Russel Wallace : Alfred Russell Wallace (sic)
As an example of this latter phase of opinion, we have the work of Dr. Rudolf Otto, Professor of Theology in the University of Göttingen, said to be in the front rank of the literature to which it belongs. According to a reviewer of this book in the Inquirer (of April 6th, 1907), the author, after a fair statement of the Darwinian theory, goes on to say:
The modern prevailing view in contrast is that these chance individual variations play but a minor part in the production of new species. New species spring from old species by a disturbance of general vital equilibrium, from which a new state results immediately. . . The struggle for existence is an unfavourable, not an advantageous factor, since it operates to prevent new developments, and the new arises, not where the struggle is severe, but where it is weak. The theory of the building up of sporadic variations is thus giving place to the theory of the development of inherent organic tendencies and characters, which are neither produced by nor dependent upon environment, but often assert themselves against it.This is the reviewer's summary, and he then gives us Dr. Otto's conclusion, as being, that Darwinism is "an unsuccessful hypothesis."
To turn from a German theologian to an English pressman, we find in the Sheffield Daily Independent of July 3rd a lengthy article [[p. 130]] on the Darwinian Jubilee of the Linnean Society, the larger portion of which consists of "Objections," among which we find such statements as the following: "The comparatively recent science of 'Heredity' has raised insuperable objections to the Darwinian assumption regarding the alleged transmission of variations"; and then, after explaining the phenomena of "Mendelism," he asks: "Where does natural selection come in then? Nobody knows exactly where. . . but the whole question of the origin of species is as much a mystery to-day as when Sir John Herschel called it 'the mystery of mysteries'." I may state in passing that the first of the above statements is simply untrue, since the "transmission" of the variations on which Darwinism rests is not an assumption, but a universally admitted fact.
Such statements as these have been going the round of the Press for the last few years, and there is probably no newspaper or magazine reader in the country who has not met with several of the same character, many of which are even more impressive, because they quote the names of British and Continental biologists, some of them professors in universities or colleges, as authorities for some of the statements made. Such readers are naturally impressed with the apparent weight of these criticisms; and as they are quite unable to detect the mis-statements or the misinterpretation of facts, and are quite unaware of the ever-growing mass of evidence and of the more weighty scientific opinion on the other side, even well-educated people are acquiring very erroneous ideas on this subject. I therefore propose now to give a short outline sketch of the theories which are claimed to be, in whole or in part, a substitute for Darwin's explanation of organic evolution by means of Natural Selection. They are commonly known as those of the Neo-Lamarckists, the Mutationists, and the Mendelians.
The great French zoologist, Lamarck, was the first who, a century ago, proposed any detailed scientific theory of the origin of the various species of organisms by a natural process of modification; but his views were never widely adopted by naturalists, because it was clear that they would not account for all the facts, especially those of adaptation to the infinitely varied conditions of existence. This theory was, briefly, that every organ or part used in satisfying a creature's wants was increased in strength or size, or otherwise modified by use and effort, that the modifications thus produced were transmitted to their offspring, and thus led in the course of time to the production of the diverse forms we see everywhere in nature. In the case of plants, it was more especially the direct influence of the environment, heat or cold, calms or storms, a moist or dry atmosphere, a rocky or an alluvial soil, that caused the modifications of form and structure, the transmission of which in time produced new species. The author tells us that otters, beavers, water-fowl, frogs [[p. 131]] and turtles were not made web-footed in order that they might swim; but, their wants having attracted them to the water in search of food, they stretched out the toes of their feet to strike the water and more rapidly move along its surface; and thus, in course of time, the broad membranes that now connect their toes were produced.
But it was soon observed by zoologists that many of the characters of animals could not possibly have been produced by use or disuse. How, for instance, could hair or wool be so produced, how could hair change into the prickles of the hedgehog, the erectile spines of the porcupine, the nose-horn of the rhinoceros? How could the hard wing-covers of beetles be modified in a thousand ways into polished or rugged surfaces, often with knobs or even spines so closely resembling those of the bark on which they rest as to serve obviously for concealment? How could any possible use of the wing-coverts of the peacock, the wings of the Argus-pheasant, or the scull-cap of the crown-pigeon or the umbrella-bird produce the wonderful developments of feathers, quite useless for either flight or covering, which we find in those birds? And, as a final example, how could the infinitely varied colours and patterns which we see upon the wings of butterflies or the feathers of birds have been possibly caused by the mere use of those organs; yet these colours are now known, in a vast number of cases, probably in all, to serve purposes of concealment, of warning, or of recognition, which are of real, often of supreme utility, but which could not possibly have been caused or even fixed in each species by any conceivable direct action, either of the organism itself or of its environment.
