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and in some other slight peculiarities; its hind legs are longer, and its habits peculiar. According to Mr. Orton (Physiology of Breeding, 1855, p. 9; quoted by Darwin, Variation, ii. 86), Dr. Wilson crossed a male Manx cat with common cats, and, out of twenty-three kittens, seventeen were destitute of tails; but when the female Manx was crossed by common male cats all the kittens had tails, though they were generally short and imperfect. Darwin gives the following in his Variation under Domestication (ii. 85): "Godina has given a curious case of a ram of a goat-like breed of sheep from the Cape of Good Hope, which produced offspring hardly to be distinguished from himself when crossed with ewes of twelve other breeds. But two of these halfbred ewes, when put to a merino ram, produced lambs closely resembling the merino breed."
I quote the following from Darwin also (p. 87): "The silk fowl breeds true, and there is reason to believe that it is a very ancient race; but when I reared a large number of mongrels from a silk hen by a Spanish cock, not one exhibited even a trace of the so-called silkiuess. Mr. Hewitt also assorts that in no instance are the silky feathers transmitted by this breed when crossed with any other variety. But three birds out of many raised by Mr. Orton from a cross between a silk cock and a bantam hen had silky feathers.
There are some instances of reciprocal crosses which seem at first sight to give directly opposite results, and therefore to contradict our theory.
Thus Darwin says that a hybrid which had for its mother a bay mare and for its father a hybrid between a male ass and a female zebra, had, when young, zebralike stripes upon its shoulders, flanks and legs. Here the only recent striped ancestor is the paternal grandmother. As the possession of stripes is a characteristic 'which distinguishes the zebra from the horse and the ass, it seems at first as if its transmission by a female ancestor is opposed to our theory. We know, however, that all the species of the horse genus are the descendants of a striped form, and the presence of stripes in the zebra is not due to recent variation, but to the fact that it has not varied. The transmission of stripes by a female zebra is therefore nothing more than we might expect. We know, too, that both the horse and the ass show a tendency to revert to the striped ancestral form, and I shall show in the next section that reversion is often excited by crossing. It is therefore quite probable that the stripes in this colt were due to reversion.
It is said that young animals born from a tigress by a male lion, as well as those born from a lioness by a male tiger, are striped, but many cat-like animals show a tendency to revert to a striped form, and in this case also we may explain the presence of stripes in the young by attributing it to reversion excited by crossing.
Darwin says that a good authority assures him that in South America, when niata cattle are crossed with common cattle, though the niata is prepotent whether males or females are used, the prepotency is strongest through the female line.
The origin of the niata breed is not known, but there is no doubt that it originated in Paraguay from common cattle; and the fact that the niata peculiarities are not shared by any other living cattle, but are very much like those of the extinct Sivatherium, seems to show that in this case also the peculiarity is due to reversion.
Difficulty of Explaining the Transmission of the Characters of Two Forms without Fusion.
A much more serious difficulty is found in the fact that while a hybrid is usually somewhat intermediate between its parents, it occasionally happens that the characteristics of one or both parents refuse to blend and are transmitted in an unmodified state. Thus Darwin states that when gray and white mice are paired the young are not piebald nor of an intermediate tint, but are pure white or of the ordinary gray color. This particular case may perhaps be explained as follows: The brown form is the ancestral form, and when no hair gemmules are transmitted the young are brown. All the hairs are homologous with each other, and are derived from the same part of the egg, and when gemmules are transmitted they may hybridize alike all the cells which are to form hairs, and the hybrid animals will therefore be entirely white or entirely brown.
It is stated that when a black game fowl is crossed with a white, the young are either pure black or pure white, but this case is precisely like that of the mice.
Darwin gives a number of interesting illustrations of this singular phenomenon, among which are the following:
When turnspit dogs and ancon sheep, both of which have dwarfed limbs, are crossed with common breeds, the offspring are not intermediate in structure, but resemble one parent only.
When tailless or hornless animals are crossed with perfect animals, it frequently but by no means invariably happens that the offspring are either perfectly furnished with these organs or are quite destitute of them.
When Dorking fowls with five toes are crossed with other breeds, the chickens often have five toes on one foot and four on the other.
When the red flowered stock of Antirrhinum is fertilized with the pollen of the purple Queen stock, about half the seedlings resemble the mother plant, while the other half bear rich purple blossoms like those of the paternal plant.
Darwin says that he fertilized the purple sweet-pea, which has a dark reddish-purple standard-petal and violet-colored wings and keel, with pollen of the paintedlady sweet-pea, which has a pale cherry-colored standard and almost white wings and keel, and from the same pod twice raised plants resembling both sorts, the greater number resembling the father.
These cases are difficult to explain, but the phenomena are so complicated that it is hardly safe to speculate upon them until they are re-examined by an observer who can devote himself to this subject especially.
Some of them may be due to the causes above indicated, and some, possibly, to fertilization by two fathers.
Crossing as a Cause of Reversion.
According to Darwin's view reversion must in all cases be due to the manifestation of a tendency which has lain dormant in the egg and has been transmitted for generations in a latent condition, for the chances against the repetition, by an accidental variation, of a characteristic of a remote ancestor, are inconceivably great.
According to our theory this is not the case, for the conditions which caused a cell in the ancestral form to throw off gemmules and thus to produce a given peculiarity may cause the corresponding cell of the parent to throw off gemmules in the same way, and these, uniting with the corresponding part of the egg, will produce variation. As the gemmule and the ovarian element are both very similar to those which produced the variation in the ancestor, the chances are not very great against the reproduction of the same peculiarity. In this case wo should have a new variation with all the characteristics of a true reversion, but due to the transmission of a gemmule, rather than to the sudden awakening of a tendency which has long lain dormant in the egg.
It is possible, therefore, that there maybe two kinds of reversion—true hereditary reappearance of features which have lain latent in the egg, and new variations which repeat again certain old characteristics of the race. There are, I think, certain reasons for believing that reversions of the latter kind are the most common, the chief one being the fact that most of the causes of variability are also causes of reversion.
Tims, crossing, which is a very efficient cause of variation, is also one of the chief causes of reversion.
Darwin gives a number of examples to sbow that, independently of the well-known tendency of hybrids and mongrels to revert, after a number of generations, to one of the parent forms, the act of crossing in itself gives an impulse towards reversion, and often results in the reappearance of long-lost characters.
The following interesting account, from Darwin's Variation (Vol. ii. p. 57), will serve to illustrate this law:
"In the chapter on the horse, reasons were assigned for believing that the primitive stock was striped and dun colored, and details were given showing that in all parts of the world stripes of a dark color frequently appear along the spine, across the legs and on the shoulders, where they are occasionally double or treble, and even sometimes on the face and body of horses of all breeds and of all colors. But the stripes appear most