512 THE WONDER OF LIFE 



ing multiplication of red blood corpuscles in the case of 

 successful human migrants to a lofty plateau, in South 

 Africa, for instance. 



Many unicellular animals are very plastic, and it seems 

 reasonable to suppose that there was a considerable prim- 

 ary plasticity in the early organisms, and that restrictions 

 were placed on this as differentiation progressed. As the 

 body became more and more complex the range of primary 

 plasticity was lessened, but a more specialized secondary 

 plasticity was gained in many cases where organisms lived 

 in environments liable to frequent vicissitudes. 



(3) Modifiability. Taking a third step we recognize 

 as a fact of life that organisms often exhibit great modifia- 

 bility. They can change for their lifetime in response to 

 changes in surroundings or habits. Thus a man's skin 

 may be so thoroughly tanned by exposure to the sun during 

 half a lifetime in the tropics, that it never becomes pale 

 again, even after migration to a far from sunny clime. This 

 change in the skin is a modification : it differs from a 

 temporary adjustment in being permanent, and from a 

 constitutional swarthiness inasmuch as it was impressed 

 from without rather than expressed from within. It is 

 exogenous, not endogenous. 



' Modifications ' may be defined as changes in the body 

 acquired during an individual lifetime as the direct result 

 of changes in function or in environment, and so transcend- 

 ing the limit of organic elasticity that they persist after 

 the inducing conditions have ceased to operate. Lack of 

 nutrition at a particular stage in development may directly 

 induce an arrest or a dwarfing, with consequences from 

 which there is no possibility of recovery. A particular 

 occupation, such as shoemaking or the old-fashioned weav- 



