animals and plants has been greatly exaggerated, though no doubt to a certain extent it exists. It would be opposed to all the principles inculcated in this work, if domestic animals, when exposed to new conditions and compelled to struggle for their own wants against a host of foreign competitors, were not in the course of time in some manner modified. It should also be remembered that many characters lie latent in all organic beings ready to be evolved under fitting conditions; and in breeds modified within recent times the tendency to reversion is particularly strong. But the antiquity of various breeds clearly proves that they remain nearly constant as long as their conditions of life remain the same.
It has been boldly maintained by some authors that the amount of variation to which our domestic productions are liable is strictly limited; but this is an assertion resting on little evidence. Whether or not the amount in any particular direction is fixed, the tendency to general variability seems unlimited. Cattle, sheep, and pigs have been domesticated and have varied from the remotest period, as shown by the researches of Rütimeyer and others, yet these animals have, within quite recent times, been improved in an unparalleled degree; and this implies continued variability of structure. Wheat, as we know from the remains found in the Swiss lake-habitations, is one of the most anciently cultivated plants, yet at the present day new and better varieties occasionally arise. It may be that an ox will never be produced of larger size or finer proportions than our present animals, or a race-horse fleeter than Eclipse, or a gooseberry larger than the London variety; but he would be a bold man who would assert that the extreme limit in these respects has been finally attained. With flowers and fruit it has repeatedly been asserted that perfection has been reached, but the standard has soon been excelled. A breed of pigeons may never be produced with a beak shorter than that of the present short-faced tumbler, or with one longer than that of the English carrier, for these birds have weak constitutions and are bad breeders; but the shortness and length of the beak are the points which have been steadily improved during at least the last 150 years; and some of the best judges deny that the goal has yet been reached. We may, also, reasonably suspect, from what
we see in natural species of the variability of extremely modified parts, that any structure, after remaining constant during a long series of generations, would, under new and changed conditions of life, recommence its course of variability, and might again be acted on by selection. Nevertheless, as Mr. Wallace[[930]] has recently remarked with much force and truth, there must be both with natural and domestic productions a limit to change in certain directions; for instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight to be carried, and the power of contraction in the muscular fibres. The English racehorse may have reached this limit; but it already surpasses in fleetness its own wild progenitor, and all other equine species.
It is not surprising, seeing the great difference between many domestic breeds, that some few naturalists have concluded that all are descended from distinct aboriginal stocks, more especially as the principle of selection has been ignored, and the high antiquity of man, as a breeder of animals, has only recently become known. Most naturalists, however, freely admit that various extremely dissimilar breeds are descended from a single stock, although they do not know much about the art of breeding, cannot show the connecting links, nor say where and when the breeds arose. Yet these same naturalists will declare, with an air of philosophical caution, that they can never admit that one natural species has given birth to another until they behold all the transitional steps. But fanciers have used exactly the same language with respect to domestic breeds; thus an author of an excellent treatise says he will never allow that carrier and fantail pigeons are the descendants of the wild rock-pigeon, until the transitions have "actually been observed, and can be repeated whenever man chooses to set about the task." No doubt it is difficult to realise that slight changes added up during long centuries can produce such results; but he who wishes to understand the origin of domestic breeds or natural species must overcome this difficulty.
The causes inducing and the laws governing variability have been so lately discussed, that I need here only enumerate the leading points. As domesticated organisms are much more
liable to slight deviations of structure and to monstrosities, than species living under their natural conditions, and as widely-ranging species vary more than those which inhabit restricted areas, we may infer that variability mainly depends on changed conditions of life. We must not overlook the effects of the unequal combination of the characters derived from both parents, nor reversion to former progenitors. Changed conditions have an especial tendency to render the reproductive organs more or less impotent, as shown in the chapter devoted to this subject; and these organs consequently often fail to transmit faithfully the parental characters. Changed conditions also act directly and definitely on the organisation, so that all or nearly all the individuals of the same species thus exposed become modified in the same manner; but why this or that part is especially affected we can seldom or never say. In most cases, however, of the direct action of changed conditions, independently of the indirect variability caused by the reproductive organs being affected, indefinite modifications are the result; in nearly the same manner as exposure to cold or the absorption of the same poison affects different individuals in various ways. We have reason to suspect that an habitual excess of highly nutritious food, or an excess relatively to the wear and tear of the organisation from exercise, is a powerful exciting cause of variability. When we see the symmetrical and complex outgrowths, caused by a minute atom of the poison of a gall-insect, we may believe that slight changes in the chemical nature of the sap or blood would lead to extraordinary modifications of structure.
The increased use of a muscle with its various attached parts, and the increased activity of a gland or other organ, lead to their increased development. Disuse has a contrary effect. With domesticated productions organs sometimes become rudimentary through abortion; but we have no reason to suppose that this has ever followed from mere disuse. With natural species, on the contrary, many organs appear to have been rendered rudimentary through disuse, aided by the principle of the economy of growth, and by the hypothetical principle discussed in the last chapter, namely, the final destruction of the germs or gemmules of such useless parts. This difference may be partly
accounted for by disuse having acted on domestic forms for an insufficient length of time, and partly from their exemption from any severe struggle for existence, entailing rigid economy in the development of each part, to which all species under nature are subjected. Nevertheless the law of compensation or balancement apparently affects, to a certain extent, our domesticated productions.
We must not exaggerate the importance of the definite action of changed conditions in modifying all the individuals of the same species in the same manner, or of use and disuse. As every part of the organisation is highly variable, and as variations are so easily selected, both consciously and unconsciously, it is very difficult to distinguish between the effects of the selection of indefinite variations, and the direct action of the conditions of life. For instance, it is possible that the feet of our water-dogs, and of the American dogs which have to travel much over the snow, may have become partially webbed from the stimulus of widely extending their toes; but it is far more probable that the webbing, like the membrane between the toes of certain pigeons, spontaneously appeared and was afterwards increased by the best swimmers and the best snow-travellers being preserved during many generations. A fancier who wished to decrease the size of his bantams or tumbler-pigeons would never think of starving them, but would select the smallest individuals which spontaneously appeared. Quadrupeds are sometimes born destitute of hair, and hairless breeds have been formed, but there is no reason to believe that this is caused by a hot climate. Within the tropics heat often causes sheep to lose their fleeces, and on the other hand wet and cold act as a direct stimulus to the growth of hair; it is, however, possible that these changes may merely be an exaggeration of the regular yearly change of coat; and who will pretend to decide how far this yearly change, or the thick fur of arctic animals, or as I may add their white colour, is due to the direct action of a severe climate, and how far to the preservation of the best protected individuals during a long succession of generations?
Of all the laws governing variability, that of correlation is the most important. In many cases of slight deviations of structure as well as of grave monstrosities, we cannot even