VIII.
ABBOT MENDEL: A NEW OUTLOOK IN HEREDITY
There is grandeur in this view of life, with its several powers having been originally breathed by the Creator into a few forms or into one; and that, while this planet has gone circling on according to the fixed law of gravity from so simple a beginning, endless forms, most beautiful and most wonderful, have been and are being evolved.– Closing sentence of DARWIN'S Origin of Species.
VIII.
ABBOT MENDEL: A NEW, OUTLOOK IN HEREDITY
13Scientific progress does not run in cycles of centuries, and as a rule it bears no relationship to the conventional arrangement of years. As has been well said–for science a new century begins every second. There are interesting coincidences, however, of epoch-making discoveries in science corresponding with the beginning of definite eras in time that are at least impressive from a mnemonic standpoint, if from no other.
The very eve of the nineteenth century saw the first definite formulation of the theory of evolution. Lamarck, the distinguished French biologist, stated a theory of development in nature which, although it attracted very little attention for many years after its publication, has come in our day to be recognized as the most suggestive advance in biology in modern times.
As we begin the twentieth century, the most interesting question in biology is undoubtedly that of heredity. Just at the dawn of the century three distinguished scientists, working in different countries, rediscovered a law with regard to heredity which promises to be even more important for the science of biology in the twentieth century than was Lamarck's work for the nineteenth century. This law, which, it is thought, will do more to simplify the problems of heredity than all the observations and theories of nineteenth-century workers, and which has already done much more to point out the methods by which observation, and the lines along which experimentation shall be best directed so as to replace elaborate but untrustworthy scientific theorizing by definite knowledge, was discovered by a member of a small religious community in the little-known town of Brünn, in Austria, some thirty-five years before the beginning of the present century.
Considering how generally, in English-speaking countries at least, it is supposed that the training of a clergyman and particularly that of a religious unfits him for any such initiative in science, Father Mendel's discovery comes with all the more emphatic surprise. There is no doubt, however, in the minds of many of the most prominent present-day workers in biology that his discoveries are of a ground-breaking character that will furnish substantial foundation for a new development of scientific knowledge with regard to heredity.
Lest it should be thought that perhaps there is a tendency to make Father Mendel's discovery appear more important here than it really is, because of his station in life, it seems desirable to quote some recent authoritative expressions of opinion with regard to the value of his observations and the importance of the law he enunciated, as well as the principle which he considered to be the explanation of that law.
In the February number of Harper's Monthly for 1903, Professor Thomas Hunt Morgan, Professor of Biology at Bryn Mawr, and one of the best known of our American biologists, whose recent work on "Regeneration" has attracted favorable notice all over the world, calls attention to the revolutionary character of Mendel's discovery. He considers that recent demonstrations of the mathematical truth of Mendel's Law absolutely confirm Mendel's original observations, and the movement thus initiated, in Professor Morgan's eyes, gives the final coup de grace to the theory of natural selection. "If," he says, "we reject Darwin's theory of natural selection as an explanation of evolution, we have at least a new and promising outlook in another direction and are in a position to answer the oft-heard but unscientific query of those who must cling to some dogma: if you reject Darwin, what better have you to offer?"
Professor Edmund B. Wilson, the Director of the Zoological Laboratory of Columbia University, called attention in Science (19 December, 1902) to the fact that studies in cytology, that is to say, observations on the formation, development, and maturation of cells, confirm Mendel's principles of inheritance and thus furnish another proof of the truth of these principles.
Two students working in Professor Wilson's laboratory have obtained definite evidence in favor of the cytological explanation of Mendel's principles, and have thus made an important step in the solution of one of the important fundamental mysteries of cell development in the very early life of organisms.
In a paper read before the American Academy of Arts and Sciences last year, Professor W. E. Castle, of Harvard University, said with regard to Mendel's Law of Heredity:–
What will doubtless rank as one of the greatest discoveries in the study of biology, and in the study of heredity, perhaps the greatest, was made by Gregor Mendel, an Austrian monk, in the garden of his cloister, some forty years ago. The discovery was announced in the proceedings of a fairly well-known scientific society, but seems to have attracted little attention, and to have been soon forgotten. The Darwinian theory then occupied the centre of the scientific stage, and Mendel's brilliant discovery was all but unnoticed for a third of a century. Meanwhile, the discussion aroused by Weissman's germ plasm theory, in particular the idea of the non-inheritance of acquired characters, put the scientific public into a more receptive frame of mind. Mendel's law was rediscovered independently by three different botanists, engaged in the study of plant hybrids–de Vries, Correns, and Tschermak, in the year 1900. It remained, however, for a zoologist, Bateson, two years later, to point out the full importance and the wide applicability of the law. Since then the Mendelian discoveries have attracted the attention of biologists generally.14
Professor Bateson, whose book on Mendel's "Principles of Heredity" is the best popular exposition in English of Mendel's work, says that an exact determination of the laws of heredity will probably produce more change in man's outlook upon the world and in his power over nature than any other advance in natural knowledge that can be clearly foreseen. No one has better opportunities of pursuing such work than horticulturists and stockbreeders. They are daily witnesses of the phenomena of heredity. Their success also depends largely on a knowledge of its laws, and obviously every increase in that knowledge is of direct and special importance to them.
