Popular Science Monthly/Volume 65/July 1904/Hugo de Vries's Theory of Mutations
|HUGO DE VRIES'S THEORY OF MUTATIONS.|
By Professor A. A. W. HUBRECHT,
UNIVERSITY OF UTRECHT.
THE theory of evolution has influenced human thought in the most various ways during the past half century. In the sphere of biological science, where Darwin sowed the seeds which have grown up with such unexpected luxuriance, there has been a continuous process of fermentation which shows no signs of subsiding.
Still it has often seemed as if the immense mass of facts, which Darwin collected and arranged with so much care and skill, were provisionally looked upon as sufficient, and as if actual experiment were no longer a primary necessity. Whenever a new observation happened to be made, it was in danger of being drowned in pailfuls of theoretical considerations. Genealogical trees were planted, grafted, transplanted and finally often committed to the flames. Statistics were brought together to demonstrate the importance of natural selection, not only in the struggle between individual organisms, but also within the organism between the elements of which it was composed. Thus Roux wrote in 1881 his 'Kampf der Theile im Organismus,' Weismann only a few years ago (1896) his 'Germinal Selection.' In a series of very remarkable publications the Freiburg professor of zoology has thrown light on a series of difficult problems and has shown himself to be not only a faithful pupil of Darwin, but one who on several occasions has been more ultra-Darwinian than perhaps Darwin himself would have been.
Those who consult the very voluminous literature of the subject will soon be convinced that the number of biologists who have preferred patient experimentation to theoretical speculation is very limited indeed. Experimentation on this subject demands a great deal of time, of patience, of devotion, and is liable to meet with many pitfalls. Yet for those who came after Darwin this should have been the task that lay closest to their heart: to test the two great groups of facts on which descent and selection are founded, by means of new and more detailed experiments.
These two groups of facts are the phenomena cf heredity and of variability. Heredity, the conservative factor, by which what was once acquired is multiplied and rendered stable; variability, by which there appear side by side with those forms that are hereditarily constant, others which, being perhaps in yet closer harmony with the environment and with the prevailing conditions of life, may thus obtain a chance to defeat the first and to supplant them, with the prospect, however, of being in turn ousted by yet more closely adapted new forms, more exactly fitted to the surroundings. The mutual interaction of these two factors, heredity and variability, shows that a continual tendency in organic nature prevails, by which life proceeds from the simple to the more complicated, from the more primitive to the more perfect. And the archives in stone that have been opened to us by the geologists contain ample proofs to convince us that during the succession of thousands and thousands of centuries, plant life and animal life all over the world have passed through a similar process of development.
When Darwin was writing his 'Origin of Species' the chapter on 'Heredity' in physiology was yet a book sealed with seven seals. During the last forty) r ears, especially during the latter twenty, several most important pages of that book have been closely studied and partly deciphered. The conviction has begun to dawn upon us that the phenomena of heredity, assimilation and growth do not belong to different categories, and that, furthermore, this so mysterious heredity can be traced to the minute material particles with which it is bound up, and which through the whole range of plants and animals show an unexpected uniformity.
The labors of such men as Hertwig, Boveri, van Beneden, Strasburger, Guignard and many others, who have made a study of heredity, are especially important, because they have succeeded in analyzing the phenomena into their component elements, thanks to careful observation and experimental testing. They have shown us how very much we may yet expect from experimental work. In the next few years we may, no doubt, look forward to a rich harvest in this extensive field of investigation.
Variability is the name of the second group of facts, on which the slow development of higher, better and more complicated types of living beings rests. Here, too, we must call for the facts and ask for the credentials of the theories we meet. There is ample proof that in the domain of variability we encounter many delusive traps and a host of difficulties. Even Darwin has not spun out to the end the thread of comparative experimentation. It has been reserved to Hugo de Vries to point this out in a very remarkable book ('Die Mutationstheorie,' Leipzig, 1901-1903) that has just been completed.
For nearly twenty years Professor de Vries has been busy making experiments on a large scale on variability in plants, and the results of those experiments have enabled him to formulate gradually his own thoughts and to compare them with the teachings of Darwin, Wallace and others. Those who are acquainted with the patient experiments of the Amsterdam professor and have assimilated his results, will agree that, by the publication of this book, a new leaf has been turned *in the history of evolution. A set of new facts has been gathered with which all those who of late years have theorized about Darwinism must reckon, and which will undoubtedly prove to be the starting-point for further experiments in the direction they so clearly indicate.
I shall now try to point out: (1) in what respects de Vries's work is such a very decided step in advance; (2) in how far it might be supposed that there is any conflict between Darwin's opinions and those of de Vries; and (3) to what extent a teleological interpretation of nature might draw upon the results of de Vries's investigations.
