NATURAL SELECTION AND ADAPTATION 401 



will eventually die. Any change in the constitution 

 of the seedling of a species under these conditions 

 which enables its shoot to grow more quickly will give 

 it a decisive advantage in the struggle for light. 



In the former case the seedlings were struggling with 

 their inorganic environment, in the latter competing with 

 other organisms. The world of organisms is the scene 

 of both these kinds of struggle at every stage of the 

 life history of individual species, but the struggle is 

 most severe in the early stages of life, when the indi- 

 viduals are most numerous and have not yet established 

 themselves. This struggle for existence necessarily leads 

 to the success of the forms which are best equipped to 

 succeed. There is a selection of such forms by the 

 elimination of those which do not succeed, and it is 

 this which Darwin called natural selection. 



One of the first things that must strike the student 

 of biology is the marvellous adaptation of the different 

 kinds of plants and animals to the conditions in which 

 they live, and this adaptation is in large part brought 

 about by the success of those forms which have changed 

 in directions advantageous to them. Since they have 

 succeeded they are the forms we actually meet with 

 the failures have disappeared. But there are many 

 degrees of success. Some species are tremendously 

 successful, spreading widely and constantly establish- 

 ing themselves in new places carrying, so to speak, 

 all before them. Others hold their own, but no more, 

 succeeding perhaps in some places, failing in others. 

 Again, there are species which are not now holding their 

 own, but are slowly, or rapidly, dying out. 



In the course of evolution different species have 

 become closely adapted to different kinds of life con- 

 ditions habitats as they are called for instance hot 



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