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DARWIN'S CENTURY -- EVOLUTION AND THE MEN WHO DISCOVERED IT |
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Chapter VIII: The Priest Who Held the Key to Evolution
I. Gregor Mendel "On a clear, cold evening in February; so his biographer states, for the record is clearer upon the weather of this particular evening of 1865 than upon the momentous event that occurred in it, "Father Gregor Mendel read before the Brunn Society for the Study of Natural Science, his paper upon 'Experiments in Plant Hybridization.'" [1] Forty people were present in the room at the schoolhouse where the lecture was given. They were not ignorant people. Botanists, a chemist, an astronomer, a geologist were among those present. In the next month Mendel spoke again to the same audience recounting before them his new theory upon the nature of inheritance. The audience listened patiently. At the end of the blue-eyed priest's eager presentation of his researches, the still existing minutes of the society indicate there was no discussion. Stolidly the audience bad listened. Just as stolidly it had risen and dispersed down the cold, moonlit streets of Brunn. No one had ventured a question, not a single heartbeat had quickened. In the little schoolroom one of the greatest scientific discoveries of the nineteenth century had just been enunciated by a professional teacher with an elaborate array of evidence. Not a solitary soul bad understood him. Thirty-five years were to flow by and the grass on the discoverer's grave would be green before the world of science comprehended that tremendous moment. Aged survivors from the little audience would then be importuned for their memories. Few would have any. In the four huge volumes in which, at the end of the century, the scientific historian John Merz records a hundred years of discovery, the name Gregor Mendel receives only footnote mention. Yet with Lamarck and Charles Darwin he shares today the biological honors of the nineteenth century. It is par excellence the century that discovered time and change. Perhaps as a consequence there is something a little symbolic about the lives of these three men. Lamarck died in forgotten poverty, but above his grave rang his daughter's defiant outcry, "The future will remember you, my father." Charles Darwin had been more fortunate in the world's adulation, yet a decade after the publication of the Origin he was to hesitate and fall back upon a theory which weakened his life's work and which would have proved unnecessary had he known what was said on that winter evening of 1865 in Brunn. Darwinism, after the rediscovery of Mendel, was to undergo a sea change. It was to be half dismissed by Mendel's first followers and then emerge once more strengthened, enriched, and rejuvenated by the discoveries which flowed from the work of the obscure priest who read the Origin of Species and carried on queer experiments with peas which he affectionately referred to as his children. From peas, dwarfed, wrinkled, yellow, tall, short, he was to derive the laws which make modem genetics one of the most exact of the biological sciences. He had probed into the mysteries of the cell without a microscope. He had done it by infinite patience alone in the solitude of a monastery garden. Although his observations were reported to the world, they lay unread. "My time will come," he said once to his friend Niessl, but it is doubtful if by then he really believed it. When he died in 1884, it was as a prelate of the church, worn out with the cares of office. His experiments had long since ceased. They had never aroused public attention and perhaps in the end, alone, confused, and ill-advised by the only botanist he knew, he had come to doubt their value. A few years after his election as prelate a visitor wishing to observe the experimental plants at the monastery reported simply, I found that I had come too late" In a similar way fame came at last to Gregor Mendel. There is perhaps no stranger story in the annals of science than the rise to international eminence of this solitary man sixteen years after his death and thirty-five years after the talk in the little hall at Brunn. It is a story which is worth perusal by all scholars, not alone because of what Mendel achieved, but also because the complete failure of communication in this particular instance was, to a major degree, the failure of professional science. It has its lessons, even though the world has changed greatly since 1865. No man who loves knowledge would want an episode like this to happen twice. Some scientists have tried to argue that the journal in which Mendel published was obscure, but his tragedy is more profound than this. He was advised by one of the great European botanists of his generation and he was betrayed, not consciously, we may say in charity, but betrayed through condescension. Mendel was an amateur and the professional scientist whom he looked up to and admired saw in him no more than an instrument for the furtherance of his own researches. It is true that the intellectual climate of the time increased his difficulties, but it is also true that Mendel, this man of buoyant good will, was denied throughout his life the solace of a single sincere professional friend who would lend an understanding ear to the account of his experiments. From first to last Mendel was dogged by ill luck in everything that mattered save just one thing: the choice of the edible pea for his experiments. Even this plant, with its luckily simple genetic structure, was eventually abandoned -- once more by professional scientific advice. indeed, at this late point in time one might readily wonder how much he really glimpsed of the significance of his own discoveries -- one might, that is, if one did not know of the well-stocked monastery library with its annotated copy of Darwin. We know, too, that he tried experiments to test the Lamarckian principle. Alone in his garden he had wrestled with the two leading theories involving organic evolution, but where Darwin and Lamarck had been fascinated by change, Mendel was fascinated by stability. Instead of attempting, as did Darwin, to determine how the characteristics of the adult organism were transferred to, or compressed into, a minute germ cell, Mendel sought to determine how it came about that the germ cell contained and transmitted the characters of the living