202 COOK 



it does not occur to us to suppose that their diversities are due 

 to the different conditions under which they have grown, for the 

 conditions are the same. We accept without debate the fact that 

 the plants are developing each according to the methods of its 

 own species. It is only when we find plants of the same species 

 following different methods of growth when under different con_ 

 ditions that we can be betrayed into supposing that the condi- 

 tions are producing the characters of the organisms. In reality 

 this reasoning has no more propriety when we compare a plant 

 or an animal with another member of its own species than when 

 we compare it with a member of a different species. 



As long as the adjustments are physiological only, we do not 

 find it necessary to marvel, but when they become appreciable 

 from the morphological standpoint our interest is aroused. And 

 when accommodations cause taxonomic difficulties by affecting 

 the characters by which we have described species, some are 

 ready to believe that environment must be responsible for evo- 

 lution because it can be alleged to change the characters of 

 species. To reach this conclusion the amassing of detailed 

 knowledge of plants and animals was superfluous. It could 

 have been based quite as logically on the fact that rain " causes " 

 us to carry umbrellas, and to wear waterproof coats. The 

 African variety of mankind adopts the reverse policy, but no 

 less appropriate to the occasion. He discards all of his scanty 

 wardrobe and gives his naked skin a coat of palm oil. The 

 birds can not change or take off their feathers, but their own 

 organization provides a convenient supply of oil, and an instinct 

 to use it when needed. Plants can neither go in when it rains 

 nor oil themselves, but many plants grow a water-shedding coat 

 of wax or of fine hairs on the upper surfaces of their leaves. 



All species of plants and animals have, as already remarked, 

 not only their general specific methods of development, but they 

 have in addition certain ranges of adjustment to the different 

 conditions under which they are able to exist. The environ- 

 mental qualifications of a species are not to be represented by a 

 single point, but by maximal and minimal boundaries, like the 

 geographical latitudes and longitudes which may be used to 

 indicate its position on the earth's surface. 