After thus insisting on the theoretic and practical importance of the subject, Professor Bateson says:–
As regards the Mendelian principles which it is the chief aim of this introduction to present clearly before the reader, it may be said that by the application of those principles we are enabled to reach and deal in a comprehensive manner with phenomena of a fundamental nature, lying at the very root of all conceptions not merely of the physiology of reproduction and heredity, but even of the essential nature of living organisms; and I think that I use no extravagant words when, in introducing Mendel's work to the notice of the Royal Horticultural Society's Journal, I ventured to declare that his experiments are worthy to rank with those which laid the foundation of the atomic laws of chemistry.
Professor L. H. Bailey, who is the Director of the Horticultural Department at Cornell University and the editor of the authoritative Encyclopedia of Horticulture, was one of the first of recent scientists to call attention to Mendel's work. It was, we believe, because of a reference to Mendel's papers by Bailey that Professor de Vries was put on the track of Mendel's discoveries and found that the Austrian monk had completely anticipated the work at which he was then engaged. In a recent issue of The Independent, of New York, Professor Bailey said:–
The teaching of Mendel strikes at the root of two or three difficult and vital problems. It presents a new conception of the proximate mechanism of heredity. The hypothesis of heredity that it suggests will focus our attention along new lines, and will, I believe, arouse as much discussion as Weissmann's hypothesis, and it is probable that it will have a wider influence. Whether it expresses the actual means of heredity or not, it is yet much too early to say. But the hypothesis (which Father Mendel evolved in order to explain the reasons for his law as he saw them) is even a greater contribution to science than the so-called Mendel's Law as to the numerical results of hybridization. In the general discussion of evolution Mendel's work will be of the greatest value because it introduces a new point of view, challenges old ideas and opinions, gives us a new theory for discussion, emphasizes the great importance of actual experiments for the solution of many questions of evolution, and then forces the necessity for giving greater attention to the real characters and attributes of plants and animals than to the vague groups that we are in the habit of calling species.
It is very evident that a man of whose work so many authorities are agreed that it is the beginning of a new era in biology, and especially in that most interesting of all questions, heredity, must be worthy of close acquaintance. Hence the present sketch of his career and personality, as far as they are ascertainable, for his modesty, and the failure of the world to recognize his worth in his lifetime, have unfortunately deprived us of many details that would have been precious.
Gregor Johann Mendel was born 27 July, 1822, at Heinzendorf, nor far from Odrau, in Austrian Silesia. He was the son of a well-to-do peasant farmer, who gave him every opportunity of getting a good education when he was young. He was educated at Olmutz, in Moravia, and after graduating from the college there, at the age of twenty-one, he entered as a novice the Augustinian Order, beginning his novitiate in 1843 in the Augustinian monastery Königen-kloster, in Altbrünn. He was very successful in his theological studies, and in 1846 he was ordained priest. He seems to have made a striking success as a teacher, especially of natural history and physics, in the higher Realschule in Brünn. He attracted the attention of his superiors, who were persuaded to give him additional opportunities for the study of the sciences, particularly of biological science, for which he had a distinct liking and special talents.
Accordingly, in 1851 he went to Vienna for the purpose of doing post-graduate work in the natural sciences at the university there. During the two years he spent at this institution he attracted attention by his serious application to study, but apparently without having given any special evidence of the talent for original observation that was in him. In 1853 he returned to the monastery in Altbrünn, and at the beginning of the school year became a teacher at the Realschule in Brünn. He remained in Brünn for the rest of his life, dying at the comparatively early age of sixty-two, in 1884. During the last sixteen years of his life he held the position of abbot of the monastery, the duties of which prevented him from applying himself as he probably would have desired, to the further investigation of scientific questions.