animal. Mendel, in other words, had intuitively grasped what seemingly no one else of his generation understood; namely, that until we had some idea of the mechanisms which .controlled organic persistence we would be ill-equipped to understand what it was that produced evolutionary change. The persistence of biological form in time is the first fact in our experience. Organic change is a far more subtle phenomenon whose detection, as we have had occasion to observe, is dependent upon a sophisticated knowledge of successive plant and animal transformations occurring throughout great stretches of the past. It is for this reason that evolution remained so long undetected, whereas the assumption of special creation of each species struck very few as being in the least illogical. It was Mendel's virtue that he concentrated with more precision than anyone before him upon the way in which already existing characters emerged or failed to emerge in the offspring of a particular union. In examining the details of his unfortunate career it will be possible to see with greater clarity why Darwin by 1871 in the Descent of Man was expressly retreating from his bold stand upon natural selection as the major factor in the production of evolutionary change, In that volume Darwin, quite in contrast with his assurance of 1859, wrote as follows: "I now admit ... that in the earlier editions of my 'Origin of Species' I perhaps attributed too much to the action of natural selection or the survival of the fittest." [2] There was a reason for this wary retreat on the part of the master. Ironically enough, two years after Mendel had actually placed a possible answer to Darwin's problem on record, a very erudite Scotch engineer brought forward in the pages of the North British Review [3] a formidable challenge to the Darwinians. It was a challenge which only a Mendelian geneticist could have answered -- and Mendel, immured in his monastery, was unknown to both parties. Darwin never attempted a direct response to Jenkin -- he always avoided public controversy -- but there is ample testimony in his letters to the effect which Jenkin's criticism had upon him. "Fleeming Jenkin has given me much trouble ..." he wrote to Hooker in January of 1869. [4] In February he confided to Wallace: "Jenkin argued in the 'North British Review' against single variations ever being perpetuated, and has convinced me...." Finally, in the sixth edition of the Origin of Species one may read his open confession: "Nevertheless, until reading an able and valuable article in the 'North British Review' (1867) I did not appreciate how rarely single variations, whether slight or strongly marked, could be perpetuated.... The justice of these remarks cannot, I think, be disputed." [5] The reader must now consider what is implied in the above statements. Fleeming Jenkin had, in actuality, well-nigh destroyed the fortuitous character of variation as it was originally visualized by Darwin. Jenkin set forth the fact that a newly emergent character possessed by one or a few rare mutants would be rapidly swamped out of existence by backcrossing with the mass of individuals that did not possess the trait in question. Only if the same trait emerged simultaneously throughout the majority of the species could it be expected to survive. An admission that numbers of animals or plants mutate simultaneously in the same direction, however, greatly reduces the significance of natural selection and suggests either some interior orthogenetic drive which is affecting the individual members of the species, or an external environmental force of Lamarckian character producing a direct effect on the germ plasm of an entire group of organisms. In either case fluctuating fortuitous individual variation has to be abandoned and with it goes much of the importance of natural selection." Jenkin's formidable mathematical attack, formidable, that is, in the light of the conception of blending inheritance prevalent at the time, seemed to Darwin largely unanswerable. The only recourse was to fall back toward the type of Lamarckianism around which he elaborated his theory of pangenesis. Darwin died with this difficulty unsolved and its consequences haunting his last years. The answer to Fleeming Jenkin had been standing on library shelves in the Proceedings of the Brunn Society for the Study of Natural Science since 1866. Jenkin, the hardheaded engineer, and the gracious, dreaming naturalist who had been forced to retreat before him would both be gone before anyone blew the dust from those forgotten pages. Mendel is a curious wraith in history. His associates, his followers, are all in the next century. That is when his influence began. Yet if we are to understand him and the way in which he eventually rescued Darwinism itself from oblivion we must go the long way back to Brunn in Moravia and stand among the green peas in a quiet garden. Gregor Mendel had a strange fate: he was destined to live one life painfully in the flesh at Brunn and another, the intellectual life of which he dreamed, in the following century. His words, his calculations were to take a sudden belated flight out of the dark tomblike volumes and be written on hundreds of university blackboards, and go spinning through innumerable heads. Before their importance can be grasped, however, it is necessary to examine the state of genetics at the time Darwin wrote the Origin of Species and to gain some idea of the nature of the menace which confronted Darwin upon the publication of Jenkin's paper. [7] II. Pre-Mendelian Genetics The earlier history of human genetics is an amazing assemblage of superstitious error and fallacious observation. Monstrous births were assumed to be the result of man-animal connections. Right down into the eighteenth century such reports continued to be printed. As I remarked on an earlier page, the fixed precision of Christian speciation really represents in no small degree a late amalgamation of Linnaean scientific taxonomy with the increasing Christian emphasis upon special creation. [8] Monstrous hybrids between men, bears, and other animals which no educated person would accept today were taken quite seriously right into De Maillet's time -- an added reason, incidentally, for not dismissing as romantics, or as unscientific, scholars who were merely repeating the common beliefs of their day. [9] Undoubtedly some of the floating beliefs that plants could change their