Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/95

Rh VARIATION 79 ditions of life, and consequently what infinitely varied diversities of structure might be of use to each being under changing condi tions of life, can it be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations, useful in some way to each being in the great and complex battle of life, should occur in the course of many generations ? And, if such do occur, can we doubt (remembering that many more indi viduals are born than can possibly survive) that individuals having any advantage, however slight, over others would have the best chance of surviving and procreating their kind ? On the other hand, we may feel sure that any variation in the least degree in jurious would be inevitably destroyed. This preservation of favour able and this destruction of injurious variations are called natural .selection, or, less metaphorically, the survival of the fittest. The probable course of natural selection may be understood from the case of a country undergoing change of climate. The proportional numbers of its species will be changed ; some will probably become extinct ; and these changes would seriously affect the others. Im migration of new forms might occur, with further serious disturb ance, or, where this is impossible, we should have places in the economy of nature which might be better filled up. In such cases slight modifications which in any way favoured the individuals of any species, by adapting them better to their altered conditions, would tend to be preserved, and natural selection would have free scope for the work of improvement. Moreover, changed conditions increase variability. As man produces a great result, what may not natural selection effect ? The former acts only for his own good, nature for that of the being itself ; man on mere external characters, nature on the whole machinery of life ; man irregularly and imperfectly for a short time, nature by accumulation during whole geological periods. Natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations, reject ing those that are bad, preserving and adding up all that are good, silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. It may operate on char acters which we are apt to consider of very trifling importance, and its accumulation of small variations may set up unexpected cor relative changes. It may modify the egg, seed, or young as easily as the adult, the structure of young to parent and of parent to young, and in social animals it will adapt the structure of each for the benefit of all. 1 The theory of natural selection is next illus trated (1) by supposing the formation of swift varieties of wolves, much as greyhounds have been developed by man ; (2) by reference to the excretion of nectar by flowers, its use to insects, the action of these in carrying pollen, its advantage in intercrossing, and the resultant modification and adaptation of flower and insect to each other through the preservation of their advantageous variations. Circumstances favourable for the production of new forms through natural selection are great variability, large numbers of individuals, the complex effects of intercrossing, isolation in confined areas (yet probably still more extension over continental ones, especially if oscillating in level), and considerable lapse of time, although this by itself must not be supposed to do anything (as if the forms of life were undergoing change by some innate law), but merely to afford increased opportunity for variation and environmental change. Extinction, to which rare species are on the way, is caused by natural selection. The divergence of character brought about by artificial selection in domestic breeds is efficiently paralleled in nature, since the more diversified the offspring of each species, the more they will seize on diverse places in the economy of nature, and so increase in numbers. The greatest amount of life can be supported by great diversification of structure. This divergence of character, with extinction of intermediate forms, explains the difficulties of tax onomy, which are then discussed in detail with the aid of a diagram. This, of course, takes the form of a genealogical tree, and suggests that of &quot; the great tree of life, which fills with its dead and broken branches the crust of the earth and covers the surface with its ever-branching and beautiful ramifications.&quot; The preceding summary of the classical statement of the doctrine of natural selection should be supplemented by reference not only to the original work, to the corrobora tive labours of its author, and to the able independent treatise (Natural Selection) of Wallace, but to the enormous mass of exposition, argument, and illustration accumulated by subsequent writers, commencing with Hooker and Asa Gray, Huxley and Haeckel, but soon becoming too numer ous even for mention. At the same time the history of the controversy which has arisen out of that statement .should also be treated, in so far as it has kept within 1 In the later editions of the Origin a brief account of sexual selec tion is given at this point, vide infra, p. 82. scientific bounds. The leading objections which have been brought against natural selection, together with the replies to them, should all be summarized, as, for instance, Fleem- ing^ Jenkin s important criticism as to the swamping of individual variations by crossing, 2 Mivart s vigorous and detailed polemic against natural selection as capable of accounting for the incipient stages of useful structures, &c.,s and so on. But this could not be done to the satis faction of either party without far exceeding the present limits; happily, however, two chapters of the Origin of Species (v. and vi.) are generally admitted to have at least made the attempt up to 1872, the date of the final edition, with great candour and fairness. The various constructive efforts to supplement the hypothesis of natural selection by introducing additional factors of evolutionary change should also be mentioned, headed of course by Darwin s own accessory hypothesis of sexual selection. We must not, however, leave natural selection without Huxley s a fuller statement of its services and claims, and this may advocacy, most appropriately be taken from the recent and, as it were, judicial deliverance of their veteran advocate, Prof. Huxley. 4 He first points out the grounds of his agnostic position (up to 1858) with respect to evolution as promul gated by Lamarck, Chambers, and even Spencer : &quot; Firstly, that up to that time the evidence in favour of transmuta tion was wholly insufficient ; and, secondly, no suggestion respecting the causes of the transmutation assumed, which had been made, was in any way adequate to explain the phenomena.&quot; He then goes on to say &quot; The suggestion that new species may result from the selective action of external conditions upon the variations from their specific type which individuals present and which we call &quot;spontaneous&quot; because we are ignorant of their causation is as wholly unknown to the historian of scientific ideas as it was to biological specialists before 1858. But that suggestion is the central idea of the Origin of Species, and contains the quintessence of Darwinism. . That which we were looking for, and could not find, was an hypothesis respecting the origin of known organic forms which assumed the operation of no causes but such as could be proved to be actually at work. We wanted, not to pin our faith to that or any other speculation, but to get hold of clear and definite con ceptions which could be brought face to face with facts and have their validity tested. The Origin provided us with the working hypothesis we sought. . . . The facts of variability, of struggle for existence, of adaptation to conditions were notorious enough, but none of us had suspected that the road to the heart of the species problem lay through them till Darwin and Wallace dispelled the darkness, and the beacon-fire of the Origin guided the be nighted. . . . The only rational course for those who had no other object than the attainment of truth was to accept &quot; Darwinism &quot; as a working hypothesis and see what could be made of it. Either it would prove its capacity to elucidate the facts of organic life or it would break down under the strain. . . . Whatever may be the ultimate fate of the particular theory put forward by Darwin, I venture to affirm that, so far as my knowledge goes, all the ingenuity and all the learning of hostile critics have not enabled them to adduce a solitary fact of which it can be said, this is irreconcilable with the Darwinian theory. &quot; 5 Darwin s own case for natural selection has also been Darwin s tersely put. 6 &quot;The belief in natural selection must at reasous - present be grounded entirely on general considerations, (1) on its being a vera causa, from the struggle for exist ence and the certain geological fact that species do somehow change ; (2) from the analogy of change under domestication by man s selection ; (3) and chiefly from this view connecting under an intelligible point of view a host of facts.&quot; Applying to natural selection the accepted tests of a good theory (1) power of explaining all phenomena and meeting all objections, (2) power of meeting new facts as they occur, (3) applicability as an instrument of research, - North British Review, 1867. 3 Genesis of Species, 1871. Cf., however, op. cit., p. 129, note 4. 6 Letter to Bentham, 1863, in Life of Darwin, iii. p. 25.
 * Life ofDanvin, vol. ii., &quot;On the Reception of the Origin of Species. &quot;