12 THE CHARACTERS OF LIVING BEINGS 



some structures, for example, the hair, teeth, nose, external ears 

 and reproductive organs, is apparently not influenced in the 

 slightest degree by use. It is plain, therefore, that the power of 

 growing under the influence of use, unlike that of responding to 

 injury, is not a property common to all or nearly all living 

 structures. It is present only in some structures, and in them 

 not equally at all periods of life. 



22. If we continue to study the matter closely we make another 

 discovery and a very important one ; we find that this power, like 

 that of repairing injury, is present only in structures where it is 

 useful, to the extent to which it is useful, and during the time it is 

 useful. In man, for example, it long persists in the muscles of the 

 limbs and in the heart and kidneys, which even in advanced age 

 hypertrophy under increased strain. But no structures are more 

 used than the joints. With the rest of the limbs they grow under 

 the influence of use during youth, after which the power of so grow- 

 ing, which would then be useless, indeed injurious, departs from 

 them. The tongue again is a structure which is immensely used. 

 We do not know whether it grows in part under the stimulus of 

 use or only under that of nutriment ; but, in any case, growth 

 ceases when adult life is attained ; for no amount of use, apparently, 

 can add anything to it subsequently. Manifestly, therefore, 

 the power of growing under the stimulus of use is also an 

 adaptation. It is not a necessary property of all living protoplasm, 

 but a thing which has been implanted in some structures by 

 evolution. 



23. Moreover we have massive evidence that it is a late and a 

 high product of evolution. It does not date back to the origin of 

 life like the faculties of growing in response to the stimuli of 

 nutriment and injury. It is observable only in the higher organisms, 

 and in its greatest development only in the highest. Thus a human 

 infant is born very immature and helpless ; it owes most of its 

 subsequent growth, its advance towards maturity, mainly to use. 

 A young foal is born better developed ; it owes less to use and more 

 to the stimulus of nutriment. There is evidence that amphibians 

 (e.g. frog) and fish owe little or nothing to use ; they seem to 

 develop perfectly, or at least very well, when so rigidly confined 

 that use has no great scope to act as a stimulus. Most insects 

 seem to grow wholly under the stimulus of nutriment (and injury). 

 Certainly use plays no part in bringing about the changes which 

 occur during their metamorphoses. Lower in the animal scale 

 there is even less probability that use plays any part in develop- 