type-ideas which survive in the pages of the Vestiges -- were derived from accidental cases of genuine plant hybridity and mutation. Anecdote and tall tale were the common data of genetics until well into the. latter part of the eighteenth century. At that time the rise of professional breeding and the growing interest in the importation of valuable food and drug plants began to place emphasis upon controlled experimentation. The idea of selective livestock breeding arose in England during the early phases of the Industrial Revolution when the multiplying towns began to demand meat and dairy produce on a large scale. What emerged, and stimulated practical improvement in livestock, was the shift from purely local subsistence farming to the profitable business of supplying the food and wool needs of the new industrial towns. All of these purely economic factors greatly stimulated experimentation among commercial breeders. Darwin, who had come from the country, early showed a shrewd instinct for merging the theoretical with the practical when he began his intensive perusal of horticultural and livestock journals. If we are to get clearly in mind the difference between the genetics of Darwin's day and the sort of problems which began to emerge toward the close of the century we must remember that all the great cytological work upon cell mechanisms was unavailable to both Darwin and Mendel. Their observations were confined to direct breeding experiments, or what they could learn from others. Mendel, as we have intimated, approached the problem in a quite different way from Darwin and proved to be the better experimentalist. Perhaps he was fortunate, so far as his experiments went, in not being a famous man already laboring under a point of view. We have already learned the general nature of Darwin's beliefs. Here we are concerned only with the contrast he was later to make with Wallace on the one hand and, later on and posthumously, with the Mendelians on the other. Just as in the case of Darwin's evolutionary thinking, it is not always easy to isolate, out of the vast mass of his accumulated examples, the precise outlines of his genetic ideas. It is very commonly stated that Darwin believed in blending inheritance, while Mendel succeeded in demonstrating the reality of particulate inheritance. This appears to me a mild oversimplification of a more complicated situation. The confusion is emphasized when one comes to remark that Romanes, in discussing Darwin's views a few years prior to the rediscovery of Mendel, classifies Darwin's theory of heredity as a particulate one. [10] Actually it would seem that the case might be better put as follows. Prior to the emergence of the critiques of A. W. Bennett and Fleeming Jenkin it would appear that Darwin had taken a great deal of the genetics of his day for granted. His primary interest, because of his evolutionary studies, lay in the field of variation. In the first edition of the Origin he simply states that the laws governing in- heritance are quite unknown, though he is vaguely aware of phenomena that today would go under such categories as sex-linked inheritance, or dominance and recessiveness. He confesses that variability is governed by unknown laws, but he realizes that this variability is without significance unless its benefits can be retained and accumulated through heredity. Drawing upon the forceful analogy of domestic breeding he professes to see no limit to the transmuting power of nature. As one studies this first edition of the Origin one can see that in spite of the author's enthusiasm for natural selection he is rather careful to mention all factors which could conceivably playa part in organic change. As we have remarked, he remains, in this sense, a transitional figure. His genetics is essentially that of the shrewd out-of-doors observer. He is neither particulate in any precise sense, nor does he incline totally toward blending conceptions of inheritance. In reality he is occupied with just two things: variation and natural selection. He is thinking about evolution and his views have not yet been proved vulnerable by means of heredity. It was the attack launched by Jenkin and Bennett that forced Darwin into a more elaborate treatment of genetic mechanisms and led eventually to a retreat down one of the. pathways he had left open for himself. The retreat was not dictated through Jenkin's criticism alone. His troubles were augmented by events in the field of geophysics which we will chronicle in the next chapter. When Jenkin penned his attack on natural selection it is quite obvious that he had found a loophole which Darwin, who was not mathematically gifted, had entirely overlooked. In brief, Jenkin simply took the position: 1. That it was not possible in domestic breeding to push a strain beyond a certain point of maximum efficiency for a given character. In his analysis of this problem Jenkin appears to have theoretically anticipated the later discoveries of Johannsen in the field of fluctuating variation. In this, however, he was ahead of his time and the debates which would later emerge around that subject. The attack which really shook Darwin was: 2. The argument that a favorable mutative sport would be "utterly outbalanced by numerical inferiority." Since the unblending character of Mendelian units was unknown, Jenkin's position was simply that a single favorable mutation would soon be swamped out and by degrees obliterated in any population group in which it occurred. Since the favored animal or plant would presumably be mating with its normal fellows, the rare variation would not long survive. As a potent example Jenkin advanced the hypothetical case of a single well- endowed white man being cast ashore on an island inhabited by Negroes. No matter how much power he might attain among them, the tribe would certainly not become white because of his presence. The only answer, ignoring for the moment Mendelian genetics, is to postulate a large group of animals mutating in a similar direction and contemporaneously. Jenkin points out this alternative, though, as he justly observes, it results in an evolution which is no longer the product of chance and selection but rather "a theory of successive creations." The fortuitous element involved in natural selection disappears and one is immediately confronted, not with accident, but an orthogenetic and controlled movement in a single direction. Darwin