THE FINAL ADJUSTMENTS III 



reasons : either so as to fit in with its surroundings, 

 or in response to the amount of work the part has 

 to do. An alteration due to the first reason is seen if 

 a small lens is grafted into a large eye: the small 

 lens grows faster than it normally would and the 

 large eye more slowly, so that together they make 

 up a harmonious but middle-sized organ. The in- 

 fluence of the second factor, the amount of work 

 required, is shown by a comparison of tadpoles 

 which have been reared on a rich diet of animal 

 matter with those reared on a poorer vegetable diet. 

 The gut of the vegetarian tadpole has to work 

 harder to absorb enough nourishment from its 

 poorer food and grows much longer than that of the 

 flesh-eating tadpoles (Fig. 28). 



The Influence of Function 



The work which an organ has to do and the move- 

 ments it makes may influence not only its size but 

 also its structure. There is a very good example of 

 this in the differentiation of the leg-bones of the 

 chick. It was found that the isolated thigh-bone 

 can produce quite a well-shaped end to fit into 

 the knee-joint even when it is lying in a culture 

 with no muscles attached to it and makes no move- 

 ments at all. If a thigh- and shin-bone are grown 

 together in this way, both sides of the joint begin to 

 form, but soon the two bones fuse together and the 

 joint becomes obliterated. If, on the other hand, the 

 two bones are exercised (by wagghng them by hand), 

 the joint goes on developing properly. Thus move- 



