USE AND DISUSE 33 



and use ; apparently they are able to grow into normal adult 

 animals in its absence. Thus, if a tadpole finds its way 

 through a crevice into a small cavity, and is able to obtain 

 sufficient food, it may develop into a normal frog though it 

 leads a purely vegetative life. Fish are said to possess a 

 similar power. Higher in the scale, in a continuously increas- 

 ing degree, among birds and mammals, and most of all in the 

 highest mammals, the animal attains its full devolopment, as 

 regards many structures, only in response to the stimulation 

 of exercise and use; thus, for instance, if the limb of an 

 infant be locked by paralysis or by a joint disease, so that it 

 cannot be used, it does not develop into an adult limb. 1 



55. Now, if a " normal " man takes a more than normal 

 amount of exercise, he gets a more than ordinary development 

 of various structures, as happens in the case of the blacksmith's 

 arm. This additional development is regarded as "abnormal," 

 and is rightly termed "acquired." But, as we see, the 

 "normal" degree of development is attained only as a re- 

 sponse to stimulation (exercise) similar in kind though less 

 in amount. Therefore it is clear that the full development of 

 the normal arm, as well as many other important structures, is 

 acquired, differing in this from hair, nose, eyes, ears, teeth, nails, 

 sexual organs, etc., which are wholly inborn, and apparently 

 do not owe their development in the least to use and exercise. 

 In fact, on consideration, I think it will be found that adult 

 man differs physically from the infant almost wholly in char- 

 acters which are acquired through use, not in those which are 

 inborn. In the features of his face, in primary and secondary 

 sexual organs, and in some other respects, he differs from the 

 infant largely in inborn characters ; but as regards nearly all 

 the structures of the trunk and limbs as well as most of those 

 of the head (e. g. brain), the difference lies in characters which 

 have been acquired by the adult in response to the stimulation 

 of exercise and use. Thus, the limbs develop almost wholly 

 in response to use ; the heart and blood-vessels develop in 

 proportion to the strain put on them ; as also do the lungs 

 and their accessory muscles, as well as the bony attachments 

 of the latter. The muscles, arteries, nerves, bones, ligaments, 

 and other structures of the head and neck develop in response 

 to similar stimulation. Moreover the normal standard of 



1 The effects of use and disuse in relation to heredity have been dis- 

 cussed at greater length in my book The Present Evolution of Man. It 

 it now admitted, probably on all hands, that higher animals are more 

 " plastic" than lower animals. See in particular Development and 

 Evolution by Professor Mark Baldwin. 



D 