was sufficiently impressed by this argument that, although he did not abandon his book, he incorporated into it the Jenkin alternative suggestion and began at the same time a retreat toward habit and use -- inheritance which it is obvious he now saw as a refuge from the corner into which he had been forced by Jenkin. A. W. Bennett pressed the same advantage in another paper three years later in Nature [11] and Herbert Spencer, one of England's pre-Darwinian evolutionists, reiterated the Jenkin position as late as 1893. [12] The final edition of the Origin contains, in the light of Jenkin's views, some quite surprising comment. "There must be some efficient cause for each slight individual difference," Darwin says, "as well as for more strongly marked variations which occasionally arise; and if the unknown cause were to act persistently, it is almost certain that all the individuals of the species would be similarly modified." [13] (Italics mine. L.E.) In those lines Darwin has assumed the Jenkin argument which permits the retention of evolution but at the price of fortuitous variation. One line further, however, and we encounter the contention that he has underrated "the frequency and importance of modifications due to spontaneous variability." Darwin with his gift for compromise has here accepted both a point of view which, if pursued, would be metaphysically fatal to his system and, at the same time, has stepped up the pace of variation to try to overcome the logic of Jenkin's argument. The number of these concealed contradictions makes the later editions of the Origin instructive but difficult reading. For clarity and reasonable consistency the first edition is by far the most satisfactory. III. Pangenesis In 1868 Darwin published the Variation of Animals and Plants under Domestication. In it, for the first time, he set forth a theory of inheritance to which he applied the term "pangenesis." This theory actually implies a type of particulate inheritance, although Darwin's concern over Jenkin's paper quite obviously reveals that this assumption of blending inheritance raised no question in his mind in 1867. Pangenesis, however, is a theory of particulate inheritance beginning at the other end, so to speak, of the problem Mendel pursued. It begins, that is, with the assemblage of another potential individual from the body cells of an existing organism. It is not an idea originating with Darwin by any means; it runs all the way back to the Greeks, [14] but Darwin's elaboration of it is an indirect escape from such problems as Bennett and Jenkin had formulated. Darwin assumed that the cells of the body throw off minute material particles and that these particles, "gemmules," he calls them, are gathered from all parts of the body into the sexual cells of the organism. Darwin thus assumes that the sexual cells contain only what is represented in the living body -- or primarily so -- and the, particles they receive upon fertilization. Every character thus comes from the somatic, or body, tissues, and the germ cells contain only what is brought to them by the blood stream from all parts of the body. The germ is merely a device to create a new body out of the mingling of the particles of the parents' bodies. Darwin's germ materials are thus developed anew with every living individual. This is in marked contradiction to later theories about the inviolability of the germ plasm. It permits any somatic modification during an individual's lifetime to be represented in his germ cells. It is, in other words, a Lamarckian device ensuring the inheritance of adaptive modifications in unending succession. That Darwin should have proposed this theory indicates, not alone how inadequate natural selection had come to seem to him, but how truly transitional, in retrospect, we can observe his thinking to be. He is half modem, half experimental, yet in times of difficulty he is capable of obscure retreats in the direction of eighteenth-century concepts. August Weismann (1834-1914), the man who reversed the trend of particulate studies, and who has been termed the first original evolutionist after Darwin, [15] has himself remarked that he would probably never have been led to deny the inheritance of acquired characters if it had not been for the impossible complications involved in "the giving off, circulation, and accumulation of gemmules." [16] In spite of the fact that Weismann remained sufficiently hypnotized by the omnipresent Darwinian shadow to postulate a "struggle" among the determiners in the germ cell, he actually diverted the study of evolution into the pathway which has led oil to the great modem advances in the field of genetics. We have seen that Darwin's determiners were supposed to arise in the body cells and to carry, in some mysterious manner, the image of their particular body region compacted into a newly produced germ cell. Weismann, on the other hand, reversed the attention which had been directed to the body as a source of variation, and concentrated his attention upon the germ itself as the source of emergent change. He postulated a germ plasm which was basically immortal and inviolable. By this he meant that the reproductive cells are isolated early and are passed along unchanged from individual to individual in the history of the race. By "unchanged" is meant unaffected by exterior environmental influences. All changes which emerge in the phylogeny of a given organism must therefore emerge from the alteration or elimination of particular hereditary determiners within the germ plasm itself, not from "messenger" determiners carried into the germ from sources in the adult body. It has been said by many modem writers that Weismann carried this inviolability principle too far, but it should be remarked in simple justice that since his works are no longer read in great detail, his own qualifications upon this point have been forgotten. He was willing to concede that the germ plasm was probably not totally isolable from influences penetrating it from the body, but that such influences -must be extremely slight. [17] It must be remembered that Weismann was combating Darwin's notion of a great stream of "messengers" entering the germ plasm from the body itself. There is no reason to think that Weismann, if he were alive today, would find it necessary to cavil over mutations produced in the germ plasm by radiation or by other similar powerful forces exerted upon