WEISMANN ON HEREDITY, VARIATION, LIFE, AND DEATH.*
IN the later editions of his Origin of Species, Darwin betrayed, in a few well-known passages the circumstances attending the insertion of which it would be interesting to know, some doubt as to the completeness of the theory of natural selection, and an inclination to give greater weight than he had at first accorded to the agency of use and disuse, and the influence of the environment in the production of specific characters in plants and animals Against this distrust shown, but only on a very few occasions, by Darwin himself of his own theory, and still more against the attempts made by some biologists, for the most part unfamiliar with morphology, to reduce the efficiency of natural selection to a minimum, if not, indeed, to eliminate it altogether from Nature's machinery, Mr. Wallace's latest work is an eloquent protest, abounding in facts and arguments, and distinguished by a minuteness of * Essays upon Heredity and Kindred Biological Problems. By Dr. August Weiemann, Prof. Univ. Freiburg. Authorised Translation, edited by E. B. Poulton, F.B.S., S. Soh6nland, Ph.D., and A. E. Shipley, M.A., F.L.S. Oxford : Clarendon Press.
knowledge and philosophical largeness of treatment worthy of the great naturalist whose cardinal doctrine it undertakes to propound and defend. But the volume before us exhibits perhaps an even more perfect defence of Darwinism than can be gathered from Mr. Wallace's pages. For if Professor Weismann's theory of the continuity of the germ-plasm (specialised reproductive protoplasm) in plants and animals —to the exposition of which the more important of these essays are devoted—should be sustained, natural selection, an indubitable vera causa in Nature, must, in the most absolute sense, be acknowledged as the causa eausans of specific differentiation. What Mr. Wallace does by induction, Weismann, in fact, seeks to effect by the process, more congenial to the German mind, of deduction, though it is not his immediate object to defend Darwinism, which he accepts as astronomers accept gravitation—Newton's proof whereof it took the French Academy forty years to understand—but to explain variation, in such a manner, however, as necessarily to involve selection as the controlling agency in the evolution of species out of variational phenomena. Like most other great principles, that of the continuity of the germ-plasm is simple enough in statement, but difficult of application, and therefore open to endless argument in details, through the complexity of the phenomena with which it is concerned. To understand it, we may go back to Darwin's "pangenesis "—which was never, by-the-bye, more than a suggestion—now generally rejected, but which, it is not improbable, has led up to the Freiburg Professor's theory, being, indeed, in some measure analogous to the latter.
Of Nature's great secrets, one of the greatest is the secret of heredity: What explanation is conceivable of the repetition down to the minutest details, of the organisation of the parents in that of the offspring through the intermediation of a single microscopic cell, the germinal vesicle, imbedded in the ovum of the mother P According to Darwin, an infinitesimal por- tion of each cell of the parental organisms is contained in the fertilised vesicle, and thus in the development of the latter the whole parental organism is again built up, with certain slight differences, due principally to variations of nutrition and environment. But the contents of the parental body-cells are not homogeneous, either chemically or structurally, and could
at best be only partially represented by the supposed "gem- mule." Pangenesis, again, unlike natural selection, is not a
vera causa ; no proof of the existence of the " gemmules," or portions of cell-substance thrown off by the cells, necessary to the theory has been, or, indeed, ex hypothesi almost, is capable of being adduced. Weismann's doctrine, on the other hand, possesses at least a basis of ascertained fact. If the germ-cell is not a sort of pangenetic extract of the whole parental body,—
" There remain," he writes, " only two other possible, physio- logically conceivable, theories as to the origin of germ-cells, manifesting such powers as we know they possess. Either the substance of the parent germ-cell is capable of undergoing a series of changes which, after the building-up of a new individual, leads back again to identical germ-cells ; or the germ-cells are not derived at all, as far as their essential and characteristic substance is concerned, from the body of the individual, but they are derived directly from the parent germ-cell."
The latter alternative is the one Weismann adopts, and explains heredity upon the theory of the continuity of the germ-plasm from generation to generation, just as unicellular organisms form a serial being whose life began with the beginning and will end with the end of organic existence.