Supposing there existed no variability in nature, life would lose a good many of its attractions. Fancy men and women resembling each other like so many drops of water, both physically and morally! Fancy all dogs constructed according to one pattern, all flowers, all trees of one species being absolutely identical as to their branches, number and shape of leaves, etc.! Fortunately, from our very childhood we have learnt to see nature in a different light, and we have all contracted the habit of giving our preference to the finest and best horses, flowers and playfellows; permanent selection is thus being exercised by us, which can add much to our happiness in life. In effecting it, we make use of what variability offers, and, consciously or unconsciously, we always tend to favor the better and to decline the worse.
Yet more intensely than in the way just sketched, the variability of living organisms is utilized by those who make their livelihood by the rigorous application of selective principles to plants and animals. Dealers in seeds of improved plants, nursery gardeners who cultivate rare varieties of flowers, breeders of birds and domestic animals, all these have a direct interest in every change for the better or for the worse, and are very keen at increasing the former and eliminating the latter. Any one who sells corn or maize, which, when sown under the same conditions, produces ears doubly full or yielding flour of a better quality, may be certain of a substantial gain. One who cultivates beet-roots containing a greater amount of sugar gains equally. Again, a man who lays out a considerable amount of money in purchasing mares and stallions beautifully fit for racing purposes, and who breeds from these with care, will not only be paid back the money he spent, but find means of quickly doubling his capital.
The improvement of our domestic animals, the development of our wheat, the diversity in color and in shape of our decorative plants, have long been objects of constant effort. Whole classes are constantly occupied in making the most of the phenomena of variability, as much for their own advantage as for the benefit of the community. If we consider the results thus obtained we can not fail to notice that the plants and animals in question have grown to differ so much from the original stock that, should we meet with them in nature, we should undoubtedly call them new species, perhaps even new genera. It follows that if man can in this way direct the phenomena of variability to his own use, the origin of species of plants and animals in nature may depend on a similar series of phenomena.
In nature, however, the selecting breeder is replaced by an automatic process—the survival of the fittest in the struggle for life. That struggle occurs in the first place between members of the same species; it is a struggle for food, light and air, for fecundation and thus for reproduction.
But such a struggle for existence, by which the fittest remain alive and gradually supplant the less fit, does not take place only between individuals of the same species; it is also waged—and perhaps more effectually—between closely allied species.
In this conception the final decision will be reached by the cooperation of very numerous circumstances. It finally leads to a sifting process, to the disappearance of many and to the selection of a few. Selection is a self-regulating phenomenon; it is nature that chooses, and the name of 'natural selection' is thus amply justified. According to Darwin and Wallace, who simultaneously formulated the principle, the origin of species is brought about by 'natural selection.' It is the counterpart of the voluntary or 'artificial selection,' to which man owes the improvement of various cultivated plants and domestic animals. The material oufr of which in both cases new races and new species are being created, is that which variability offers: the struggle for existence in nature, the breeder in his hothouses or in his kennels, shapes the material into new races, varieties and species.
We thus find ourselves compelled, whenever we wish to penetrate more deeply into nature's laboratories where new species are being fabricated, to sift most carefully the whole and complicated set of phenomena which we call variability. Only in this way may we hope to approach by means of our imagination the coming and going of different forms of organized life that have succeeded one another since the cooling of our planet, and of which only a small portion have been preserved as fossils since the Silurian epoch.
Darwin inaugurated the sifting process with wonderful sagacity. Wallace has continued the work, but has wandered away from reality (as de Vries will teach us) to a considerable extent. Darwin was well acquainted with the fact that two kinds of variability should be distinguished: one, which is called fluctuating variability, oscillates round a mean value; we shall consider it a little more closely. Whichever characteristic of a species we happen to choose, we shall always find, in considering a number of specimens, that individual differences, individual variations, can be noticed which, when tabulated according to size or to number, do not exceed two opposite extremes. Half-way between these extremes we find the 'norm' for that particular characteristic. The fluctuation may be represented by a curve, the culminating point of which corresponds to the norms just mentioned, whereas to the right and to the left of it the curve gradually approaches the horizontal line and has a symmetrical shape. Quetelet and Galton have insisted on the great significance of the fact, that fluctuating variation remains enclosed within the limits of such a curve of regular shape; the curve itself is, therefore, often called Galton 's curve.
Not only for plants, but also for animals and especially for man, the existence of such Galton curves, expressing the amount of variability, has been definitely established by different observers in very numerous instances. Thus, for example, Ammon has obtained his material from South German recruits. We need not insist on the fact that the greater the number of cases, the more reliable the curve.
The different degrees of fluctuating variability can undoubtedly be seized upon by any one who wishes to make them the starting-point for the breeding of certain distinct variations. Thus, for instance, by constantly selecting for the reproductive process those plants in which a given deviation is strongly marked, after a certain time and after a series of generations, a plant can be obtained for which the Galton curve would indicate a displacement of its culminating point in the direction of the selected variation. In this way an increase in the yield of sugar obtained from the beet roots has been arrived at from about 7 per cent, to 13 or 14 per cent. Thus also ears of maize have been produced that bore 20 rows of grain, whereas the kind from which the experiment had started always bore 12 to 14 rows.