The experiments on which his great discoveries were founded were carried out in the garden of the monastery during the sixteen years from 1853 to 1868. How serious was his scientific devotion may be gathered from the fact that in establishing the law which now bears his name, and which was founded on observations on peas, some 10,000 plants were carefully examined, their various peculiarities noted, their ancestry carefully traced, the seeds kept in definite order and entirely separate, so as to be used for the study of certain qualities in their descendants, and the whole scheme of experimentation planned with such detail that for the first time in the history of studies in heredity, no extraneous and inexplicable data were allowed to enter the problem. Besides his work on plants, Mendel occupied himself with other observations of a scientific character on two subjects which were at that time attracting considerable attention. These were the state and condition of the ground-water–a subject which was thought to stand at the basis of hygienic principles at the time and which had occupied the attention of the distinguished Professor Pettenkofer and the Munich School of Hygiene for many years–and weather observations. At that time Pettenkofer, the most widely known of sanitary scientists, thought that he was able to show that the curve of frequency of typhoid fever in the different seasons of the year depended upon the closeness with which the ground-water came to the surface. Authorities in hygiene generally do not now accept this supposed law, for other factors have been found which are so much more important that, if the ground-water has any influence, it can be neglected. Mendel's observations in the matter were, however, in line with the scientific ideas of the time and undoubtedly must be considered of value.
The other subject in which Mendel interested himself was meteorology. He published in the journal of the Brünn Society of Naturalists a series of statistical observations with regard to the weather. Besides this he organized in connexion with the Realschule in Brünn a series of observation stations in different parts of the country around; and at the time when most scientists considered meteorological problems to be too complex for hopeful solution, Mendel seems to have realized that the questions involved depended rather on the collation of a sufficient number of observations and the deduction of definite laws from them than on any theoretic principles of a supposed science of the weather.
The man evidently had a genius for scientific observations. His personal character was of the highest. The fact that his fellow-monks selected him as abbot of the monastery shows the consideration in which he was held for tact and true religious feeling. There are many still alive in Brünn who remember him well and cannot say enough of his kindly disposition, the fröliche Liebenswürdigkeit (which means even more than our personal magnetism), that won for him respect and reverence from all. He is remembered, not only for his successful discoveries, and not alone by his friends and the fellow-members of the Naturalist Society, but by practically all his contemporaries in the town; and it is his lovable personal character that seems to have most impressed itself on them.
He was for a time the president of the Brünn Society of Naturalists, while also abbot of the monastery. This is, perhaps, a combination that would strike English-speaking people as rather curious, but seems to have been considered not out of the regular course of events in Austria.
Father Mendel's introduction to his paper on plant hybridization, which describes the result of the experiments made by him in deducing the law which he announces, is a model of simple straightforwardness. It breathes the spirit of the loftiest science in its clear-eyed vision of the nature of the problem he had to solve, the factors which make up the problem, and the experimental observations necessary to elucidate it. We reproduce the introductory remarks here from the translations made of them by the Royal Horticultural Society of England.15 Father Mendel said at the beginning of his paper as read 8 February, 1865:–
Experience of artificial fertilization such as is affected with ornamental plants in order to obtain new variations in color, has led to the experiments, the details of which I am about to discuss. The striking regularity with which the same hybrid forms always reappeared whenever fertilization took place between the same species, induced further experiments to be undertaken, the object of which was to follow up the developments of the hybrid in a number of successive generations of their progeny.
Those who survey the work that has been done in this department up to the present time will arrive at the conviction that among all the numerous experiments made 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 to ascertain definitely their statistical relations.
These three primary necessities for the solution of the problem of heredity–namely, first, the number of different forms under which the offspring of hybrids appear; secondly, the arrangement of these forms, with definiteness and certainty, as regards their relations in the separate generation; and thirdly, the statistical results of the hybridization of the plants in successive generations, are the secret of the success of Mendel's work, as has been very well said by Bateson, in commenting on this paragraph in his work on Mendel's "Principles of Heredity." This was the first time that any one had ever realized exactly the nature of the problems presented in, their naked simplicity. "To see a problem well is more than half to solve it," and this proved to be the case with Mendel's straightforward vision of the nature of the experiments required for advance in our knowledge of heredity.
While Mendel was beginning his experiments almost absolutely under the guidance of his own scientific spirit, and undertaking his series of observations in the monastery garden without any reference to other work in this line, he knew very well what distinguished botanists were doing in this line and was by no means presumptuously following a study of the deepest of nature's problems without knowing what others had accomplished in the matter in recent years. In the second paragraph of his introduction he quotes the men whose work in this science was attracting attention, and says that to this object numerous careful observers, such a Kölreuter, Gärtner, Herbert, Lecoq, Wichura and others, had devoted a part of their lives with inexhaustible perseverance.
