PART IEVOLUTION IN GENERAL
CHAPTER ITHE PRESENT CRISIS IN EVOLUTIONARY THOUGHT
Three prominent men, a scientist, a publicist, and an orator, have recently made pronouncements on the theory of Evolution. The trio, of course, to whom allusion is made, are Bateson, Wells, and Bryan. As a result of their utterances, there has been a general reawakening of interest in the problem to which they drew attention. Again and again, in popular as well as scientific publications, men are raising and answering the question: “Is Darwinism dead?” Manifold and various are the answers given, but none of them appears to take the form of an unqualified affirmation or negation. Some reply by drawing a distinction between Darwinism, as a synonym for the theory of evolution in general, and Darwinism, in the sense of the particular form of that theory which had Darwin for its author. Modern research, they assure us, has not affected the former, but has necessitated a revision of ideas with respect to the latter. There are other forms of evolution besides Darwinism, and, as a matter of fact, not Darwin, but Lamarck was the originator of the scientific theory of evolution. Others, though imitating the prudence of the first group in their avoidance of a categorical answer, prefer to reply by means of a distinction based upon their interpretation of the realities of the problem rather than upon any mere terminological consideration.
Of the second group, some, like Osborn, distinguish between thelawof evolution and the theoreticalexplanationsof this law proposed by individual scientists. The existence of the law itself, they insist, is not open to question; it is only with respect to hypotheses explanatory of the aforesaid law that doubt and disagreement exist. The obvious objection to such a solution is that, if evolution is really a law of nature, it ought to be reducible to some clear-cut mathematical formula comparable to the formulations of the laws of constant, multiple, and reciprocal proportion in chemistry, or of the laws of segregation, assortment, and linkage in genetics. Assuming, then, that it is a genuine law, how is it that today no one ventures to formulate this evolutional law in definite and quantitative terms?
Others, comprising, perhaps, a majority, prefer to distinguish between thefactand thecausesof evolution. Practically all scientists, they aver, agree in accepting evolution as an established fact; it is only with reference to the agencies of evolution that controversy and uncertainty are permissible. To this contention one may justly reply that, by all the canons of linguistic usage, a fact is an observed or experienced event, and that hitherto no one in the past or present has ever been privileged to witness with his senses even so elemental a phenomenon in the evolutionary process as the actual origin of a new and genuine organic species. If, however, the admission be made that the term “fact” is here used in an untechnical sense to denote an inferred event postulated for the purpose of interpreting certain natural phenomena, then the statement that the majority of modern scientists agree as to the “fact” of evolution may be allowed to stand, with no further comment than to note that the formidable number and prestige of the advocates fail to intimidate us. Considerations of this sort are wholly irrelevant, for in science no less than in philosophy authority is worth as much as its arguments and no more.
The limited knowledge of the facts possessed by the biologists of the nineteenth century left their imaginations perilously unfettered and permitted them to indulge in a veritable orgy of theorizing. Now, however, that the trail blazed by the great Augustinian Abbot, Mendel, has been rediscovered, work of real value is being done with the seed pan, the incubator, the microtome, etc., and the wings of irresponsible speculation are clipped. Recent advances in this new field of Mendelian genetics have made it possible to subject to critical examination all that formerly went under the name of “experimental evidence” of evolution. Even with respect to the inferential or circumstantial evidence from palæontology, the enormous deluge of fossils unearthed by the tireless zeal of modern investigators has annihilated, by its sheer complexity, the hasty generalizations and facile simplifications of a generation ago, forcing the adoption of a more critical attitude. Formerly, a graded series of fossil genera sufficed for the construction of a “palæontological pedigree”; now, the worker in this field demands that the chain of descent shall be constructed with species, instead of genera, for links—“Not till we have linked species into lineages, can we group them into genera.” (F. A. Bather,Science, Sept. 17, 1920, p. 264.) This remarkable progress in scientific studies has tended to precipitate the crisis in evolutionary thought, which we propose to consider in the present chapter. Before doing so, however, it will be of advantage to formulate a clear statement of the problem at issue.
Evolution, or transformism, as it is more properly called, may be defined as the theory which regards the present species of plants and animals as modified descendants of earlier forms of life. Nowadays, therefore, the principal use of the term evolution is to denote the developmental theory of organic species. It is, however, a word of many senses. In the eighteenth century, for example, it was employed in a sense at variance with the present usage, that is, to designate the non-developmental theory of embryological encasement or preformation as opposed to the developmental theory of epigenesis. According to the theory of encasement, the adult organism did not arise by the generation of new parts (epigenesis), but by a mere “unfolding” (evolutio) of preëxistent parts. At present, however, evolution is used as a synonym for transformism, though it has other meanings, besides, being sometimes used to signify the formation of inorganic nature as well as the transformation of organic species.
Evolution, in the sense of transformism, is opposed to fixism, the older theory of Linné, according to whom nospecificchange is possible in plants and animals, all organisms being assumed to have persisted in essential sameness of type from the dawn of organic life down to the present day. The latter theory admits the possibility of environmentally-induced modifications, which are non-germinal and therefore non-inheritable. It also admits the possibility of germinal changes of the varietal, as opposed to the specific, order, but it maintains that all such changes are confined within the limits of the species, and that the boundaries of an organic species are impassable. Transformism, on the contrary, affirms the possibility of specific change, and assumes that the boundaries of organic species have actually been traversed.
What, then, is an organic species? It may be defined as a group of organisms endowed with thehardihoodnecessary to survive and propagate themselves under natural conditions (i.e.in the wild state), exhibiting a common inheritable type, differing from one another by no major germinal difference, perfectly interfertile with one another, butsexually incompatiblewith members of an alien specific group, in such wise that they produce hybrids wholly, or partially, sterile, when crossed with organisms outside their own specific group.
David Starr Jordan has wisely called attention to the requisite of viability and survival under natural conditions. “A species,” he says, “is not merely a form or group of individuals distinguished from other groups by definable features. A complete definition involves longevity. A species is a kind of animal or plant which has run the gauntlet of the ages andpersisted.... A form is not a species until it has ‘stood.’” (Science, Oct. 20, 1922, p. 448.)
