286 CAUSES OF VARIATION 



Upper limits of development. In what sense is development 

 conditioned upon use ? Does use simply enable the part to 

 attain its normal and proper development, to which it is entitled 

 under the laws of heredity, or does it stimulate development 

 beyond the normal? 



Some biologists at once assume the latter to be impossible 

 and that any unusual appearance is a case of atavism. It is true 

 that in times long past there may have existed ducks that walked 

 and others that flew more and better than those which Darwin 

 examined ; but when did nature produce a running or a trotting 

 horse as good as the one of to-day ? To what remote ancestor 

 do our violinists and our pianists owe their skill, and what was 

 the instrument on which they acquired it ? 



The accompanying cut is a facsimile of a properly attested let- 

 ter written with the feet by a young woman twenty-three years 

 old who lost both arms at the age of ten. Among her other ac- 

 plishments she numbers cutting, sewing (threading her own 

 needle), drawing, sweeping, and a great variety of housework. 1 



Here is a case of putting parts to an entirely new use, 

 demanding a nicety of adjustment that was never acquired even 

 in one out of a million of the ancestors. Could there be better 

 evidence of the fact that few individuals ever use, and there- 

 fore few ever develop, more than a fraction of the capacities born 

 in them ; that the possibilities of life are seldom realized, and 

 that never are all faculties developed to their utmost in any 

 single individual ? 



All this is clear but it is not so easy to determine where to 

 draw a line and say, " All development below this is due to 

 inheritance and all above to use." The truth would seem to be 

 that development depends upon both inherent tendencies and 

 external conditions affording opportunities for their exercise, 

 and that the maximum of development is reached only when 

 both are at their optimum. There is much force in the word 

 " optimum." Too much exercise, too much food, too much 

 temperature, or too much of any of the conditions of life is 

 as unfortunate as too little. 



1 The author saw one man who wrote with his feet, but they were attached 

 directly to the body, with no legs whatever. 