In the case of plants, the botanist found that the direct action of the causes adduced by Lamarck could not possibly have produced the infinite variety of structural diversities in leaf, in flower, and especially in fruit, that are everywhere to be met with.
Yet though Lamarckism had been dropped as inadequate for more than half a century, at a later period, after Darwin had shown how every difficulty it presented was obviated by his own theory of natural selection, the discarded view was revived, largely by the influence of the palæontologist Cope in America. This was, in part, due to the circumstance that one of Lamarck's fundamental assumptions--the inheritance of whatever changes were produced in the individual either by the use of its own organs or by the direct agency of the environment--was believed to be true, and was actually accepted by Darwin, though he always maintained that it had very little or no effect in producing modification of species. Cope supported Lamarckism for nearly a quarter of a century, and in his last work, "The Primary Factors of Organic Evolution," published in 1896, he tells us that "the stimuli of chemical and physical forces, and also molar motion, or use or its absence, are abundantly sufficient to produce variations of all kinds in organic beings." But as he deals [[p. 132]] solely with extinct animals, and carefully avoids any attempt to account for the phenomena of living things already referred to, the accuracy of the above statement can hardly be admitted.
It is a remarkable thing that so many American biologists had more or less adopted Cope's theory, to the extent at least of minimising the influence of natural selection, while none of them seem to have recognised the very small portion of the phenomena it can possibly be made to explain. But since Cope's death in 1897 a decided change of opinion has taken place, and some very valuable experiments and observations, showing the universal action of natural selection, have been carried out.
Perhaps the most important are those recorded in the fine volume by Mr. William Lawrence Tower, giving the results of many years' experiment and observation on the habits, increase and distribution of several species of Leptinotarsa, a Chrysomelid genus of beetles. Among his results we find it stated that "there exists at present not one single fact to show the inheritance of acquired somatic variations or their incorporation in the germ-plasm." This is a strong additional proof that the fundamental assumption of the Lamarckian theory--that such characters are inherited--is untrue; while the fact that not one satisfactory case of such inheritance has ever been proved, notwithstanding all the allegations of the Lamarckians, renders a theory based on such foundation absolutely untenable.
Mr. Tower's conclusion is thus stated: "I am, therefore, of the opinion that the evolution of the genus Leptinotarsa, and of animals in general, has been continuous and direct, developing new species in migrating races by direct response to the conditions of existence. In this evolution natural selection has acted to determine the persistence of new variations."
When we further consider that in every one of the numerous cases in which the Lamarckian theory wholly fails to account for the facts, that of Darwin fully explains them, and usually in their minutest details, we shall not be surprised that every close observer who makes himself acquainted with these facts by direct observation becomes absolutely certain of the complete breakdown of the former theory as elucidating the origin of species.
* * *
We have next to consider the more modern theory of Mutation, founded in 1901 by Dr. Hugo de Vries, of Amsterdam, in his work, entitled "Die Mutations Theorie." This consists of two large volumes devoted to the elaborate study of the various forms of a species of evening primrose (nothera Lamarckiana), which has run wild in Holland, as the common species . biennis has in England and elsewhere. By growing this plant from seed in large quantities [[p. 133]] for several years, a few individuals were found of such different appearance in foliage, mode of growth, size, &c., as to appear like distinct species. These are termed "mutations," and are said to come as true from seed as the parent plant usually does. De Vries, therefore, maintains that they are new species which he has actually seen produced; and from this experiment with a single species of plant, he comes to the tremendous conclusion that it is in this way that new species are produced, per saltum, not by the slower process of variation and selection as maintained by Darwin. Yet all his efforts to find a wild European plant behaving in the same way have been so far in vain.
