MAN CONSIDERED PHYSIOLOGICALLY. 663 
animals, either bred by man to attain great speed, as the race- 
horse and greyhound, or those that have become so by the pro- 
cess of natural selection, the entire conformation of the body 
has been modified in harmony with the changes that have takén 
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 se- 
lection as a whole, but the subordinate one of correlated 
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 
prefers one gait, as pacing, for his easy, slow movements, will 
break into a trot when pushed to a higher rate of speed. 
Trotting can not be considered the natural gait for high 
speed in the horse, yet, by a process of “artificial selection” 
(by man) from horses that have shown capacity for great speed 
by this mode of progression, strains of racers have been bred, 
showing that even an acquired mode of locomotion may be 
hereditary; while that galloping is the more natural mode of 
locomotion of the horse is evident, among other things, by the 
tendency of even the best trotting racers to break into a gallop 
when unduly pushed—an instance also of an hereditary tend- 
ency of more ancient origin prevailing over one more recent. 
The bipedal modes of progression of birds are naturally 
very like those of man. 
MAN CONSIDERED PHYSIOLOGICALLY AT THE DIFFERENT 
PERIODS OF HIS EXISTENCE. 
Growth.—As a result of the intra-uterine development of 
two cells, neither of which is visible by the naked eye, the 
human being reaches about one third of its total length and 
one twentieth of its maximum weight. In the infant the rela- 
tively larger size of the head and face is obvious, while among 
internal organs the liver is especially large. The child’s future 
increase in weight is chiefly from growth of muscles. Increase 
