40 EVOLUTION OF LIVING ORGANISMS. 



generations green plants have reproduced themselves on 

 earth. Yet take the seed of the greenest plant you like, 

 and grow it in a dark cellar the new plant will come up 

 not green at all. The stimulus of light has been denied 

 it, so the green " character " does not appear ; it is not 

 transmitted. But the capacity to become green is never- 

 theless transmitted, as can be shown at once by bringing 

 the pale seedling into the light, when it will soon turn 

 green. This capacity depends on the factors constituting 

 the inheritance. Just in the same way, if the seed be 

 sown in soil devoid of iron the young plant will not be 

 green, however much light there may be ; the stimulus 

 of iron has been removed. Add a trace of iron to the 

 soil, and the plant will turn green. Again, Englishmen 

 in the tropics become sunburnt ; this " character," dark 

 pigmentation, is due to the interaction between the en- 

 vironmental stimulus, sunlight, and the transmitted 

 germinal constitution. Their children will be likewise 

 sunburnt, provided they are exposed to the rays of the 

 tropical sun. But they will not be sunburnt if they are 

 sent home to England. Once the maximum effect of the 

 stimulus has been reached, once the maximum response 

 has taken place, they will not be increasingly burnt, 

 however many generations of their parents may have 

 lived in the tropics ; and the children, if removed from 

 the action of the sun, will not be sunburnt at all. The 

 negro, on the other hand, will continue to be black and 

 his children will be black, even when he lives in a tem- 

 perate climate among white fellow-men. For the black 

 character of the negro is called forth not by the external 

 stimulus of sunlight, but by some other external or in- 

 ternal stimuli which are not removed when he changes 

 his abode. 



This example illustrates a most important principle in 

 the evolution of organisms. Obviously those characters 

 will be most constant which depend for their appearance 

 on ever present stimuli. And, except in so far as an 



