« ПредыдущаяПродолжить »
VARIATION DIRECT OR LATERAL.
The whole subject of inheritance is wonderful. Its laws appear to be extremely complex, and, as yet, are so little understood that their operation strikes us as capricious. Mr Darwin has sought to account for some of the facts by the aid of the hypothesis of Pangenesis, according to which every unit or cell of the body throws off gemmules, which are transmitted to the offspring of both sexes, and are multiplied by self-division. These may remain undeveloped during the early years of life, or during successive generations; their development into units like those from which they were derived, depending on their affinity for, and union with, other units previously developed in the due order of growth. This hypothesis is professedly only provisional, and may at any time be superseded by a better; still it may help us to remember the facts, and it is upon the wide foundation of the facts of inheritance, of variation, &c., &c., that the theory of natural selection is reared.
As stated above, the living thing having varied from its parent, produces offspring which vary again. The grandchild, in varying from its immediate parent, may revert to the grandparent in some particulars; or may take a second step in the same direction as its parent, so that its points of difference from the grandparent will be more pronounced; or thirdly, it may vary laterally, and show peculiarities which distinguish it from parent and grandparent equally. This will be repeated through all generations; and it is obvious that if the variations should happen to be continuously of the second kind— that is, in the way of adding up the variations in the same direction-the animal or plant at one end of the series will differ very widely from that at the other, from which, nevertheless, it is lineally descended. It
would seem like denying that two and two are four, and that four and two are six, to deny this; and the only question is, Whether the variations do ever accumulate in one direction to such an extent as to lead naturalists to class ancestor and descendant as distinct species ?
The Accumulation of Variations by Artificial Selection. -First, methodically: Variations which, as we have seen, are supplied by nature, have been made to accumulate by artificial selection or the choice of man, who adds them up in certain directions useful to himself or pleasing to his fancy. When a pigeon-fancier observes in a bird some peculiarity which he would like to preserve, he selects from among the offspring those individuals which show the peculiarity in the most conspicuous manner, carefully matches them, and selects from their offspring again, till in the course of successive generations he has obtained a new breed. The differences thus selected are commonly so slight as to be inappreciable by an uneducated eye, and it is only where a man is naturally gifted with accuracy of eye and judgment, and devotes his life to the study of the subject, that he succeeds in becoming an eminent breeder. The diversity of the breeds of pigeons is something astonishing. "Compare the English carrier and the short-faced tumbler, and see the wonderful difference in their beaks, entailing corresponding differences in their skulls. The carrier, more especially the male bird, is also remarkable from the wonderful development of the carunculated skin about the head; and this is accompanied by greatly elongated eyelids, very large external orifices to the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak in outline almost like that of a finch; and the common tumbler
has the singular inherited habit of flying at a great height in a compact flock, and tumbling in the air head over heels. The runt is a bird of great size, with long massive beak, and large feet; some of the sub-breeds of runts have very long necks, others very long wings and tails, others singularly short tails. The barb is allied. to the carrier, but instead of a very long beak has a very short and very broad one. The pouter has a much elongated body, wings, and legs; and its enormously developed crop, which it glories in inflating, may well excite astonishment, and even laughter. The turbit has a very short and conical beak, with a line of reversed feathers down the breast; and it has the habit of continually expanding slightly the upper part of the œsophagus. The jacobin has the feathers so much reversed along the back of the neck that they form a hood; and it has, proportionally to its size, much elongated wing and tail feathers. The trumpeter and laugher, as their names express, utter a very different coo from the other breeds. The fantail has thirty or even forty tail-feathers, instead of twelve or fourteenthe normal number in all members of the great pigeon family; and these feathers are kept expanded, and are carried so erect, that in good birds the head and tail touch the oil-gland is quite aborted. Several other less distinct breeds might be specified." Yet it is the common opinion of naturalists, that all these domestic varieties have descended from the wild rock-pigeon (Columba livia), a bird of a beautiful blue colour, with white crop, double black wing-bars, and barred and white-edged tail-feathers.
In a similar way it is certain that some of our eminent breeders have, even within a single lifetime,
modified to a large extent some breeds of cattle and sheep. With poultry again, man has produced almost every variety of colour, curious modifications of plumage, and the capacity of perpetual egg-laying. Breeders habitually speak of an animal's organization as something quite plastic, which they can model almost as they please.
Of course the variations must occur before they can be selected, but they always do occur-size, speed, form. and colour, instincts, habits, intelligence, have always varied, so as to admit of the production of the very races which the w ts or fancies, or passions of men, may have led them to desire.
It is the same with plants as with animals. The experience of cultivators shows that when a sufficient number of individuals are examined, variations of any required kind can always be met with, and can be made to accumulate by selection, without materially affecting the other characters of the species. When fashion demands any particular change in the form or size or colour of a flower, sufficient variation always occurs in the right direction, as is shown by our roses, auriculas, and geraniums; and when, as recently, ornamental leaves come into fashion, sufficient variation is found to meet the demand, and we have zoned pelargoniums and variegated ivy. This variation is not confined to old and well-known plants, subjected for a long series of generations to cultivation; but the Sikim rhododendrons, the fuchsias and calceolarias from the Andes, and the pelargoniums from the Cape, are equally accommodating, their variations accumulating just where and when and how we require them.
These are but instances, but they are sufficient to
SELECTION CONSCIOUS AND UNCONSCIOUS.
indicate the power man possesses of modifying animal and vegetable forms, by methodically selecting those variations which naturally occur, and which he wishes to perpetuate and increase. The principle of Selection, observes Mr Youatt, is the magician's wand by means of which the agriculturist may summon into life whatever form and mould he pleases.
Secondly, unconsciously: Even more important than this methodical selection, is that kind of selection which may be called unconscious, and which results from every one trying to procure and rear the best animals he can, according to his skill and the prevailing standard of excellence. Thus, a man who intends keeping pointers naturally tries to get as good dogs as he can, and afterwards breeds from his own best dogs; but he has no wish to modify the breed: he does not look to the distant future, or speculate on the final result of the slow accumulation during many generations of successive slight changes. In this way there is reason to believe that King Charles's spaniel has been modified to a large extent since the time of that monarch. By a similar process of selection, and by careful training, the whole body of English race-horses have come to surpass in fleetness and size the parent Arab stock; so that the latter, by the regulations of the Goodwood races, are favoured in the weights they carry. Youatt gives an excellent illustration of the effects of a course of selection, which may be considered as unconsciously followed, in so far that the breeders could never have expected, or even have wished, to have produced the result which ensued—namely, the production of two distinct strains. The two flocks of Leicester sheep kept by Mr Buckley and Mr Burgess have been purely bred from the original