Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/99

VARIATION 83 direct and definite effects upon the mass. The facts and considera tions for and against the belief that the conditions of existence act in a potent manner in causing definite modifications of structure are confronted in detail. In some few instances a marked effect has been rapidly produced, c.y., on European men in the United States, European dogs in India, horses in the Falkland Islands, oysters in the Mediterranean, &c. The chemical compounds of some plants and the state of their tissues are readily affected by changed conditions. The production of galls, &c., shows how great changes in structure and colour may result from chemical changes. Ye almost certainly know that organic beings in a state of nature maybe modified in various definite ways by the conditions to which the} 7 have been long exposed ; but it is difficult to distinguish be tween the definite result of changed conditions and the accumula tion through natural selection of indefinite variations which have proved serviceable. But, even granting the utmost weight to con ditions, we can rarely see the precise relation between cause and effect. Moreover, many animals and plants of wide range and great diversity of experience yet remain uniform in character. Again, the degree to which domesticated birds, c. , have varied does not stand in any close relation to the amount of change to which they have been subjected. In fact, we may have similar modifications under different conditions, different modifications under similar change of conditions, or no modifications at all. Closely parallel varieties are often produced from distinct races or even species without ascertainable unity of conditions. Bird-varia tions, too, seem conspicuously independent of circumstances. All these considerations tend to force on our minds the conviction that what we call the external conditions of life are in many cases quite insignificant in comparison with the organization or constitution of the being which varies. No doubt each variation may have its efficient cause, but it is as hopeless to search for the cause of each as to say why a chill or a poison affects one man differently from another. With respect to acclimatization, although habit does something towards the success of the process, yet the appearance of constitu tionally different individuals is a far more effective agent. Increased use adds to the size of muscles, together with the blood-vessels, nerves, bony crests of origin, and even the whole bones ; it also increases glands and strengthens sense-organs. In creased and intermittent pressure thickens the epidermis ; change of food modifies the coats of the stomach and alters the length of the intestine. Disuse weakens and diminishes all parts of the organization, lungs and chest, wings and their associated bones, &e. Although in domesticated animals this never goes so far that a mere rudiment is left, it seems often to have occurred in nature, the effects of disuse being aided by economy of growth with inter crossing. Changed habits may lead to use or disuse of organs, and consequently to their modification ; yet the effects of habit, use, and disuse have often been largely combined with the natural selec tion of innate variations and sometimes overmastered by it. Correlated variation means that the whole organization is so tied together during its growth and development that, when slight variations in any part occur, and are accumulated through natural selection, other parts become modified, apparently irrespective of advantage iu the change. Variations of structure in the young often affect those of the mature animal ; the influence of hard parts, mechanical pressure, the relative position of parts, and the size of the whole body all have important influences. Homologous tissues may exhibit associated variations, e.g., hoofs, hair, and teeth. In most cases the correlation is, however, quite obscure, and may seem to be of no utility to the species, as with various monstrosities and diseases. Colour maybe associated with other constitutional pecu liarities. Although correlation is of much importance, we may also falsely attribute to it structures which are simply due to in heritance or natural selection, or its effects may be inextricably commingled with those of increased use and of accumulation by natural selection, e.g. , the gigantic horns of the Irish elk with the changes necessarily associated with the acquirement of them. Homologous and multiple parts are peculiarly variable, and often tend to cohere. Rudimentary and lowly organized structures are variable. The law of compensation of Goethe and Geoffrey, " that in order to spend on one side nature is forced to economize on the other," holds true to a certain extent with domestic production, but more doubtfully in nature. Mechanical pressure and relative position of parts seem to be of some importance in determining variations ; but such changes are often due to reversion to long-lost characters, which may frequently occur. A part extraordinarily developed in any species tends to be highly variable. Specific characters are more variable than generic characters. Distinct species present analogous variations ; but this may arise either from analogous constitution or from re version. Secondary sexual characters are highly variable. "What ever the cause may be of each slight difference between the offspring and their parents and a cause for each must exist we have reason to believe that it is the steady accumulation of beneficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species." The preceding outline of Darwin s main positions (which Results are in harmony with his essential doctrine of indefinite of recent variability) prepares us for the discussion of more recent re! * ari - R research and opinion. But for our present concrete know- opirri on ledge of the influence of environment, use and disuse, in cluding all such researches as those of Semper, 1 or the peculiarly brilliant and luminous investigations of Poulton,- the recent valuable summary of Arthur Thomson 3 may conveniently be referred to. The corresponding theoretic argument for the definite causation of most variations by these agencies has been recently re-stated by Spencer, 4 along with his proposed limitation of natural selection. This should be taken along with the testimony of the American Neo-Lamarckian school, among which the learned and suggestive, though too undigested, essays of Cope are especially prominent. The views of Nageli, Mivart, and other advocates of internal variation here present themselves anew, along with the criticisms and replies to them, as also Weismann s doctrine of variability as being ultimately germinal. But space precludes the survey of this voluminous and unfinished controversy, which, more over, would not at present yield any general result, since neither the various inductive and deductive arguments, nor the organismal, functional, etc., and environmental explanations which these variously favour, have been as yet exhaustively stated, still less properly confronted, and least of all reconciled, by any author. It may be more profitable to attempt, though necessarily Proposed in barest outline, a fresh re-examination of the entire field. fre - sn This may be most appropriately introduced, and the initial tlie ? r y of conception of the present part of the discussion re-stated, t ional by a passage from Weismann, whose substantial acceptance laws, of the doctrine of natural selection has already been noticed (p. 81). "We certainly cannot remain at the purely em pirical conception of variability and heredity as laid down by Darwin in his admirable work. If the theory of selec tion is to furnish a method of mechanical explanation, it is essential that its factors should be formulated in a precise mechanical sense. But, as soon as we attempt to do this, it is seen that, in the first enthusiasm over the newly dis covered principle of selection, the one factor of transform ation contained in this principle has been unduly pushed into the background to make way for the other more ap parent and better known factors. The first indispensable factor, and perhaps the most important in any case, in every transformation is the physical nature of the organism itself." Let us briefly summarize, therefore, the main results of a fresh survey of the leading variations presented by plants and animals, i.e., no longer commencing with the analogy of human selection upon the smallest varietal and specific distinctions, and arguing on LyelPs principle for the cumu lative origin of the characters of larger groups, but consider ing these larger differences from the standpoint of general physiology, without any hypothesis at all. The physiological principle invoked is simply that antithesis be- Prepon- tween reproductive functions and individual ones which has been deranre of familiar since the dawn of physiology, and which, when reduced species to its physical terms, is obviously deducible from the principle of main- conservation of energy. Instead of the generally received doctrine, taining summarized above, that of indefinite variation, with progress by ends over means of struggle for existence among individuals, its systematic indi- application furnishes a detailed re-interpretation of the forms pre- vidual sen ted by plants and animals comparable to that afforded by the struggle received hypothesis, but with an essentially altered view of the and de- process and factors of organic evolution as a whole. Briefly stated, velop- ment. 1 Animal Life, Int. Sci. Series, 1881. 2 Proc. Roy. Soc., 1884-88. 3 "A Synthetic Summary of the Influence of Environment," in Trans. Roy. Phys. Soc. Etlin., 1887-88. 4 Factors of Organic Evolution, London, 1886 (reprinted from Nine teenth Century),

5 The Origin of the Fittest, London and New York, 1887.