S4 DEVELOPMENT AND HEREDITY. 



of mucous membrane into skin, when exposed per- 

 manently to the air, is a case of this kind, showing 

 the effect of a change of medium. If now we 

 admit the effect of a change of forces, or the intro- 

 duction of new forces, as a cause of disease, then 

 we must attribute an equal power to the unchanged 

 forces, for the normal life and growth of tissues 

 is as much a phenomenon of organic forces and 

 reactions as is the diseased growth. 



A striking illustration of the effects of abnormal 

 action of forces on growth is shown in the change 

 in the shape of the human foot which results from 

 the pressure of the shoe. We see how the foot fits 

 itself into the shoe as though pressed into a mould, 

 and toward middle life, becomes thus permanently 

 distorted. This manner of growth under pressure 

 throws light on the problem of ho* the internal 

 organs of the body, and the muscles and bones 

 which press against each other, are moulded to fit 

 with a perfect contact of surfaces. So we must 

 conclude that mechanical pressures and strains play 

 a large part in modelling the shape of the body. 



These illustrations, drawn from the more readily 

 observable phenomena of animal life, might be multi- 

 plied indefinitely, both in kind and number. While 

 we can observe these facts more readily among the 

 higher animals, we cannot suppose that the lower 