the body. In summary then, we may say that while it has long since been disproved that the determiners engage in a struggle for existence within the germ cell, the main features of Weismann's system have been retained as the actual basis of modern genetics. Germ cells come from other germ cel1s and are not derived from body cells. Germinal continuity Is complete, but not somatic continuity. This is the reverse of Darwin's position, and Weismann's victory over the conception of pangenesis marked the declining influence of Lamarckian theories of inheritance. As Weismann himself commented, "The transmission of acquired characters Is an impossibility, for if the germ plasm is not formed anew in each individual but is derived from that which preceded it, its structure and above all its molecular constitution cannot depend upon the individual in which it happens to occur...." [18] He also correctly recognized that sexual reproduction with its reshuffling of hereditary characters in every generation Is really a remarkable device for promoting variability -- new character combinations which may have selective value in the struggle for life. This observation was made possible by the slowly growing knowledge of cell mechanics to which the German workers of this period made such notable contributions. [19] So greatly does the sexual division promote new and individual combinations of characters that, without including any new mutations at all, it still contributes greatly to the potential evolutionary variability of any species. Weismann's centering of emphasis upon a germ plasm out of which arose variation which was manifested in the living organism, and the failure of experiment to validate Darwin's pangenesis, led directly to the renewed experimentation which eventually culminated in the rediscovery of the lost work of Gregor Mendel. Before discussing the nature of that work, however, it is necessary to examine in a brief way just what Darwin, Wallace, and Weismann meant by variation. As we will see a little later, modern genetics, beginning with Mendel, has envisaged this problem differently from the way it was treated earlier in the century. The truth is that the Darwinists lumped under the term "variation" a great range of bodily differences about which they knew nothing whatever. They assumed that these characteristics were heritable -- natural selection has no meaning without such inheritance -- and that "variation and heredity," as Hogben says, "were coextensive processes." [20] Offspring were always a little different from their parents, the line of evolution was constantly in motion and constantly subjected to the selective attrition of the struggle for existence. As someone cleverly remarked, the species was always swallowing its tail. The normal curve of distribution for a given character was constantly being advanced on one side toward greater efficiency, and similarly suffering erosion from the side of the less effective. A stable species, in other words, was merely an illusion created by the constant, slow pruning effect of natural selection. This idea, in spite of other differences, is common to Darwin, Wallace, and Weismann. There was no clear comprehension that not all somatic variation is heritable. Thus the Darwinists tended to conceive of evolution as a continuous process. Even an organism which appears to be standing still. like some living fossils, is actually in a kind of dynamic balance. Its apparent resting state is really produced by the fact that selection is holding the norm of the species at a given spot instead of thrusting it forward. The modem interpretation of evolution and variation does not totally equate with this point of view. When we use the term "variation," our meaning is somewhat different from that of the Darwinists. IV. Artificial Selection and the Evolutionists All through the earlier portion of the nineteenth century,
and indeed the latter portion of the eighteenth
century as well, evolutionists had had recourse to domesticated
animals and plants as suggesting the mutability of
biological form. Special creationists, even, had had to
recognize a certain degree of plasticity in life whether
wild or tame, but they had regarded this plasticity as
being confined and demarcated. Species, sammelarten, as
the Germans would say, were receptacles containing a
range of varieties, but the species was the original created
entity. The evolutionists, by contrast, had insisted that the
species barrier was an illusion, that given time and opportunity
the species, in Wallace's convenient phrase,
would "depart indefinitely" from its original appearance.
Buffon hinted at the possibility; Lamarck expressed it;
Darwin used the whole process of artificial selection from
which to develop, by analogy, his principle of natural
selection. "The possibility of continued divergence," he
remarked, "rests on the tendency in each part or organ
to go on varying in the same manner in which it has al
ready varied; and that this occurs is proved by the steady
and gradual improvement of many animals and plants
during lengthened periods." [21] While Darwin was not unaware
of what today we would call macro-mutations, or
saltations, he was inclined to believe that in a state of
nature, particularly, smaller changes operating by degrees
were the main instrument of change." Wallace, in a rather
unguarded moment when he was attempting to counter
the weight of the Jenkin-Bennett argument, speaks of the
"powerful influence of heredity, which actually increases
the tendency to produce the favorable variations with
each succeeding generation...." [23] The metaphysical implications