As soon, however, as such conscious and voluntary selection ceases, the next generations successively return to the original curve. In order to prevent this retrograde process, without a constant and repeated application of the artificial selective process, we are obliged to prevent the appearance of new generations, by forcing the plant to reproduce itself not by seed, but asexually by means of buds. It is well known that definite kinds of delicate fruit are reproduced in this fashion, because if multiplied by seed, they would always tend to fall back into their former state of less value. Transposing the culmination of the curve of variability artificially, as explained, or breeding variations to the right or to the left of the norm, can never exceed certain limits. Agencies are at work there which prevent the fluctuating variability from going any further. The existence of such limits compels us to acknowledge that there is no possibility that species might arise in nature according to the same plan by which certain breeds originate under artificial selection.
On this point de Vries and Wallace differ essentially. The latter is convinced that the fluctuating variability is the only source from which new species have gradually originated; de Vries, however, is quite justified in claiming that the examples of the increase and the accumulation of certain variable characters do not prove that a new species or subspecies has ever arisen in that way in its natural environment.
But in addition to the fluctuating variability we have now to consider another variability, regarding which both Darwin and Wallace have collected numerous data, the so-called 'single variations,' which do not follow the Galton curve. They are not connected with their starting-point by very gradual transformations, but are separated from it by a measurable distance which they have overcome, not by degrees, but by starts. They have, therefore, been named 'sports' or 'saults,' the leap being in different cases larger or smaller.
We have seen that fluctuating variability leads to slow changes and furnishes farmers with the material to improve the races of animals and plants. The 'chance variations' in their turn are valued quite especially by horticulturists and nursery-gardeners.
The English name 'single variations' expresses very well, indeed, the difference between the two kinds of variability. Fluctuating variability shows us simultaneously all the different degrees between extremes, as represented by the descendants of a single parental pair. The single variations, on the contrary, stand isolated; they are discontinuous; between them and the original parent form we do not observe any gradation. This difference has long been noticed, and on several occasions the difference between these 'single variations' and fluctuating' or 'oscillating variations' has been insisted upon.De Vries has accepted the name 'mutation' and has submitted the phenomenon to a severe experimental test. The chief result of this has been the conclusion which has at the same time become the basis of his own mutation theory—that by means of fluctuating variability certain local and improved races may indeed be bred, but that in nature new species never arise through its agency. These latter owe their origin exclusively to mutation, to 'discontinuous' variability. He is here entirely opposed to Wallace, who looked upon fluctuating variability as the real source from which species gradually originated. With Darwin, de Vries is less at variance, and a quotation from the 'Origin of Species' leaves no doubt that Darwin fully appreciated the value of the single variations for the formation of new species. We read on page 66 of the edition of 1872:
Those lines contain an abstract of de Vries's mutation theory. And then further, on page 72:
Again, when Darwin denies having said that time alone plays a part
in the process of modification which changes one species into another, he writes (p. 82, l. c.):
In the fragments which I have quoted Darwin appears to have had before his mind mutation, not fluctuating variation. And I must insist on the fact that de Vries makes a point of showing that Darwin was decidedly inclined to accept the process of mutation. De Vries quotes (p. 25) from Darwin's 'Life and Letters' (p. 87, Vol. II.) and from the 'Origin of Species,' e. g., the following words:
The chance variations were not for Darwin the extreme cases of fluctuating variability, that can be everywhere observed; they were fortuitous phenomena. For these natural selection is always on the lookout, or as Darwin has it, metaphorically, 'He catches hold of them, whenever and wherever opportunity offers.' Darwin must have been inclined to think that these variations, these mutations, arise in accordance with certain laws which are entirely unknown to us. In consequence of the operation of these laws at least a certain number of favorable modifications must inevitably arise after a given lapse of time. Hence the gradual evolution which most living organisms have undergone in the course of ages.
Darwin also undoubtedly suspected the existence of a certain periodicity. 'Nascent species are more plastic,' he says; and he thereby intends to imply that they form more numerous single variations and have thus a better opportunity to split up again into further species—so far de Vries (pp. 24-26).
I have purposely insisted on these points, because here and there a tendency seems to prevail to look upon Darwin's views on the origin of species as unsatisfactory and obsolete, and to proclaim the necessity of replacing them by a brand new hypothesis with which the name of de Vries should be coupled. These tendencies are in great favor with those that bear a grudge to the so-called Darwinism for other than scientific reasons, and who in their innermost heart would at the same time like to see a similar fate reserved for de Vries's demonstrations, and even for the whole theory of evolution.
We have, however, seen in de Vries's own words how little he considers himself an antagonist of Darwin. On the contrary, his great and imperishable merit consists in this, that his important and extensive experiments have provided us with a reliable basis concerning a subject about which Darwin had not fully made up his mind.