It must be remembered that the genus nothera is almost peculiar to America, and is altogether absent from Europe and temperate Asia. Moreover, the plant named . Lamarckiana is totally unknown in America (or anywhere else) in a wild state, and is said to have originated in the Jardin des Plantes at Paris; it is, therefore, thought to be a hybrid between two or more species cultivated there, and it is known that hybrids and mongrels not unfrequently produce new and eccentric forms. There is, therefore, no proof whatever that in a state of nature such mutations are produced, except, perhaps, very rarely; while the assumption that they have been, and are, produced so frequently as to constitute the mode by which all existing species have come into existence is a most illogical conclusion to draw from the phenomena presented by one species of plant of totally unknown parentage!
To show what claims are made for the theory thus founded, I will refer to a recent article by Prof. A. A. W. Hubrecht, in the Popular Science Monthly. He maintains that De Vries' work proves that in nature new species never arise through fluctuating variability, but are exclusively due to "mutations." He tells us that, forty years after Darwin, "the birth of a new species has actually been observed by him"--referring only to the abnormal "sports" of the notheræ of unknown parentage, as already described. He declares that "as long as the mutation has not appeared there can be no origin of a new species; the species is constant, and only subject to fluctuating variability, which never leads to the formation of a species." He concludes with the statement that De Vries "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." These positive assertions as to what has occurred throughout the whole realm of organic nature in the whole course of its development rest wholly on experiments with one plant, although those experiments are rendered comparatively valueless owing to its not being itself a known wild species, but probably a hybrid. [[p. 134]] Was there ever such a mountain of theory reared upon such an almost infinitesimal basis of fact!
Let us now see what a few biologists of eminence think of these amazing claims. There is probably no greater living authority on the subject of the variations among wild and cultivated plants than Sir W. T. Thiselton Dyer, the late Director of Kew Gardens, and at the end of a careful discussion of this subject last year in Nature, he concludes with the following weighty remarks:
That mutations inevitably appear sooner or later under cultural conditions is not an assumption but a fact. If, as Mr. Lock seems to argue, there is an equal chance of their occurrence in either case, then their appearance should be more frequent in nature than in cultivation, because the former has a larger population to work with. But it is not so. I therefore conclude with Darwin that cultivation introduces some provocative condition which is lacking (or latent) in nature.In a previous letter he had shown that such mutations are unable to hold their own in nature, and are thus constitutionally unfitted to become new species. And this is just what we should expect. For these large accidental variations or sports, occurring rarely, would have enormous chances against their being in exact adaptation to the whole inorganic and organic environment at the time and in the place where they happen to appear. They would thus necessarily die out.
Turning to the animal world, Mr. Power found several examples of natural "mutants" during his long investigation of the genus Leptinotarsa, and he studied them in the same complete way as all other variations. His conclusion is "that all inheritable variations behave alike, and in no case is there any evidence of a fundamental difference between 'mutants' and any other heritable variations. . . There is then no necessary incongruity between gradual small variation and rapid large variation in the origin of species, but the two are the extremes of the same process."
This entirely cuts away the basis of the mutation theory. For as the fluctuating variations are enormously frequent and in every direction, while mutations are admittedly very rare, and in only one or very few directions in each case, it is clear that the former present materials for adaptation to changed conditions which the latter do not afford.
But perhaps the most complete proof of the absolute incompetence of the mutation theory is that adduced by Professor E. B. Poulton in the Introduction to his volume of Essays on Evolution, just published. This is afforded by the innumerable examples of Protective Resemblance and Mimicry, which form such a striking feature of the insects of every part of the world, but especially of the tropics. In many cases these resemblances in outward form, and especially in the exact patterns and colours of the wings of butterflies and moths, [[p. 135]] which are very different in structure, and often belong to widely separated groups--moths sometimes resembling butterflies, at other times wasps--are so accurate to the smallest detail that, as he remarks, it is as impossible that they can be produced by sudden mutation as that a piece of iron could be struck blindfold with a chisel and hammer so as to produce a key that would open an elaborate safety lock.
Only those who have seen these marvellous copies in nature or who have carefully studied them in collections or by means of coloured illustrations, can properly appreciate the force of this argument, but it may perhaps serve to show how wonderfully minute they are to state that even the late Professor Westwood had for some months in his cabinet a heteromerous beetle and one of the Cicindelidæ placed side by side as specimens of one species; while almost every collector in the tropics has occasionally captured and brought home as specimens of the same species, butterflies which, when closely examined, proved to belong not only to different genera, but often to distinct families.
