312 
THE PRINCIPLES OF BIOLOGY 
To explain the second truth—that in the increase of size 
a limit is soon reached—we have only to follow a little farther 
the processes explained above. It was found that the imme¬ 
diate arteries and veins had increased work to do. To perform 
this for any length of time they must grow, i.e., increase in 
diameter and contractile power. The growth of these arteries 
results in growth of others which supply them, and so on. 
All the other organs are similarly affected. It is like a wave 
of increased growth passing through the whole of the body, 
affecting the remotest parts last. We find, therefore, that an 
organ will grow rapidly as long as it does not cause any 
considerable alterations in other organs ; but when this point 
is reached further growth can take place only very slowly, as 
it depends on the remodelling of numerous parts only slightly 
and remotely affected. 
But we find in this also an explanation of the fact that in 
growing individuals adaptive changes take place more easily 
than in adults. In an adult animal assimilation and expen¬ 
diture have reached a balance, so that increased nutrition of 
one part implies decreased nutrition of some other part; in 
other words, besides a considerable amount of building up 
there must be an equivalent amount of unbuilding of less 
important parts. But in the young and growing animal 
there is always excess of assimilation over expenditure, and 
this excess can be utilised on the necessity of increased 
nutrition arising in one part through increased function 
without unbuilding becoming necessary. We may compare 
an adult organism to a person who just uses all his income 
for his customary expenses. When occasion arises to increase 
expenses in a certain direction it can only be done by 
decreasing them in another, and if increased expenditure in 
one direction becomes permanent a complete rearrangement 
of expenses would become necessary. But.the young and 
growing animal resembles a person who earns more 
than he spends, and an increased expense in any direction 
can be met by the surplus without interfering with the 
other items. 
In the consideration of the above processes we find 
also an explanation of the third truth, that after the 
excess of function has ceased the original state is reached 
again after a longer or shorter time. The modifications 
which have to occur in the remotest parts of an organism 
on continued excess of function of one organ can take 
place only exceedingly slowly, and while these modifications 
are going on the part that was modified most recently is 
not in equilibrium with the rest, namely, those which are 