of this remark are about as "unDarwinian" as Neither Wallace nor Darwin had any experimental data which would enable them to distinguish between purely somatic, non-heritable variation and change of the genuine mutative variety. Darwin did have some notion of the complexities of inheritance, and it is not quite accurate to say that his notions of heredity were as simple as mixing water and ink. His knowledge, he well knew, was clouded and obscure: "The germ ... becomes a ... marvelous object, for besides the visible changes to which it is subjected, we must believe that it is crowded with invisible characters, proper to both sexes, to both the right and left side of the body, and to a long line of male and female ancestors separated by hundreds or even thousands of generations from the present time; and these characters, like those written on paper with invisible ink, all lie ready to be evolved under certain known or unknown conditions." [24] Arguments for a lessened antiquity for the globe began to mount as nineteenth-century physicists applied their calculations to the age of the earth. It is interesting to see that Darwin, who had once been quite casual as to time, shows an increasing interest in stories which suggest visible change in the present. He quotes, in the Descent of Man, the story of an American hunter who asserted that in a certain region male deer with single unbranched antlers were becoming more numerous than the normal variety. In reality the bucks were all yearlings ,with their first antlers, and the observer had been self-deceived. [25] The story is less important than the glimpse it affords into Darwin's mind. Although he had written much about the minute, age-long increments involved in evolutionary change, it is clearly apparent that some of these apocryphal anecdotes possessed a strong appeal for Darwin. There was an understandable desire to show the process of evolution in operation, even as one tried to explain why it could not' actually be seen. It is not surprising that Darwin occasionally succumbed to this temptation and was, in spite of a judicious temperament, a little too easily tempted by "spiked buck" stories. They fitted in well with his notions of the way in which domestic animals were altered. We come now, however, to a peculiar fact. It would appear that careful domestic breeding, whatever it may do to improve the quality of race horses and cabbages, is not actually in itself the road to the endless biological deviation which is evolution. There is great irony in this situation; for more than almost any other single factor, domestic breeding had been used as an argument for the reality of evolution. Its significance, however, is somewhat deceptive and capable of misinterpretation. V. Mendel's Contribution In 1900 Correns, Tschermak, and De Vries, all working independently along the lines which Weismann and others had brought under examination, rediscovered the lost principles and lost paper of Mendel. The mere fact that three workers, after the long lapse of years, turned the little document up at the same time suggests that biological science was just reaching the point where Mendel's work could be appreciated. We have seen that Weismann had dealt with the germ plasm from "inside," that he did not accept pangenesis. Mendel, though cyt0logical methods were unknown to him, had, years earlier, used essentially the same approach. By carefully controlled experiment he sought to trace particular characters of the adult through successive generations, to find out whether such characters remained the same, mixed, or disappeared. As he himself commented in the introduction to his paper, "Among all the numerous experiments made [prior to his time] not one has been carried out to such an extent and in such a way as to make it possible to determine the number of different forms under which the offspring of hybrids appear, or to arrange these forms with certainty according to their separate generations, or definitely to ascertain their statistical relations." [26] Bateson observed that these primary conceptions of Mendel were absolutely new in his day. There is a surgical precision about Menders procedures which is in marked contrast to the bunglesome anecdotal literature which fills so much even of Darwin's treatment of the subject. By selecting from a variety of pea plants a series of easily observable and identifiable characters, Mendel began his. experiments with attention focused upon what happened to these characters in the course of their passage through several generations. The details of the experiments need not concern us here, but the results, from the standpoint of evolution, were spectacular. Mendel had established for a series of plant characters the fact that they passed through the germ cell as units. Such units did not mix with other units, though it was found that certain characters might be suppressed in a heterozygous individual and re-emerge only in a homozygous one. All of these facts depended on gametic segregation. They had nothing to do with pangenesis, nothing to do with the kind of selection Darwin and Wallace had been largely concerned with. Jenkin's "swamping out" of a new mutant character could not take place so long as the individual had offspring. The units were particulate and unalterable except by actual mutation. A character could be carried and could be spread even if recessive. If it had survival value, its diffusion could be rapid. Mendel challenged directly the Darwinian idea that cultivated plants had, in some manner, been made more "plastic" and variable. "Nothing," he says, "justifies the assumption that the tendency to the formation of varieties is so extraordinarily increased that the species speedily lose all stability." Instead of this assumption, Mendel draws upon his new discoveries to suggest that most cultivated plants are actually hybrids, mixing back and forth and showing the unit character ratios which such origins would suggest. The close proximity of domesticated forms promotes the opportunities for hybridism. Thus the fluctuating variability which Darwin sometimes attributed to the indirect factors of climate, soil, and other influences could not all be regarded as due to the emergence of new evolutionary characters. Much of the supposed new was old, but variable in its phenotypic expression. Mendel had shown that the vast array of living characteristics was controlled by mathematical laws of assortment, and biological units (genes) were transmitted independently. "The course of development," he remarked, "consists simply in this, that in each successive generation the two primal characters issue distinct and unaltered out of the hybridized form, there being nothing whatever to show that either of them has inherited or taken over anything from the other." [27] Heredity and variation in the old Darwinian sense could, therefore, not be synonymous. The unit factors had a constancy which the Darwinians had failed to guess. [28] VI. Johannsen and Variation We have seen that the Darwinian evolutionary mechanism was one involving the constant selection of small variations which were assumed to be numerous and inheritable. For a long time they were pretty much taken as