WEISMANN AND THE EVOLUTION THEORY.* THERE is a touch of
sublimity as well as of pathos when a great student of Nature comes to gather up the fragments that remain after a long life's fearless work. No one will to-day deny that Dr. Weismann is one of Nature's great and ardent students. These two massive volumes are in them- selves a monument of research and speculation that will out- live much of the perennial brass of the nineteenth century. They represent, as he says, the main results of " a life of pleasant labour," and they are a permanent addition to the intellectual wealth of the world, however much their final conclusions may suffer at the hands of later investiga- tion. Born in 1834, Dr. Weismann eventually took up biology under Leuckart, and specialised later on zoology. Compelled as early as 1868 by weakness in the eyes to abandon microscopic work, he nevertheless from the first took a prominent part in the Darwinian controversy ; and in various lectures between 1867 and 1876 he dealt at length with variability of organisations. These lectures were translated into English by Professor R. Meldola and published in 1882, with a preface by Darwin dealing with the importance of the nature and cause of variability in individuals. From 1880 onwards Dr. Weismann worked at the subject of evolution unceasingly, adding to and elaborating what he calls " the Darwinian Edifice," and in particular developing his remarkable germ-plasm theory of heredity, which involves the denial of the transmissibility of acquired characteristics. Whatever may be the future of this theory, no one can peruse these volumes without feeling that he has reared a great logical structure, which, if overthrown, will only be overthrown by a more penetrating appeal to the facts of Nature than has yet been made. As an intellectual effort the germ-plasm theory is as remarkable as it is' The Evolution Theory. By Dr. August Weismann, Professor of Zoology in the University of Freiburg-in-Breisgau. Translated, with the author's co- operation. by J. Arthur Thomson, Regius Professor of Natural History in the University of Aberdeen, and Margaret B. Thomson. Illustrated. 2 viols. London: Edward Arnold. [32s. net.] fascinating. Dr. Weismands mature views on this theory were given to the English reading public in two volumes published in 1889 and 1892, and in 1902 he issued in Germany the'volumes before us. " When a life of pleasant labour. is drawing towards a close, the wish naturally asserts itself to gather together the main results, and to combine them in. a well-defined and harmonious picture which may be left as a legacy to succeeding generations." Professor Thomson and
his wife—who has borne the burden of the translation—now give us, with Dr. Weismann's personal co-operation, an incom- parably good English version of the second German edition (1904). It is only fair to say that it is impossible to detect from the English that it had a German original.
It will be convenient here to make some general comment on the Weismanu position. Dr. Weismanu states the broad Darwinian position as follows :—
" Charles Darwin championed, in the main, the same funda- mental ideas as had been promulgated by his grandfather, Erasmus Darwin, by Treviranus, and by Lamarck : species only seem to us immutable; in reality they can vary, and beconie transformed into other species, and the living world of our day has arisen through such transformations, through a sublime process of evolution which began with the lowest forms of life, but by degrees, in the course of unthinkably long ages, progressed to organisms more and more complex in structure, more and more effective in function Darwin' assumes that processes of transformations quite similar to those which take place under the guidance of Man occur also in nature, and that it is mainly these which have brought about and guided the transformations of species which have taken place in the course of the world's history. This process he calls natural selection.
Darwin did not ascribe to natural selection by any means all the changes which have taken place in organisms in the course of time. On the one hand, he ascribed a not incon- siderable importance to the correlated variations we have already mentioned ; still more, however, he relied on the direct influence of altered conditions of life, whether these consist in climatic or other changes in the environment, or in the assumption of new habits, and the increased or diminished use of individual parts and organs thereby induced. He reimognised the principle so .strongly emphasised by Lamarck, of use and disuse as a cause of heritable increase or decrease of the exorcised or. neglected. part, though he did so with a certain reserve."
Dr. Weismann rejects all these complementary causes of change in species. He refers every change to processes of
selection "of a different order from the phenomena which the Darwin-Wallace principle of natural selection serves to interpret."
