THE STIMULI TO DEVELOPMENT 207 



the same. If, then, two varieties (e.g. of man) arise in different 

 localities, it does not follow necessarily that in each locality they 

 have diverged progressively in the most useful direction conceivable, 

 but only that each has diverged in a useful direction. 



339. Except actual injuries, as distinguished from vital re- 

 actions to injury, all the characters of living beings result from an 

 interaction between the hereditary tendencies or potentialities of 

 the individual and stimuli which awaken them to activity. In 

 essence, all evolution consists in the evolution of these hereditary 

 tendencies that is, in the evolution of the germ-plasm. 



340. There are many such stimuli to development. Amongst 

 the principal are nutriment, use, and injury. In order to avoid 

 unnecessary complications in the argument, I have ignored all 

 stimuli save the three named, or rather I have roughly grouped 

 under the head of nutriment all stimuli save use and injury. 

 Here I have merely followed established biological custom, which 

 distinguishes characters as inborn and acquired, and separates the 

 latter into those resulting, on the one hand, from use, and on the 

 other, from injury. Nutriment supplies, not only a stimulus to 

 development, but the materials for all growth. All the ' normal ' 

 structures of many species appear to develop wholly under the 

 influence of this stimulus, and many of the structures of all species 

 so develop. In every case the beginnings of all growth are 

 attributable to it, for, of necessity, it is the first stimulus to which 

 each organism and structure responds. The capacity to develop 

 under the influence of use or injury is present only in structures 

 where it is useful, at a time when it is useful, and only to an 

 extent which is useful. The power of responding by growth to 

 injury (regeneration, healing) has been closely studied by biolo- 

 gists. That of responding to the stimulus of use has been more 

 neglected. The utmost confusion prevails, for example, as to 

 which characters in each species develop under the influence of 

 nutrition, and which (if any) under that of use. When, however, 

 we study mind we shall see that this question is the most interest- 

 ing and practically important of all the problems of biology. 

 Probably the great Lamarckian controversy, which still lingers, 

 would have ended as soon as it began, if biologists had carefully 

 defined their terms and noted in what species, in what characters, 

 and to what extent the power of responding to the stimulus of 

 use was developed : in other words, if they had studied this power 

 as what it really is, an adaptation, a product of evolution, not a 

 chance or necessary property of living protoplasm. 



