THAT ‘half-breeds have the worst qualities of both races,’ is, perhaps, the commonest saying about hybridisation, and nothing could be farther from the truth. Cross the wild Drosophila with the stumpy-winged variety, and the offspring will be decidedly longer-lived than either parent stock. A mule has the toughness, disease-resistance, and sure-footedness of its donkey father, with the intelligence, size, and strength of the horse. Mate the English meat or dairy breeds of cattle with the zebu-type of India and East Africa, and the hybrids will inherit their native parents’ ‘thriftiness ‘and hardiness in difficult conditions, with a large share of the size and other qualities of their English ancestry.
The hybrid usually inherits the best qualities of both stocks, for the reason that useful qualities are generally dominant to their opposite numbers, and the offspring of a cross is therefore the fortunate exhibitor of a double set of the characters that evolution has found valuable. The phrase ‘hybrid vigour,’ indeed, was a commonplace among practical breeders for many years before the science of genetics started to explain it as the result of dominance.
On the other hand, hybrids are often sterile, like the mule, and for the very same reason that they are vigorous ! Look at it in terms of chromosomes, and you will see why. The egg-cell of the horse-donkey cross contains two sets of chromosomes which are complete strangers to each other; and since two heads are better than one, the members of a chromosome-pair tend to remedy each other’s deficiencies. But when partnerships are hastily, not to say violently, formed, there are apt, from time to time, to be quarrels between conflicting temperaments.
The chief quarrel that occurs when the chromosomes of two different species are forced into intimate co-operation is over the rates of growth of the various parts of the body. The donkey, in some ways, matures earlier, and in other ways later than the horse. The result is that the two sets of chromosomes, each insisting that its own is the better way, between them manage to bungle the delicate mechanism of the reproductive organs.
If the two species are only very distantly related, egg and sperm will refuse to unite, or will quarrel fatally at an early stage. If they are very nearly akin—as, for instance the red and white Shorthorns—perfect harmony will prevail. But hybrid vigour will not then be noticeable, since neither parent variety has any particular^ useful dominant qualities that the other does not possess.
Beyond this general rule, hybridisation is a matter of detail. Each kind of cross is different from the others, and every new one is to a large extent a gamble. The really big gamble comes, however, when the hybrids (if fertile) are crossed in their turn—or mated back to one of the parent stocks, as is often done—for an immense number of new combinations of chromosomes thus becomes possible. Think of ringing the changes on the 28 pairs in cattle, for instance! Here is the point where only the breeder of genius can rise to the occasion:
(1)by breeding large numbers, to give him a wide choice;
(2)by knowing which two or three beasts among them have just the right combination of grandparental qualities to enable him to use them as the parents of a new variety.
‘But surely,’ a critic may interject, ‘there must be some foundation for the prejudice against human hybrids? ‘There certainly is; but it is not intrinsically connected with genetics. Man is the only creature with a social tradition, and that tradition is very much opposed to racial crosses. Usually, therefore, only the social outcasts of each race are willing to break so important a taboo, and you can scarcely expect such parents to produce a good type of child. Secondly, again owing to the social tradition, half-breeds find themselves from the very outset the objects of suspicion and dislike. Whatever good qualities they may possess have little chance to show themselves.
On the few occasions when proper studies have been made of crosses that carry no social stigma, the children have been found to be as sound and normal as their pure-bred companions. The English-Chinese community in Liverpool is an example, and another is the extraordinarily mixed population of Kisar, an island in the Dutch East Indies, where the people are a mixture of Native, English, French, Dutch, and German, with a sprinkling from India, the neighbouring islands, and some negroid types !
The ‘quarrelling ‘between the opposing sets of chromosomes in a hybrid, by the way, often makes itself felt in other parts besides the sexual organs. When the Canadians tried, for instance, to cross the native bison with English cattle, the disharmony between mother and child was so great as to cause high mortality at parturition. The double experiment that followed, though, was successful. The Asiatic yak was crossed with the bison and with English cattle (Hereford), yielding healthy offspring in each case. What is more, they were fertile. But there was about them all a comic clumsiness, a disproportion of one part with another, that betrayed their hybrid origin. When the two kinds of hybrids were mated together, the combination of cattle, yak and bison in the same animal had an effect that was at least equally odd.
The same sort of disproportion occurred when the Russians crossed yaks with zebus (the humped Indian cattle). The males of this match were sterile, too; though the females were fertile when crossed back either to yak or zebu.
One biologist (Bond) has gone so far as to say that in a hybrid the two sets of chromosomes tend to keep themselves to themselves, so to speak, and each to take charge of a different side of the body—so that the left side takes after the mother’s family, for example, the right after the father’s. He has produced much sound evidence, from birds, animals, and humans, to support this view. But human racial crosses are nothing like as drastic as the animal ones mentioned; and any disharmonies are too slight and dubious to be mentioned here.
A MAN MAY MARRY HIS COUSIN—WITH CAUTION INBREEDING is the opposite of outbreeding—there is more in that truism than meets the eye—and the object of quite as much prejudice. Cousin marriage is said to be the cause of mental deficiency, insanity, tuberculosis, and most of the other ills in the medical dictionary. And, it is true, they very often follow it.
On the other hand, our cattle, horses, swine, sheep, and other domestic animals have all been brought to their present state by a system of inbreeding much closer than an occasional cousin marriage. Moreover, the Pharaohs usually married their sisters; their successors, the Ptolemies, did much the same; and there were several cousin marriages in the Wedge-wood-Darwin-Galton group of families that gave us some of our greatest Victorians.
What is the explanation of these discordant results? There is nothing either vicious or virtuous in inbreeding in itself. To repeat the truism, ‘Inbreeding is the opposite of Outbreeding.’ Instead of mixing widely different characters, it combines two sets of chromosomes whose genes reinforce each other in every way. If the stock is healthy, strong, clever, inbreeding will intensify those qualities. If it is weak and foolish, or has a number of recessive or semi-recessive defects, the results will be disastrous. The whole point about inbreeding is that it intensifies all the qualities of the stock, good and bad, known and unknown.