« PreviousContinue »
miss methods of fancy breeding, and England has "mice clubs" whose members, just for the fun of it, have long been interested in obtaining fancy house mice with pink eyes and variously colored overcoats instead of the uniform gray costume of the usual victim of the domestic mouse trap.
All these differences, secured apparently by chance but really, it now appears, in obedience to a law of heredity, are of direct value in still further testing the uniformity of the law and in getting new lines on the problems of inheritance. In this scientific menagerie the guinea pigs, rabbits, rats, and mice include specimens of practically all the varieties yet produced by experimental breeding. Each has a pedigree, so far back as it can be traced, neatly recorded in a card catalogue, and, as the experiments inspire no fear or distrust of their keepers, they are most of them tame and easy to handle. The rabbits in particular behave very much like those that inhabit little Johnny's coop in a suburban back yard.
A typical working out of the Mendelian law of heredity may be seen in the mating of an albino and a black guinea pig. If a pure bred black guinea pig is mated with a white one all the young guinea pigs in the first litter are black. These baby pigs are apparently the offspring of black parents, although, as a matter of fact, the white parent has contributed something of her own color characteristic and the contribution is unseen because the black color characteristic is dominant and conceals it. According to the Mendelian law the white color characteristic, being recessive, can appear visibly only from the union of two germ cells in both of which this characteristic predominates—and that is exactly what happens when the black children of this black guinea pig with an
albino partner are in turn mated. Two black guinea pigs are presumably as surprised as a hen that has hatched ducklings by the appearance of one white guinea pig for about every three black ones in the family circle. The original cross had brought together two characteristics, B (black) and W (white), to make a single individual, B W, which showed only the dominant black characteristic. When these individual B Ws formed germ cells for the continuation of their species therewas a return to original conditions; B became the sole characteristic of some of these germ cells and W the sole characteristic of others. When these germ cells united to create a new individual there were necessarily three possible combinations—B B, B W. and WW, the combination BW evidently occurring twice as often as either of the
others. And, as any individual containing the dominant characteristic B would be black in color it follows that the combination W VV would alone produce a white individual and would be likely to occur on an average of about once in four offspring. Evidently, too, in the case of individuals represented by B 1! and W W one characteristic had been doubled and the other eliminated.
By applying the Mendelian law in scientific breeding new combinations of individuals or races can be obtained in the course of a few generations. Thus when a guinea pig with a dark and smooth coat is mated with one whose coat is smooth and white, the young will exhibit a wjiolly new combination, dark and
rough; and if these dark and rough guinea pigs are bred together a fourth combination will appear in the grandchildren of the original couple. Some of these grandchildren will be white and smooth while others will represent the combinations seen respectively in the parents and grandparents. By proper selection of parents any one of these combinations may be established as a pure race of guinea pigs in which the children will invariably resemble their father and mother. Often also a new combination of characteristics obtained through experimental crosses will coincide with some long lost racial combination. The yellow rabbit mated with a black one may produce a certain proportion of little rabbits with the characteristic gray color of the wild progenitors of both father and mother. In the same way occur presumably those occasional surprising cases in human families when a son or daughter developes racial peculiarities not visible in either parent. If their genealogies could be carried back far enough each parent would probably be found to possess an ancestor of the race to which the child had reverted— so, at any rate, we may deduce from the guinea pig.
A case of such reversion, for example, has resulted in Dr. Castle's laboratory in the evolution of a race of four-toed guinea pigs. The laboratory some years ago came into possession of a male guinea pig whose four-toed feet made Mm unique among all the guinea pigs in the collection. There was no other like him. And yet the four-toed guinea pig one day surprised observers by becoming the father of a four-toed descendant. The mother was apparently normal. The only explanation, therefore, was that she had inherited what might be called the four-toed tendency from some distant
ancestor; that this tendency was recessive; and that it could only be perpetuated by union with another recessive tendency of the same kind, supplied in this case by the visibly four-toed father. Selective breeding among the descendants has since "fixed" the type so that it breeds true and adds yet another striking characteristic for the study of inheritance.
There are some characteristics, however, in which so far the Mendelian law does not altogether coincide with the observed results of scientific breeding, although it is now believed that further investigation will discover another application of the same principle. Size is the most important of these exceptions. Whereas color characteristics have been proved to vary with predicable uniformity, size is apparently a permanent blend
of inheritances. In a typical rabbit family the size of the offspring is intermediate between that of the parents and none of the descendants revert to the extreme proportions of either grandparent. The practical result of this condition is that scientific breeding may produce at will a race of rabbits of any desired size between the known limits of size in rabbits, and with any conceivable combination of color characteristics. The variation in size is apparently continuous and depends upon blending or striking a compromise between the sizes of the parent rabbits. The variation in color depends upon the scientific selection of parent rabbits in which the desired color characteristic is not counteracted by any other.
