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DARWIN ON CAUSES OF VARIATION.
motion. It is but fair to Mr Darwin that we should say he seems to be quite aware of the existence of this theory, though too cautious to commit himself fully to it; and it is but fair to the theory to say, that Mr Darwin adduces many facts which give it support. A few of these facts may here be brought together.
Mr Darwin expresses his opinion that the great variability of animals and plants under domestication is simply due to their having been raised under conditions of life, not the same and not so uniform as those to which the parent species have been exposed under nature. In conformity with this, when the artificial conditions are removed, and the animals return to the natural world of their ancestors, those which survive appear often to resume the ancestral peculiarities. Thus the horses which have run wild in South America are generally brownish-bay, and in the East dun-coloured ; while their heads have become larger and coarser. When variously-coloured tame rabbits are turned out in Europe, they generally re-acquire the colouring of the wild animal. The pigs which have run wild in the West Indies, South America, and the Falkland Islands, have everywhere acquired the dark colour, the thick bristles, and great tusks of the wild boar, and the young have longitudinal stripes.1 This is the rebounding of the spring when the pressure is taken off, and goes to prove that the spring will yield to every form of pressure; and that when external conditions put the same kind of pressure on many individuals at once, they will all tend to vary in the same direction.
There is, Mr Darwin thinks, some probability in the view propounded by Andrew Knight, that the variability 1 Animals and Plants under Domestication, ii. 33.
of animals and plants under domestication may be partly connected with excess of food.1 The long-continued use or disuse of organs would also have its effect, as in our domestic quadrupeds, which are all descended, so far as is known, from species having erect ears; but being no longer under the same necessity as wild animals to catch every passing sound and ascertain the direction whence it comes, have, many of them, lost the power to erect the ears. Cats in China, horses in parts of Russia, sheep in Italy and elsewhere, the guinea-pig in Germany, goats and cattle in India, rabbits, pigs, and dogs in all long-civilised countries, have drooping ears. This is the converse case to that of the blacksmith's arm becoming stronger through continual exercise, excepting that the effect has accumulated from generation to generation. Some amount of variation, it is thought, may be attributed to the effects of mechanical pressure of one part on another, some authors believing that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape of their kidneys, and that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child.
The effects of "station," climate, &c., are not left out of view by Mr Darwin. There can be no doubt, he says, that horses become greatly reduced in size and altered in appearance by living on mountains and islands: every one knows how small and rugged the ponies are on the northern islands and on the mountains of Europe; and this apparently is due to want of nutritious and varied food. It would appear that climate to a certain extent directly modifies the forms 1 Origin of Species, chapter i.
CAUSES OF VARIATION.
of dogs. English bull-dogs imported into India have been known to pin down even an elephant by its trunk; but after two or three generations not only fall off in pluck and ferocity, but lose the under-hung character of their lower jaws, and attain finer muzzles and lighter bodies. Several accounts have been published of the change which sheep imported from Europe undergo in the West Indies: great heat seems to act directly on the fleece. Dr Nicholson of Antigua informed Mr Darwin that after the third generation the wool disappears from the whole body, except over the loins, and the animal appears like a goat with a dirty door-mat on its back.1
All those, says Mr Darwin, who have closely attended to the subject, insist on the close adaptation of numerous varieties of wheat to various soils and climates, even within the same country. Thus, Colonel Le Conteur says, "It is the suitableness of each sort to each soil that will enable the farmer to pay his rent by sowing one variety, where he would be unable to do so by attempting to grow another of a seemingly better sort." We must not forget that at each successive period the state of agriculture, and the quantity of manure supplied to the land, will have determined the maximum degree of productiveness; for it would be impossible to cultivate a highly productive variety, unless the land contained a sufficient supply of the necessary chemical elements. The effect of the climate of Europe on the American varieties of maize is highly remarkable. Metzger, who cultivated maize in Germany, observed the following changes with a tall kind brought from the warmer parts of America. During the first year the
1 Animals and Plants under Domestication, i. 37, 52, 53, 91, 92, 98, 99.
plants were twelve feet high, and few seeds were perfected; the lower seeds in the ear kept true to their proper form, but the upper seeds became slightly changed. In the second generation the plants were from nine to ten feet high, and ripened their seed better; the depression on the outer side of the seed had almost disappeared, and the original beautiful white colour had become duskier; some of the seeds had even become yellow, and in their now rounded form they approached common European maize. In the third generation, nearly all resemblance to the original and very distinct American parent-form was lost; in the sixth the maize perfectly resembled a European variety, described as the second sub-variety of the fifth race. Analogous results were obtained by the cultivation of another American race, the "white-tooth corn," in which the tooth nearly disappeared even in the second generation.1
Such instances might be indefinitely added to from Mr Darwin's works; and that he had a perception of their meaning may be inferred from passages like the following: Changes of any kind in the conditions of life, even extremely slight changes, often suffice to cause variability. Excess of nutriment is perhaps the most efficient single exciting cause. The causes which induce variability act on the mature organism, on the embryo, and, as we have good reason to believe, on both sexual elements before impregnation has been effected.2 As geology plainly proclaims that each land has undergone great physical changes, we might have expected that organic beings would have varied under nature, in the same way as they generally have varied under the
1 Animals and Plants under Domestication, i. 316, 322, 325. 2 Ibid., ii. 270.
VARIABILITY: DARWIN AND SPENCER.
changed conditions of domestication. Why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products?
As, however, Mr Darwin says-in almost his latest work1-that "with respect to the causes of variability we are in all cases very ignorant," we must not make him responsible for this part of the theory of Evolution, but must look to Mr Spencer.2
Mr Spencer's view of the cause of variations may be epitomised as follows: The sum of the forces within an organism must balance the sum of the forces which press upon it from without, or else disintegration and death will ensue; but the outward forces continually vary (necessitating corresponding inward variation), and life may be defined as the continuous adjustment of internal relations to external relations; that is to say, the variations in the environment produce the organic variations, the organism in its turn reacting on the environment.
(1.) Inward forces balance outward forces.-There is invariably and necessarily a conformity between the vital functions of any organism and the conditions in which it is placed, between the processes going on inside of it and the processes going on outside of it. A fish cannot live in the air, nor a man in water. Oaks do not grow in the ocean, nor sea-weeds on the top of a hill. Every animal is limited to a certain range of climate, every plant to certain zones of latitude and elevation. Of the marine fauna and flora, each species is found exclusively between such and such depths.
1 Descent of Man, i. 111.
2 Herbert Spencer: "First Principles," and "Principles of Biology."