been expected on mechanical principles it is a fact that 

 some of the fleetest horses that have ever run on the course have 

 not in all respects been built in conformity with them. But 

 it is to be remembered that a vital mechanism differs from all 

 others in that the whole consists of parts dependent not only as 

 one portion of any machine is on the other, but that every part 

 is energized and directed by a governing nervous system ; that 

 every cell is being in a sense constantly renewed, so that the 

 comparison between any ordinary mechanism and the body of a 

 living animal holds only to a limited extent. Moreover, apart 

 from peculiarities in the muscles of anima;ls, to which atten- 

 tion has been drawn (page 205), it is well to bear in mind that not 

 only every animal, but every tissue has its own functional indi- 

 viduality ; and to this especially (as exemplified in the most im- 

 portant of all the tissues, the nervous) must we attribute the 

 undoubted fact that the speed, endurance, etc., of animals can 

 not be explained on mechanical principles alone — a truth to 

 which too little attention has hitherto been drawn. These 

 principles have, however, been unconsciously recognized prac- 

 tically, hence the great attention paid by breeders to using ani- 

 mals for stock purposes that have actually shown merit by their 

 performances. 



Evolution. — It is noteworthy that with almost all quadru- 

 peds the gallop is the natural method for rapid propulsion. In 

 all animals, either bred by man to attain great speed, as the 

 race-horse and greyhound, or those that have become so by the 

 process of natural selection, the entire conformation of the 

 body has been modified in harmony with the changes that have 

 taken place in the legs and feet. This is seen in the greyhound 

 among domestic animals, and in the wild deer of the plain and 

 forest. Such instances illustrate not only the principle of 

 natural selection as a whole, but the subordinate one of corre- 

 lated growth. 



Any one observing the modes of locomotion of quadrupeds, 

 especially horses and dogs, will perceive the advantages of the 

 four-legged arrangement. Not only is there a variety of modes 

 of progression, as walking, trotting, galloping, cantering, the 

 alternations of which permit of rest to certain groups of mus- 

 cles, with their corresponding nervous connections, etc., but on 

 occasion some of these animals can progress fairly well with 

 three legs. Sometimes it may also be noticed that a horse that 