Sexual (gametic) incompatibility as a criterion of specific distinction, presupposes the bisexual or biparental mode of reproduction, namely, syngamy, and is therefore chiefly applicable to the metista, although, if the view tentatively proposed by the protozoölogist, E. A. Minchin, be correct, it would also be applicable to the protista. According to this view, no protist type is a true species, unless it is maintained by syngamy (i.e.bisexual reproduction)—“Not until syngamy was acquired,” says Minchin, “could true species exist among the Protista.” (“An Introduction to the Study of the Protozoa,” p. 141.)
To return, however, to the metista, the horse (Equus caballus) and the ass (Equus asinus) represent two distinct species under a common genus. This is indicated by the fact that the mule, which is the hybrid offspring of their cross, is entirely sterile, producing no offspring whatever, when mated with ass, horse, or mule. Such total sterility, however, is not essential to the proof of specific differentiation; it suffices that the hybrid be less fertile than its parents. As early as 1686, sterility (total or partial) of the hybrid was laid down by John Ray as the fundamental criterion of specific distinction. Hence Bateson complains that Darwinian philosophy flagrantly “ignored the chief attribute of species first pointed out by John Ray that the product of their crosses is frequently sterile in a greater or lesser degree.” (Science, Jan. 20, 1922, p. 58.)
Accordingly, the sameness of type required in members of the same species refers rather to the genotype, that is, the sum-total of internal hereditary factors latent in the germ, than to the phenotype, that is, the expressed somatic characters, viz. the color, structure, size, weight, and all other perceptible properties, in terms of which a given plant or animal is described. Thus it sometimes happens that two distinct species, like the pear-tree and the apple-tree, resemble each other more closely, as regards their external or somatic characters, than two varieties belonging to one and the same species. Nevertheless, the pear-tree and the apple-tree are so unlike in their germinal (genetic) composition that they cannot even be crossed.
According to all theories of transformism, new species arise through the transformation of old species, and hence evolutionists are at one in affirming the occurrence of specific change. When it comes, however, to assigning the agencies or factors, which are supposed to have brought about this transmutation of organic species, there is a wide divergence of opinion. The older systems of transformism, namely, Lamarckism and Darwinism, ascribed the modification of organic species to the operation of the external factors of the environment, while the later school of orthogenesis attributed it to the exclusive operation of factors residing within the organism itself.
Lamarckism, for example, made the formation of organs a response to external conditions imposed by the environment. The elephant, according to this view, being maladjusted to its environment by reason of its clumsy bulk, developed a trunk by using its nose to compensate for its lack of pliancy and agility. Here the use or function precedes the organ and molds the latter to its need. Darwinism agrees with Lamarckism in making the environment the chief arbiter of modification. Its explanation of the elephant’s trunk, however, is negative rather than positive. This animal, it tells us, developed a trunk, because failure to vary in that useful direction would have been penalized by extermination.
Wilson presents, in a very graphic manner, the appalling problem which confronts evolutionists who seek to explain the adaptations of organisms by means of environmental factors. Referring, apparently, to Henderson’s “Fitness of the Environment,” he says: “It has been urged in a recent valuable work ... that fitness is a reciprocal relation, involvingthe environment no less than the organism. This is both a true and suggestive thought; but does it not leave the naturalist floundering amid the same old quicksands? The historical problem with which he has to deal must be grappled at closer quarters. He is everywhere confronted with specific devices in the organism that must have arisen long after the conditions of environment to which they are adjusted. Animals that live in water are provided with gills. Were this all, we could probably muddle along with the notion that gills are no more than lucky accidents. But we encounter a sticking point in the fact that gills are so often accompanied by a variety of ingenious devices, such as reservoirs, tubes, valves, pumps, strainers, scrubbing brushes, and the like, that are obviously tributary to the main function of breathing. Given water, asks the naturalist, how has all this come into existence and been perfected? The question is an inevitable product of our common sense.” (Smithson. Inst. Rpt. for 1915, p. 405.)
Impressed with the difficulty of accounting for the phenomena of organic adaptation by means of the far too general and unspecific influence of the environment, the orthogenetic school of transformism inaugurated by Nägeli, Eimer, and Kölliker repudiated this explanation, and sought to explain organic evolution on the sole basis of internal factors, such as “directive principles,” or germinal determinants. According to this conception, the elephant first developed his trunk under the drive of some internal agency, and afterwards sought out an environment in which the newly-developed trunk would be useful. In other words, orthogenesis makes the organ precede the function, and is therefore the exact reverse of Lamarckism.
Evolutionists in general, as we have said, regard our present plants and animals as the modified progeny of earlier forms, understanding by “modified” that which is the product of a trans-specific, as distinguished from a varietal or intra-specific, change. To substantiate the claim that changes of specific magnitude have actually taken place, they appeal to two principal kinds of evidence, namely: (a) empirical evidence basedon such variations as are now observed to occur among living organisms; (b) inferential evidence, which aposterioristically deduces the common ancestry of allied organic types from their resemblances and their sequence in geological time. Hence, if we omit as negligible certain subsidiary arguments, the whole evidence for organic evolution may be summed up under three heads: (1) the genetic evidence grounded on the facts of variation; (2) the zoölogical evidence based on homology, that is, on structural resemblance together with all further resemblances (physiological and embryological), which such similarity entails; (3) the palæontological evidence which rests on the gradual approximation of fossil types to modern types, when the former are ranged in a series corresponding to the alleged chronological order of their occurrence in the geological strata. It is the bearing of recent genetical research upon the first of these three lines of evidence that we propose to examine in the present chapter, an objective to which a brief and rather eclectic historical survey of evolutionary thought appears to offer the easiest avenue of approach.
While many bizarre speculations on the subject of transformism had been hazarded in centuries prior to the nineteenth, the history of this conception, as a scientific hypothesis, dates from the publication of Lamarck’s “Philosophie Zoologique” in 1809. According to Lamarck, organic species are changed as a result of theindirectinfluence of the external conditions of life. A change in environment forces a change of habit on the part of the animal. A change in the animal’s habits results in adaptation, that is, in the development or suppression of organs through use or disuse. The adaptation, therefore, thus acquired was not directly imposed by the environment, but only indirectly—that is, through the mediation of habit. Once acquired by the individual animal, however, the adaptation was, so Lamarck thought, taken up by the process of inheritance and perpetuated by being transmitted to the animal’s offspring. The net result would be a progressive differentiation of species due to this indirect influence of a varying environment.