Now, this continuity of the germ-plasm is not wholly theoretic, for in certain organisms it has been actually observed,— as, for instance, in dipterous insects (flies, gnats, &a.), where the first cells formed in the egg become the reproductive glands of the embryo. It is therefore a vera causa, though it cannot be directly observed to be such throughout the world of life. But as such, and so far as it explains the phenomena of heredity, it must be accepted in preference to mere hypo- theses, even where these give a possible theory of those phenomena. The theory, however, claims much more than this. It sweeps away entirely almost all the theories which have been so diligently put forward of late in depreciation of Darwinism. These all depend upon the transmission of acquired characters, which is impossible if the somatic (body) plasm and germ-plasm are not only different from each other, but are not mutually productive of each other. In a word, continuity of germ-plasm replaces inheritance from the indi- vidual by inheritance from the racial ancestry, and wholly eliminates from Darwinism every trace of Lamarckism, and much at least of what may be termed Semperism (acquired and accumulated results of the influence of the environment).
To explain Darwinism, however, it is not enough to discover a principle of heredity. Selection acts through heredity upon materials furnished by variation, and any theory of heredity must therefore be supplemented by a theory of variation: Weis- mann's theory aims at showing that variation has nothing to do with characters acquired by the individual; and to explain varia- tion he points to the commingling of the male and female cells, or rather nuclei—that is, of the two sexual germ-plasma—in the offspring. This commingling must bring different proportions of different elements together in each generation; but the real difficulty is to explain the origination of variation in the germ- cells themselves (which is necessary to the theory), or in what replaces the germ-cells on the lowest horizons of life,—in unicellular organisms. Weismann finds himself "driven to the conclusion that the ultimate origin of hereditary indi- vidual differences lies in the direct action of external influences upon the organism," thus founding his theory of variation upon a particular form of Semperism. This initial variation is intensified by sexual reproduction, and under the guidance of natural selection (itself, perhaps, more or less influenced by various agencies, but in a wholly subordinate manner in free nature) produces all the varied forms of organic existence.
From every ovum, before metamorphosis begins, one or more masses of nucleolar matter, the so-called "polar bodies," are extruded. In parthenogenetic or non-sexual ova—that is, ova not requiring fertilisation, such as the eggs of aphides or plant-lice—one polar body is found ; in eggs requiring fertilisation, two are, according to Weismann, always dis- coverable. These strange phenomena have long puzzled biologists, and Weismann's explanation of them, if not itself a sufficient one, is at least of a highly interesting and suggestive character. He supposes the parthenogenetic
polar body to represent an excess of body-plasm, and the second polar body of fertilisable ova to represent an excess of germ-plasm. In this way, parthenogenesis is explained by the retention in the parthenogenetic ovum of the germ. plasm, which is extruded as the second polar body from the fertilisable ovum ; while in the latter, the body thus ex- truded is replaced by the male germ-plasm, and thus the process of variation is enormously facilitated. The essays dealing with this subject are not easy reading, and can only be appreciated by those who are acquainted with the principal facts of embryology. The remaining essays contained in this volume, owing in great measure to the admirable way in which the translators have performed their task, will be readily understood by any intelligent reader. Among them, two of the most interesting are those which deal with the duration of life, and explain the significance of death. They are interesting as showing how apparently inexplicable phenomena yield their secret to the key which Darwin forged, but sometimes hesitated to use. In the first of the essays in question, Weismann shows that the duration of life—which varies from a few hours in may-flies to two hundred years in whales, and perhaps thousands in some trees, while unicellular organisms, which are propagated by mere self-division, are practically immortal —is governed by the needs not of the individual, but of the race, and is determined by "precisely the same mechanical process of regulation—i.e., selection—as that by which the structure and functions of an organism are adapted to its environment."
The essay on " Death " is really a corollary to the one on "Life." The life of the individual being prolonged through selection until the needs of the race (production and care of offspring) are provided for, the further life of the individual is a matter of indifference, so far as the continuance of the race is concerned, and therefore withdrawn from the con- servative influence of selection. Not only so, but selection is directly adverse to the continued life of the individual, for Nature aims at the improvement of the race, not of the in- dividual; and to give play to the race, it is clear that the sojourn of the individual upon her stage must be limited. Without death, therefore, as without the struggle for existence—the supposed cruelty of which Mr. Wallace denies—progress would be impossible in a world constituted as ours is.
No doubt Weismann's theories are in a measure tentative. He admits himself that they are so, and, indeed, finality in
these matters is not attainable. But their great merit will be admitted by all biologists, and especially by Darwinians, to whose doctrines they lend such a powerful support. Every year the number of those who are qualified to take an intelli- gent pleasure in the study of the philosophy of natural history increases, and we cannot doubt that these profoundly inter- esting and fruitful papers will meet with the attention and welcome their intrinsic qualities call for, and the name of their author entitles them to receive.
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