Habit has a decided influence towards inducing variation. As the blacksmith's right arm becomes more muscular from the habit of exercise induced by his vocation, so we find in domestic animals that use, or the demand created by habit, is met by a development or change in the organization adapted to the requirement. For instance, with cows in a state of nature or where required only to suckle their young, the supply of milk is barely fitted to the requirement. If more is desired, and if the milk be drawn completely and regularly, the yield is increased and continued longer. By keeping up the demand there is induced in the next generation a greater development of the secreting organs, and more milk is given. By continuing the practice, by furnishing the needful conditions of suitable food, &c., and by selecting in each generation those animals showing the greatest tendency towards milk, a breed specially adapted for the dairy may be established. It is just by this mode that the Ayrshires have, in the past eighty or a hundred years, been brought to be what they are, a breed giving more good milk upon a given quantity of food than any other.
It is because the English breeders of modern Short-horns altogether prefer beef-making to milk-giving properties that they have constantly fostered variation in favor of the one at the expense of the other until the milking quality in many families is nearly bred out. It was not so formerly—thirty years ago the Short-horns (or as they were then usually called, the Durhams) were not deficient in dairy qualities, and some families were famous for large yield. By properly directed efforts they might, doubtless, be bred back to milk, but of this there is no probability, at least in England, for the tendency of modern practice is very strong toward having each breed specially fitted to its use—the dairy breeds for milk and the beef breeds for meat only. The requirements of the English breeder are in some respects quite unlike those of New England farmers—for instance, as they employ no oxen for labor there is no inducement to cultivate working qualities even, in connection with beef.
As an illustration of the effect of habit, Darwin[5] cites the domestic duck, of which he says, "I find that the bones of the wing weigh less, and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild duck; and I presume that this change may be safely attributed to the domestic duck flying much less and walking more than its wild parent." And again, "not a single domestic animal can be named which has not in some country drooping ears, and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable."
Climate, food and habit are the principal causes of variation which are known to be in any marked degree under the control of man; and the effect of these is, doubtless, in some measure indirect and subservient to other laws, of reproduction, growth and inheritance, of which we have at present very imperfect knowledge. This is shown by the fact that the young of the same litter sometimes differ considerably from each other, though both the young and their parents have apparently been exposed to exactly the same conditions of life; for had the action of these conditions been specific or direct and independent of other laws, if any of the young had varied, the whole would probably have varied in the same manner.
Numberless hypotheses have been started to account for variation. Some hold that it is as much the function of the reproductive system to produce individual differences as it is to make the child like the parents. Darwin says "the reproductive system is eminently susceptible to changes in the conditions of life; and to this system being functionally disturbed in the parents I chiefly attribute the varying or plastic condition of the offspring. The male and female sexual elements seem to be affected before that union takes place which is to form a new being. But why, because the re-productive system is disturbed this or that part should vary more or less, we are profoundly ignorant. Nevertheless we can here and there dimly catch a faint ray of light, and we may feel sure that there must be some cause for each deviation of structure however slight."
It may be useless for us to speculate here upon the laws which govern variations. The fact that these exist is what the breeder has to deal with, and a most important one it is, for it is this chiefly, which makes hereditary transmission the problem which it is. His aim should ever be to grasp and render permanent and increase so far as practicable, every variation for the better, and to reject for breeding purposes such as show a downward tendency.
That this may be done, there is abundant proof in the success which has in many instances attended the well directed efforts of intelligent breeders. A remarkable instance is furnished in the new Mauchamp-Merino sheep of Mons. Graux, which originated in a single animal, a product of the law of variation, and which by skillful breeding and selection has become an established breed of a peculiar type and possessing valuable properties. Samples of the wool of these sheep were shown at the great exhibition in London, in 1851, and attracted much attention. It was also shown at the great recent Agricultural Exhibition at Paris. A correspondent of the Mark Lane Express, says:
"One of the most interesting portions of the sheep-show is that of the Mauchamp variety of Merinos, having a new kind of wool, glossy and silky, similar to mohair. This is an instance of an entirely new breed being as it were created from a mere sport of nature. It was originated by Mons. J.L. Graux. In the year 1828, a Merino ewe produced a peculiar ram lamb, having a different shape from the usual Merino, and possessing a long, straight, and silky character of wool. In 1830, M. Graux obtained by this ram one ram and one ewe, having the silky character of wool. In 1831, among the produce were four rams and one ewe with similar fleeces; and in 1833 there were rams enough of the new sort to serve the whole flock of ewes. In each subsequent year the lambs were of two kinds; one possessing the curled elastic wool of the old Merinos, only a little longer and finer; the other like the new breed. At last, the skillful breeder obtained a flock combining the fine silky fleece with a smaller head, broader flanks, and more capacious chest; and several flocks being crossed with the Mauchamp variety, have produced also the Mauchamp-Merino breed. The pure Mauchamp wool is remarkable for its qualities as a combing-wool, owing to the strength, as well as the length and fineness of the fibre. It is found of great value by the manufacturers of Cashmere shawls and similar goods, being second only to the true Cashmere fleece, in the fine flexible delicacy of the fibre; and when in combination with Cashmere wool, imparting strength and consistency. The quantity of the wool has now become as great or greater than from ordinary Merinos, while the quality commands for it twenty-five per cent. higher price in the French market. Surely breeders cannot watch too closely any accidental peculiarity of conformation or characteristic in their flocks or herds."
Mons. Vilmorin, the eminent horticulturist of Paris, has likened the law of similarity to the centripetal force, and the law of variation to the centrifugal force; and in truth their operations seem analogous, and possibly they may be the same in kind, though certainly unlike in this, that they are not reducible to arithmetical calculation and cannot be subjected to definite measurement. His thought is at least a highly suggestive one and may be pursued with profit.
Among the "faint rays" alluded to by Mr. Darwin as throwing light upon the changes dependent on the laws of reproduction, there is one, perhaps the brightest yet seen, which deserves our notice. It is the apparent influence of the male first having fruitful intercourse with a female upon her subsequent offspring by other males. Attention was first directed to this by the following circumstance, related by Sir Everard Home: A young chestnut mare, seven-eighths Arabian, belonging to the Earl of Morton, was covered in 1815 by a Quagga, which is a species of wild ass from Africa, and marked somewhat in the style of a Zebra. The mare was covered but once by the Quagga, and after a pregnancy of eleven months and four days gave birth to a hybrid, which had, as was expected, distinct marks of the Quagga, in the shape of its head, black bars on the legs and shoulders, &c. In 1817, 1818 and 1821, the same mare was covered by a very fine black Arabian horse, and produced successively three foals, and although she had not seen the Quagga since 1816, they all bore his curious and unequivocal markings.