In the Hope Museum, at Oxford, Professor Poulton has now got together an extensive collection, so arranged as to show these strange phenomena in their wonderful complexity and abundance. To anyone sufficiently interested in the problems here discussed it would be worth a journey to Oxford to see and carefully examine this unique collection under Professor Poulton's guidance. It would, I believe, completely set at rest any doubts he may have had as to the relative values of the Mutationist and the Darwinian theories in furnishing an adequate explanation of the specific modifications of plants and animals.
* * *
We now come to the third modern theory that is so generally supposed to have replaced Darwinism in the opinion of the majority of Evolutionists, and which is known as Mendelism. For the information of the general reader, it may be well here to state, as briefly and simply as possible, what Mendelism is.
J. G. Mendel, the son of well-to-do Silesian peasants, became a priest in 1847, studied physics and natural science at Vienna in 1851-53, then returned to his monastery at Brünn, of which he afterwards became Abbot. For many years he carried on systematic experiments on hybridisation in the monastery garden, and discovered a new law relating to certain crossed plants, now known as "Mendel's Law," an account of which he published (in two papers) in the Proceedings of the Natural History Society of Brünn, in 1866, where they remained buried till attention was called to them by several Continental botanists in 1900.
[[p. 136]] His chief experiments were with common garden peas, which, as most people know, are now usually classed in two distinct forms or varieties, those whose ripe seeds are yellow and those in which they are bluish-green. The former comprise the split-peas of grocers and many of the older and hardier sorts of garden peas, while the latter are known as marrow-fat peas, from their superior eating qualities. There are, however, many other distinctive characters, both in seed, in pod, in foliage, stature, &c. Each of these varieties, as is well known, when sown comes true to name.
But when two different varieties were crossed, Mendel found that in each case the hybrids were of one kind only. In the case of yellow and blue peas, for instance, the hybrids were all yellow. Hence in this pair he termed the yellow the "dominant" or (as Darwin would have termed it) the "prepotent" form. But the special discovery of Mendel was that when these yellow peas, the produce of the first cross, were grown by themselves, instead of producing all yellow peas, the result was both kinds in the proportion, approximately, of three yellow to one green, both colours occurring in the same pod. Then, in the third generation, another surprise occurred, for the "greens" bred true for several successive generations; but the yellows produced only one yellow which continued to breed true, while the other two broke up into yellows and greens in the same proportions as before. The final result of a large number of generations is found to be that yellows and greens, each breeding true, occur in equal proportions; whence it follows that what are termed "dominant" characters are not permanently so, but constitute a purely temporary feature which, after a large number of generations, has only a minute numerical superiority. The ultimate result of the Mendelian law seems to be a tendency to perpetuate certain pairs of characters without intermixture.
This fixity of character may at first sight be supposed to favour the evolution of a new species, as strongly urged by many of the Mendelians, but this will entirely depend on what is the nature of these characters. It is clear that if they are in the slightest degree, and in any way, injurious, they will interfere with adaptation, and therefore by the constant effect of natural selection will rapidly die out. Professor J. A. Thomson tells us that Mendelian phenomena have been observed in peas, mice, rabbits, poultry, snails, and several other plants and animals. But in every case one of the parents at least is either a sport which has occurred under domestication or a doubtful natural variety or "mutation." One of the mice is an albino; of the rabbits, albino or long-haired; of the poultry, a breed with abnormal combs; of the cattle, hornless, &c.; while of two European nettles (Urtica pilulifera and U. Dodartii) the latter is [[p. 137]] an almost entire-leaved form, apparently of unknown origin, and not admitted as a species either by Hooker or Nyman.1
It is evident that we have here to do with abnormal forms that rarely or never occur in nature as permanent self-sustaining species. The curious phenomena that present themselves when these are crossed with allied forms either domesticated or natural, though they may be of considerable interest as furnishing materials for the study of the theory of heredity, have absolutely nothing whatever to do with the origin or modification of species.
It must not be forgotten also that the results of Mendelian experiments are not constant, some observers obtaining different results with the same or very similar forms, of which Prof. Thomson gives many examples; and much ingenious reasoning is devoted to explaining (or explaining away) these diverse results.
