It may perhaps be objected that the differences in the appendages may quite well have had their origin simply during the development of the animal, while the primary constituents were the same for all, so that a single determinant in the germ-plasm would suffice. But this could only be the case if the differences depended not on internal but on external causes, that is, if the same primary constituents gave rise to a set of appendages which became different because they were subject in the course of their development to different modifying influences. But this is not the case, at least not to the extent that this supposition would necessitate. Can it be supposed that, for instance, the jumping legs of the water-flea (Gammarus) are a necessary consequence of the somewhat divergent form of the segments from which they grow? A direct proof to the contrary may be found in 'Heterotopia,' for in the place where a posterior limb, modified for holding the eggs, normally occurs in the crab an ordinary walking leg may exceptionally develop (Fig. 90, Bethe), or an appendage resembling an antenna may take the place of an extirpated eye (Herbst). But if there were really only one determinant in the germ-plasm for all the appendages these would of necessity be all alike, apart from the larger or smaller differences which might be stamped upon them by growing from segments different in size and in nutrition. Such differences, however, are far from being sufficient to explain the great deviations seen among the appendages of most kinds of Crustaceans, and still less to explain their adaptation to quite different functions.
Fig. 90. The Common Shore-Crab (Carcinus mænas), seen from below, with the abdomen forced back. In place of the swimmeret, which ought to be borne by the fifth abdominal swimmeret, a walking leg has grown on the left side, and one which properly should belong to the right side (6). 1-5, thoracic limbs, ps1-4, swimmerets of the right side. s6, s7, posterior segments of the abdomen. After Bethe.
It need not be imagined that my argument can be controverted by saying that one appendage-determinant in the germ may split itself in the course of development into a series of different appendage-determinants. The question would then arise, How is it able to do so? And the answer can be no other than that the single first determinant had within it several different kinds of elements, which subsequently separated to determine in different ways the various appendages. But that is just another way of saying that this single determinant actually includes within itself several different determinants. For a determinant means nothing more than an element of the germ-substance by whose presence in the germ the specific development of a particular part of the body is conditioned. If we could remove the determinants of a particular appendage from the germ-plasm this appendage would not develop; if we could cause it to vary the appendage also would turn out differently.
In this general sense the determinants of the germ-plasm are not hypothetical, but actual; just as surely as if we had seen them with our eyes, and followed their development. Hypothesis begins when we attempt to make creatures of flesh and blood out of these mere symbols, and to say how they are constituted. But even here there are some things which may be maintained with certainty; for instance, that they are not miniature models, in Bonnet's sense, of the parts which they determine; and, further, that they are not lifeless material, mere substances, but living parts, vital units. If this were not so they would not remain as they are throughout the course of development, but would be displaced and destroyed by the metabolism, instead of dominating it as living matter alone can do—doubtless undergoing oxidation, but at the same time assimilating material from without, and thereby growing. There cannot be lifeless determinants; they must be living units capable of nutrition, growth, and multiplication by division.
And now we have arrived at the point at which a discussion of the organization of the living substance in general can best be interpolated.
The Viennese physiologist, Ernst Brücke, forty years ago promulgated the theory that living matter could not be a mere mixture of chemical molecules of any kind whatever; it must be 'organized,' that is, it must be composed of small, invisible, vital units. If, as we must certainly assume, the mechanical theory of life is correct, if there is no vital force in the sense of the 'Natur-Philosophie,' Brücke's pronouncement is undoubtedly true; for a fortuitous mixture of molecules could no more produce the phenomena of life than a single molecule could, because, as far as our experience goes, molecules do not live; they neither assimilate, nor grow, nor multiply. Life can therefore arise only through a particular combination of diverse molecules, and all living substance must consist of such definite groups of molecules. Shortly after Brücke, Herbert Spencer likewise assumed the reality of such vital 'units,' and the same assumption has been made in more recent times by Wiesner, De Vries, and myself. In the meantime we can say nothing more definite about the composition of these bearers of life, or 'biophors,' as I call them, than that albumen-molecules, water, salts, and some other substances play the chief part in their composition. This has been found out by analysis of dead protoplasm; but in what form these substances are contained in the biophors, and how they affect each other in order to produce the phenomena of life by going through a ceaseless cycle of disruptions and reconstructions, is still entirely hidden from us.
We have, however, nothing to do with that here; we content ourselves with recognizing in the biophors the characteristics of life, and picturing to ourselves that all living substance, cell-substance, and nuclear substance, muscle-, nerve-, and gland-substance, in all their diverse forms, consist of biophors, though, of course, of the most varied composition. There must be innumerable kinds of biophors in all the diverse parts of the millions of forms of life which now live upon the earth; but all must be constructed on a certain fundamental plan, which conditions their marvellous capacity for life; all possess the fundamental characters of life—dissimilation, assimilation, growth, and multiplication by division. We must also ascribe to them in some degree the power of movement and sensibility.
As to their size, we can only say that they are far below the limits of visibility, and that even the minutest granules which we can barely perceive by means of our most powerful microscopes cannot be small individual biophors, but must be aggregates of these. On the other hand, the biophors must be larger than any chemical molecule, because they themselves consist of a group of molecules, among which are some of complex composition, and therefore of relatively considerable size.
It may now be asked whether the determinants, whose existence we have already inferred, are not identical with these 'biophors' or smallest living particles; but that is not the case, at least not generally. We called determinants those parts of the germ-substance which determine a 'hereditary character' of the body; that is, whose presence in the germ determines that a particular part of the body, whether it consists of a group of cells, a single cell, or a part of a cell, shall develop in a specific manner, and whose variations cause the variations of these particular parts alone.