Such was the theory of Lamarck, and it is sound and plausible in all respects save one, namely, the unwarranted assumption that acquired adaptations are inheritable, since these, to quote the words of the Harvard zoölogist, G. H. Parker, “are as a matter of fact just the class of changes in favor of the inheritance of which there is the least evidence.” (“Biology and Social Problems,” 1914, p. 103.)
The next contribution to the philosophy of transformism was made by Charles Darwin, when, in the year 1859, he published his celebrated “Origin of Species.” In this work, the English naturalist bases the evolution of organic species upon the assumed spontaneous tendency of organisms to vary minutely from their normal type in every possible direction. This spontaneous variability gives rise to slight variations, some of which are advantageous, others disadvantageous to the organism. The enormous fecundity of organisms multiplies them in excess of the available food supply, and more, accordingly, are born than can possibly survive. In the ensuing competition or struggle for existence, individuals favorably modified survive and propagate their kind, those unfavorably modified perish without progeny. This process of elimination Darwin termed natural selection. Only individuals favored by it were privileged to propagate their kind, and thus it happened that these minute variations of a useful character were seized upon and perpetuated “by the strong principle ofinheritance.” In this way, these slight but useful modifications would tend gradually to accumulate from generation to generation in the direction favored by “natural selection,” until, by the ensuing summation of innumerable minor differences verging in the same direction, a major difference would be produced. The end-result would be a progressivedivergenceof posterity from the common ancestral type, whence they originally sprang, ending in a multiplicity of new forms or species, all differing to a greater or lesser extent fromthe primitive type. The contrary hypothesis of a possibleconvergenceof two originally diverse types towards eventual similarity Darwin rejected as an extremely improbable explanation of the observed resemblance of organic forms, which, not without reason, he thought it more credible to ascribe to their assumed divergence from a common ancestral type.
Such was the scheme of evolution elaborated by Charles Darwin. His hypothesis leaves the origin of variations an unsolved mystery. It assumes what has never been proved, namely, the efficacy of “natural selection.” It rests on what has been definitely disproved by factual evidence, namely, the inheritability of the slight variations, now called fluctuations, which, not being transmitted even, by the hereditary process, cannot possibly accumulate from generation to generation, as Darwin imagined. Moreover, fluctuations owe their origin to variability in the external conditions of life (e.g.in temperature, moisture, altitude, exposure, soil, food, etc.), being due to thedirectinfluence or pressure of the environment, and not to any spontaneous tendency within the organism itself. Hence Darwin erred no less with respect to the spontaneity, than with respect to the inheritability and summation, of his “slight variations.”
The subsequent history of Lamarckian and Darwinian Transformism is briefly told. That both should pass into the discard was inevitable, but, thanks to repeated revisions undertaken by loyal adherents, their demise was somewhat retarded. In vain, however, did the Neo-Darwinians attempt to do for Darwinism what the Neo-Lamarckians had as futilely striven to do for Lamarckism. The revisers succeeded only in precipitating a lethal duel between these two rival systems, which has proved disastrous to both. The controversy begun in 1891 between Herbert Spencer and August Weismann marked the climax of this fatal conflict.
Spencer refused to see any value whatever in Darwin’s principle of natural selection, while other Neo-Lamarckians, less extreme, were content to relegate it to the status of a subordinate factor in evolution. Darwin had considered it “the most important means of modification,” but it is safe to say that no modern biologist attaches very much importance to natural selection as a means of accounting for the differences which mark off one species from another. In fact, if natural selection has enjoyed, or still continues to enjoy, any vogue at all, it is not due to its value in natural science (which, for all practical intents and purposes, is nil), but solely to its appeal as “mechanistic solution”; for nothing further is needed to commend it to modern thinkers infected with what Wasmann callsTheophobia. Natural selection, in making the organism a product of the concurrence of blind forces unguided by Divine intelligence, a mere fortuitous result, and not the realization of purpose, has furnished the agnostic with a miserable pretext for omitting God from his attempted explanation of the universe. “Here is the knot,” exclaims Du Bois-Reymond, “here the great difficulty that tortures the intellect which would understand the world. Whoever does not place all activity wholesale under the sway of Epicurean chance, whoever gives only his little finger to teleology, will inevitably arrive at Paley’s discarded ‘Natural Theology,’ and so much the more necessarily, the more clearly he thinks and the more independent his judgment.... The possibility, ever so distant, of banishing from nature its seeming purpose, and putting a blind necessity everywhere in the place of final causes, appears, therefore, as one of the greatest advances in the world of thought, from which a new era will be dated in the treatment of these problems. To have somewhat eased the torture of the intellect which ponders over the world-problem will, as long as philosophical naturalists exist, be Charles Darwin’s greatest title to glory.” (Darwin versus Galiani, “Reden,” Vol. I, p. 211.)
But however indispensable the selection principle may be to a philosophy which proposes to banish the Creator from creation, its scientific insolvency has become so painfully apparent that biologists have lost all confidence in its power to solvethe problem of organic origins. It is recognized, for example, that natural selection would suppress, rather than promote, development, seeing that organs have utility only in the state of perfection and are destitute of selection-value while in the imperfect state of transition. Again, the specific differences that diversify the various types of plants and animals are notoriously deficient in selection-value, and therefore the present differentiation of species cannot be accounted for by means of the principle of natural selection. Finally, unless one is prepared to make the preposterous assumption that the environment is a telic mechanism expressly designed for shaping organisms, he is under logical necessity of admitting that the influence of natural selection cannot be anything else than purely destructive. There is, as Wilson points out, no aprioristic ground for supposing that natural selection could do anything more than maintain thestatus quo, and as for factual proofs of its effectiveness in a positive sense, they are wholly wanting. Professor Caullery of the Sorbonne, in his Harvard lecture of Feb. 24, 1916, assures us that, “since the time of Darwin, natural selection has remained a purely speculative idea and that no one has been able to show its efficacy in concrete indisputable examples.”