Darwin seems to have suspended his judgment; at all events, he has not drawn a hard and fast line between the results which artificial selection can attain when applied to fluctuating variations, on the one hand, and to mutations, on the other.
The experiments which de Vries has continued during many years on the two divergent processes, which Darwin has not sufficiently kept separate, have justified him in claiming that now, for the first time—forty years after the appearance of the 'Origin of Species'—the actual birth of a species has been observed by him. He has thus opened up a most extensive field for further investigations by other naturalists, and he has undoubtedly put an end to useless polemics which often threaten to become yet further burdened by subtleties.
Far from having undermined Darwin's Darwinism, de Vries has completed, purified and simplified it. To Wallace's Darwinism, however, de Vries has dealt a severe blow, Wallace having attached no significance to 'single variations' as possible sources of new species; whereas Darwin has always continued to acknowledge their importance as such, even though he did not thoroughly understand the laws to which fluctuating variability is subject. Even Weismann, who has only partially appreciated Darwin's philosophic indecision and who has, without wavering, followed a road which has now landed him in his 'germinal selection,' has undoubtedly taken notice of de Vries's experimental treatment of the subject with interest, though probably not with personal satisfaction.
Let us now try to picture to ourselves what conclusions de Vries has been able to reach experimentally with respect to the phenomena of mutation, and what he has taught us concerning the all-important question: How have species originated? De Vries has started from the phenomenon above mentioned, so well known to nursery-gardeners and which is often of considerable financial importance to them; the phenomenon that suddenly in some of their flower-beds single variations appear which are constant when reproduced by seed. By systematically propagating these exceptional specimens a quantity of seed may be obtained in a few years that can be brought into the market. Soon, however, the variety loses its mercantile value; seed may now be obtained by anybody in increased quantities from the plants that have already been sold.
Are there any of our native species of plants in which the same phenomenon produces itself naturally? was the question which de Vries set himself. And if so, can they teach anything about the formation of species? In commencing the inquiry he started with about a hundred different species. Of all these, only one exhibited the property sought for—this, however, in such a way as to throw full light on the subject in most unexpected directions.
The species of plant which de Vries actually managed to detect in the act of 'mutation' on certain fields in Graveland, and which has continued to do so with perfect distinctness during many years in the Amsterdam Botanical Garden, bears the name of Œnothera Lamarckiana. It is one of three species of the genus that have been brought over from the United States and is now running wild in Europe.
De Vries has thus convinced himself that the great majority of plants about us do not show cases of 'chance variations,' 'mutation variation' per saltum; in other words, that the species that have been observed for many centuries may be said to be stable, invariable. But they are stable in so far only as—perhaps with very long pauses—periods of mutability appear, during which, next to the stable central species, new sub-species appear that are also stable when propagated by seed. Further experiments, however, are required to throw light on the periodicity.
Another conclusion was this, that the species which do produce mutations bring forth not a single mutation, but quite a number of them, varying among themselves. The mutations occur both in plants growing in the wild state and in those samples of Œnothera Lamarckiana that are bred under supervision. Their frequency, as determined by exact statistical tables drawn up by de Vries, varies between one and two per cent. Of the 50,000 Œnothera which de Vries has observed during ten years' culture, there were 800 that could not be designated by the name Œnothera Lamarckiana.
A somewhat skeptically disposed person may claim that this number of 800 indicates the number of the most marked deviations which were noticed among 50,000 plants; that, in other words, they are individual, fluctuating variations that would also be found in the same quantity among 50,000 other plants. To this it may be replied that the phenomenon of fluctuating variation, as it appears in Œnothera, has been studied in detail by de Vries and has been exactly determined both for the central species and for the different subspecies (mutations). In all of them it occurs on a large scale, but not one of the specimens above mentioned belongs to it. These 800 have very special characteristics, by which they can be sharply distinguished from the fluctuating variations. And, as is especially remarkable, they are not in every respect different from each other, but may be arranged in seven natural groups, each of which possesses exactly the same systematic value as that particular combination of specific characteristics to which the name of Œnothera Lamarckiana has been applied.
The number of individuals of those seven groups, of which de Vries has observed the spontaneous appearance, is, however, most unequal and varies between 1 (Œnothera gigas), 56 (Œnothera albida), 350 (Œnothera oblonga), 32 (Œnothera rubrinervis), 150 (Œnothera nanella), 221 (Œnothera lata) and 8 (Œenothera scintillans).
De Vries has studied the mutations thus arising, some of which are rare and some more common, with the utmost care, and has followed them during the whole of their existence.
