§ 97a. Since the foregoing four chapters were written, thirty-four years ago, the topics with which they deal have been widely discussed and many views propounded. Ancient hypotheses have been abandoned, and other hypotheses, referring tacitly or avowedly to the cell-doctrine, have been set forth. Before proceeding it will be well to describe the chief among these.
Most if not all of them proceed on the assumption, shown in [§ 66] to be needful, that the structural characters of organisms are determined by the special natures of units which are intermediate between the chemical units and the morphological units—between the invisible molecules of proteid-substances and the visible tissue-components called cells.
Four years after the first edition of this volume was published, appeared Mr. Darwin's work, The Variation of Animals and Plants under Domestication; and in this he set forth his doctrine of Pangenesis. Referring to the doctrine of physiological units which the preceding chapters work out, he at first expressed a doubt whether his own was or was not the same, but finally concluded that it was different. He was right in so concluding. Throughout my argument the implication everywhere is that the physiological units are all of one kind; whereas Mr. Darwin regards his component units, or "gemmules," as being of innumerable unlike kinds. He supposes that every cell of every tissue gives off gemmules special to itself, and capable of developing into similar cells. We may here, in passing, note that this view implies a fundamental distinction between unicellular organisms and the component cells of multicellular organisms, which are otherwise homologous with them. For while in their essential structures, their essential internal changes, and their essential processes of division, the Protozoa and the component units of the Metazoa are alike, the doctrine of Pangenesis implies that though the units when separate do not give off invisible gemmules the grouped units do.
Much more recently have been enunciated the hypotheses of Prof. Weismann, differing from the foregoing hypotheses in two respects. In the first place it is assumed that the fragment of matter out of which each organism arises consists of two portions—one of them, the germ-plasm, reserved within the generative organ of the incipient individual, representing in its components the structure of the species, and gives origin to the germs of future individuals; and the other of them, similarly representative of the specific structure, giving origin to the rest of the body, or soma, but contains in its components none of those latent powers possessed by those of the germ-plasm. In the second place the germ-plasm, in common with the soma-plasm, consists of multitudinous kinds of units portioned out to originate the various organs. Of these there are groups, sub-groups, and sub-sub-groups. The largest of them, called "idants," are supposed each to contain a number of "ids"; within each id there are numerous "determinants"; and each determinant is made up of many "biophors"—the smallest elements possessing vitality. Passing over details, the essential assumption is that there exists a separate determinant for each part of the organism capable of independent variation; and Prof. Weismann infers that while there may be but one for the blood and but one for a considerable area of skin (as a stripe of the zebra) there must be a determinant for each scale on a butterfly's wing: the number on the four wings being over two hundred thousand. And then each cluster of biophors composing a determinant has to find its way to the place where there is to be formed the part it represents.
Here it is needless to specify the modifications of these hypotheses espoused by various biologists—all of them assuming that the structural traits of each species are expressed in certain units intermediate between morphological units and chemical units.
§ 97b. A true theory of heredity must be one which recognizes the relevant phenomena displayed by all classes of organism. We cannot assume two kinds of heredity, one for plants and another for animals. Hence a theory of heredity may be first tested by observing whether it is equally applicable to both kingdoms of living things. Genesis, heredity, and variation, as seen in plants, are simpler and more accessible than as seen in animals. Let us then note what these imply.
Already in [§ 77] I have illustrated the power which some plants possess of developing new individuals from mere fragments of leaves and even from detached scales. Striking as are the facts there instanced, they are scarcely more significant than some which are familiar. The formation of cauline buds, presently growing into shoots, shows us a kind of inheritance which a true theory must explain. As described by Kerner, such buds arise in Pimpernel, Toad-flax, etc., below the seed-leaves, even while yet there are no axils in which buds usually grow; and in many plants they arise from intermediate places on the stem: that is, without definite relations to pre-existing structures. How fortuitous is their origin is shown when a branch is induced to bud by keeping it wrapped round with a wet cloth. Even still better proved is the absence of any relation between cauline buds and normal germs by the frequent growth of them out of "callus"—the tissue which spreads over wounds and the cut ends of branches. It is not easy to reconcile these facts with Mr. Darwin's hypothesis of gemmules. We have to assume that where a cauline bud emerges there are present in due proportions gemmules of all the parts which will presently arise from it—leaves, stipules, bracts, petals, stamens, anthers, etc. We have to assume this though, at the time the bud originates, sundry of these organs, as the parts of flowers, do not exist on the plant or tree. And we have to assume that the gemmules of such parts are duly provided in a portion of adventitious callus, far away from the normal places of fructification. Moreover, the resulting shoot may or may not produce all the parts which the gemmules represent; and when, perhaps after years, flowers are produced on its side shoots, there must exist at each point the needful proportion of the required gemmules; though there have been no cells continually giving them off.
Still less does the hypothesis of Prof. Weismann harmonize with the evidence as plants display it. Plant-embryogeny yields no sign of separation between germ-plasm and soma-plasm; and, indeed, the absence of such separation is admitted. After instancing cases among certain of the lower animals, in which no differentiation of the two arises in the first generation resulting from a fertilized ovum, Prof. Weismann continues:—
"The same is true as regards the higher plants, in which the first shoot arising from the seed never contains germ-cells, or even cells which subsequently become differentiated into germ cells. In all these last-mentioned cases the germ-cells are not present in the first person arising by embryogeny as special cells, but are only formed in much later cell-generations from the offspring of certain cells of which this first person was composed." (Germ-Plasm, p. 185.)
How this admission consists with the general theory it is difficult to understand. The units of the soma-plasm are here recognized as having the same generative powers as the units of the germ-plasm. In so far as one organic kingdom and a considerable part of the other are concerned the doctrine is relinquished. Relinquishment is, indeed, necessitated even by the ordinary facts, and still more by the facts just instanced. Defence of it involves the assertion that where buds arise, normal or cauline, there exist in due proportion the various ids with their contained determinants—that these are diffused throughout the growing part of the soma; and this implies that the somatic tissue does not differ in generative power from the germ-plasm.