Neither is the whole conception quite so novel as usually stated, since most of the facts were known to Darwin. He states that so long ago as 1729 it was observed that blue and white peas planted near each other mutually crossed, and in the Autumn peas of both colours were found in one pod, while none were of an intermediate colour. Wiegmann, Gärtner, and the Rev. J. M. Berkeley all repeated this experiment, and found the fact to be correct. ("Animals and Plants under Domestication," vol. I., p. 397) The fundamental fact of Mendelism, that certain characters in a few special cases do not blend when crossed, was not only well known to Darwin, but carefully discussed by him. He has an interesting section headed: On certain characters not blending, which begins thus:
When two breeds are crossed their characters usually become intimately fused together; but some characters refuse to blend, and are transmitted in an unmodified state, either from both parents or from one. When grey and white mice are paired the young are not piebald nor of an intermediate tint, but are pure white or of the ordinary grey colour; so it is when white and common turtledoves are paired. In game fowls, a great authority, Mr. J. Douglas, remarks: "I may here state a strange fact: that if you cross a black with a white game, you get birds of both breeds of the clearest colour." Sir R. Heron crossed during many years white, black, brown and fawn-coloured Angora rabbits, and never once got these colours mingled in the same animal, but often all four colours in the same litter. Additional cases could be given, but this form of inheritance is very far from being universal even with respect to the most distinct colours. When turnspit dogs and Ancon sheep, both of which have dwarfed limbs, are crossed with common breeds, the offspring are [[p. 138]] not intermediate in structure, but take after either parent. When tailless or hornless animals are crossed with perfect individuals it frequently, but by no means invariably, happens that the offspring are either perfectly furnished with these organs or are quite destitute of them.He then gives cases of partially intermediate forms when Dorking fowls, hairless dogs, or solid-hoofed pigs are crossed with normal types.
He then discusses similar phenomena in plants in which still more curious facts had been observed. Major Trevor Clarke crossed a small glabrous-leaved annual stock with pollen of a large, red-flowered, rough-leaved biennial stock, the seeds of which produced half glabrous and half rough-leaved forms, but none intermediate.
"In the succeeding generations raised from rough-leaved crossed seedlings some glabrous plants appeared, showing that the glabrous character, though incapable of blending with or modifying the rough leaves, was all the time latent in this family of plants. The numerous plants, formerly referred to, which I raised from reciprocal crosses between the peloric and common Antirrhinum, offer a nearly parallel case; for in the first generation all the plants resembled the common form, and in the next generation, out of one hundred and thirty-seven plants, two alone were in an intermediate condition, the others perfectly resembling either the peloric or the common form."In these last two cases we have the Mendelian law clearly brought out that the product of a first cross, presenting the characters of one of the parents only, produced seeds which, in the next generation, gave plants with the characters of the two original forms. If, therefore, Mendel had never made his experiments, or had never published them, here was the sufficient basis for what is now known as Mendelism, published by Darwin in 1868, and remaining practically unnoticed by the host of workers, some of whom declare Mendel's paper to be "the most important contribution of its size ever made to biological science."
The reason why Darwin did not prosecute the research further, so as to detect the numerical law of successive generations, is sufficiently shown in his closing remarks on the subject. In the first place, he was quite satisfied, from the large mass of facts he had accumulated during more than twenty years of research, that hybridisation or the intercrossing of very distinct forms had no place whatever in the natural process of species-formation. This is intimated by his concluding remarks to this section, which I will now quote:
"It is remarkable, as has been strongly insisted on by Isidore Geoffroy St. Hilaire in regard to animals, that the transmission of characters without fusion occurs most rarely when species are crossed; I know of one exception alone, namely, with the hybrids naturally produced between the common and hooded crow (Corvus corone and C. cornix), which, however, are closely allied species [[p. 139]] differing in nothing except colour.2 Nor have I met with any well-ascertained cases of transmission of this kind, even when one form is strongly prepotent over another, when two races are crossed which have been slowly formed by man's selection, and therefore resemble to a certain extent natural species. All the characters above enumerated, which are transmitted in a perfect state to some of the offspring and not to others--such as distinct colours, nakedness of skin, smoothness of leaves, absence of horns or tail, additional toes, pelorism, dwarfed structure, &c.--have all been known to appear suddenly in individual animals and plants. From this fact, and from the several slightly aggregated differences which distinguish domestic races and species from each other not being liable to this peculiar form of transmission, we may conclude that it is in some way connected with the sudden appearance of the characters in question." ("Animals and Plants," vol. ii. pp. 92-95.)It appears to me that in the three pages from which I have quoted, Darwin has given us a better idea of the real nature and bearing of Mendelian inheritance, as regards any possible influence on the problem of the origin of species, than any amount of study of the complex diagrams and tabular statements which the Mendelians are for ever putting before us with great flourish of trumpets and reiterated assertions of their importance.