Considerations of this sort induced not only Neo-Lamarckians, but many non-partisans as well, to take the field against the Darwinian Selection Principle. Thus Spencer’s caustic attack became a forerunner of others, and eminent biologists, like Fleischmann, Driesch, T. H. Morgan, and Bateson, have in turn poured the vials of their satire upon the attempts of Neo-Darwinians to rehabilitate the philosophy of natural selection. Wm. Bateson warns those, who persist in their credulity with reference to the Darwinian account of organic teleology, that they “will be wise henceforth to base this faith frankly on the impregnable rock of superstition and to abstain from direct appeals to natural fact.” This admonition forms the conclusion of a scathing criticism of what he styles the “fustian of Victorian philosophy.” “In the face of what weknow,” it runs, “of the distribution of variability in nature, the scope claimed for natural selection must be greatly reduced. The doctrine of the survival of the fittest is undeniable so long as it is applied to the organism as a whole, but to attempt by this principle to find value in all definiteness of parts and functions, and in the name of science to see fitness everywhere, is mere eighteenth century optimism. Yet it was in its application to the parts, to the details of specific difference, to the spots on the peacock’s tail, to the coloring of an orchid flower, and hosts of such examples, that the potency of natural selection was urged with greatest emphasis. Shorn of these pretensions the doctrine of the survival of favored races is a truism, helping scarcely at all to account for the diversity of species. Tolerance plays almost as considerable a part. By these admissions the last shred of that teleological fustian with which Victorian philosophy loved to clothe the theory of evolution is destroyed.” (Heredity, “Presidential Address to Brit. Ass’n. for Advanc. of Science,” Aug. 14, 1914.) Nor is this all. The Darwinian Selection Principle is reproached with having retarded the progress of science. It is justly accused of having discouraged profound and painstaking analysis by putting into currency its shallow and spurious solution of biological problems. “Too often in the past,” says Edmund Wilson, “the facile formulas of natural selection have been made use of to carry us lightly over the surface of unsuspected depths that would have richly repaid serious exploration.” (Smithson. Inst. Rpt. for 1915, p. 406.)
In retaliation for the destructive criticism of natural selection, the Neo-Darwinians have proceeded to pulverize the Lamarckian tenet concerning the inheritability of acquired adaptations. Weismann, having laid down his classic distinction between thesoma(comprising the vegetative or tissue cells in contact with the environment) and thegerm(i.e.the sequestered reproductive cells or gametes, which are sheltered from environmental vicissitudes), showed that the Lamarckian assumption that a change in the somatic cells (whichconstitute the organism of the individual) is registered in the germ cells (which constitute the vehicle of racial inheritance), is supported neither bya prioriprobability nor by any facts of observation. Germ cells give rise by division to somatic or tissue cells, but the converse is not true; for, once a cell has become differentiated and specialized into a tissue cell, it can never again give rise by division to germ cells, but only to other tissue cells of its own kind. Hence the possibility of a change in the tissue being transmitted to the germ has no antecedent probability in its favor. Neither is it grounded on the facts of observation. Bodily mutilations of the parent are not transmitted to the offspring. The child of a blacksmith is not born with a more developed right arm than that of a tailor’s child. When the ovaries from a white rabbit are grafted into a black rabbit, whose own ovaries have been previously removed, the latter, if mated to a white male, will produce spotlessly white young. Hence the offspring inherit the characters of the germ track of the white female, whence the ovaries were derived, without being influenced in the least by the pigmented somatic cells of the nurse-body (i.e.the black female), into which the ovaries were grafted. Kammerer’s experiments, in which young salamanders were found to exhibit at birth the coloration, which their parents had acquired through the action of sunlight, fail to convince, because, in this case, the bodies of the parents are not sufficiently impervious to light to preclude its direct action upon the gametes while in the reproductive organs of the parents. Hence we cannot be sure but that the coloration of the offspring derived from these gametes is due to the direct agency of sunlight rather than to the intermediate influence of the modified somatic cells upon the germ plasm.
The same objection holds true of the recent experiments, in which the germ cells have been modified by modifying the interior medium or internal environment by means of antibodies and hormones. No one doubts the possibility of influencing heredity by a direct modification of the germ cells,especially when, as is always the case in these experiments, the modification produced is destructive rather than constructive. The experiments, therefore, of Prof. M. F. Guyer of Wisconsin University, in which a germinally-transmitted eye defect was produced by injecting pregnant female rabbits with an antilens serum derived from fowls immunized to the crystalline lens of rabbits as antigen, are beside the mark. To demonstrate the Lamarckian thesis one must furnish evidence of a constructive addition to inheritance by means of prior somatic acquisition. The transmission of defects artificially produced is not so much a process of inheritance (transmission of type) as rather one of degeneracy (failure to equate the parental type).[1]Commenting on Guyer’s suggestion that an organism capable of producing antibodies that are germinally-destructive, may also be able to produce constructive bodies, Prof. Edwin S. Goodrich says: “The real weakness of the theory is that it does not escape from the fundamental objections we have already put forward as fatal to Lamarckism. If an effect has been produced, either the supposed constructive substance was present from the first, as an ordinary internal environmental condition necessary for the normal development of the character, or it must have been introduced from without by the application of a new stimulus. The same objection does not apply to the destructive effect. No one doubts that if a factor could be destroyed by a hot needle or picked out with a fine forceps the effect of the operation would persist throughout subsequent generations.” (Science, Dec. 2, 1921, p. 535.)
