2i6 A STUDY IN HEREDITY 



athlete. In the plant world characters cannot, of course, be 

 acquired to any extent as a response to the stimulation of 

 exercise and use. Plants, therefore, of necessity, attain their 

 full development in the absence of almost all stimulation other 

 than such as are supplied by food and warmth. Of such plant- 

 like animals as sponges, the same also, of necessity, is true. It 

 is true, with possible exceptions, even of such active animals as 

 insects. Thus, a pupa may develop into a perfect insect while 

 lying quiescent. The lower vertebrates, such as fish and reptiles, 

 have also little or no power of developing in response to the 

 stimulation of use and exercise ; 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 develops into a normal frog, though it leads a 

 purely vegetative life. Higher yet in the scale among birds and 

 mammals, and most of all amongst the highest mammals, 

 the animal attains its full development, 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. Now, if a " normal " man takes 

 a more than ordinary amount of exercise, he gets a more than 

 ordinary development of various structures, as happens in the 

 case of the blacksmith's arm. This extra development is 

 regarded by biologists as "abnormal" and is rightly termed 

 "acquired." But, as we see, the "normal" degree of develop- 

 ment is attained only as a response to exercise (t.e. stimulation), 

 similar in kind though less in amount. Therefore, it is dear 

 that the full development of the normal adult arm, as well as 

 many other important structures, is acquired, differing in this from 

 eyes, ears, teeth, nails, etc., which are wholly inborn, and 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 characters which are 

 acquired, not in those which are inborn. In teeth, hair, skull-bones, 

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



