GROWTH IN RESPONSE TO USE n 



reproduce the whole individual from a fragment. Great powers of 

 regeneration are the rule in plants or parts of plants which are 

 exposed to injury from animals, for example in grasses. 



19. The power of responding to the stimulus of injury, then, is 

 clearly allied to and is derived from the power of responding to 

 the stimulus of nutriment. The germ-plasm of every type of 

 animal and plant has so changed from the primitive kind that 

 under the stimulus of nutriment the individual tends to develop 

 the characteristics which evolution conferred on his immediate 

 predecessors ; it has also so changed that wherever the power of 

 regeneration is useful, and to the extent to which it is useful, injury 

 stimulates to the replacement or repair of lost or damaged parts. 

 Like the power of growing under the stimulus of nutriment, the 

 power of repairing injury is always an adaptation. The kind of 

 repair that normally occurs is ever the most useful and extensive 

 possible under the conditions. 



20. The characters which develop under the stimulus of injury 

 are interesting and important, but vastly more interesting, both 

 from the theoretical and the practical standpoint, are those which 

 develop under the stimulus of use. If we study almost any work 

 on the sister subjects, heredity and evolution, published more than 

 a decade ago, and a great many published since, we frequently 

 meet an entirely unwarrantable assumption the assumption that 

 all structures and organs in all types of living beings tend to grow 

 and develop if used. For example, the celebrated Lamarck, the 

 predecessor and in a way almost the peer of Darwin, who during 

 the nineteenth century exerted an immense influence on biological 

 thought, founded his theory of racial change on the supposition 

 that all structures and qualities are capable of increase under the 

 influence of use and decrease under that of disuse. The same 

 idea bulks largely in the writings of Spencer, Lewes, Romanes, 

 Cope, and a multitude of others. 



21. In man, as we have seen, certain structures grow under the 

 influence of use, but the amount even of this growth (i.e. multi- 

 plication of cells) is by no means strictly proportionate to the 

 amount of use, and is always rigidly limited. Different men 

 respond in different degrees to the stimulus of use, but even a 

 1 natural ' athlete can increase the size of his muscles only up to a 

 certain point, after which cell-multiplication no longer occurs, no 

 matter how much he uses his muscles. All growth due to use 

 occurs mainly during early life ; in old age it practically ceases in 

 most structures though they are still used. Lastly, the growth of 



