150 DEVELOPMENT AND HEREDITY. 



ent stress on the bones ; and a different pressure 

 of the surrounding medium would affect both circu- 

 lation and respiration. In short, the sum total of 

 the forces acting upon the organism would be 

 radically changed. Can we imagine, then, in the 

 interplay of the internal and external forces which 

 mould the structure of the animal, that no change 

 will appear in the structure as the result of the 

 changes in the moulding forces .-' Let us follow 

 the development of one of the young after the 

 species has made the change in its mode of life. 

 The germ begins existence with its inherited 

 impulse of development, and up to the moment 

 of its birth is subject to the same conditions and 

 the same stimuli that have been customary in its 

 race ; therefore, through this period of its life it 

 grows and develops in the same way that its ances- 

 tors developed during the corresponding period 

 of their lives. But soon after it is born, and Ions: 

 before it has attained its full growth and develop- 

 ment, it follows its mother into the water and 

 begins to swim. The change of condition chiefly 

 affects the legs. In the first place the extremities 

 of the legs, which are deepest in the water, are 

 subjected to the greatest pressure of the medium, 

 and therefore the blood circulation in those parts 

 meets with greatest resistance. Many of the 



