Chapter XIV

, p. 313).

The Origin of Variations.--The way in which Weismann accounts for the origin of variation among higher animals is both ingenious and interesting. In all higher organisms the sexes are separate, and the reproduction of their kind is a sexual process. The germinal elements involved are seeds and pollen, eggs and sperms. In animals the egg bears all the hereditary qualities from the maternal side, and the sperm those from the paternal side. The intimate mixture of these in fertilization gives great possibilities of variations arising from the different combinations and permutations of the vital units within the germ-plasm.

This union of two germ-plasms Weismann calls amphimixis, and for a long time he maintained that the purpose of sexual reproduction in nature is to give origin to variations. Later he extended his idea to include a selection, mainly on the basis of nutrition, among the vital elements composing the germ-plasm. This is germinal selection, which aids in the production of variations.

In _The Evolution Theory_, volume II, page 196, he says: "Now that I understand these processes more clearly, my opinion is that the roots of all heritable variation lie in the germ-plasm; and, furthermore, that the determinants are continually oscillating hither and thither in response to very minute nutritive changes and are readily compelled to _variation in a definite direction_, which may ultimately lead to considerable variations in the structure of the species, if they are favored by personal selection, or at least if they are not suppressed by it as prejudicial."

But while sexual reproduction may be evoked to explain the origin of variation in higher animals, Weismann thought it was not applicable to the lower ones, and he found himself driven to assume that variation in single-celled organisms is owing to the direct influence of environment upon them, and thus he had an awkward assumption of variations arising in a different manner in the higher and in the simplest organisms. If I correctly understand his present position, the conception of variation as due to the direct influence of environment is being surrendered in favor of the action of germinal selection among the simplest organisms.

Extension of the Principle of Natural Selection.--These variations, once started, will be fostered by natural selection provided they are of advantage to the organism in its struggle for existence. It should be pointed out that Weismann is a consistent Darwinian; he not only adopts the principle of natural selection, but he extends the field of its operation from externals to the internal parts of the germinal elements.

"Roux and others have elaborated the idea of a struggle of the parts within the organism, and of a corresponding intra-selection; ... but Weismann, after his manner, has carried the selection-idea a step farther, and has pictured the struggle among the determining elements of the germ-cell's organization. It is at least conceivable that the stronger 'determinants,' _i.e._, the particles embodying the rudiments of certain qualities, will make more of the food-supply than those which are weaker, and that a selective process will ensue" (Thomson). This is the conception of germinal selection.

He has also extended the application of the general doctrine of natural selection by supplying a great number of new illustrations.

The whole theory of Weismann is so well constructed that it is very alluring. Each successive position is worked out with such detail and apt illustration that if one follows him step by step without dissent on some fundamental principle, his conclusion seems justified. As a system it has been elaborated until it makes a coherent appeal to the intellect.

Inheritance of Acquired Characters.--Another fundamental point in Weismann's theory is the denial that acquired characters are transmitted from parent to offspring. Probably the best single discussion of this subject is contained in his book on _The Evolution Theory_, 1904, to which readers are referred.

A few illustrations will be in place. Acquired characters are any acquisitions made by the body-cells during the lifetime of an individual. They may be obvious, as skill in piano-playing, bicycle-riding, etc.; or they may be very recondite, as turns of the intellect, acquired beliefs, etc. Acquired bodily characters may be forcibly impressed upon the organism, as the facial mutilations practiced by certain savage tribes, the docking of the tails of horses, of dogs, etc. The question is, Are any acquired characters, physical or mental, transmitted by inheritance?

Manifestly, it will be difficult to determine on a scientific basis whether or not such qualities are inheritable. One would naturally think first of applying the test of experiment to supposed cases of such inheritances, and this is the best ground to proceed on.

It has been maintained on the basis of the classical experiments of Brown-Séquard on guinea-pigs that induced epilepsy is transmitted to offspring; and, also, on the basis of general observations, that certain bodily mutilations are inherited. Weismann's analysis of the whole situation is very incisive. He experimented by cutting off the tails of both parents of breeding mice. The experiments were carried through twenty-two generations, both parents being deprived of their tails, without yielding any evidence that the mutilations were inheritable.

To take one other case that is less superficial, it is generally believed that the thirst for alcoholic liquors has been transmitted to the children of drunkards, and while Weismann admits the possibility of this, he maintains that it is owing to the germinal elements being exposed to the influence of the alcohol circulating in the blood of the parent or parents; and if this be the case it would not be the inheritance of an acquired character, but the response of the organism to a drug producing directly a variation in the germ-plasm.