given, and little or no attempt was made to determine what lay back of them, or whether all variation actually arose from the same cause. William Bateson, one of the first active Mendelian researchers, put the matter succinctly when he said: "The indiscriminate confounding of all divergences from type into one heterogeneous heap under the name 'variation' effectually concealed those features of order which the phenomena severally present. creating an enduring obstacle to the progress of evolutionary science." [29] It was Menders contribution to have revealed that not all variation was new in the sense of just emerging. Furthermore, the revelation that discrete unblending hereditary units existed which might be studied cytologically as well as through breeding experiments swung interest in new directions. Hugo De Vries, whom we shall discuss in the following chapter, seized public attention by his advocacy of rapid species alteration through sizable changes, speciation really, by sudden saltations or jumps. This doctrine in its extreme form was fated to be modified, but it cannot be denied that his emphasis upon the distinction between minor "fluctuating variations" and "discontinuous" variability, to which he applied the term "mutation; greatly stimulated research. Among the results of that research was the discovery of the Danish scientist W. L. Johannsen that the more or less constant somatic variations upon which Darwin and Wallace had placed their emphasis in species change cannot be selectively pushed beyond a certain point, that such variability does not contain the secret of "indefinite departure." The Belgian anthropologist Lambert Quetelet (1796-1874) observed in 1871 that for almost any biological character, height for example, one could erect a frequency distribution curve, provided a statistically adequate sample was available. There would be a scattering of individuals on either side of the norm and the extreme variants would lie at either end of the frequency curve. There is, in other words, an oscillation in a given population group around a mean value for any biological characteristic that we may choose to examine. It was this type of fluctuating variation which the Darwinian school had assumed might be "selected," either artificially or naturally, by the simple expedient of eliminating organisms at the lower end of the curve and selecting the individuals at the upper end of the curve for breeding purposes until the norm was moved forward. The breeder, it is true, can do certain things in this regard, but his effects are limited in a way the Darwinians were not in a position to foresee. By selecting pure lines of beans, Johannsen anticipated that by raising beans from large bean seeds and from small and intermediate types he would obtain a series of different norms of size from his several plants. In this he failed. Whatever the size of the bean used, the progeny continued to fluctuate about a norm. Selection had had no effect in modifying the character of the norm. These variations in bean size were purely somatic, that is, they had no connection with genetic factors, but instead apparently represented accidentally favorable or unfavorable growth conditions. There is another factor which is concerned in the successful artificial breeding of both animals and plants. Johannsen did find that in spite of the somatic norm indicated by the frequency distribution of his pure lines of beans, there were also distinct means in separate lines of beans. This represented a true hereditary component. If we breed for large beans, say, or the fastest race horses, we are selecting out a stock which contains hereditary unit factors favorable to our intent. By constant selection we perfect a relatively pure line for the given effect we wish to produce. Through judicious mating we may even introduce new elements into the complex. Basically, however, our efforts are limited to what exists genetically in the stock. By careful manipulation we may draw certain characters to the surface or combine them with others. [30] We can, however, produce only what is potentially contained within a given line. Beyond this the breeder can do nothing but wait upon those incalculable events known as mutations, which appear spontaneously. For example, Johannsen at one point in his experiments observed that the range shifted in an unexplainable man ner in one of his true lines. It was a true mutative event -- a new factor had been introduced. The result of Johannsen's studies of 1903 and later was to demonstrate conclusively (1) that organisms with the same genotype (i.e., genetic composition) could differ phenotypically, that is, in their physical appearance; (2) that the selection of phenotypic characters without a genetic base would not yield hereditary change; (3) that selection of hereditary characters could induce some degree of physical alteration but the effect would attenuate and halt unless there were added mutations which are sometimes forthcoming and sometimes not. For a time there was an understandable feeling that Darwinism was moribund. This was due partly to the discovery that certain of the variations upon which Darwin had depended were non-heritable, partly to the feeling that new changes emerged suddenly and were not the result of a slow accretion of characters. By degrees, however, the latter notion gave way. It began to be realized that there were small mutations as well as large, which would produce an effect not greatly different from the kind of continuous evolution Darwin had visualized. Thus the word "mutation" began to take on its modern meaning. [31] The word "macro-mutation" fits better today the kind of evolutionary leaps which, under De Vries's influence, were heavily popularized in the first few years of the twentieth century. In this period there was, for a brief time, a line drawn between the significance of large and small variations, but it was a line which could not be maintained. As the century progressed, biological thought swung around to the opinion that however wrong Darwin may have been in certain details, he had been justified in his view that small changes are less apt to be detrimental to the organism and are the more likely mode of evolutionary change. [32] Nevertheless, in contemplating the Darwinian rejuvenation, it is well to remember a forgotten observation of Jacques Loeb, one of the finest experimental biologists of the early