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, the tetraploid 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. (See [Addenda].)
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 an elementary step in 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 final coup de grace to 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 the cytological 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 not breed 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 of balanced 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 the lethals 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 delicate and 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 by random assortment of 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 for Œnothera material and are not content to accept as evidence of mutations the behavior of Lamarckiana and 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 it comes 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 it Darwin’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.)