But in demonstrating against the Neo-Lamarckians that somatic modifications unrepresented in the germ plasm could have no significance in the process of racial evolution, Weismann hadproved too much. His argument was no less tellingagainst Darwinism than it was against Lamarckism. Darwin’s “individual differences” or “slight variations,” now spoken of as fluctuations, were quite as unrepresented and unrecorded in the germ cells as Lamarck’s “acquired adaptations.” There can be no “summation of individual differences” for the simple reason that fluctuations have no germinal basis and are therefore uninheritable—“We must bear in mind the fact,” says Prof. Edmund Wilson, “that Darwin often failed to distinguish between non-inheritable fluctuations and hereditary mutations of small degree.” (Smithson. Inst. Rpt. for 1915, p. 406.) Fluctuations, as we have seen, are due to variability in the environmental conditions,e.g.in access to soil nutrients, etc. As an instance of fluctuational variation the seeds of the ragweed may be cited. Normally these seeds have six spines, but around this average there is considerable fluctuation in individual seeds, some having as many as nine spines and others no more than one. Yet the plants reared from nine-spine seeds, even when similarly mated, show no greater tendency to produce nine-spine seeds than do plants reared from one-spine seeds.
To meet the difficulty presented by the non-inheritability of the Lamarckian adaptation and the Darwinian fluctuation, De Vries substituted for them those rare and abruptly-appearing inheritable variations, which he called mutations[2]and regarded as elementary steps in the evolutionary process. This new version of transformism was announced by De Vries in 1901, and more fully explained in his “Die Mutations-Theorie” (Leipzig, 1902-1903). Renner has shown that De Vries’ new forms of Œnothera were cases of complex hybridization rather than real mutants, as the forms produced by mutation are now called. Nevertheless, the work of Morgan, Bateson, and others leaves little doubt as to the actualoccurrence offactorialmutants, while Dr. Albert F. Blakeslee has demonstrated the existence ofchromosomalmutants. When unqualified, the term mutant usually denotes the factorial mutant, which arises from a change in one or more of the concatenated genes (hereditary factors) of a single chromosome (nuclear thread) in the germinal (i.e.gametic) complex. All such changes are called factorial mutations. They are hereditarily transmissible, and affect the somatic characters of the race permanently, although, in rare cases, such as that of the bar-eyed Drosophila mutant, the phenomenon ofreversionhas been observed. The chromosomal mutant, on the contrary, is not due to changes in the single factors or genes, but to duplication of one or more entire chromosomes (linkage-groups) in the gametic complex. Like the factorial mutant, it produces a permanent and heritable modification. The increase in nuclear material involved in chromosomal mutation (i.e.duplication) seems to cause a proportionate increase in the cytoplasmic mass of the single somatic cells, which manifests itself in the phenotype as giantism. De Vries’Œnothera gigasis a chromosomal mutant illustrative of this phenomenon. Besides the foregoing, there is thepseudomutantproduced by the factorial recombination, which results from acrossover,i.e.an exchange of genes or factors between two germinal chromosomes of the same synaptic pair. This reciprocal transfer of genes from one homologous chromosome to another happens, in a certain percentage of cases, during synapsis. The percentage can be artificially increased by exposing young female hybrids to special conditions of temperature.
If these new mutant forms could be regarded as genuine new species, then the fact that such variations are heritable and come within the range of actual observation, would constitute the long-sought empirical proof of the reality of evolution. Consciously or subconsciously, however, De Vries recognized that this was not the case; for he refers to mutants as “elementary species,” and does not venture to present them as authentic organic species.
The factorial mutant answers neither the endurance test nor the intersterility test of a genuine species. It would, doubtless, be going too far to regard all such mutant forms as examples of germinal degeneracy, but it cannot be denied that all of them, when compared to the wild type, are in the direction of unfitness, none of them being viable and prosperous under the severe conditions obtaining in the wild state. Bateson, who seems to regard all mutant characters as recessive and due to germinal loss, declares: “Even in Drosophila, where hundreds of genetically distinct factors have been identified, very few new dominants, that is to say positive additions, have been seen, and I am assured that none of them are of a class which could be expected to be viable under natural conditions. I understand even that none are certainly viable in the homozygous state.” (Toronto Address,Science, Jan. 20, 1922, p. 59.) “Garden or greenhouse products,” says D. S. Jordan, “are immensely interesting and instructive, but they throw little light on the origin of species. To call them species is like calling dress-parade cadets ‘soldiers.’ I have heard this definition of a soldier, ‘one that has stood.’ It is easy to trick out a group of boys to look like soldiers, but you can not define them as such until they have ‘stood.’” (Science, Oct. 20, 1922.) In a word, factorial mutants, owing, as they do, their survival exclusively to the protection of artificial conditions, could never become the hardy pioneers of new species.
Bateson insists that the mutational variation represents a change of loss. “Almost all that we have seen,” he says, “are variations in which we recognize that elements have been lost.” (Science, Jan. 20, 1922, p. 59.) In his Address to the British Association (1914), he cites numerous examples tending to show that mutant characters are but diminutions or intensifications of characters pre-existent in the wild or normal stock, all of which are explicable as effects of the loss (total or partial) of either positive, or inhibitive (epistatic) hereditary factors (genes). One of these instances illustrating thesubtractive nature of the factorial mutation is that of the Primula “Coral King,” a salmon-colored mutant, which was suddenly given off by a red variety of Primula called “Crimson King.” Such a mutation is obviously based on the loss of a germinal factor for color. The loss, however, is sometimes partial rather than total, as instanced in the case of the purple-edged Picotee sweet pea, which arose from the wholly purple wild variety by fractionation of the genetic factor for purple pigment. Even where the mutational variation appears to be one of gain, as happens when a positive character appearsde novoin the phenotype, or when a dilute parental character is intensified in the offspring, it is, nevertheless, interpretable as a result of germinal loss, the loss, namely, total or partial, of a genetic inhibitor. Such inhibitive genes or factors are known to exist. Bateson has shown, for example, that the whiteness of White Leghorn chickens is due, not to the absence of color-factors, but to the presence of a genetic inhibitor—“The white of White Leghorns,” he says, “is not, as white in nature often is, due to the loss of the color elements, but to the action of something which inhibits their expression.” (Address to the Brit. Ass’n., Smithson. Inst. Rpt. for 1915, p. 368.) Thus the sudden appearance in the offspring of a character not visibly represented in the parents may be due, not to germinal acquisition, but the loss of an inhibitory gene, whose elimination allows the somatic character previously suppressed by it to appear. Hence Bateson concludes: “In spite of seeming perversity, therefore, we have to admit that there is no evolutionary change which in the present state of our knowledge we can positively declare to be not due to loss.” (Loc. cit., p. 375.)
