Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/93

Rh VARIATION 77 perfect genera are kept down by the more perfect. The less definite view of Goethe included, besides recognition of the conservative or centripetal force of heredity, that of a progressive or centrifugal tendency to adaptation to environment. Oken (1809) similarly regarded all pro gress from his primeval &quot; Urschleim &quot; as having been in terms of its interaction with the external conditions of life. In 1813 Wells made his now well-known suggestion of the importance of natural selection in determining the varieties of the human species; and in 1831 Patrick Matthew published his much more developed, yet equally disregarded, statement of the same doctrine in its more general applications. In 1828-30 Geoffrey St-Hilaire, afterwards ably succeeded by his son Isidore, denied inde finite variation, regarding function as of secondary im portance, and laying special stress on the direct influence of the environment : for instance, it was not so much the effort to fly as the (supposed) diminished proportion of carbonic acid in the atmosphere which determined the evolution of birds from saurians. The veteran geographer Von Buch naturally inclined to emphasize the influence of geographical isolation (locality, climate, soil, food, &c.), and laid stress on the restriction of the area of possible sexual union as bearing upon the origin of varieties. The embryologist Yon Baer (1834) dwelt especially upon the organismal nature of variation, on the unfolding, as it were, of new structures; and Schleiden (1838) and other naturalists more or less distinctly advanced similar opinions. In 1844 appeared the Vestic/es of Creation, which in its later editions (1853) formulated an hypothesis of progress (1) by rhythmic impulse through grades of organization, (2) by another impulse tending to modify organic structures in accordance with external circumstances. In 1852 Naudin argued for the formation of new species in nature in a similar way to that of varieties under cultivation, further attaching great importance to an assumed &quot; prin ciple of finality,&quot; apparently a kind of organismal fate. Herbert Spencer, whose weighty arguments for evolution date from 1852, laid special stress upon the modifying influences of environment, this involving changes of function, and so ultimately of structure. Finally, in 1853 Victor Carus argued for a progressive adaptation to chang ing external conditions. An undefined hypothesis of in ternal modifiability appears also to have floated before the mind of Owen. In this succession of variational theories we recognize the repeated general insistence upon every separate factor in the problem, organism, function, and environment, the successive authors, in fact, almost falling into as many schools. Yet these are singularly unprogressive : the same hypothesis is stated again and again, but always fails to carry conviction ; many employ the merest abstractional explanations, in terms of what is simply the old pre- physiological &quot; vital force &quot; (see BIOLOGY), or attempt to cut the knot by the more or less open introduction of a deus ex machina. Even the more definite and scientific theories of Erasmus Darwin, Lamarck, Geoffroy, and .Spencer fail adequately to establish in a sufficient number of concrete cases the supposed potency of environment or function, much less of internal or volitional moods, in de termining extensive changes ; moreover, they are con stantly confronted by cases in which any one set of changes seems to take place independently of the other. Hence, despite the deep and unanimous conviction of so many biological thinkers as to the origin of species by evolution, the essential problem remained unsolved. For, until some explanation, rational and concrete, detailed yet universal, of the origin of species should be reached, the cautious naturalist could not feel justified in accepting as a scientific certainty a doctrine which rested so largely upon unde- monstrable grounds. Any really new attempt at estab lishing the doctrine of evolution had thus, not simply to reorganize and strengthen the empirical evidence, by marshalling beside the generalizations of morphological unity of type the facts of more strictly historic bearing, distributional, pakeontological, and embryological, but also to meet this deeper theoretic want by showing not only that evolution has taken place but how. Hence the import ance and rapid acceptance of the Origin of Species (1859), Charles for which the time was fully ripe. For it is evident from Darwin, the preceding historical outline that the view still fre quently promulgated of the exclusive and, as it were, catastrophic importance of this work in the establishment of the doctrine of evolution must be replaced by a more uniformitarian and, so to speak, less creationist view. It should not seriously be supposed that the theory of descent which had been held in one form or another by the leading biological thinkers of that and the two preced ing generations, with almost the individual exception of Cuvier, had been decisively abandoned by men of science, or that the public which had purchased ten editions of the Vestiges of Creation in as many years, and which took up the first edition of the Origin of Species on the day of publication, could be quite unprepared for the acceptance of the major thesis of both works. Yet that a lull had taken place is also unquestionable, for the failure of wave after wave of speculative effort had produced a widespread feeling of discouragement, and a tendency to concentration upon more concrete and smaller (for the most part Linnaean or Cuvierean) problems alone, since of these the solution was comparatively sure. The dramatic coincidence between Darwin s and Wallace s conclusions l soon helped to gain immediate attention to the new theory ; and this was thoroughly developed and retained by the prompt appear ance of the first and second editions of the Origin of Species. Without entering upon a review of this classic treatise, much less a retrospect of the controversy it pro voked, it is evident that it was upon almost all sides so much stronger than preceding attempts, its central doctrine so lucid and obvious a statement of the everyday facts of life, its mode of presentation so temperate yet so forcible, its self-criticism so frank and unconcealed, that its marvellous success was owing far more to intrinsic merit than favour able circumstances. The required re -statement of the evidence for the historic fact of evolution was at last learnedly and forcibly done, and the salient difficulties in its way, like that of the imperfection of the geological record, admirably met ; yet this portion of the work sinks into comparative ineffectiveness beside the long- sought modal explanation of the process proffered in the theory of natural selection, which was soon seen to throw a new and searching light upon the smallest details of structure, function, and distribution, in fact, to afford the instru ment of a new and systematic interpretation of organic nature. Of this doctrine, then, a brief account is needed, and this may be best given by following as closely as pos sible upon the lines of the magnum opus itself, although the full title of this The Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life is its own best and briefest summary. THEORY OF NATURAL SELECTION. After mentioning that his first light upon the origin of Analysis species was derived from his early distributional studies, of Origin Darwin points out that &quot;a naturalist, reflecting on the ^ pecies mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succes sion, and such other facts, might come to the conclusion that species had not been independently created, but had 1 Journ. and 1 roc. Linn. Soc., 1858.