Œnothera gigas, which has only once arisen in the Lamarckiana group and which is characterized by much larger-sized flowers, a different shape of the leaves that form a rosette at the root, more thickly set leaves along the stem, etc., was sown by de Vries in 1897, after he had obtained seed, thanks to artificial fecundation, the possibility of self-fertilization being excluded. He thus obtained 450 plants, which, with the exception of a single one, exhibit all the characteristics of the Œnothera gigas with perfect constancy. The exception was not a retrogression towards O. Lamarckiana, but a new deviation, provisionally indicated as O. gigas nanella. From 1898 to 1900 further propagation by seed has been effected during three generations and under very strict precautions; and until now all the descendants of that one mutation of 1895 remain perfectly constant; de Vries has actually seen the species O. gigas come forth out of O. Lamarckiana, first in nature, afterwards in his own nursery-garden. It appears to be a very strong plant, which, if it had to fight for its existence against O. Lamarckiana, in equal numbers and under the same circumstances, would probably prove to be the winner.
The second mutation, Œnothera albida, which occurred 56 times during the experiments, shows another character. It is a feeble plant, and was originally considered a pathological deviation, which, however, in the later generations has proved itself to be none the less constant, and, though but little fertile, produced 86 plants in 1898 and 36 in 1899.
The third mutation, called Œnothera oblonga, repeatedly occurred (350 specimens) in the series of generations of Œnothera Lamarckiana that were successively cultivated. Many hundreds were cultivated later. It may be recognized with certainty as soon as the sixth leaflet unfolds, and has remained unchanged, with the exception of two specimens, which, however, have not retroceded towards O. Lamarckiana, but showed the characteristics of O. albida and O. rubrinervis. This mutation has thus, although perfectly stable, retained the power to further mutate.
The fourth mutation, Œnothera rubrinervis, again shows other interesting peculiarities. It is a strong plant, not less rich, both in pollen and in seed, than O. Lamarckiana, which was more or less the case with the other mutations. It has been obtained in very great numbers (2,976 specimens) by de Vries, and the stability of Œnothera rubrinervis has asserted itself most distinctly also for all those specimens that had descended from different mother plants.
The fifth mutation, O. nanella, differs from the others in the fact that its deviation from the original O. Lamarckiana does not show itself in a given number of sharply determined and constant characteristics, but only in one, its dwarfed dimensions. Thus we should be inclined to look upon O. nanella rather as a variety than as an elementary species. However, in this case the smaller dimensions do not come under the head of fluctuating variability, but we have undoubtedly an unmistakable mutation that can be recognized as such with certainty as soon as the second leaf begins to show itself, and which, when fecundated in 1893 by its own pollen, directly produced 440, and in 1895 2,463 germinating plants, all of them, without exception, Œnothera nanella. In 1896 the seeds of 36 other plants of O. nanella were again planted and 18,000 seedlings were obtained, which again showed, with perfect precision, the characteristics of the species, with the exception of three mutating plants that bore at the same time the distinctive characters of O. oblonga, and thus formed an elementary species of the second degree, O. nanella oblonga.
We have yet to mention two mutations, O. lata and O. scintillans. The first only consists of female plants; there is never any fertile pollen produced, so that its stability can not be made out with certainty. The second, a dark green plant with shining leaves, is a rare mutation, which especially differs from those we have been studying, by the fact that even when artificially fertilized, with the utmost precautions, the mutation is not stable. The majority of its descendants belong to three groups: O. scintillans, O. oblonga, O. Lamarckiana. As compared with the wonderful stability which we encountered in the preceding mutations, the lability of this one—which at the same time seems to follow a certain law—is a most remarkable phenomenon, the origin and the significance of which have still to be traced.
We have now seen that for a number of years de Vries has been able, by his careful and skilful experiments, actually to witness the very process of the origin of species in nature. On the particular spot near Graveland, where he first noticed the process of mutation in nature, it had, of course, been going on even before his first observations. He here encountered, besides the O. Lamarckiana, a second species, the O. laivifolia, with which he also made experiments in the Amsterdam hortus, and which also produced numerous mutations, some of them identical with those obtained from O. Lamarckiana.
Especially important was the irrefutable demonstration of the fact that the process of mutation does not appear as one single specimen which gives rise to a constant variety, but that it again and again repeats itself in every generation in a certain percentage, and that absolutely the same 'mutants' appear—even though in varying quantities. The individual mutants thus find their chance of surviving considerably increased and, as soon as a slight change in the outward circumstances occurs, any mutation, which at the outset was in the minority as compared with the parent species, may slowly but surely become the majority. It is constantly getting a fresh supply from the parent species and, as soon as it shows itself better adapted to the circumstances, it may finally supplant the parent species entirely.
This struggle for existence does not occur between individuals of the same species, but between the mutations and the parent species. As long as the mutation has not appeared, there can be no question of the origin of a new species; the species is then constant, and only submitted to fluctuating variability, which can produce local races (not elementary species) under the constant cooperation (either artificial or natural) of selection, but which never leads to the formation of species. During a period of mutation a modification of the species is not always a necessary consequence; for in many cases the parent species will prove to be the fittest, and the mutations will then none of them be permanent.
The fact has been established by de Vries that in the natural life of a wild plant a series of phenomena occurs which justifies us in saying that he has made us see and actually touch the origin of species, whereas Darwin had made us understand it.