Another consideration, which disqualifies the factorial mutant for the rôle of a new species, is its failure to pass the test of interspecific sterility. When individuals from two distinct species are crossed, the offspring of the cross is either completely sterile, as instanced in the mule, or at least partially so. But when, for example, the sepia-eyed mutant of the vinegar fly isback-crossed with the red-eyed wild type, whence it originally sprang, the product of the cross is a red-eyed hybrid, which is perfectly fertile with other sepia-wild hybrids, with wild flies, and with sepia mutants. This proves that the sepia-eyed mutant has departed, so to speak, only a varietal, and not a specific, distance away from the parent stock. Ordinary or factorial mutation does not, therefore, as De Vries imagined, produce new species. These mutants do, indeed, meet the requirement of permanent transmissibility, for their distinctive characters cannot be obliterated by any amount of crossing. Nevertheless, the factorial mutation falls short of being an empirical proof of evolution, because it is a varietal, and not a specific, change. In other words, factorial mutants are new varieties and not new species. Only a heritable change based on germinal acquisition of sufficient magnitude to produce gametic incompatibility between the variant and the parent type would constitute direct evidence of the transmutation of species, provided, of course, that the variant were also capable of survival under the natural conditions of the wild state.
In his Toronto address of December 28, 1921, Wm. Bateson announced the failure of De Vries’ Mutation Theory, when he said: “But that particular and essential bit of the theory of evolution, which is concerned with the origin and nature of species remains utterly mysterious. We no longer feel as we used to do, that the process of variation, now contemporaneously occurring, is the beginning of a work which needs merely the element of time for its completion; for even time cannot complete that which has not yet begun. The conclusion in which we were brought up that species are a product of a summation of variations ignored the chief attribute of species first pointed out by John Ray that the product of their crosses is frequently sterile in greater or less degree. Huxley, very early in the debate, pointed out this grave defect in the evidence, but before breeding researches had been made on a large scale no one felt the objection to be serious. Extendedwork might be trusted to supply the deficiency. It has not done so, and the significance of the negative evidence can no longer be denied....
“If species have a common origin where did they pick up the ingredients which produce this sexual incompatibility? Almost certainly it is a variation in which something has been added. We have come to see that variations can very commonly—I do not say always—be distinguished as positive and negative.... Now we have no difficulty in finding evidence of variation by loss, but variations by addition are rarities, even if there are any such which must be so accounted. The variations to which interspecific sterility is due are obviously variations in which something is apparently added to the stock of ingredients. It is one of the common experiences of the breeder that when a hybrid is partially sterile, and from it any fertile offspring can be obtained, the sterility, once lost, disappears. This has been the history of many, perhaps most, of our cultivated plants of hybrid origin.
“The production of an indubitably sterile hybrid from completely fertile parents which has arisen under critical observation is the event for which we wait. Until this event is witnessed, our knowledge of evolution is incomplete in a vital respect. From time to time such an observation is published, but none has yet survived criticism.” (Science, Jan. 20, 1922, pp. 58, 59.)
But what of the chromosomal mutant? For our knowledge of this type of mutation we are largely indebted to Blakeslee’s researches and experiments on the Jimson weed (Datura stramonium). According to Blakeslee, chromosomal mutants result from duplication, or from reduction, of the chromosomes, and they are classified asbalancedorunbalancedtypes according as all, or only some, of the chromosomal linkage-groups are similarly doubled or reduced. If only one of the homologous chromosomes of a synaptic pair is doubled, the mutant is termed atriploidform. It is balanced when one homologous chromosome is doubled in every synapticpair, but if one or more chromosomes be added to, or subtracted from, this balanced triploid complex, the mutant is termed an unbalanced triploid. When all the chromosomes of the normal diploid complex are uniformly doubled, we have a balancedtetraploidrace. The subtraction or addition of one or more chromosomes in the case of a balanced tetraploid complex renders it an unbalanced tetraploid mutant. The retention in somatic cells of the haploid number of chromosomes characteristic of gametes and gametophytes gives a balancedhaploidmutant, from which hitherto no unbalanced haploids have been obtained. The normal diploid type and the balanced tetraploid type are said to constitute anevenbalance, while balanced triploids and haploids constitute anoddbalance. The odd balances and all the unbalanced mutants are largely sterile. Thus, for example, more than 80% of the pollen of the haploid mutant is bad. “The normal Jimson Weed,” says Blakeslee, “is diploid (2n) with a total of 24 chromosomes in somatic cells. In previous papers the finding of tetraploids (4n) with 48 chromosomes and triploids (3n) with 36 was reported, as well as unbalanced mutants with 25 chromosomes represented by the formula (2n + 1). The finding of two haploid or 1n plants, which we are now able to report, adds a new chromosomal type to the balanced series of mutants inDatura. This series now stands: 1n, 2n, 3n, 4n. Since a series of unbalanced mutants has been obtained from each of the other balanced types by the addition or subtraction of one or more chromosomes, it is possible that a similar series of unbalanced mutants may be obtainable from our new haploid plants, despite the great unbalance which would thereby result.” (Science, June 16, 1923, p. 646.) The haploid mutant, of which Blakeslee speaks, has, of course, 12 unpaired chromosomes in its somatic cells.
