810 
THE PRINCIPLES OF BIOLOGY. 
Adaptation being a consequence of function, we may 
expect to find only very insignificant adaptive changes in 
plants where function is very limited, and the few examples we 
find are all confined to parts actually in the process of growth. 
With animals it is different. All through the organism 
a continuous process of decomposition and rebuilding of the 
tissues is going on ; the material of all parts is continually 
renewed, and thus a certain pliability or modifiability is 
produced, so that any part even of the adult organism will 
undergo adaptive changes, although they are produced even 
more easily during the period of growth. The phenomena of 
adaptation consist mainly of three general truths, which can 
easily be verified by our every-day experiences. 
The first general truth is that extra function of any organ 
is followed by extra growth of that organ ; muscles unusually 
exercised grow to an unusual degree, of which the blacksmith’s 
arm and the dancer’s leg are examples, as also the thickening 
of the epidermis on hands which have to do hard or long- 
continued work. In fact, examples may be found in all parts 
of the body, and we find similar facts in connection with the 
nerves, as the increased delicacy of any of our senses on 
continued practice or when one of them has partly to take 
the place of the other, as in the case of blind or deaf people. 
The second general truth is that after a certain limit is 
reached, no present, or only very little further, modification 
can be produced, and that little only very slowly. Athletes 
very soon reach a limit to the increase of their skill and 
of their strength, and no possible further exercise will 
make any difference. Singers in the beginning of their 
training can increase the compass of their voice a tone or 
two at each end, but after that no training will increase the 
range. Very clearly is this second truth recognised in the 
intellectual faculties. Everyone has a certain capability for 
drawing, music, mathematics, &c., and everyone can improve 
this capability, but only to a certain degree or limit, and this 
limit is different for different individuals, and a special talent 
for any of these arts or sciences seems to be caused by a 
special facility of adaptation of the organs which are exercised 
by them. 
The third general truth is that the increase of size by 
increased function of any organ is not permanent unless the 
increase of function is permanent. Muscles increased by 
exercise are soon reduced to the original size when the 
exercise is not continued, and the rapidity of reduction is 
proportional to the shortness of the time during which the 
exercise was continued. If the exercise has lasted for years, 
