THE PRINCIPLES OF BIOLOGY. 
311 
it will be a long time before the original size of the organ is 
restored. In games of skill, as cricket, billiards, chess, and 
in the playing of musical instruments, we all know that 
continued practice is necessary to keep up the increased skill 
gained by it. 
In trying to explain all these facts deductively we 
experience the greatest difficulty in the first of the above 
three truths. The second and third follow as necessary 
consequences from the first. That over-exertion of an organ 
should be met by an extra supply of food is a fact which you 
would not expect a priori. We know from mechanical 
principles that every action produces an equal reaction, or 
as Mr. Spencer says, “ the rhythmical changes produced by 
antagonistic organic actions cannot any of them he carried 
to an excess in one direction without there being an 
equivalent excess in the opposite direction.” But in the 
phenomena of adaptation we have more than that. The 
excess in the opposite direction is not only equivalent, but 
it is more than equivalent, and hence the mean state between 
the oscillations is altered. A leaden bullet suspended by a 
string hangs quietly in a vertical line ; if drawn out to a 
certain distance, on being released it will go to the other side 
to the same distance, come back, and repeat so on. Without 
friction it would go on for ever, and the mean between the 
two farthest points is exactly the point at which it was at 
rest. It would be inexplicable if the pendulum were to go 
farther on the one side than on the other, and so alter the 
mean ; but what we see in the muscle growing by increased 
exertion is exactly analogous. But then the processes 
occurring during the growth of the muscle are not so simple 
as the processes of an oscillating pendulum. There are 
primary, secondary, tertiary, &c., processes, and it is by the 
actions of these that we must try to explain the seemingly 
contradictory facts. Let us see exactly what occurs when 
a muscle grows through increased work. Additional work of 
the muscles necessitates additional supply of blood. This 
can only be done by additional work being thrown upon the 
arteries which supply the muscle. The increased supply of 
blood necessitates increased work given to the veins which 
carry it off again. And also arteries and veins not in direct 
communication with the muscle will be similarly influenced. 
The muscle is excited by the nervous centres, and therefore 
the nerves will have to carry increased nervous force to the 
muscle, and this increased nervous force and extra supply of 
blood will produce an increase of the power of assimilation, 
which again results in the increase of the size of the muscle. 
