Now, it can scarcely fail that such sports occur in nature. And if they are stable compounds, they will not be readily swamped by intercrossing. It only requires some further isolation to convert the sporting individuals into a distinct and separate variety. Now, Darwin tells us that the ancons have been observed to keep together, separating themselves from the rest of the flock when put into enclosures with other sheep. Here, then, we have preferential mating as the further isolating factor. I feel disposed, therefore, to agree with Mr. Galton when he says,[DZ] "The theory of natural selection might dispense with a restriction for which it is difficult to see either the need or the justification, namely, that the course of evolution always proceeds by steps that are severally minute, and that become effective only through accumulation. That the steps may be small, and that they must be small, are very different views; it is only to the latter that I object, and only when the indefinite word 'small' is used in the sense of 'barely discernible,' or as small as compared with such large sports as are known to have been the origins of new races."
Connected, perhaps, with the phenomena we have just been considering is that of prepotency.[EA] It is found that, when two individuals of the same race or of different races are crossed, one has a preponderant influence in determining the character of the offspring. Thus the famous bull Favourite is believed to have had a prepotent influence on the short-horn race; and the improved short-horns possess great power in impressing their likeness on other breeds. The phenomena are in some respects curiously variable. In fowls, silkiness of feathers seems to be at once bred out by intercrossing between silk-fowl and any other breed. But in the silky variety of the fan-tail pigeon this character seems prepotent; for, when the variety is crossed with any other small-sized race, the silkiness is invariably transmitted. One may fairly suppose that prepotent characters have unusual stability; but to what causes this stability is due we are at present ignorant.
Lastly, we have to consider the phenomenon of latency. This is the lying hid of characters and their subsequent emergence. We may distinguish three forms of latency.
1. Where characters lie hid till a certain period of life, and then normally emerge.
2. Where the characters normally lie hid throughout life, but are, under certain circumstances, abnormally developed.
3. Where the characters lie hid throughout life, but appear in the offspring or (sometimes distant) descendants.
Latency is often closely connected with correlated variations. Secondary sexual characters, for example, are correlated with the functional maturity or activity of the reproductive organs. They therefore lie hid until these organs are mature and ready for activity. When they are restricted to the male, they normally remain latent throughout the life of the female, but reappear in her male offspring. Under abnormal conditions, such as the removal of the essentially male organs, the secondary sexual characters correlated with them do not appear, or appear in a lessened and modified form. The males may even, under such circumstances, acquire female characters. Thus capons take to sitting, and will bring up young chickens. Conversely, females which have lost their ovaries through disease or from other causes sometimes acquire secondary sexual characters proper to the male. Characters thus normally latent abnormally emerge. Mr. Bland Sutton[EB] gives a case of a hen golden pheasant which "presented the resplendent dress of the cock, but her plumage was not quite so brilliant; she had no spurs, and the iris was not encircled by the ring of white so conspicuous in the male." Her ovary was no larger than a split pea.
A curious instance of latent characters correlated with sex is seen in hive bees. The worker bee differs from the female in the rudimentary condition of the sexual organs, in size and form, and in the higher development of the sense-organs. But it is well known that, if a very young worker grub be fed on "royal jelly," she will develop into a perfect queen. Not only are the sexual organs stimulated to increased growth and functional activity, but the correlated size and condition of the sense-organs are likewise acquired. The characters of queen and worker are latent in the grub. According to the nature of the food it receives, the one set of characters or the other emerges. Professor Yung's tadpoles and Mrs. Treat's butterflies (ante, [p. 59]) afford similar instances.
We come now to those cases of latency in which this obvious correlation does not occur. They afford examples of reversion to more or less remote ancestral characters. In some cases the cause of such reversion—such unexpected emergence of characters, which have remained latent through several, perhaps many, generations—is quite unknown. In others, at any rate among domesticated animals, the determining condition of such reversion is the crossing of distinct breeds.
Darwin gives[EC] an instance of reversion, on the authority of Mr. R. Walker. He bought a black bull, the son of a black cow with white legs, white belly, and part of the tail white; and in 1870 a calf, the gr-gr-gr-gr-grandchild of this cow, was born, coloured in the same very peculiar manner, all the intermediate offspring having been black. In man partial reversions are not infrequent. An additional pair of lumbar ribs is sometimes developed, and in such cases the fan-shaped tendons which are normally connected with the transverse processes of the vertebræ are replaced by functional levator muscles. Since it is probable that the ancestor of man had more than the twelve pairs of ribs that are normally present in the human species, we may, perhaps, fairly regard the supernumerary rib as a reversion. But it may be a new sport on old lines.