Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/289

Rh identical with a wild prototype either living or extinct, and that man has merely deprived them of liberty and re gulated their environment and propagation in the manner most advantageous to each kind. At the present day, when the whole range of biological thought is so largely permeated by the principles of the doctrine of evolution, this objection will probably not be advanced. Yet, when it is remembered that such an authority as Col. Hamilton Smith held the belief that each breed of dog had its wild prototype, it appears necessary to modify the definition above given. Let it be said that the art of breeding consists in changing the conditions of life and regulating the repro duction of animals and plants. Since a breed is a domestic variety, it implies the existence of a group of individuals marked off from their congeners by the possession of certain characters which are transmitted to their offspring. It is this transmission of peculiarities which is the essential characteristic of a breed ; for any collection of domesticated organisms could be divided into groups of individuals distinguished by certain points, but such groups would not necessarily form breeds. It is evi dent, then, that the law of heredity which asserts that like begets like&quot; must hold good, or the existence of breeds will be an impossibility. Again, if it were absolutely true that like begets like, that is, if the offspiing w r ere in all cases iden tical with the parent, it is evident that neither by man s inter ference, nor by the operation of nature, could a breed or race arise. It seems, then, that were it not in the nature of all organic beings to reproduce their kind in the manner formu lated in the principle of heredity, and were it not for the continuous slight infringement of it expressed by the prin ciple of variability, breeds could not have arisen. It is therefore necessary to examine these two principles as part of the subject under consideration. Whatever views we may entertain respecting the origin of our domestic animals and plants, there can be no doubt as to the matter of fact that breeders have always proceeded on one principle, select the best individuals in each genera tion and pair them. Now we have found that the qualities of organic beings (forming in a certain sense the material on which the breeder has to work) can be generalized under two principles heredity and variability. And in the same way the art of breeding is itself capable of a kind of gene ralization under the principle of selection. There are thus three great principles or laws heredity, variability, and selection, the last relating to the art of man, the other two to those qualities of organic beings which render the art practicable.

Heredity.—The simplest form of heredity is found amongst those organisms which reproduce their kind by division into two parts similar to each other. This process is illustrated by the fission of a Moneron. The next ad vance in complexity of reproduction occurs when the two portions into which the organism divides are dissimilar to one another ; here the process by which both portions ultimately assume the form of the parent is not one of simple nutrition, i.e., of formation of tissue like that already formed. The process by which man propagates some of his cultivated plants is one of artificial reproduction by fission. For instance, a cutting or part of a shoot, or even a leaf (as with Begonia), if placed in suitable soil, will re produce the original plant in all its minute details. We are here face to face with the mystery of reproduction ; for we have the ever wonderful fact that in a few cells lies dormant the vital impetus which enables them to produce from inor ganic pabulum a most complicated structure, which in its totality is utterly unlike themselves. And this example shows us, moreover, how essentially the same are sexual and asexual reproduction ; for there is no intrinsic difference between reproduction from a small part artificially separated from a simple foliar organ (a leaf) and the same sequence of growth originating in a small portion naturally segregated from a transformed foliar organ (the ovary). The conditions of growth are not the same in the two cases, and there all essential difference ends ; for the broad distinction which the congress of two individuals in one case appears to make is swept away by the facts of Parthenogenesis. In the lowest of living things we have seen that growth and reproduction are almost identical aspects of life. And this connection is not less close among higher organisms ; as Mr Herbert Spencer observes, &quot; When in place of its lost claw a lobster puts forth from the same spot a cellular mass, which, while increasing in bulk, assumes the form and structure of the original claw, we can have no hesitation in ascribing this result to a play of forces like that which moulds the materials contained in a piece of Begonia leaf into the shape of a young Begonia. In the one case as in the other the vitalized molecules composing the tissues show their proclivity towards a particular arrangement ; and whether such proclivity is exhibited in reproducing the entire form or in completing it when rendered imperfect matters not.&quot; The main fact of inheritance is so obvious that it is apt to be forgotten. Mr Darwin remarks, &quot; It is hardly possible, within a moderate compass, to impress on those who have nob attended to the subject the full conviction of the force of inheritance, which is slowly acquired by rearing animals, by studying the various treatises which have been published on the various domestic animals, and by con versing with breeders.&quot; Certain peculiarities have appeared only once or twice in the history of the world, but have reappeared in children or grandchildren of the individuals so characterized. Thus Lambert &quot; the porcupine man,&quot; whose skin was covered with warty projections, which were periodically moulted, had all his six children and two grandsons similarly affected. The mcst striking cases of inheritance have, as in this instance, been observed in man ; but the very existence of the numerous breeds of domestic animals is clear evidence of the possibility of the transmission of every kind of peculiarity. For instance, it is believed that the varieties of the domestic pigeon amount to at least 150, and these races differ from each other in many ways, and all breed true to their kind. Some very curious peculiarities have been perpetuated. A race of cattle called &quot; Dutch buttocked &quot; was formed in Yorkshire by selecting in each generation the animals with the largest hinder-quarters. When the breed began to be established it was found that the large size of the calves hind-quarters increased the dangers of parturition to a considerable extent. This case is interesting as showing that hurtful peculiarities may be inherited just as readily as those which are beneficial, and as bearing witness to the improbability of the view that there is an innate tendency to vary in the right direction. The terrible strength of inheritance exhibited by disease is a fact which is only too well established in the case of man ; and in the maladies of domestic animals the same law holds good. It appears that nearly all the diseases to which the horse is subject are hereditary, for instance, contracted feet, curbs, splints, spavin, founder, and weakness of the fore legs, roaring or broken and thick wind, melanosis, specific ophthalmia, and blindness, and even such habits as crib-biting and jibbing, are all plainly hereditary. The fact that any, even the most complex combinations of qualities are capable of hereditary transmission, is, perhaps, more forcibly brought home by considering the monetary aspect of the art of breeding, than by the fullest collection of special instances. As Mr Herbert Spencer remarks : &quot; Excluding those inductions 