While Darwin supplemented natural by sexual selection Weismann would further supplement the theory by a doctrine of germinal selection which is intended to sweep away those outstanding difficulties that Darwin and his school met by a doctrine of external forces producing heritable effects, and thus to supply a logical explanation of the entire phenomena of life. Dr. Weismann put to himself the problem, " How can purely local changes, not based in the germ, but called forth by the chances of life, be transmitted to descendants ?" A consideration of this problem has led him " to an absolute denial of the transmission" of individual characteristics acquired through use or disuse. He bases the denial on his conception of " the mechanism which enables the germ-cell to reproduce the whole organism, and not merely, like other cells, others like itself." Each new life springs, we are told, from a unified nucleus which exhibits " the combination of the hereditary tendencies of two individuals in one." These two equal and unified com- plements form an hereditary substance, called the germ- plasm, which is the chromatin substance of the nucleus of a germ-cell.. This chromatin is made up of a varying number of small but visible units, called ids, each of which contains within itself, as primary constituents (whatever that may mean), all the parts of a perfect animal, and represents the ancestry of the animal, though Dr. Weismann disclaims—the disclaimer is difficult to follow—the identification of any particular id with any particular ancestor. Each id of the germ-plasm consists of a vast number of different living parts, each of which is " an element of the germ-substance by whose presence in the germ the specific development of a particular part of the body is conditioned," and which " determine a ` hereditary character' of the body and whose variations cause the variations of those particular parts alone." These parts of the id are called determinants, which may be male or female as the case requires, and the determinant itself is (in multi-cellular organisms) "a group of biophors "—the ultimate vital unit—" which are bound together by internal forces to form a higher vital unity. This determinant must live as a whole, that is assimilate, grow, and multiply by division, like every vital unit, and its biophors must be individually variable, so that the separate parts of a cell controlled by them may also be capable of transmissible variation."
Professor Weismann has, in fact, supplied machinery to explain the classes of variations that are apparent. But there seems no reason why we should stop at the biophor. The Greeks heaped epicycle upon epicycle to explain the phenomena of the planets. Dr. Weismann has, indeed, to complete his theory by supplying a system of mysterious internal forces operating within the id, the determinant and the biophor, and setting up a doctrine of selection within the germ as between specific determinants. In this way he obtains a theory which certainly appears to offer on purely selectional lines explana- tions of groups of phenomena that seemed to Darwin to require a doctrine of external forces. Dr. Weismann is completely satisfied that there is no such thing as the acquisition of heritable characteristics. He feels justified in declaring dogmatically with respect to the inheritance of functional modifications that he has demonstrated its non-existence, and can therefore on that ground rule out Hering's admirable expansion of the Lamarckian position. The extraordinary complexity of the machinery by which Dr. Weismann explains biologic evolution—a machinery that has grown more and more complex as new facts have demanded new explanations—is in itself a subject for criticism, since the fundamental fact of fundamental laws is simplicity. But even if we assume that the theory is a satisfactory working hypothesis, it appears (despite its elaboration) only to carry the rejected position of Lamarck and Darwin one step back. Dr. Weismann tells us that a characteristic only becomes in- heritable if it is due to " a definite variation in the constitution of the germ." It is, however, admitted that the germ-plasm does alter constitutionally as the result of germinal selec- tion, and that this germinal selection is dependent upon external conditions of life which affect the struggle for existence between the various determinants of the ids. If this is so, it appears reasonable to believe with Hering that the germ experiences in some degree all that the complete organism experiences, and carries forward into its next manifestation the results of a persistent environment. The direct influence of external conditions on the germ- plasm is not denied by Dr. Weismann, and such an influence is not logically distinguishable in its nature from the acquisi- tion of characteristics that are inheritable. The dictum that " all variations depend upon internal causes" is clearly inconclusive. A doctrine of conservation of energy within the individual id may be compared with the same doctrine with respect to the atom. Modern physics teaches us that every atom is a gravitational system, and is slowly changing its nature as the result of internal forces. It remains to be seen if, in fact, this change is not really due to external conditions, such as temperature or pressure, playing upon varying degrees of internal stability.
The most remarkable fact in Dr. Weismann'a remarkable hypothesis is the way in which it suggests the identity of the ultimate problems in physics and biology. Here, we believe, a great truth is slowly becoming apparent. If the organic and the inorganic have, in fact, a common origin, we seem to come face to face with that Directive Force which the modern physicist is no longer afraid to conceive. It is left to the biologist to assume the role of St. Thomas. Dr. Weis- mann—though he still feels, as all great Nature-lovers have felt, the need for religion—can only conceive of two possible origins for the first organisms : "either that they have been borne to our earth from outside, from somewhere else in the universe, or that they have originated upon our earth itself through what is called ' spontaneous generation,' generatio spontanea." He rightly dismisses Justus Liebig's idea that life came hither out of space, as merely moving the problem a stage back, and adopts the idea of generatio spontanea. The conception that tellurian life arose from a chance combination of ultimate atoms seems to us unworthy of a great scientist. We prefer to feel that an Almighty and self-conscions Power, when He created out of ultimate simplicity the first organic and the first inorganic structures, had within His consciousness the whole scope of Creation. The Book of Genesis, when it describes the organic and inorganic universe as created within the mind of God
before it was translated into fact, seems to have described with a poetic truth which transcends the imaginings of the biologist the infinite possibilities and potentialities that the Creator gave to the infinitesimal beginning of things. Organisation,. as Dr. Weismann says with quite another meaning, is still at the basis of life.
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