THE STIMULI FOR GROWTH 7 



11. Living beings develop mainly under the influence of three 

 distinct kinds of stimuli nutriment, use, and injury. To take the 

 example of the human being : up to the time of birth the infant 

 develops wholly, or at least principally, under the stimulus of 

 nutriment. After birth some of his structures continue to grow 

 under this stimulus, for example his hair, his teeth, his external 

 ears, and his organs of reproduction. But, as regards other of his 

 structures, though nutriment continues to supply the materials for 

 growth, it ceases to supply the stimulus. Thus, no matter how 

 well the child is fed, the muscles of his limbs do not develop unless 

 they are used. It is a noticeable fact that structures and those 

 structures only which grow under the influence of use, tend to 

 atrophy when disused. Thus disuse, or lessened use, diminishes 

 the size of the muscles in a human limb, while under similar con- 

 ditions the external ears and the reproductive organs persist 

 unchanged. Since the development which occurs under the 

 stimulus of use is always a mere extension though often a most 

 important extension of that which occurs under the stimulus of 

 nutriment, it is frequently difficult to distinguish structures which 

 owe their growth wholly to nutriment from those which owe it in 

 part to use. But the tendency of the latter class to atrophy when 

 disused supplies us with a rough criterion. 1 Lastly, if the individual 

 be injured, as by a cut, the wound supplies the stimulus for the 

 growth which occurs during the process of healing. 



12. Other stimuli to growth besides nutriment, use, and injury, 

 exist, though we need not pause to discuss them at length. Thus 

 all organs develop within a certain range of temperature, and best 



or injury ; here nutriment furnishes only the materials. The reader will see 

 later that, for the sake of convenience, I include under ' nutriment ' a number 

 of other stimuli, but that I sharply distinguish from it use and injury. He will 

 find that the reason why I include a number of stimuli under the heading of 

 nutriment, but exclude from that category use and injury, is because biologists 

 are accustomed to term all characters which arise under the stimulus of use and 

 injury ' acquirements,' whereas all other characters are termed by them ' inborn ' 

 or ' innate.' The effect of distinguishing characters by means of the stimuli 

 under which they arise, rather than by the words ' innate ' or ' acquired,' will be 

 to make us take a view of heredity and evolution, especially mental heredity and 

 evolution, immensely different, but I think, demonstrably more correct and 

 comprehensive than the view hitherto accepted. 



1 The criterion is very rough, however. If a structure atrophies under disuse 

 then, as far as I am aware, we may be sure it has grown under the influence of 

 use. But it is not true that all structures which have grown under the influence 

 of use atrophy when disused. Thus the bones of the limbs grow under the influence 

 of use, but they do not atrophy like the muscles, at least to the same extent, when 

 disused. Apparently this is due to the fact that bones are not to the same extent 

 composed of living cells. 



