DARWINISM AND HEREDITY. 41 



organism can choose its environment, the so-called in- 

 ternal stimuli are those whose presence is most assured. 

 An organism, therefore, must have such a germinal 

 transmitted constitution that it can develop fully under 

 the ordinary stimuli of its environment, and it will 

 develop fully with the greater certainty the more it does 

 so with the help of internal stimuli. Hence, in the 

 struggle for existence, those may succeed best which are, 

 so to speak, freed from the dependence on merely ex- 

 ternal stimuli. This explains the importance of self- 

 regulating processes. A familiar instance is that of the 

 warm-blooded mammals. In them has been evolved an 

 automatic mechanism for keeping the temperature of the 

 body at the most favourable point for the carrying on of 

 the process of metabolism, and they have become thus 

 to a great extent independent of the changes in their 

 environment ; while the cold-blooded reptiles and in- 

 sects, for instance, are at the mercy of the surrounding 

 temperature, unable to live an active life except in warm 

 weather. 



Two more very instructive examples may be mentioned 

 to illustrate our discussion on the factors of inheritance. 

 Many plants live both in the alpine heights and in the 

 low plains, and acquire a characteristic structure in these 

 two different habitats. So different in appearance may 

 the two forms become, that a botanist, not knowing of 

 their common origin, would certainly place them in 

 separate species. Now, the French botanist Bonnier 

 divided a common dandelion (Taraxacum wdgare), and 

 grew one half in the lowlands and the other half in the 

 mountains. While the former grew into a tall and slender 

 plant, the half raised in the alpine heights grew into a 

 plant of very different appearance, with longer roots, 

 much shorter stems, smaller and more hairy leaves, 

 larger and brighter flowers. Each variety will reproduce 

 its like in its own locality ; but seeds of the alpine plant 

 will produce only the lowland form if sown there, and 