The balanced triploid is, like the haploid mutant, largely sterile, and is only obtainable by crossing the tetraploid race with the normal diploid plant. Since, then, the product of the cross of the diploid and tetraploid races is sterile, thetetraploid race fulfills the sterility test of a distinct species. Whether or not it fulfills the endurance test of survival under natural condition is doubtful, inasmuch as diploid Daturas are about three times as prolific as the tetraploid race. Moreover, as Blakeslee himself confessed in a lecture at Woods Hole attended by the present writer in the summer of 1923, the origin of a balanced tetraploid form from the normal diploid type by simultaneous duplication of all the chromosomes in the diploid complex, is an event that has yet to be witnessed. Nor is any gradual transition from the diploid to the tetraploid race, by way of unbalanced types and triploids, conceivable, seeing that such forms are too sterile to maintain themselves, and are, in fact, incapable of transmitting their own type in the absence of artificial intervention. There are, it is true, some instances, in which diploid and tetraploid races and species occur together in cultivation and in nature. In certain cases, this tetraploidy is merely apparent, being due to fragmentation of the chromosomes; in other cases, it is really due to chromosomal duplication, giving rise to genuine tetraploid forms. The question is often hard to decide, the mere number of the chromosomes being not, in itself, a safe criterion. Of the actual origin, however, of tetraploid from diploid races we have as yet no observational evidence. Hence Blakeslee’s researches on the chromosomal mutant have so far failed to furnish experimental proof of the origin of a genuine new species. Besides, waiving all other considerations, the limits within which chromosomal duplication is possible are of necessity so narrow, that, at best, this phenomenon can only be invoked to explain a very small range of variation. In fact, it is doubtful whether haploidy, triploidy, and tetraploidy have any important bearing whatever upon the problem of the origin of species. (SeeAddenda.)
The mutation, then, in so far as we have experimental knowledge of it, does not fulfill requirements of a specific change. It cannot even be regarded as anelementary stepin the direction of such a change. With this admission, De-Vriesianism becomes obsolete, descending like its predecessors, Lamarckism and Darwinism, into the charnel-house of discarded systems whose value is historic, but no longer scientific. When we enquire into the reason of this common demise of all the classic systems of transformism, we find it to reside in the progress of the new science of Mendelian genetics, whose foundations were laid by an Augustinian monk of the nineteenth century. Six years after the appearance of Darwin’s “Origin of Species,” Gregor Johann Mendel published a short paper entitled “Versuche über Pflanzen-hybriden,” which, unnoticed at the time by a scientific world preoccupied with Darwinian fantasies, was destined, on its coming to light at the beginning of the present century, to administer the finalcoup de graceto all the elaborate schemes of evolution that had preceded or followed its initial publication. It took half a century, however, before the dust of Darwinian sensationalism subsided sufficiently, to permit the “rediscovery” of Mendel’s solid and genuine contribution to biological science. But the Prälat of the abbey at Brünn never lived to see the day of his triumph. The true genius of his century, he died unhonored and unsung, a pretender being crowned in his stead. For Coulter says of Darwin: “He died April 19, 1882, probably the most honored scientific man in the world.” (Evolution, 1916, p. 35.)
Within the small dimensions of the paper, of which we have spoken, Mendel had compressed the results of years of carefully conceived and accurately executed experimentation reduced to precise statistical form and interpreted with a penetrating sagacity of the highest order. It is no exaggeration to say that his discovery has revolutionized the science of biology, giving it, for the first time, mathematical formulas comparable to those of chemistry. His two laws of inheritance, namely, the law of segregation and the law of independent assortment of characters, have, as previously intimated, become the basis of the new science of Genetics. His analysis of biparental reproduction has interpreted for us thecytological phenomena of synapsis, meiosis, and syngamy, has explained for us the instability of hybrids, has placed Weismann’s speculations concerning the autonomy and continuity of the germ plasm on a firm basis of experimental fact, has clarified all our notions respecting the mode and range of hereditary transmission, and has, in a word, opened our eyes to that new and hitherto unexplored realm of nature which Bateson calls “the world of gametes.”
Efforts have been made to construct systems of transformism along Mendelian lines, but none of them has met with notable success. Lotsy, for example, sought to explain all variation on the basis of the rearrangement of preëxistent genetic factors brought about by crossing. But such a solution of the problem is very unsatisfactory. In the first place, the generality of hybrid (heterozygous) forms are ruled out on the score of instability. The phenotype of hybrids is directly dependent, not on the genes themselves, but on the diploid combination of genes contained in the zygote. This combination, however, is always dissolved in the process of gamete-formation, by the segregative reduction division which occurs in the reproductive organs of the hybrid. Hybrids, therefore, do notbreed true, if propagated by sexual reproduction. To maintain constancy of type in hybrids, one must resort to somatogenic reproduction (i.e.vegetative growth from stems, etc.). Certain violets, in fact, as well as blackberries, are maintained in a state of constant hybridism by means of this sort of reproduction, even in nature. In the case ofbalanced lethals(i.e.factors causing death in the pure or homozygous state), the hybrid phenotype may be maintained even by sexual reproduction, inasmuch as all the pure (homozygous) offspring are non-viable. Two lethals are said to be balanced, when they occur, the first in one and the second in the other homologous chromosome of the same synaptic pair. “Such a factorial situation would maintain a state of constant heterozygosis, the fixed hybridism of an impure species ... the hybrid will breed true until the relative position of thelethals are changed by a crossover, or the genetical constitution in these respects is altered by a mutation.” (Davis,Science, Feb. 3, 1922, p. 111.) As is evident, however, the condition of balanced lethals involves a considerable reduction in fertility.
Hybridization, moreover, is successful between varieties of the same species rather than between distinct species. Interspecific crosses are in some cases entirely unproductive, in other cases productive of wholly-sterile, hybrids, and in still other cases productive of semisterile hybrids. When semisterile hybrids are obtainable from an interspecific cross, the phenotype can be kept constant by somatogenic reproduction, but, as we shall see in a later chapter, this kind of reproduction does not counteract senescence, and stock thus propagated usually plays out within a determinate period. Finally, the mixture of incompatible germinal elements involved in an interspecific cross tends to produce forms, which are subnormal in their viability and vitality. The conclusions of Goodspeed and Clausen are the following: “(1) As a consequence of modern Mendelian developments, the Mendelian factors may be considered as making up a reaction system, the elements of which exhibit more or less specific relations to one another; (2) strictly Mendelian results are to be expected only when the contrast is between factor differences within a common Mendelian reaction system as is ordinarily the case in varietal hybrids; (3) when distinct reaction systems are involved, as in species crosses, the phenomena must be viewed in the light of a contrast between systems rather than between specific factor differences, and the results will depend upon the degree of mutual compatibility displayed between the specific elements of the two systems.” (Amer. Nat., 51 (1917), p. 99.) To these conclusions may be added the pertinent observation of Bradley Moore Davis: “Of particular import,” he says, “is the expectation that lethals most frequently owe their presence to the heterozygous condition since the mixing of diverse germ plasms seems likely to lead to the breaking down of delicateand vital adjustments in proportion relative to the degree of protoplasmic confusion, and this means chemical and physical disturbance.” (Science, Feb. 3, 1923, p. 111.)
