TWO DECADES OF GENETIC PROGRESS.1

By E. M. EAST,

Harvard University, Bussey Institution, Forest Hills, Mass.

Genetics was born 21 years ago when there came the first real appreciation of the studies on heredity made in the little garden at Brunn. Now that it has reached full manhood and is ready to assume the toga virilis, the time seems fitting to call back the yesterdays, to cast up accounts, and to judge whether the performance of maturity promises to repay the cost of infancy and childhood.

Perhaps it will serve our purpose to contrast the status of affairs toward the close of the long prenatal period previous to the twentieth century with that of to-day. When one does this, he is convinced that our chance metaphor is really rather apt. When the chick breaks through the egg, when the butterfly bursts the chrysalis, when the rosebud opens, the change is superficially so revolutionary that one is likely to forget the intensive energy expended in preparation for the natal day. So also with the study of heredity. Genetics was born and christened because of Gregor Mendel. Not because he diligently gathered facts regarding the heredity of the garden pea; rather because he was able to analyze and correlate these facts. Others had gathered facts galore. Indeed the growth-curve of knowledge had been rising steadily for many years. Yet metamorphosis came only when mathematics began to be applied effectively to the efforts of physiologist and morphologist. Change in method rather than a single great discovery gave the first real insight into the master riddle of the ages.

EARLY STUDIES OF HEREDITY.

Previous to the beginning of the twentieth century, isolated observations on heredity had been made by many types of workers. It was only natural that this should be the case. Such a seemingly mysterious force could hardly have failed to fascinate mankind from the very beginning of his speculative history. But isolated observations on subjects wherein are numerous complex variables usually

'Reprinted by permission, with change of title, from Journal of Heredity, May, 1922.

wait long for the keystone with which the generalizing mind can support an edifice of useful theory. And in this particular instance the time was undoubtedly extended by a striking aloofness and lack of a spirit of cooperation among the laborers in the various guilds. The first tier of foundation stones was laid by the breeder. As was to have been expected, the empiricism of a practical art led the judicial classification and the inductive reasoning of science. One has only to study the wonderful domestic animals in the paintings and reliefs of Babylonia, of Assyria, of Egypt, to realize that knowledge of the effects of selection has been extant for at least 6,000 years, perhaps for 10,000 or 20,000 years. And Jacob's little scheme to mulet his father-in-law of the ring-straked and spotted cattle shows us somewhat of the older theoretical beliefs. Jacob, in fact, seems to have been as advanced a geneticist as many of the animal breeders of the nineteenth century, since the textbooks of this period express a similar belief in maternal impressions and other fables and contain not a single conclusion that one can now point out as having a permanent value.

Generally speaking, the history of plant breeding gives a little more cause for pride. True, the early Semitic knowledge of plant sexuality was actually lost until the latter part of the seventeenth century; but having rediscovered this fundamental truth through the work of Camerarius, the eighteenth and nineteenth century hybridizers did leave behind them several legacies well worth while. Kölreuter established the fact that reciprocal crosses give very similar results. A little later the efforts of such men as Sageret, Wiegmann, Gärtner, and Naudin placed three other conclusions on a firm foundation of experiment-the variability of hybrids of the second generation when compared with that of the first, the dominance of certain individual characteristics, and the occasional reappearance of the qualities lost to sight. Possibly analogous observations had been made previously by animal breeders; but it is certainly within the truth to say that even at the beginning of the twentieth century these were not accepted with anything like the unanimity which existed in the botanical field.

CONTRIBUTIONS OF MORPHOLOGY.

Morphology, with a much later start, got down to essentials a great deal more quickly than experimental breeding. Indeed, morphologists built so rapidly during the Victorian era they nearly reached a pinnacle of success that would have given us a different day to celebrate. They lacked but the inspiration to put their "ifs" to the test of calm experiment.

Logically it followed from the theory of genetic continuity by cell division that a material substance passed from cell to cell is the basis of all heredity. Naturally, then, the mechanics of cell division was the subject of intense investigation. The result was the discovery that in building up the tissues of the individual organism, in the preparation of the reproductive cells for their special work, and in the behavior of these cells in carrying out that work there was an essential similarity of the two processes in both animals and plants. As these studies progressed it became apparent that the cell nucleus was the controlling agent of inheritance, and that within the nucleus the chromosomes played the star rôle. This hypothesis, put forth as a speculation by Haeckel in 1866, within 15 years gained the support of such eminent investigators as Hertwig, Strasburger, and Van Beneden, largely because of the similar elaborate preparations within the nucleus of egg and of sperm during maturation and of their apparently identical contribution of nuclear material in biparental inheritance. Numerous investigations on artificial fertilization were made by adherents and opponents of this view; but owing to the experimental difficulties involved, they were not conclusive. Polemic dissertations on the part played by nucleus and cytoplasm followed that were reminiscent of discussions in the realm of religion or of politics. Gradually the proponents of the view gained more and more converts, not because they were able to demonstrate a monopoly of directive action by the nucleus in development and heredity, not because they could prove that the intricate organization of so many unfertilized eggs was controlled by nuclear behavior, for such was not the case; it came through small increments to cytological knowledge which gradually wove a mesh so fine that there was no loophole of escape from the conclusion. Belief in the importance of the chromosomes grew, as in the case of organic evolution, not because of direct proof, but because of circumstantial evidence. Without going into an extended argument on the subject, one may recall the constancy of chromosome number in each species, their individuality in size and shape, the exactitude of their division during growth, and their peculiar behavior at the maturation of the germ cells.