To quote Mendel's own words:–
Gärtner, especially in his work, "Die Bastarderzeugung im Pflanzenreiche,"16 has recorded very valuable observations; and quite recently Wichura published the results of some profound observations on the hybrids of the willow. That so far no generally applicable law governing the formation and development of hybrids has been successfully formulated can hardly be wondered at by anyone who is acquainted with the extent of the task and can appreciate the difficulties with which experiments of this class have to contend. A final decision can only be arrived at when we shall have before us the results of the changed detailed experiments made on plants belonging to the most diverse orders. It requires some courage indeed to undertake a labor of such 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 can not be overestimated in connexion with the history of the evolution of organic forms.
The paper now presented records the results of such a detailed experiment. This experiment was practically confined to a small plant group, and is now after eight years' pursuit concluded in all essentials. Whether the plan upon which the separate experiments were conducted and carried out was the best suited to attain the desired end is left to the friendly decision of the reader.
Mendel's discoveries with regard to peas and the influence of heredity on them, were founded on very simple, but very interesting, observations. He found that if peas of different colors were taken, that is to say, if, for instance, yellow-colored peas were crossed with green, the resulting pea seeds were, in the great majority of cases, of yellow color. If the yellow-colored peas obtained from such crossing were planted and allowed to be fertilized only by pollen from plants raised from similar seeds, the succeeding generation, however, did not give all yellow peas, but a definite number of yellow and a definite number of green. In other words, while there might have been expected a permanence of the yellow color, there was really a reversion in a number of the plants apparently to the type of the grandparent. Mendel tried the same experiment with seeds of different shape. Certain peas are rounded and certain others are wrinkled. When these were crossed, the next generation consisted of wrinkled peas, but the next succeeding generation presented a definite number of round peas besides the wrinkled ones, and so on as before. He next bred peas with regard to other single qualities, such as the color of the seed coat, the inflation or constriction of the pod, as to the coloring of the pod, as to the distribution of the flowers along the stem, as to the length of the stem, finding always, no matter what the quality tested, the laws of heredity he had formulated always held true.
What he thus discovered he formulated somewhat as follows: In the case of each of the crosses the hybrid character, that is, the quality of the resultant seed, resembles one of the parental forms so closely that the other escapes observation completely or cannot be detected with certainty. This quality thus impressed on the next generation, Mendel called the dominant quality. As, however, the reversion of a definite proportion of the peas in the third generation to that quality of the original parent which did not appear in the second generation was found to occur, thus showing that, though it cannot be detected, it is present, Mendel called it the recessive quality. He did not find transitional forms in any of his experiments, but constantly observed that when plants were bred with regard to two special qualities, one of those qualities became dominant in the resultant hybrid, and the other became recessive, that is, present though latent and ready to produce its effects upon a definite proportion of the succeeding generation.
Remembering, then, that Mendel means by hybrid the result of the crossing of two distinct species, his significant discovery has been stated thus: The hybrid, whatever its own character, produces ripe germ cells, which bear only the pure character of one parent or the other. Thus, when one parent has the character "A," in peas, for example, a green color, and the other the character "B," in peas once more a yellow color, the hybrid will have in cases of simple dominance the character "AB" or "BA," but with the second quality in either case not noticeable. Whatever the character of the hybrid may be, that is to say, to revert to the example of the peas, whether it be green or yellow, its germ cells when mature will bear either the character "A" (green), or the character "B" (yellow), but not both.
As Professor Castle says: "This perfectly simple principle is known as the law of segregation, or the law of the purity of the germ cells. It bids fair to prove as fundamental to a right understanding of the facts of heredity as is the law of definite proportions in chemistry. From it follow many important consequences."
To follow this acute observer's work still further by letting the crossbreds fertilize themselves, Mendel raised a third generation. In this generation were individuals which showed the dominant character and also individuals which presented the recessive character. Such an observation had of course been made in a good many instances before.
But Mendel noted–and this is the essence of the new discovery in his observations–that in this third generation the numerical proportion of dominants to recessives is in the average of a series of cases approximately constant–being, in fact, as three to one. With almost absolute regularity this proportion was maintained in every case of crossing of pairs of characters, quite opposed to one another, in his pea plants. In the first generation, raised from his crossbreds, or, as he calls them, hybrids, there were seventy-five per cent dominants and twenty-five per cent recessives.