But crossing produces, in the second filial generation (F2), pure (homozygous) as well as hybrid (heterozygous) forms.⅖ In some cases these pure forms are new, the phenotype being different from that of either pure grandparent. Such a result is produced byrandom assortmentof the chromosomes in gamete and zygote formation, and occurs when the genes for two or more pairs of contrasted characters are located in different chromosome pairs. The phenomenon is formulated in Mendel’s Second Law, the law of independent assortment. The novelty, however, of the true-breeding forms thus produced is not absolute, but relative. There is no origination of new hereditary factors. It is simply a recombination of the old genes of different stocks, the genes themselves undergoing no intrinsic alteration. The combination is new, but not the elements combined. In addition to chromosomal recombination, we have factorial recombination by means of crossovers. This, too, can produce new and true-breeding forms of a fixed nature, but here, likewise, it is the combination, and not the elements combined, which is new. The “new” forms thus produced are called, as we have seen, pseudomutants. When pseudomutations, that is, crossovers, occur in conjunction with the condition of balanced lethals, they closely simulate genuine factorial mutations. This is exemplified in the case of De Vries’Œnothera Lamarckiana, which is the product of a crossover supervening upon a situation of balanced lethals. In cases of this kind, the crossover releases hitherto suppressed recessive characters, giving the appearance of real mutation. “The workers with Drosophila,” says Davis, “seem inclined to believe that much of the phenomena simulating mutation in their material is in reality the appearance of characters set free by the breaking of lethal adjustments which held the characters latent. Well-known workers have arrived at similar conclusions forŒnotheramaterial and are not content toaccept as evidence of mutations the behavior ofLamarckianaand some other forms when they throw their marked variants.” (Science, Feb. 3, 1922, p. 111.)
The new forms, however, resulting from random assortment and crossovers cannot be regarded as new species. “Analysis,” says Bateson, “has revealed hosts of transferable characters. Their combinations suffice to supply in abundance series of types which might pass for new species, and certainly would be so classed if they were met with in nature. Yet critically tested, we find that they are not distinct species and we have no reason to suppose any accumulation of characters of the same order would culminate in the production of distinct species. Specific difference therefore must be regarded as probably attaching to the base upon which these transferables are implanted, of which we know absolutely nothing at all. Nothing that we have witnessed in the contemporary world can colorably be interpreted as providing the sort of evidence required.” (Science, Jan. 20, 1922, pp. 59, 60.)
Anyone thoroughly acquainted with the results of genetical analysis and research will find it impossible to escape the conviction that there is no such thing as experimental evidence for evolution. In spite of the enormous advances made in the fields of genetics and cytology, the problem of the origin of species is, scientifically speaking, as mysterious as ever. No variation of which we have experience is interpretable as the transmutation of a specific type, and David Starr Jordan voices an inevitable conclusion when he says: “None of the created ‘new species’ of plant or animal I know of would last five years in the open, nor is there the slightest evidence that any new species of field or forest or ocean ever originated from mutation, discontinuous variation, or hybridization.” (Science, Oct. 20, 1922, p. 448.)
“In any case,” as Professor Caullery tells us in his Harvard lecture on the “Problem of Evolution,” “we do not see in the facts emerging from Mendelism, how evolution, in the sense that morphology suggests, can have come about. And itcomes to pass that some of the biologists of greatest authority in the study of Mendelian heredity are led, with regard to evolution, either to a more or less complete agnosticism, or to the expression of ideas quite opposed to those of the preceding generation; ideas which would almost take us back to creationism.” (Smithson. Inst. Rpt. for 1916, p. 334.) It is, of course, impossible within the limits of a single chapter to convey any adequate impression of all that Mendel’s epoch-making achievement portends, but what has been said is sufficient to give some idea of the acuteness of the crisis through which the theory of organic evolution is passing as a result of his discovery. In its classic forms of Lamarckism, Darwinism and De-Vriesianism, the survival of the theory is out of the question. Whether or not it can be rehabilitated in any form whatever is a matter open to doubt. Transfixed by the innumerable spears of modern objections, its extremity calls to mind the plight of the Lion of Lucerne. Possibly, it is destined to find a rescuer in some great genius of the future, but of one thing, at least, we may be perfectly certain, namely, that, even if rejuvenated, it will never again resume the lineaments traced by Charles Darwin. In the face of this certainty, it is almost pitiful to hear the die-hards of Darwinism bolstering up a lost cause with the wretched quibble that, though natural selection has been discredited as an explanation of the differentiation of species, Darwinism “in its essentials” survives intact. For, if there is any feature which, beyond all else, deserves to be called an essential of Darwin’s system, surely it is natural selection. For Darwin it was “the most important” agency of transformation (cf. “Origin of Species,” 6th ed., p. 5). Apart from his hypothesis of the summation through inheritance of slight variations (“fluctuations”), now completely demolished by the new science of genetics, it represented his sole contribution to the philosophy of transformism. It alone distinguishes Darwinism from Lamarckism, its prototype. Without it the “Origin of Species” would be Hamlet without the Prince of Denmark. With itDarwin’s fame should stand or fall. Therefore, since Darwin erred in making it “the most important means of modification,” Darwinism is dead, and no grief of mourners can resuscitate the corpse. “Through the last fifty years,” says Bateson, “this theme of the natural selection of favored races has been developed and expounded in writings innumerable. Favored races certainly can replace others. The argument is sound, but we are doubtful of its value. For us that debate stands adjourned. We go to Darwin for his incomparable collection of facts. We would fain emulate his scholarship, his width, and his power of exposition, but to us he speaks no more with philosophical authority. We read his scheme of evolution as we would those of Lucretius or of Lamarck, delighting in their simplicity and their courage.” (Heredity, Presid. Add. to British Assoc. for Advanc. of Science, Smith. Inst. Rpt. for 1915, p. 365.)