NINETEENTH CENTURY THEORIES OF HEREDITY.

These facts, together with numerous minor discoveries, were the basis of nineteenth century theories of heredity. But besides the efforts of the practical breeders and of the morphologists, a serious attempt was made by Francis Galton and Karl Pearson to put genetic studies on a firm groundwork of quantitative experiment. Essentially their method was to measure the degree of association

between parents and offspring for any particular character. It was wholly a group method, and by its very nature precluded both the analysis of individual cases and the utilization of biological facts among the premises. Its chief generalization, the law of ancestral heredity, wherein the correlation of characters among blood relatives was interpreted as showing the inheritance of an individual to be made up of a series of contributions, one-half from the parents, one-fourth from the grandparents, and so on, has been shown to be erroneous. Having proved no stimulant to productive investigation, its discussion has passed from the genetic literature of to-day; but the mathematical procedure evolved by the Galtonian school has proved to be extremely helpful.

The earlier genetic theories of the period under consideration necessarily were highly speculative because of the paucity of known facts; but the fundamental postulate of each, active ultramicroscopic living units, has been retained in the genetic theory of to-day.

Darwin's provisional hypothesis of pangenesis (1868), for example, assumed that such particles, the gemmules, were given off at all times by every cell, and passed to all parts of the body including the germ cells. He thus accounted vaguely for the inheritance of acquired characters and for regeneration of parts, as well as for ordinary heredity.

Among several contemporary modifications of this type of theory was that of De Vries (1889), who assumed that the corpuscles, which he called pangens, represented potential elementary body characters rather than cell qualities, and that the universe of their activity was the cell rather than the body.

It is clear, even with only a glimpse of such theories, that they could satisfy none but the philosophically inclined. They did little or nothing toward stimulating work designed to test the points involved.

A different fate met the speculations of Nägeli (1884). Here was postulated two types of protoplasm built up of physiological units, the micella: The one was nutritive in its functions, and required no particular architecture; the other, the idioplasm, a structure of elaborate constitution, was built of units which represented the potential elementary characters of the organism.

WEISMANN'S THEORY.

Utilizing this conception, Weismann (1892) evolved a theory which more nearly fulfilled the requirements of an experimental working hypothesis than any of those previously outlined. The

idioplasm or germplasm he identified with the chromatin of the nucleus. His ultimate physiological unit, the biophore, was the biological atom active in building up organic characters. Grouped together into higher units, the determinants, these corpuscles controlled the specialization of cells. The various determinants of an organism made up the ids contributed by past generations. The ids, if more than one, might differ slightly among themselves, thus governing variation within the species. They formed the chromosomes, or idants, by arrangement in a linear series.

Denying the inheritance of acquired characters, and doing much toward demolishing the fallacious logic put forth as proof at that time by adherents in the belief, Weismann outlined a very stimulating conception of heredity on this basis. The immortal germplasm was assumed to be set apart at a very early cell division and passed along unchanged to the next generation, except as the activities of the living units produced occasional changes in its constitution. A provision for accurate equational division of the chromosomes and their reduction in number at the maturation of the germ cells was thus demanded, predicted and afterwards realized though not precisely in the way he supposed-by discoveries in the field of cytology.

Weismann further accounted for evolution by a selective struggle between the determinants of the germ cells, and for individual development by a qualitative distribution of the determinants of those cells set apart to build up the bodies which were to act as hostelries for the immortal germplasm.

With Weismann is reached the peak of genetic generalization at the beginning of the twentieth century. To-day we have parted company with him in many particulars, nevertheless if modern genetic theory can be said to be the outgrowth of any earlier school, the Weismannian school must be given the preference. As Wilson has said, he brought "the cell theory and the evolution theory into organic connection." His work, besides dispelling many old wives' notions by its cutting logic, was second only to that of Mendel in making genetics an experimental science. Morgan credits him with "the basis of our present attempt to explain heredity in terms of the cell" in that he propounded three of the principles upon which the modern Chromosome Theory is founded.

WEISMANN AND MENDEL.

Some may see an inconsistency in ascribing the ground-work of current ideas of heredity to Weismann, and yet celebrating the rediscovery of Mendel's papers as the true break between the old