These transmitted characteristics, it will be seen, are invariably associated with the parent at birth and are never "acquired" by the necessities of individual existence and then transmitted to offspring. Dr. Castle's experiments, like those of many modern students of biology, tend to disprove the theory that acquired characteristics can be transmitted. Rather they confirm the conclusions of Weissmann, which for twenty years have been the battle ground of biological opinion, that the germ plasm, or reproductive cells on which the continuity of all organic life depends, is independent of the body containing it, and that it is not influenced by the characteristics acquired by that body during the space of a single lifetime. This theory of inheritance casts an interesting and hopeful light on the statistics that are every now and then published to show that the physical condition of the average city-bred individual of today is inferior to that of his immediate ancestors. It would seem to indicate that such retrogression is not the result of inheritance but of environment, and that healthier living conditions in the large cities, together with a wider distribution of population back to the country, would counteract the tendency and produce a corresponding physical improvement. But it is also true, judging by these lower animals, that the statistics indicate breeding from inferior family and racial stocks as a serious factor in the retrogression of a startling number of individuals.
It is still a far journey from guinea pig, mouse, rat or rabbit to a human
being, and the thoughtful observer naturally wonders about subtler inheritances than those of size and color. There is the question of inherited intelligence and disposition—and this, in fact, is the next step in the study of the lower animals. Work is now in progress in Dr. Castle's laboratory to investigate the effects of a union between wild and tame animals; the wild rat, for example, mated with a rat that has been domesticated by several generations of captivity. These experiments are recent, but, so far as they have gone, they seem to indicate that the strength and ferocity of the wild creature is the dominant characteristic according to the Mendelian law. The offspring of the wild and the tame rat are in the first generation all apparently as wild as if they had been born of two wild parents, but when the offspring are mated the characteristics of the wild rat appear in modified form in some of the descendants while others seem to lack it entirely. These qualities are readily recognizable and the work of the psychologists with the various kinds of apparatus they have invented for the study of intelligence in the lower animals fortunately offers a means of examination for mental characteristics that are more difficult to determine. The psychological study of individual animals and the psychological examination of their offspring and descendants for inherited traits of intelligence is the next step to be taken by these trained searchers in the investigation of heredity by scientific breeding.
CAPTAIN MAYNE REID, whose books have made naturalists of more than one American boy, once wrote a story called, "The Plant Hunters." Its title was not particularly alluring to the lads of the generation who looked to their favorite for tales of pure adventure, but before they had read long they knew that they were to get excitement with their botany.
The heroes, there were three of them in the Captain's book, if memory's long shadow does not obscure the facts, were on a plant hunting expedition in a valley of the Himalaya Mountains. The scene of action was laid largely in wild places virtually geographically identical with those which are being searched today for species of plant life by Frank N. Meyer, an American field explorer working under commission of the Bureau of Plant Industry of the Department of Agricul
ture. Some of Mr. Meyer's experiences are akin to those of the three hunter naturalists whom Captain Reid represented as having been sent out on like errand by the authorities of the Kew Gardens of London.
The adventurous botanists of the old tale were seeking specimens to be dried and preserved for museum purposes. The adventurous botanist of the present tale cares nothing for the cut and dried. His is what David Fairchild of Washington, the Agricultural Explorer in Charge of Foreign Seed and Plant Introduction, calls "a living work."
It is the duty of the man now in a great desert of the Himalayan region to secure seeds or cuttings of plants, shrubs and trees which he considers worthy of introduction into the United States, and to get them quickly to Washington where the work of propagation almost instantly is started.
Explorer Meyer is today in a lonely land and before his mission is ended he will pass through still lonelier lands. His collecting journey began at St. Petersburg and it will end at some sea coast port of Eastern China. His trip already lias been successful enough to make it worth much more than the money it has cost. He has frozen and melted alternately as the altitudes have changed: he has encountered wild beasts and men nearly as wild; he has scaled glaciers and crossed chasms of dizzying depths; he has been the subject of the always alert suspicions of government officials and of strange peoples jealous of intrusions into their land—but he has found what he was sent for.
A plant hunter! Official and peasant are accustomed to the coming of hunters of wild beasts. They understand the lust of killing and the desire for danger which make men take long journeys into strange places. But a plant hunter!—It seems to them the thinnest pretense
to hide some design on the peace of the government or the community. The specimen bag must hold some strange instrument of destruction, the more deadly because it is unknown. The experiences of botanists in the Eastern mountains, though with an added element of real clanger, are like those of the peaceful opera glass ornithologist whose sanity is doubted and whose arrest is threatened by the country folk because he prefers to study the living bird rather than to kill it, fill it with cotton and arsenic, and to pierce it with wires for mounting in painful and grotesque attitude.
Admittedly the expression falls within the limits of what the objectors call the bromides, but it is the desire of David Fairchild, the Agricultural Explorer in Charge, and of his fellow laborers in field and Capital, to make such deserts as the United States has to blossom like the rose or, if not the rose, the pear, the apple, the orange, the pomegranate or the olive. The nature