De Vries has not deviated from the teachings of the master, but he has developed them; he has brought us a further and a most important step forward and he has paved the way for later investigators. Theirs will be the task to make out in how far the laws of the mutation process, which de Vries has for the present only been able to make out for one genus of plants, also apply to the other plants and to animals. These laws are: (1) New elementary species arise suddenly, without transitions. (2) New elementary species are generally perfectly stable from the very first. (3) Most of the new types have all the qualities of elementary species, not of varieties. (4) The elementary species usually appear in a considerable number of individuals simultaneously, or at least within the same period. (5) No important relation whatever exists between individual variability and the new qualities of the elementary species. (6) The mutations, which give rise to new elementary species, take place in the most various and divergent directions. The modifications concern all the organs and are of the most varied descriptions. Part of the new types perish without descendants. Among the others, natural selection must slowly decide. (7) The phenomenon of mutability appears periodically.
Some of these laws of the mutation theory require further explanation. In the first, the sadden appearance of new elementary species is formulated. The characteristic qualities of the species thus arise per saltum, without transitions, such as are always observed in fluctuating variations. The ancestral forms of the different Œnothera 'mutantes' were perfectly well known. It is a fact that every mutant has been obtained from seed of normal and carefully examined O. Lamarckiana. On every occasion the new mutation suddenly appeared in all its details. The name 'elementary species' is given to the new form, and here we enter the domain of terminology and must necessarily furnish some explanation.
What is a species, what is a new species? What is an elementary species, which has also been called a subspecies? Are these last different from races and varieties? If so, how? For those who are not naturalists all these questions seem to be frivolous. They know that Darwin has written a celebrated work on the 'Origin of Species' and that a century earlier Linnaeus had instituted for species in nature the 'binary nomenclature,' so that 'Bellis perennis' stands for daisy, Elephas indicus for the Asiatic, and Elephas africanus for the African elephant. And so, according to their lights, zoologists and botanists will, by this time, have agreed on what a species is. This, alas, is far from being the case! The Mosaic belief in the separate and independent creation of every species at least furnished us with a distinct definition, even though transcendental. But as the idea of a slow and gradual evolution in nature has come to predominate in the course of time, sharp boundary lines have been effaced, and species have become both of an artificial and of a temporary nature. They have become a compartment into which man temporarily brings together a larger or smaller number of individuals, knowing that in times gone by the contents were confluent with those of another such compartment and that in the far future there will be other changes.
And when in later years the ideas of Wallace, which we mentioned above, found increasing sympathy, the lines of separation grew dimmer yet, and the idea of species became exclusively an artificial limit comparable in musical terms to so many thin lines that mark the bars in the continuous symphony of the evolution of life upon earth.
Thanks to de Vries's experiments, which have enabled him to formulate his mutation theory, he has now provided us with the means to define species more strictly. The species is limited by space and time. By time, for it begins whenever, by a process of mutation, its peculiar combination of specific characters springs into existence, even though this mutation, unobserved by the untrained eye, can as yet only be detected by the specialist. Whenever mutation appears, the combination of specific characters is modified in the mutants, and at the same time a new species has appeared side by side with the mother species, which itself remains stable.
The distribution of a species in space can be very varied; some are known only from a very limited area, others may be cosmopolitan. The species thus limited in time and space is what de Vries calls an elementary species. It is with these elementary species that the next generation of naturalists will have to grapple when they wish to elucidate evolutionary problems experimentally. The existence of such elementary species is no novelty which de Vries has been the first to make us acquainted with. Linnæus knew these elementary species perfectly well, but he called them varieties and forbade his pupils to waste their time on them. 'Varietates levissimas non curat botanicus.' From his point of view this was perfectly justified. He came forward to restore order in the chaos of classification, and as such he strove to combine the material then available into not too small bundles. His species were what the Germans have called by an expressive name 'Sammelarten,' receptacles, into which the so-called 'varietates minores' were thrown together. According to his idea, the species had been created in the beginning as an entity, the 'varietates' had gradually arisen from it, even though he could not prove this experimentally. With regard to them, Linnaeus was an evolutionist, just as, among his predecessors, the idea had long predominated that the genera had been created, whereas the species had come from these, as so many local deviations.
Who would deny that Linnæus 's work has facilitated the task of those that have come after him? Nevertheless, many species have been repeatedly subdivided. We must henceforth admit that when a species goes through a period of mutation, or has just gone through it, the number of elementary species that keep up their independent existence by the side of the parent species may be considerable, as we have seen with Œnothera. And it is easily understood that a tendency arises to denominate for convenience those numerous elementary species not by their own names, but by a collective name. This happens in most handbooks of systematic botany for well-known European plants, as, e. g., Draba verna, of which not less than 200 perfectly stable elementary species are known, Viola tricolor, etc.
