THE PRINCIPLES OF BIOLOGY. 
313 
not yet modified. Should the increase of function he carried 
on till every part, every cell, one might almost say, has 
undergone the required amount of change and a new and 
perfect equilibrium is established, then the change would be 
a permanent one. But the time required for this is more 
than the lifetime of one generation. If the increase of 
function ceases before the new equilibrium has been estab¬ 
lished, then the last affected parts are changed only very 
slightly, and are not in equilibrium with the rest; they, 
therefore, resume their former state in a short time. The 
parts that depend on them immediately will do the same, and 
so the process of restoration of the former state goes on the 
reverse way, till at last the originally affected organ is again 
reduced to its former size. It is easy to understand that the 
longer the increase of function in that organ lasted, or, in 
other words, the farther these adaptive changes have proceeded 
towards the remoter parts of the body, the longer time will 
be required in regaining the original state. 
Mr. Spencer illustrates- the whole of the process of adapta¬ 
tion by an analogy from commercial processes. He assumes 
a suddenly increased demand for iron ships, this causes an 
increase in the demand for iron, this again affects the demand 
for coal, &c., and he shows that in every detail the process is 
exactly analogous to the process of an organ growing by 
increased exertion, and also that the restoration of the original 
state takes place in both cases in an exactly analogous way. 
EXPOSITION OF CHAPTER VI.--INDIVIDUALITY. 
Chapter VI. treats of individuality, and nearly the whole 
of it is devoted to the solution of the question: What con¬ 
stitutes an individual ? One might think that was a task 
which would hardly require a whole chapter, but Mr. Spencer 
shows that it is not only a very difficult matter, but that it is 
impossible to give a perfectly satisfactory definition of an 
individual. The case is clear enough as long as we consider 
only the higher animals, but when we extend the word 
Individual to the whole organic world, an exact definition of 
its meaning becomes a matter of great difficulty. 
To begin with plants, although it is a rule to speak of a 
tree or any whole plant as singular, we must not forget that 
every branch and every bud has to a certain extent an 
independent life. If cut off' and planted it will make roots, 
grow, and become a whole plant like the one it was derived 
from. Are we to consider both as parts of one individual or 
