335 
We see an illustration of the effect of function when a 
new habit is adopted and a new correlation of the parts 
is the necessary result. Speaking of the American trotters, 
Mr. Lupton says :— 
“ Their tibiae, or long bones from their stifle joints to their hocks, 
are unusually long as compared with an English horse, and this 
gives them their magnificent hind action that causes them to be such 
fast trotters. . . . This powerful hind gait, always present in 
the American trotter, is seldom manifested in English horses. . . . 
The incentive to their propagation in the United States has been 
the trotting-track, similarly as the race-course has caused the crea 
tion of the English thoroughbred.”—(The Nineteenth Century, 
vol. xxxv., pf>. 936-7.) 
There is here evidently a correlated variation correspond 
ing to the different kinds of action to which the horses 
are habituated ; and surely when we take the influence 
of the trotting track into consideration, we shall be ready 
to believe not simply that the American horse can trot 
because it has specially modified hind legs, but also be 
cause the practice of trotting has modified the organism 
so as to make it most efficient for that purpose. 
It also follows from the idea of a structure of co 
ordinated parts that greater use in one direction, followed 
by an enlargement of the parts used, will bring about 
a decrease of other parts which are no longer used, or not 
used so much as heretofore. As the amount of food 
which an animal can take is limited, and the consequent 
amount of support derived from it is limited, a great 
demand made by one organ or set of organs must be met 
at the expense of others. This law is known as the 
principle of “compensation of growth” and “balancement.” 
“As Goethe expressed it, ‘ In order to spend on one side, nature is 
forced to economise on the other side.’ ... If nourishment flows 
to one part or organ in excess, it rarely flows, at least in excess, 
to another part. Thus it is difficult to get a cow to give much milk