Henceforth, however, we may no longer allow ourselves to be guided by opportunism. Systematic botany will have to take her watchword from physiology. De Vries has combined the qualities of the experimenter, who dares to look the physiological problem in the face, with those of the systematist, who observes and appreciates with uncommon sagacity the slightest shades of difference, and who with utmost delicacy of touch sifts and deals with species and races, mutations and variations.
The elementary species are stable. Selection calls forth different races within the limits of these species, but whenever selection ceases the races turn back to the parent form. The maximum deviation in these races is generally obtained after three or four generations of continuous selection; it takes about as many generations to bring back the parent form.
It is superfluous to say that many of these phenomena must be yet submitted to experimental investigation. De Vries has started this, and both in the domain of fluctuating variation and formation of races and in that of crossing and hybridizing he has already partly completed, partly only just commenced, elaborate experiments. Others besides himself have of late years analyzed the phenomenon of variety closely. The Cambridge zoologist, Bateson, has attempted to trace in his well-known work, 'Materials for the Study of Variation,' what it really is that variability offers towards the making of species, both in the most different species of animals and with respect to their divergent organs. He has, however, not seen his way out of the labyrinth, and although he came to the conclusion that it is not fluctuating variability which presides over the formation of species, but that a discontinuity must necessarily play a part, yet he, too, has committed himself to the assumption that the determination of the width of the fluctuations can furnish us with valuable data for understanding the gradual formation of species. He, too has not yet succeeded in analyzing and distinguishing from each other—what has been de Vries's merit—variability within the boundaries of constant species and mutability which does not fluctuate, but which by a sudden bound leads to the new species.
Much closer to this valuable discovery we find two students of the fossil-animal kingdom, two paleontologists, one of whom (Waagen), as long as twenty-five years ago, understood the importance of the phenomenon of mutation, even without the support which the series of de Vries's experiments would, of course, have afforded him, while the second (W. B. Scott, of Princeton) has most clearly expressed himself (American Journal of Science, 1894) that the formation of species by selection of fluctuating mutations, such as Wallace maintains, is rendered most improbable by what the fossil-animal world teaches us.
This world of fossil animals exhibits in certain regions of the earth, where the successive geological formations have been retained in undisturbed order, a similarly undisturbed ascending series. Far from finding in that series the divergent fluctuations which Bateson had accepted for so many animals, Scott has shown (and has strengthened his argument by referring to the results of many other paleontologists) that these fluctuations are indeed—though exceptionally—found among fossil animals as so many individual deviations (thus proving that also in that time the fluctuating variability existed within the limits of the species)—but he is at the same time convinced that this phenomenon has nothing to do with the slow modification of species, which takes a straight line and not a zigzag one.
Scott, although he was not at that time acquainted with de Vries's experimental evidence, staunchly holds to the idea that species have not grown out of the gradual selection of deviating individuals, but have appeared by mutation, by very small but sudden starts from one stage to the next.
We see before our eyes how the species of the deeper layers are gradually modified as we reach the higher layers; we find that all individuals simultaneously underwent this modification; in other terms, the phenomenon can hardly be described otherwise than by saying that the older species tends directly towards an aim, which the younger species that has descended from it has attained.
Many paleontologists even go so far as to admit a previously determined direction in gradual evolution. There is, of course, close affinity between such a predetermined direction in evolution and the teleological idea of design presiding at the creation of species. Clerical opponents of evolution may here have their chance of adapting the newest results in the study of that process to their personal principles.
Still, although the mutation experiments of de Vries have considerably strengthened the argument of Scott and other paleontologists, that slow, simultaneous mutation has also taken place among those fossil animals—the same experiments have, moreover, proved beyond any doubt that there is no such thing in nature as predetermined mutation in one special direction, but that, on the contrary, mutation occurs in very different and very divergent directions.
When once the mutation process leading to the formation of species has begun, the most different mutations, as we have seen above, arise. From our point of view, some of these may be called good, others bad or indifferent. About the permanence of any of them, it is, however, the surrounding conditions, acting by means of selection, that decide. And often the decision lies in another direction than would have been surmised from the human adjectives just named.
By the phenomenon of mutation the possibility exists that useless, and even to a certain extent prejudicial or noxious, specific characters may appear, a phenomenon which could never be reconciled with the views of Wallace.
For the greater part these characters are sure to be eliminated, but if other circumstances happen to be or to become favorable to a mutation, which was originally without any particular significance, it can then gradually develop and become adapted to certain modifications in the surrounding factors of life. The majority of the mutations, however, soon perish in the struggle for existence. Of those many elementary species that were doomed from the first, nothing has, of course, come down to us. in the archives of the fossil remains; only when their number has considerably increased in comparison with the parent species will it have been possible for them to survive, but then they have already risen to be a side branch, or may even be supplanting the parent stock.
