THE PLASTICITY OF LIFE 201 



changes in habits and surroundings. The changes which 

 crop up in a brood of jelly fishes or in the progeny of an 

 evening-primrose, seem to arise in the mysterious arcana of 

 the germ-cells. They are born, not made ; endogenous, not 

 exogenous. Modifications, on the other hand, are novel 

 responses to peculiarities in nurture ; they are physiologic- 

 ally traceable to changes in environment or in function, and 

 it is to them that our phrase, " plasticity of life," refers. 



When an Englishman on foreign service becomes, in 

 the course of half a lifetime of work under a tropical sun, 

 so thoroughly tanned that the browning never disappears 

 through all his years of pension-life in a Scottish Highland 

 retreat where the sun never scorches we call that a 

 " modification/ 1 It is a structural change in the body 

 which transcends the limits of organic elasticity, and has 

 been so saturating that it persists after the originally 

 inducing causes have ceased to operate. We get clear of 

 vagueness if we adopt some such definition, and it is to the 

 capacity of undergoing such " modifications" that we wish 

 to restrict the term plasticity. 



Sun-burning does not happen to be of much practical 

 importance, but it illustrates clearly what we mean by a 

 bodily modification directly due to environmental change ; 

 just as the formation of callosities or hard skin pads on 

 hands or feet may serve as an example of one which is 

 rather due to change in function than to change in sur- 

 roundings. What we wish to do is to show how broad a 

 biological basis this doctrine of modifiability or plasticity 

 has. 



The fact that we wish to illustrate is the influence 

 of the environment in producing modifications. " In a 

 smithy we see a bar of hot iron being hammered into useful 

 form. Around a great anvil are four smiths with their 

 hammers. Each smites in his own fashion as the bar 