Notwithstanding the well-defined opposition of Weismann, the inheritance of acquired characters is still a mooted question. Herbert Spencer argued in favor of it, and during his lifetime had many a pointed controversy with Weismann. Eimer stands unalterably against Weismann's position, and the Neo-Lamarckians stand for the direct inheritance of useful variations in bodily structure. The question is still undetermined and is open to experimental observation. In its present state there are competent observers maintaining both sides, but it must be confessed that there is not a single case in which the supposed inheritance of an acquired character has stood the test of critical examination.

The basis of Weismann's argument is not difficult to understand. Acquired characters affect the body-cells, and according to his view the latter are simply a vehicle for the germinal elements, which are the only things concerned in the transmission of hereditary qualities. Inheritance, therefore, must come through alterations in the germ-plasm, and not directly through changes in the body-cells.

[Illustration: Fig. 114.--August Weismann, Born 1834.

Permission of Charles Scribner's Sons.]

Weismann, the Man.--The man who for more than forty years has been elaborating this theory (Fig. 114) is still living and actively at work in the University of Freiburg. August Weismann was born at Frankfort-on-the-Main in 1834. He was graduated at Göttingen in 1856, and for a short time thereafter engaged in the practice of medicine. This line of activity did not, however, satisfy his nature, and he turned to the pursuit of microscopic investigations in embryology and morphology, being encouraged in this work by Leuckart, whose name we have already met in this history. In 1863 he settled in Freiburg as _privat-docent_, and has remained connected with the university ever since. From 1867 onward he has occupied the chair of zoölogy in that institution. He has made his department famous, especially by his lectures on the theory of descent.

He is a forceful and interesting lecturer. One of his hearers in 1896 wrote: "His lecture-room is always full, and his popularity among his students fully equals his fame among scientists."

It is quite generally known that Weismann since he reached the age of thirty has been afflicted with an eye-trouble, but the inference sometimes made by those unacquainted with his work as an investigator, that he has been obliged to forego practical work in the field in which he has speculated, is wrong. At intervals his eyes have strengthened so that he has been able to apply himself to microscopic observations, and he has a distinguished record as an observer. In embryology his studies on the development of the diptera, and of the eggs of daphnid crustacea, are well known, as are also his observations on variations in butterflies and other arthropods.

He is an accomplished musician, and during the period of his enforced inactivity in scientific work he found much solace in playing "a good deal of music." "His continuous eye trouble must have been a terrible obstacle, but may have been the prime cause of turning him to the theories with which his name is connected."

In a short autobiography published in _The Lamp_ in 1903, although written several years earlier, he gives a glimpse of his family life. "During the ten years (1864-1874) of my enforced inactivity and rest occurred my marriage with Fräulein Marie Gruber, who became the mother of my children and was my true companion for twenty years, until her death. Of her now I think only with love and gratitude. She was the one who, more than any one else, helped me through the gloom of this period. She read much to me at this time, for she read aloud excellently, and she not only took an interest in my theoretical and experimental work, but she also gave practical assistance in it."

In 1893 he published _The Germ-Plasm, A Theory of Heredity_, a treatise which elicited much discussion. From that time on he has been actively engaged in replying to his critics and in perfecting his system of thought.

The Mutation-Theory of De Vries.--Hugo de Vries (Fig. 115), director of the Botanical Garden in Amsterdam, has experimented widely with the growth of plants, especially the evening primrose, and has shown that different species appear to rise suddenly. The sudden variations that breed true, and thus give rise to new forms, he calls mutations, and this indicates the source of the name applied to his theory.

In his _Die Mutationstheorie_, published in 1901, he argues for the recognition of mutations as the universal source of the origin of species. Although he evokes natural selection for the perpetuation and improvement of variations, and points out that his theory is not antagonistic to that of natural selection, it is nevertheless directly at variance with Darwin's fundamental conception--that slight individual variations "are probably the sole differences which are effective in the production of new species" and that "as natural selection acts solely by accumulating slight, successive, favorable variations, it can produce no great or sudden modifications." The foundation of De Vries's theory is that "species have not arisen through gradual selection, continued for hundreds or thousands of years, but by jumps through sudden, through small transformations." (Whitman's translation.)

