Importance of Selection 61 
could be regarded as a serious rival to it. That selection is a factor, 
and a very powerful factor in the evolution of organisms, can no 
longer be doubted. Even although we cannot bring forward formal 
proofs of it in detail, cannot calculate definitely the size of the 
variations which present themselves, and their selection-value, cannot, 
in short, reduce the whole process to a mathematical formula, yet we 
must assume selection, because it is the only possible explanation 
applicable to whole classes of phenomena, and because, on the other 
hand, it is made up of factors which we know can be proved actually 
to exist, and which, if they exist, must of logical necessity cooperate 
in the manner required by the theory. We must accept it because 
the phenomena of evolution and adaptation must have a natural 
basis, and because it is the only possible explanation of them’. 
Many people are willing to admit that selection explains adapta- 
tions, but they maintain that only a part of the phenomena are thus 
explained, because everything does not depend upon adaptation. 
They regard adaptation as, so to speak, a special effort on the part 
of Nature, which she keeps in readiness to meet particularly difficult 
claims of the external world on organisms. But if we look at the 
matter more carefully we shall find that adaptations are by no means 
exceptional, but that they are present everywhere in such enormous 
numbers, that it would be difficult in regard to any structure what- 
ever, to prove that adaptation had noé played a part in its evolution. 
How often has the senseless objection been urged against selection 
that it can create nothing, it can only reject. It is true that it can- 
not create either the living substance or the variations of it; both 
must be given. But in rejecting one thing it preserves another, 
intensifies it, combines it, and in this way creates what is new. 
Everything in organisms depends on adaptation; that is to say, 
everything must be admitted through the narrow door of selection, 
otherwise it can take no part in the building up of the whole. But, 
it is asked, what of the direct effect of external conditions, tempe- 
rature, nutrition, climate and the like? Undoubtedly these can give 
rise to variations, but they too must pass through the door of selec- 
tion, and if they cannot do this they are rejected, eliminated from 
the constitution of the species. 
It may, perhaps, be objected that such external influences are 
often of a compelling power, and that every animal must submit to 
them, and that thus selection has no choice and can neither select 
nor reject. There may be such cases; let us assume for instance 
that the effect of the cold of the Arctic regions was to make all the 
mammals become black; the result would be that they would all 
1 This has been discussed in many of my earlier works. See for instance The All- 
Sufficiency of Natural Selection, a reply to Herbert Spencer, London, 1893. 
