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o simple that Huxley could say of it later； 〃How extremely stupid not to have thought of that。〃 As Darwin was led to the general doctrine of descent； not through the labours of his predecessors in the early years of the 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 undergone 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 observations； 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 special features of the Darwinian conception of nature。 That there are carnivorous animals which take heavy toll in every generation of the progeny of the animals on which they prey； and that there are herbivores which decimate the plants in every generation had long been known； but it is only since Darwin's time that sufficient attention has been paid to the facts that； in addition to this regular destruction； there exists between the members of a species a keen competition for space and food； which limits multiplication； and that numerous individuals of each species perish because of unfavourable climatic conditions。 The 〃struggle for existence；〃 which Darwin regarded as taking the place of the human breeder in free nature； is not a direct struggle between carnivores and their prey； but is the assumed competition for survival between individuals OF THE SAME species； of which； on an average； only those survive to reproduce which have the greatest power of resistance； while the others； less favourably constituted； perish early。 This struggle is so keen； that； within a limited area； where the conditions of life have long remained unchanged； of every species； whatever be the degree of fertility； only two； ON AN AVERAGE； of the descendants of each pair survive； the others succumb either to enemies； or to disadvantages of climate； or to accident。 A high degree of fertility is thus not an indication of the special success of a species； but of the numerous dangers that have attended its evolution。 Of the six young brought forth by a pair of elephants in the course of their lives only two survive in a given area； similarly； of the millions of eggs which two thread…worms leave behind them only two survive。 It is thus possible to estimate the dangers which threaten a species by its ratio of elimination； or； since this cannot be done directly； by its fertility。
Although a great number of the descendants of each generation fall victims to accident； among those that remain it is still the greater or lesser fitness of the organism that determines the 〃selection for breeding purposes；〃 and it would be incomprehensible if； in this competition； it were not ultimately； that is； on an average； the best equipped which survive； in the sense of living long enough to reproduce。
Thus the principle of natural selection is THE SELECTION OF THE BEST FOR REPRODUCTION； whether the 〃best〃 refers to the whole constitution； to one or more parts of the organism； or to one or more stages of development。 Every organ； every part； every character of an animal； fertility and intelligence included； must be improved in this manner； and be gradually brought up in the course of generations to its highest attainable state of perfection。 And not only may improvement of parts be brought about in this way； but new parts and organs may arise； since； through the slow and minute steps of individual or 〃fluctuating〃 variations； a part may be added here or dropped out there； and thus something new is produced。
The principle of selection solved the riddle as to how what was purposive could conceivably be brought about without the intervention of a directing power； the riddle which animate nature presents to our intelligence at every turn； and in face of which the mind of a Kant could find no way out； for he regarded a solution of it as not to be hoped for。 For； even if we were to assume an evolutionary force that is continually transforming the most primitive and the simplest forms of life into ever higher forms； and the homogeneity of primitive times into the infinite variety of the present； we should still be unable to infer from this alone how each of the numberless forms adapted to particular conditions of life should have appeared PRECISELY AT THE RIGHT MOMENT IN THE HISTORY OF THE EARTH to which their adaptations were appropriate； and precisely at the proper place in which all the conditions of life to which they were adapted occurred： the humming…birds at the same time as the flowers； the trichina at the same time as the pig； the bark…coloured moth at the same time as the oak； and the wasp…like moth at the same time as the wasp which protects it。 Without processes of selection we should be obliged to assume a 〃pre…establis