The work of De Vries is a most important contribution to the study of the origin of species, and is indicative of the fact that many factors must be taken into consideration when one attempts to analyze the process of organic evolution. One great value of his work is that it is based on experiments, and that it has given a great stimulus to experimental studies. Experiment was likewise a dominant feature in Darwin's work, but that seems to have been almost overlooked in the discussions aroused by his conclusions; De Vries, by building upon experimental evidence, has led naturalists to realize that the method of evolution is not a subject for argumentative discussion, but for experimental investigation. This is most commendable.

[Illustration: Fig. 115.--Hugo de Vries.]

De Vries's theory tends also to widen the field of exploration. Davenport, Tower, and others have made it clear that species may arise by slow accumulations of trivial variations, and that, while the formation of species by mutation may be admitted, there is still abundant evidence of evolution without mutation.

Reconciliation of Different Theories.--All this is leading to a clearer appreciation of the points involved in the discussion of the theories of evolution; the tendency is not for the breach between the different theories to be widened, but for evolutionists to realize more fully the great complexity of the process they are trying to explain, and to see that no single factor can carry the burden of an explanation. Mutation is not a substitute for natural selection, but a coöperating factor; and neither mutation nor natural selection is a substitute for the doctrine of the continuity of the germ-plasm. Thus we may look forward to a reconciliation between apparently conflicting views, when naturalists by sifting shall have determined the truth embodied in the various theories. One conviction that is looming into prominence is that this will be promoted by less argument and more experimental observation.

That the solution of the underlying question in evolution will still require a long time is evident; as Whitman said in his address before the Congress of Arts and Science in St. Louis in 1904: "The problem of problems in biology to-day, the problem which promises to sweep through the present century as it has the past one, with cumulative interest and correspondingly important results, is the one which became the life-work of Charles Darwin, and which can not be better or more simply expressed than in the title of his epoch-making book, _The Origin of Species_."

Summary.--The number of points involved in the four theories considered above is likely to be rather confusing, and we may now bring them into close juxtaposition. The salient features of these theories are as follows:

I. Lamarck's Theory of Evolution.

1. Variation is explained on the principle of use and disuse.

2. Heredity: The variations are inherited directly and improved in succeeding generations.

A long time and favorable conditions are required for the production of new species.

II. Darwin's Theory of Natural Selection.

1. Variations assumed.

2. Heredity: Those slight variations which are of use to the organism will be perpetuated by inheritance.

3. Natural selection is the distinguishing feature of the theory. Through the struggle for existence nature selects those best fitted to survive. The selection of trivial variations that are of advantage to the organism, and their gradual improvement, leads to the production of new species.

III. Weismann's Theory of Continuity of the Germ-plasm.

1. The germ-plasm has had unbroken continuity from the beginning of life. Owing to its impressionable nature, it has an inherited organization of great complexity.

2. Heredity is accounted for on the principle that the offspring is composed of some of the same stuff as its parents. The body-cells are not inherited, _i.e._,

3. There is no inheritance of acquired characters.

4. Variations arise from the union of the germinal elements, giving rise to varied combinations and permutations of the qualities of the germ-plasm. The purpose of amphimixis is to give rise to variations. The direct influence of environment has produced variations in unicellular organisms.

5. Weismann adopts and extends the principle of natural selection. Germinal selection is exhibited in the germ-plasm.

IV. De Vries's Theory of Mutations.

1. The formation of species is due not to gradual changes, but to sudden mutations.

2. Natural selection presides over and improves variations arising from mutation.

Among the other theories of evolution that of Eimer is the most notable. He maintains that variations in organisms take place not fortuitously or accidentally, but follow a perfectly determinate direction. This definitely directed evolution is called orthogenesis. He insists that there is continuous inheritance of acquired characters, and he is radically opposed to the belief that natural selection plays an important part in evolution. The title of his pamphlet published in 1898, _On Orthogenesis and the Impotence of Natural Selection in Species-Formation_, gives an indication of his position in reference to natural selection. A consideration of Eimer's argument would be beyond the purpose of this book.

The cause for the general confusion in the popular mind regarding any distinction between organic evolution and Darwinism is not far to seek. As has been shown, Lamarck launched the doctrine of organic evolution, but his views did not even get a public hearing. Then, after a period of temporary disappearance, the doctrine of evolution emerged again in 1859. And this time the discussion of the general theory centered around Darwin's hypothesis of natural selection. It is quite natural, therefore, that people should think that Darwinism and organic evolution are synonymous terms. The distinction between the general theory and any particular explanation of it has, I trust, been made sufficiently clear in the preceding pages.

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