When these plants were again self-fertilized and the offspring of each plant separately sown, a new surprise awaited the observer. The progeny of the recessives remained pure recessive; and in any number of subsequent generations never produced the dominant type again, that is, never reverted to the original parent, whose qualities had failed to appear in the second generation. When the seeds obtained by self-fertilizing the plants with the dominant characteristics were sown, it was found by the test of progeny that the dominants were not all of like nature, but consisted of two classes–first, some which gave rise to pure dominants; and secondly, others which gave a mixed offspring, composed partly of recessives, partly of dominants. Once more, however, the ratio of heredity asserted itself and it was found that the average numerical proportions were constant–those with pure dominant offspring being to those with mixed offspring as one to two. Hence, it was seen that the seventy-five per cent of dominants are not really of identical constitution, but consist of twenty-five per cent which are pure dominants and fifty per cent which are really crossbreds, though like most of the crossbreds raised by crossing the two original varieties, they exhibit the dominant character only.
These fifty crossbreds have mixed offspring; these offspring again in their numerical proportion follow the same law, namely, three dominants to one recessive. The recessives are pure like those of the last generation, but the dominants can, by further self-fertilization and cultivation of the seeds produced, be again shown to be made up of pure dominant and crossbreds in the same proportion of one dominant to two crossbreds.
The process of breaking up into the parent forms is thus continued in each successive generation, the same numerical laws being followed so far as observation has gone. As Mendel's observations have now been confirmed by workers in many parts of the world, investigating many different kinds of plants, it would seem that this law which he discovered has a basis in the nature of things and is to furnish the foundation for a new and scientific theory of heredity, while at the same time affording scope for the collection of observations of the most valuable character with a definite purpose and without any theoretic bias.
The task of the practical breeder who seeks to establish or fix a new variety produced by cross-breeding in a case involving two variable characters is simply the isolation and propagation of that one in each sixteen of the second generation offspring which will be pure as regards the desired combination of characters. Mendel's discovery, by putting the breeder in possession of this information enables him to attack this problem systematically with confidence in the outcome, whereas hitherto his work, important and fascinating as it is, has consisted largely of groping for a treasure in the dark. The greater the number of separately variable characters involved in a cross, the greater will be the number of new combinations obtainable; the greater too will be the number of individuals which it will be necessary to raise in order to secure all the possible combinations; and the greater again will be the difficulty of isolating the pure, that is, the stable forms in such as are similar to them in appearance, but still hybrid in one or more characters.
The law of Mendel reduces to an exact science the art of breeding in the case most carefully studied by him, that of entire dominance. It gives to the breeder a new conception of "purity." No animal or plant is "pure," simply because it is descended from a long line of ancestors, possessing a desired combination of characters; but any animal or plant is pure if it produces gametes--that is, particles for conjugation of only one sort–even though its grandparents may among themselves have possessed opposite characters. The existence of purity can be established with certainty only by suitable breeding tests, especially by crossing with recessives; but it may be safely assumed for any animal or plant, descended from parents which were like each other and had been shown by breeding tests to be pure.
This naturally leads us to what some biologists have considered to be the most important part of his work–the theory which he elaborated to explain his results, the principle which he considers to be the basis of the laws he discovered. Mendel suggests as following logically from the results of his experiments and observations a certain theory of the constitution of germinal particles. He has put this important matter so clearly himself and with such little waste of words that it seems better to quote the translation of the passage as given by Professor Bateson,17 than to attempt to explain it in other words. Mendel says:–
The results of the previously described experiments induced further experiments, the results of which appear fitted to afford some conclusions as regards the composition of the egg and pollen-cells of hybrids. An important matter for consideration is afforded in peas (pisum) by the circumstance that among the progeny of the hybrids constant forms appear, and that this occurs, too, in all combinations of the associated characters. So far as experience goes, we find it in every case confirmed that constant progeny can only be formed when the egg-cells and the fertilizing pollen are of like character, so that both are provided with the material for creating quite similar individuals, as is the case with the normal fertilization of pure species.
We must therefore regard it as essential that exactly similar factors are at work also in the production of the constant forms in the hybrid plants. Since the various constant forms are produced in one plant, or even in one flower of a plant, the conclusion appears logical that in the ovaries of the hybrids there are formed as many sorts of egg-cells and in the anthers as many sorts of pollen-cells as there are possible constant combination forms, and that these egg and pollen-cells agree in their internal composition with those of the separate forms.
In point of fact, it is possible to demonstrate theoretically that this hypothesis would fully suffice to account for the development of the hybrids in the separate generations, if we might at the same time assume that the various kinds of egg and pollen-cells were formed in the hybrids on the average in equal numbers.
Bateson says in a note on this passage that this last and the preceding paragraph contain the essence of the Mendelian principles of heredity. Mendel himself, after stating this hypothesis, gives the details of a series of experiments by which he was able to decide that the theoretic considerations suggested were founded in the nature of plants and their germinal cells.