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 
2—2 
