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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…established harmony〃 after the famous Leibnitzian model； by means of which the clock of the evolution of organisms is so regulated as to strike in exact synchronism with that of the history of the earth！ All forms of life are strictly adapted to the conditions of their life； and can persist under these conditions alone。
There must therefore be an intrinsic connection between the conditions and the structural adaptations of the organism； and； SINCE THE CONDITIONS OF LIFE CANNOT BE DETERMINED BY THE ANIMAL ITSELF； THE ADAPTATIONS MUST BE CALLED FORTH BY THE CONDITIONS。
The selection theory teaches us how this is conceivable； since it enables us to understand that there is a continual production of what is non… purposive as well as of what is purposive； but the purposive alone survives； while the non…purposive perishes in the very act of arising。 This is the old wisdom taught long ago by Empedocles。
II。 THE LAMARCKIAN PRINCIPLE。
Lamarck； as is well known； formulated a definite theory of evolution at the beginning of the nineteenth century； exactly fifty years before the Darwin… Wallace principle of selection was given to the world。 This brilliant investigator also endeavoured to support his theory by demonstrating forces which might have brought about the transformations of the organic world in the course of the ages。 In addition to other factors； he laid special emphasis on the increased or diminished use of the parts of the body； assuming that the strengthening or weakening which takes place from this cause during the individual life； could be handed on to the offspring； and thus intensified and raised to the rank of a specific character。 Darwin also regarded this LAMARCKIAN PRINCIPLE； as it is now generally called； as a factor in evolution； but he was not fully convinced of the transmissibility of acquired characters。
As I have here to deal only with the theory of selection； I need not discuss the Lamarckian hypothesis； but I must express my opinion that there is room for much doubt as to the cooperation of this principle in evolution。 Not only is it difficult to imagine how the transmission of functional modifications could take place； but； up to the present time； notwithstanding the endeavours of many excellent investigators； not a single actual proof of such inheritance has been brought forward。 Semon's experiments on plants are； according to the botanist Pfeffer； not to be relied on； and even the recent； beautiful experiments made by Dr Kammerer on salamanders； cannot； as I hope to show elsewhere； be regarded as proof； if only because they do not deal at all with functional modifications； that is； with modifications brought about by use； and it is to these ALONE that the Lamarckian principle refers。
III。 OBJECTIONS TO THE THEORY OF SELECTION。
（a） Saltatory evolution。
The Darwinian doctrine of evolution depends essentially on THE CUMULATIVE AUGMENTATION of minute variations in the direction of utility。 But can such minute variations； which are undoubtedly continually appearing among the individuals of the same species； possess any selection…value； can they determine which individuals are to survive； and which are to succumb； can they be increased by natural selection till they attain to the highest development of a purposive variation？
To many this seems so improbable that they have urged a theory of evolution by leaps from species to species。 Kolliker； in 1872； compared the evolution of species with the processes which we can observe in the individual life in cases of alternation of generations。 But a polyp only gives rise to a medusa because it has itself arisen from one； and there can be no question of a medusa ever having arisen suddenly and de novo from a polyp…bud； if only because both forms are adapted in their structure as a whole； and in every detail to the conditions of their life。 A sudden origin； in a natural way； of numerous adaptations is inconceivable。 Even the degeneration of a medusoid from a free…swimming animal to a mere brood… sac （gonophore） is not sudden and salt