Selection 19 



century, but by his own observations, so it was in regard to the 

 principle of selection. He was struck by the innumerable cases of 

 adaptation, as, for instance, that of the woodpeckers and tree-frogs 

 to climbing, or the hooks and feather-like appendages of seeds, which 

 aid in the distribution of plants, and he said to himself that an 

 explanation of adaptations was the first thing to be sought for in 

 attempting to formulate a theory of evolution. 



But since adaptations point to changes which have been under- 

 gone by the ancestral forms of existing species, it is necessary, first 

 of all, to inquire how far species in general are variable. Thus 

 Darwin's attention was directed in the first place to the phenomenon 

 of variability, and the use man has made of this, from very early 

 times, in the breeding of his domesticated animals and cultivated 

 plants. He inquired carefully how breeders set to work, when they 

 wished to modify the structure and appearance of a species to their 

 own ends, and it was soon clear to him that selection for breeding 

 purposes played the chief part. 



But how was it possible that such processes should occur in free 

 nature ? Who is here the breeder, making the selection, choosing 

 out one individual to bring forth offspring and rejecting others? 

 That was the problem that for a long time remained a riddle to 

 him. 



Darwin himself relates how illumination suddenly came to him. 

 He had been reading, for his own pleasure, Malthus' book on 

 Population, and, as he had long known from numerous observa- 

 tions, that every species gives rise to many more descendants than 

 ever attain to maturity, and that, therefore, the greater number of 

 the descendants of a species perish without reproducing, the idea 

 came to him that the decision as to which member of a species was 

 to perish, and which was to attain to maturity and reproduction 

 might not be a matter of chance, but might be determined by the 

 constitution of the individuals themselves, according as they were 

 more or less fitted for survival. With this idea the foundation of 

 the theory of selection was laid. 



In artificial selection the breeder chooses out for pairing only 

 such individuals as possess the character desired by him in a 

 somewhat higher degree than the rest of the race. Some of the 

 descendants inherit this character, often in a still higher degree, and 

 if this method be pursued throughout several generations, the race 

 is transformed in respect of that particular character. 



Natural selection depends on the same three factors as artificial 

 selection : on variability, inheritance, and selection for breeding, but 

 this last is here carried out not by a breeder but by what Darwin 

 called the "struggle for existence." This last factor is one of the 



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