Thomas Hunt Morgan

“ Within the last five or six years, however, from a common wild species of fly, the fruit fly, Drosophila ampelophila, which we have brought into the laboratory, have arisen over a hundred and twenty-five new types whose origin is completely known. Let me call attention to a few of the more interesting of these types and their modes of inheritance, comparing them with wild types in order to show that the kinds of inheritance found in domesticated races occur also in wild types. ”

“ To be consistent, this same objection should be extended to the use of the spectroscope in the study of the evolution of the stars, to the use of the test tube and the balance by the chemist, of the galvanometer by the physicist. All these are unnatural instruments used to torture Nature's secrets from her. I venture to think that the real antithesis is not between unnatural and natural treatment of Nature, but rather between controlled or verifiable data on the one hand, and unrestrained generalization on the other. ”

“ There are two mutant races in our cultures of the fruit fly Drosophila that have dark body color, one called sooty, another which is even blacker, called ebony (fig. 20) . Sooty crossed to ebony gives offspring that are intermediate in color. Some of them are so much like sooty that they cannot be distinguished from sooty. At the other extreme some of the hybrids are as dark as the lightest of the ebony flies. If these hybrids are inbred there is a continuous series of individuals, sooties, intermediates and ebonies. Which color here shall we call the dominant? ”

“ One may venture the guess that some of the specific and varietal differences that are characteristic of wild types and which at the same time appear to have no survival value, are only by-products of factors whose most important effect is on another part of the organism where their influence is of vital importance. ”

“ Evolution of wild species appears to have taken place by modifying and improving bit by bit the structures and habits that the animal or plant already possessed. We have seen that there are thirty mutant factors at least that have an influence on eye color, and it is probable that there are at least as many normal factors that are involved in the production of the red eye of the wild fly. ”

“ There is a class of cases of inheritance, due to the XY chromosomes, that is called sex linked inheritance. It is shown both by mutant characters and characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance. If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. ”

“ The results will show beyond dispute that the characters of wild types are inherited in precisely the same way as are the characters of the mutant types—a fact that is not generally appreciated except by students of genetics, although it is of the most far-reaching significance for the theory of evolution.
A mutant appeared in which the eye color of the female was different from that of the male. The eye color of the mutant female is a dark eosin color, that of the male yellowish eosin. ”

“ It is well known that systematists make use of characters that are constant for groups of species, but which do not appear in themselves to have an adaptive significance. If we may suppose that the constancy of such characters may be only an index of the presence of a factor whose chief influence is in some other direction or directions, some physiological influence, for example, we can give at least a reasonable explanation of the constancy of such characters. ”

“ In Papilio turnus there exist, in the southern states, two kinds of females, one yellow like the male, one black (fig. 27) . The evidence here is not so certain, but it seems probable that a single factor difference determines whether the female shall be yellow or black.
Finally in Papilio polytes of Ceylon and India three different types of females appear, (fig. 28 to right) only one of which is like the male. Here the analysis of the breeding data shows the possibility of explaining this case as due to two pairs Mendelian factors which give in combination the three types of female. ”

“ The result of their combined action is to produce flowers intermediate in color.
When the hybrids mature and their germ cells (eggs or pollen) ripen, each carries only one of these factors, either the red or the white, but not both. In other words, the two factors that have been brought together in the hybrid separate in its germ cells. Half of the egg cells are white bearing, half red bearing. Half of the pollen cells are white bearing, half red bearing. Chance combinations at fertilization give the three classes of individuals of the second generation. ”

“ It requires indeed some courage to undertake a labor of such a far-reaching extent; it appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be over-estimated in connection with the history of the evolution of organic forms. ”

“ If the dominant is the sooty then we count three sooties to one ebony, putting the hybrids with the sooties. The important fact to find out is whether there actually exist three classes in the second generation. This can be ascertained even when, as in this case, there is a perfectly graded series from one end to the other, by testing out individually enough of the flies to show that one-fourth of them never produce any descendants but ebonies, one-fourth never any but sooties, and one-half of them give rise to both ebony and sooty. ”

“ Diagram showing history of germ cells of cross of Fig. 15. The larger circles indicate the color of the birds; their enclosed small circles the nature of the factors in the germ cells of such birds. The explanation of the failure to produce a pure race of Andalusians is that they are like the pink flowers of the four o'clock, i.e., they are a hybrid type formed by the meeting of the white and the black germ cells. If the whites produced by the Andalusians are bred to the blacks (both being pure strains) , all the offspring will be blue (fig. 15) ”

“ In rabbits there is probably a triple system, that includes the albino, the Himalayan, and the black races. In the silkworm moth there have been described four types of larvae, distinguished by different color markings, that form a system of quadruple allelomorphs. In Drosophila there is a quintuple system of factors in the sex chromosome represented by eye colors, a triple system of body colors, and a triple system of factors for eye colors in the third chromosome. ”

“ It is, of course, hardly to be expected that any random change in as complex a mechanism as an insect would improve the mechanism, and as a matter of fact it is doubtful whether any of the mutant types so far discovered are better adapted to those conditions to which a fly of this structure and habits is already adjusted. ”

“ It would have been more accurate to have stated that the chief effect of the factor was observed in a particular part of the body. Most students of genetics realize that a factor difference usually affects more than a single character. ”

“ Constant but trivial effects may be the product of factors having other vital aspects g. Sex-linked inheritance in Drosophila ampelophila in the wild species D. repleta in man in domesticated Fowls in the wild moth, Abraxas h. Multiple allelomorphs in the wild Grouse Locust in domesticated mice and rabbits in Drosophila ampelophila 4. Mutation and Evolution
Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied the heredity of certain characters of the common edible pea, in the garden of the monastery at Brünn. ”

“ Bred to wild females the new character was found to dominate, at least to the extent that the eyes of all its offspring were narrower than the normal eye, although not so narrow as the eye of the pure stock. ”

“ The females are almost completely sterile, while the males are fertile. The viability of the stock is poor. When flies with rudimentary wings are put into competition with wild flies relatively few of the rudimentary flies come through, especially if the culture is crowded. ”

“ The only difference between them is that the hybrid which contains both the yellow and the green factors is in appearance not intermediate, but like the yellow parent stock. Yellow is said therefore to be dominant and green to be recessive.
Fig. 18. Diagram illustrating the history of the factors in the cross shown in Fig. 17. Another example where one of the contrasted characters is dominant is shown by the cross of Drosophila with vestigial wings to the wild type with long wings (fig. 19) . The F1 flies have long wings not differing from those of the wild fly, so far as can be observed. ”

“ These inbred give black hens and black cocks, barred hens and barred cocks.
There is a case comparable to this found in a wild species of moth, Abraxas grossulariata. A wild variation of this type is lighter in color and is known as A. lacticolor. When these two types are crossed they exhibit exactly the same type of heredity as does the black-barred combination in the domestic fowl. As shown in figure 41, lacticolor female bred to grossulariata male gives grossulariata sons and daughters. These inbred give grossulariata males and females and lacticolor females. ”

“ All the flies of this stock however, cannot be said to be eyeless, since many of them show pieces of the eye—indeed the variation is so wide that the eye may even appear like a normal eye unless carefully examined. Formerly we were taught that eyeless animals arose in caves. ”

“ The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection." Why do biologists throughout the world to-day agree that Mendel's discovery is one of first rank? ”

“ It is smaller than the normal mesothorax and its wings are imperfectly developed, but the bristles on the upper surface may have the typical arrangement of the normal mesothorax. The mutant shows how great a change may result from a single factor difference. ”