decades of this century. He commented that one of the greatest peculiarities of the Darwinian period was the seeming scientific indifference to the actual visible demonstration of specific change. The draft of limitless time at the Darwinists' command led them to assume that the process was too slow to be observed at all. That this troubled Darwin, particularly after the time scale began to be shortened, we can see from stories such as the account of the spiked buck. The literature, however, remained largely polemical. It was therefore an enormous leap forward when Hugo De Vries proposed his "mutation" theory and demonstrated hereditary changes of form. The rediscovery of Mendel at this time with his evidence for the actual existence of specific hereditary determiners marked, as Loeb says, "the beginning of a real theory of heredity and evolution." Even though some of De Vries's thought was later to be repudiated, and though Loeb was writing in the period of uncritical enthusiasm for De Vries's discoveries, we may, I think, with little reservation, endorse this final remark: "If it is at all possible to produce new species artificially I think that the discoveries of Mendel and De Vries must be the starting point." [33] In the next fifty years Mendel's principles were expanded to cover many organisms, both plant and animal. Mathematical tools elaborated by such men as Fisher, Sewall Wright, and others were introduced to handle the theoretical genetics of entire populations. It was discovered that certain types of mutations occur over and over again in particular stocks, and thus by inference it was possible to assume that a certain reservoir of variability was always at hand in particular species-a reservoir possibly contributing to organic change in times of shifting conditions. Certain kinds of genetic mutation were found more likely to occur than others. [34] Cytology continued to press farther and farther into the mysterious mechanics of the nucleus and the cytoplasm. Finally, today, mutations are being artificially induced by various types of radiation and chemical agents. All this, however, is a book-long story in itself. There is still much that is unknown: the cellular location and nature of the great mechanisms that control the structure of phyla and classes escape us still; we know far more about fruit flies than men. It is strange, now, to walk through the laboratories and encounter the warning signs before radiation experiments, and to think of Mendel among the droning bees and flowers in the monastery at Brunn. "My time will come," he had said to his friend Niessl. "My time will come." Perhaps, as others had heard the sound of change and the flow of waters in the night, Mendel had learned from those tiny intricate units that shape a flower's heart something of the elemental patience that holds a living organism to its course while mountains wear away. "My time will come," he said. It was the indefinable echo of another century in the air. _______________ Notes: 1. Hugo Iltis, Life of Mendel, New York, 1932. 2. C. Darwin, Descent of Man, 1871, Modern Library ed., p. 441. 3. FIeeming Jenkin, "The Origin of Species," North British Review, 1867, Vol. 46, pp. 149-71. 4. LLD, Vol. 2, p. 379. 5. Modern Library ed., p. 71. 6. J. C. Willis, The Course of Evolution, Cambridge University Press, 1940, pp. 5, 165-66. Also H. J. Muller. "The Views of Haeckel in the Light of Genetics," Philosophy of Science, 1934, Vol. 1, p. 318. 7. It can also be found in his Papers, Literary, scientific, Etc., ed. by Sidney Colvin and J. A. Ewing, London, 1887, Vol. 1. 8. E. B. Poulton in Essays on Evolution, Oxford, 1908, p. 56, suggests seventeenth-century Puritan influence. 9. Conway Zirkle in The Beginnings of Plant Hybridization, Philadelphia, 1935, gives an extended historical account of fantastic animal combinations. 10. G. J. Romanes, Darwin and after Darwin, Chicago, 1897, Vol. 2, p. 45. E. S. Russell in The Interpretation of Development and Heredity, Oxford, 1930, p. 63, similarly expresses himself and cites Johannsen to the same effect. 11. "The Theory of Selection from a Mathematical Point of View," Nature, 1870, Vol. 3, pp. 30-31. 12. "The Inadequacy of Natural Selection," Popular Science Monthly, 1893, Vol. 42, p. 807. 13. Modern Library ed., p. 155-56. 14. M. J. Sirks, General Genetics, The Hague. 1956, p. 49 ff. 15. Mendel, of course, being unknown. 16. Essays upon Heredity, Oxford, 1892, Vol. 2, pp. 80-81. 17. Op. cit. edition of 1889, p. 170. 18. Ibid., p. 266. 19. The advances in cell-staining techniques in Germany were responsible for major advances in cytology. Roux had observed the behavior of chromatin and examined mitosis. He believed that the secret of heredity was incorporated in a particulate manner within the nucleus. Following Roux's lead Weismann glimpsed the role of the chromosomes in carrying what today we would call genes. He also predicted In 1887 the reduction division which was later on to be established for meiosis. 20. L. Hogbon, Genetic Principles in Medicine and Social Science, New York, 1932, p. 167. 21. Charles Darwin, Variations of Animals and Plants under Domestication, New York: Orange Judd & Co., 1868, Vol. 2 p. 300. 22. Ibid., pp. 306-7. 23. A. R. Wallace, "Natural Selection -- Mr. Wallace's Reply to Mr. Bennett," Nature, 1870, Vol. 3, p. 49. 24. VAP, Vol. 2, p. 80. 25. J. T. Cunningham, "Organic Variations and Their Interpretation: Nature, 1898, Vol. 58, p. 594. 26. Mendel's paper is reproduced in W. Bateson's Mendel's Principles of Heredity, Cambridge University Press, 1913. 27. Cited by Hugo Iltis, Life of Mendel, New York, 1932, pp. 147-48. 28. Ibid., pp. 178-79. 29. "Heredity and Evolution," Popular Science Monthly, 1904, Vol. 65, p. 524. 30. Raymond Pearl, "The Selection Problem," American Naturalist, 1917, Vol. 51, pp. 65-91. 31. T. H. Morgan, "For Darwin," Popular Science Monthly, 1909, Vol. 74, p. 375. 32. H. J. Muller, "On the Relation Between Chromosome Changes and Gene Mutations" in Mutation, Report of Symposium held June 15-17, 1955, Brookhaven National Laboratory, Upton, N.Y., pp. 134, 142. 33. "The Recent Development of Biology,"
Science, 1904, n.s. Vol. 20, 34. Thomas Hunt Morgan, "The Bearing of Mendelism on the Origin of Species," Scientific Monthly, 1923, Vol. 16, p. 247. See also W. E. Castle, "Mendel's Laws of Heredity," Science, 1903, n.s. Vol. 18, p. 404.
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