The theory of mutation, as well as that which ascribes the origin of species to the selection of fluctuating varieties, enables us to understand how efficiency and adaptation in organic nature have come about by the mutual interaction of natural processes without the aid of supernatural intervention. The struggle for existence between species and mutations comes about in the same way as does the struggle for existence between individuals in the older view. Spencer's expression, however, 'the survival of the fittest,' must henceforth be interpreted as meaning 'the survival of the fittest species.' When we agree with de Vries that the gradual mutation of species is not necessarily the revelation of a foreordained design, this should be interpreted in the spirit of greater humility which befits the naturalist when he is confronted by the gigantic problems of organic nature. As long as a natural coordination of facts furnishes us with an intelligible causal connection, he does not feel justified in agreeing with those who are ready to accept explanations outside the pale of science. The same naturalist, however, will always be found ready to admit that he is yet exceedingly far from being able to give an 'explanation' of the inner meaning of the real significance of the mutation process.
In order to penetrate into this it is necessary to analyze further the phenomenon of heredity. Concerning this, de Vries has already on a previous occasion published theoretical views which follow in the footsteps of Darwin's celebrated theory of pangenesis. As the chemist operates with molecules and atoms, for the reconstruction of the processes of inorganic nature, so the biologist, when trying to represent to himself living matter, has to take into account the smallest entities, which have received various names from various naturalists, and to which de Vries gives that of 'pangens.'
Pangens are something different from complicated molecules; they can assimilate and they can reproduce themselves. Not only does all living matter, wherever found, consist of them, but those smallest living particles must at the same time be considered, either individually or grouped together, as being bearers of single or of mutually correlated properties of living matter.
An augmentation or a diminution of the number of pangens which represent a certain property will call forth the phenomenon which we have named fluctuating variations; while a modification in the composition of the pangen, for example, by division in two unequal parts, or by substitution—using a term well known in chemistry—will be equivalent to a mutation (progressive mutation), as will also the disappearance of a determined pangen (regressive mutation). Thus, according to these abstract representations which we form of the mysteries of heredity, the fluctuating variation depends on quite a different category of phenomena from those of the chance variation. And we understand directly that the chance variation obeys a more complicated mechanism than fluctuating variation, which depends only on the greater or lesser numerical importance of the preexisting elements, while the chance variation, the formation of species, implies a change of the existent elements.
As to how this change in the pangens periodically takes place, both simultaneously and successively, among a certain number of individuals, or might be aroused or caused; as to how the unequal division or substitution obeys fixed laws in such a way that the mutants, arranged in groups, are alike—all this for the present can not be explained by us.
If we aim to understand the conditions we shall be able to create species, as we can now breed improved races. And as we gradually learn to analyze the elements of the phenomenon, the probability grows that sometime we shall master the art of actually directing the series of natural phenomena. A new and limitless field of work would then be opened. Provisionally we can guess only from what we have as yet observed that certain processes are able to call forth or to accelerate the phenomena of mutation. Thus de Vries suggests the idea that a rapid succession of periods of reproduction might facilitate the reappearance of a period of mutation, whilst others think that transportation into quite different surroundings or transplantation might produce it. Others, again, appear to believe that increased nutrition, either combined with the conditions mentioned or not, would call forth mutation. All this, however, is no more than guesswork and hypothesis. We have as yet no means of fully knowing and of understanding.
For the present it is safer to recognize our absolute ignorance, and at the same time to define more exactly how far de Vries has brought us, and what is the important step for which we have to thank him. We can warmly recommend the reading and studying of de Vries 's clearly written and beautiful book. He has been the first to show us the sharp distinction that exists between chance variation and fluctuating variation, and to prove that it is not the latter, but the former, that calls forth in nature the origin of species. He has not yet been able to tell us whether, and, if so, how, chance variation could be called forth artificially by man. The fact that artificial selection of fluctuating varieties, as well as hybridizing, etc., has already led to such indisputable improvements in the different races of animals and plants may, however, give us hope that a conscientious experimenter and close observer, such as de Vries, has still a full store of important pioneer's work before him and may yet succeed in finding how to direct the mutation process. Thus, the origin of species would not only have been studied more closely, but would be subjugated to the human will. After having seen species originate in nature, man would then be able to call them forth. Then only the 'Origin of Species,' to which Darwin has given us such a marvelous introduction, would be revealed in all its details.
- This article was written in English by Professor Hubrecht, the eminent Dutch zoologist, who has an equal command of the French and German languages. Professor de Vries is at present in the United States in order to lecture at the University of California and other institutions.—Editor.
- It should here be mentioned that de Vries has noticed (l. c., p. 186) that the seeds of mutating plants generally retain the power of germinating for a longer period than the seeds of the normal O. Lamarckiana. Upon this fact he bases the expectation that perhaps later it will be possible to utilize this peculiarity and to find means to increase the percentage of mutating plants in a given series of sowing experiments by artificially somewhat accelerating the dying off of the seeds. This might also prove important when searching for mutations.
- "Many of my readers," says de Vries, "will be inclined to call my new species varieties, just because I was able to trace their origin. This is a mere verbal contention, of no importance at all for science."
- H. de Vries, 'Intracelluläre Pangenesis,' Jena, 1880.