The Evolution Theory, Vol. 1 of 2

by August Weismann

But organisms also are machines which perform a particular and purposeful kind of work, and they are only capable of doing so because the energies which perform the work are forced into definite paths by superior forces; these superior forces are thus 'the steersmen of the energies.' There is undoubtedly a kernel of truth in this view, and I shall return to it. Reinke, however, uses it in a way which I cannot follow; that is, he infers from it a 'cosmic intelligence' which puts these superior forces into the organisms, and thus controls these machines to purposeful work, as the watchmaker puts 'superior forces' into the watch by means of wheels, cylinders, and levers. In one case it is human intelligence which controls the 'superior forces,' in the other 'cosmic' intelligence. I cannot regard this reasoning from analogy as convincing, because, in the first place, these 'superior forces' are not 'forces' at all. They are constellations of energy, co-ordinations of matter and the energies immanent therein under complex and precisely defined conditions, and it is a matter of indifference whether chance or human intelligence has brought them together. If we take Reinke's own example of carbohydrates it is certain that our coal-gas is due to the intelligence of man, which brings together the carbon and the water in such a way that coal-gas must arise. The 'superior forces' must here be looked for in the arrangements of the coke-stove, and, in the second place, in the intelligence of man. But when decaying plants in the marsh form another carbon-compound, marsh-gas, where do the directing 'superior forces' come in? Surely only in the fortuitous concomitance of the necessary materials and the necessary conditions. Or may 'cosmic' intelligence have established this laboratory in the marsh? If not, what can compel us to refer the formation of dextrin or starch in the cells of the green leaves of plants to 'superior forces' which are placed in them by 'cosmic' intelligence? I am far from believing that the great and deep problem here touched upon can be put aside in any off-hand manner, but I feel sure that it will never be solved by word-play about energies and 'superior forces.'

Let us return to the kernel of truth in Reinke's thesis; it lies in this, that, while the working of a machine does really depend on the forces or energies which are bound up with the stuffs of which it consists, it also depends on a particular combination of these stuffs and forces, on a particular 'constellation' of them, as Fechner expressed it. In the watch these 'constellations' are the springs, the wheels, &c., and their position in relation to each other; but in the organism they are the organs, down to the cells and cell-parts; for the cell too is a machine, indeed a very complex one, as its functions prove. There are thousands of kinds of 'constellations' of elementary substances and forces which condition the activity of the living machine, and only when all these constellations are present in the proper manner and in the proper relations to each other can the functions of the organism be properly discharged.

But the living machine differs essentially from other machines in the fact that it constructs itself; it arises by development from a cell, by going through numerous 'stages of development.' But none of these stages is a dead thing, each is itself a living organism whose chief function is to give rise to the next stage. Thus each stage of the development may be compared to a machine whose function consists in producing a similar but more complex machine. Each stage is thus composed, just like the complete organism, of a number of such 'constellations' of elementary substances and elementary forces, whose number in the beginning is relatively small, but increases rapidly with each new stage.

But whence come these 'constellations' or, to keep to our metaphor, the levers, wheels, and cranks of each successive stage in the making of the organic machine? The epigenetic theory of a germ-plasm without primary constituents answers by pointing to internal and external influences which cause the germ-plasm, originally homogeneous, to differentiate gradually more and more, bringing it into the most diverse 'constellations.' But how can such influences introduce new springs, levers, and wheels of a quite specific kind, as must be the case if apparently similar germinal substances are to give rise to two such different animals as a domestic duck and a teal? The cause must lie in the invisible differences in the protoplasm, opponents will answer, and we with them. But our studies up to this point have shown us that the differences cannot be merely elementary differences, cannot be merely of a physico-chemical nature depending on the composition of the raw material and the implicated energies; they must depend on the definite co-ordination of substances and energies, in other words, on the occurrence of 'constellations' of these. Thus the germ-plasm must be composed of definite and very diverse combinations of living units, which are themselves bound up in a higher 'constellation,' so that they act as a living machine at the first stage of development, and liberate into activity the already existing constellations of the second stage. The second stage in the series of living machines which arise successively from each other liberates the sleeping 'constellations' for the third, and so on.