As playing any essential part in the scheme of organic development, the phenomena seem to me to be of the very slightest importance. They arise out of what are essentially abnormalities, whether called varieties, "mutations," or sports. These abnormalities are very rare in a state of nature, as compared with the ever-present individual variability ample in amount and affecting every part and organ which furnishes the material for both man's and for nature's selection. The very fixity of these abnormalities and the small number of the characters affected by them, as well as their rarity, all show them to be the refuse material of nature's workshop, as proved by the fact that none of them ever maintain themselves in a state of nature. It seems to me that the most probable interpretation of the phenomena is, that both the fixity and the rarity of such characters are beneficial to the species in which they appear, because it renders their extinction under natural conditions more certain and more rapid, thus preventing the injurious effects that might result from their competing with the normal form while undergoing slow adaptive modification, or from checking that adaptation by the continued production of less adapted forms through intercrossing. It is, I think, clear that any species which gave birth to a large number of such abnormal and unchangeable individuals would be so hampered by them whenever adaptive modification became necessary that the [[p. 140]] whole species might be in danger of extinction. Hence, among allied species, those which gave rise to fewest of such abnormal forms would be best adapted to become the parents of new species, whenever the need for adaptation to new conditions arose.
But the absolute denial of any part in the process of organic evolution for these abnormal outcasts of nature does not imply that their study may not have a certain value for a comprehension of the mysterious phenomena of inheritance. The study of disease in all its strange forms, together with that of the numerous deformities to which the human body is subject, may in many cases have thrown light on obscure physiological processes and upon the conditions essential to health; but we should hardly claim them as being essential to the processes of a healthy and normal growth.
To anyone who has devoted a considerable portion of his life to the study of nature, both in field and in cabinet, both at home and in distant regions, the vast complex of phenomena presented by the organic world, with its endless specific forms, their myriad relations and adaptations, the laws of their development in the past and their distribution in the present, is almost overwhelming in its grandeur and its beauty. Almost all such loving students of nature have found in the theory of Darwin, in his many stimulating works and in those of his friends and followers, the only intelligible clue to the mighty labyrinth of nature. To such students of nature the claims of the Mutationists and the Mendelians, as made by many of their ill-informed supporters, are ludicrous in their exaggeration and total misapprehension of the problem they profess to have solved. To set upon a pinnacle this mere side-issue of biological research, as if it comprised within itself all the phenomena and problems presented by the organic cosmos, is calculated to bring ridicule upon what, in its place, may be an interesting and perhaps useful line of study. To myself these monstrous claims suggest a comparison with those of the perhaps equally enthusiastic and equally ill-informed admirers of the immortal Pickwick, who believed his "Speculations on the Source of the Hampstead ponds with some Observations on the Theory of Tittlebats," to have been a most important contribution to the science of that period.
* * *
In conclusion, I would suggest to those of my readers who are interested in the great questions associated with the name of Darwin, but who have not had the means of studying the facts either in the field or the library, that in order to obtain some real comprehension of the issue involved in the controversy now going on, they should read at least one book on each side. The first I would recommend is a volume by Mr. R. H. Lock, on "Variation, Heredity, and Evolution" (1906), as the only recent English work giving an account of the whole subject from the point of view of the Mendelians and [[p. 141]] Mutationists. When they have mastered this, I ask them to read my own book on "Darwinism" (1901), which, though published before Mendelism became prominent, gives some idea, in popular language, of the vast range of subjects which the Darwinian theory explains, and adduces a sufficient body of facts to show the inadequacy of any other explanation of the whole series of phenomena yet made public.
Having read these two works, and again considered
the arguments adduced in this article, I leave them to form their conclusions
as to whether Darwinism is or is not "an unsuccessful hypothesis."
Notes Appearing in the Original Work
clearest exposition of Mendel's law and of the germ theory elaborated
to explain it, well illustrated by diagrams and tabular formulæ,
is to be found in Prof. J. A. Thomson's recent work on "Heredity." A good
explanation of the Mendelian germ theory is also given in Prof. E. B.
Poulton's "Essays on Evolution," already referred to. [[on