These 'constellations' of matter and energy are the biophors, the determinants, and the 'groups of determinants' which we may think of as disposed in a manifold overlapping series. That they do not enter into activity all at once, but successively take their part in development, seems to me a necessary consequence of their successive origin in the phylogeny; and the ontogeny, as we shall see later, arises through a modified condensation of the phylogeny. Now since every new determinant that arises in the course of phylogeny can only develop by division and subsequent variation from the determinants which were previously active at the same place in the organism, it is quite intelligible that later on, when the phylogeny has been condensed in the ontogeny, they should not enter upon their active stage at the same time as their phyletic predecessors, but after them. The theory of Oscar Hertwig, who starts from a germ-plasm without primary constituents, that all parts of the germ-plasm become active at the same time, seems to me quite untenable. How could the wheels, levers, and springs of the complete vital machine, which arose so very slowly in the course of phylogeny, arise to-day in the ontogeny in such rapid succession unless they were already present in the germ-plasm and only required to be incited to activity, that is, liberated by the stage preceding them? Even Fechner supported this view when he supposed that the interaction and mutual influences of the parts in the organism, that is, of the 'constellations,' gave rise of themselves to the succeeding stage, that is to say, to the new constellations peculiar to the succeeding stage. To this Reinke reasonably objected that it was like expecting the window frames of a house in process of building to produce the panes of glass. The panes in the organism only develop in the window frames if their determinants have been present in the germ-plasm from the beginning, and are liberated by the development of the frames, just as the activity of the glazier is liberated by the sight of the completed frames. Neither new panes nor new determinants could be produced rapidly; the former must be manufactured in the glass factory, the latter in the developmental workshop of the form of life in question, which workshop we call its phylogeny. But just as it is unnecessary to erect a new glass factory for each new house that is built, so the development of each individual does not require the establishment each time of those numberless life-factories—the constellations—whose business it is to produce anew the wheels, levers, springs, and cylinders of the developmental machinery at each stage, for they are all provided for in the germ-plasm, and it is only on this account that they are capable of hereditary variation.

I have already directed attention to some embryological facts which seem to be contradictory, if not to the germ-plasm theory itself, at least to the assumption it makes that the germ-plasm is analysed out during the ontogeny; and something more must be said on this head. I refer to the numerous facts brought to light through the science of developmental mechanics founded by Wilhelm Roux, and particularly to the investigations as to the prospective significance of the segmentation-cells of the animal ovum.

Among these investigations we find experiments in compressing certain eggs (sea-urchin's) in the early stages of segmentation. The blastomeres are prevented by artificial pressure from grouping themselves in the normal manner; they are compelled to spread out side by side in the same plane. If the pressure is removed, they change their grouping, and yield a normal embryo. I will not here discuss whether these results can only be interpreted as showing that each segmentation-cell has the same prospective significance, and that it is only its relative position which decides what part of the embryo is to be formed from it; this could not be done without going into great detail; I therefore assume it to be true, and confine my survey to the second group of experiments, those on isolated segmentation-cells.

It has been shown that in the eggs of the most diverse animals, for instance in the sea-urchin once more, each of the two first blastomeres, if separated from one another, can develop into a complete larva. Indeed, in the eggs of sea-urchin and some other animals each of the first four, or any of the first eight, blastomeres, and indeed any segmentation-cell during the earlier stages, possesses the power of developing to a certain point, namely, as far as the so-called 'blastula-larva.' This seems to contradict a theory which assumes that the primary constituents become separated in the successive stages of ontogeny. But in the first place the blastomeres of all animals do not behave in this way, and, moreover, the facts can be quite well explained without entirely renouncing the assumption of the segregation of the determinant-complexes. It is only necessary to assume that the segmentation-cells, which develop in the isolated condition as if they were intact eggs, still contain the complete germ-plasm, and that the differential segregation into groups of determinants with dissimilar hereditary tendencies takes place later. This would certainly load the theory with further complications, and I shall not enter into the question here, since the facts which we should have to consider are as yet by no means undisputed.

But in any case the facts of developmental mechanics referred to, which we owe to numerous excellent observers of the last decade,—I need only name W. Roux, O. Hertwig, Chun, Driesch, Barfurth, Morgan, Conklin, Wilson, Crampton, and Fischel—not only leave the essential part of the germ-plasm theory untouched, but rather strengthen than endanger its more subordinate points, such as the assumption of a segregation of the components of the germ-plasm in the course of ontogenesis.