An Examination of Weismannism

by George John Romanes

We now proceed to examine Weismann’s theory of germ-plasm, and as this in its various developments has now become a highly complex theory, we had best begin by marking out the lines on which the examination will be conducted.

As I have already pointed out, the Weismannian system is not concerned only with the physiology of reproduction: it is concerned also—and in an even larger measure—with the doctrine of descent. The theory of germ-plasm as a whole is very much more than a theory of heredity; it is a new theory of evolution. The latter, indeed, is deduced from the former; but although the two are thus intimately related, they are nevertheless not mutually dependent. For the relationship is such that the new theory of evolution stands upon the basis supplied by the new theory of heredity, and although it follows from this that if the latter were disproved the former would collapse, it does not follow that if the former were to be found untenable the latter must necessarily be negatived. Hence, for the sake of clearness, and also for the sake of doing justice to both theories, we had best deal with them separately. The present chapter, then, will be devoted to examining Weismann’s theory of heredity, while the ensuing chapter will be concerned with his sequent theory of evolution.

Again, Weismann’s theory of heredity stands on his fundamental postulate—the continuity of germ-plasm; and also on a fact well recognized by all other theories of heredity, which he calls the stability of germ-plasm. But his sequent theory of evolution stands not only on this fundamental postulate, and on this well-recognized fact; it requires for its logical basis two further postulates—viz., that germ-plasm has been perpetually continuous “since the first origin of life,” and unalterably stable “since the first origin of sexual propagation.” That these things are so, a very few words will be sufficient to prove.

Any theory of heredity which supposes the material of heredity to occupy a more or less separate “sphere” of its own, is not obliged further to suppose that this material has always been thus isolated, or even that it is now invariably so. There have been one or two such theories prior to Weismann’s, and they were founded on the well-known fact of congenital characters being at any rate much more heritable than are acquired characters. But it has not been needful for these theories to assume that the “continuity” thus postulated has been perpetually unbroken. Even if it has been frequently to some extent interrupted, all the facts of heredity could be equally well comprised under such theories—and this even if it be supposed that acquired characters are but rarely, or never, transmitted to progeny. For, in as far as the continuity may have been interrupted, it does not follow that the acquired characters (body-changes), which by hypothesis caused the interruption, must be inherited by progeny exactly as they occurred in the parents. Or, in other words and adopting Weismann’s terminology, so far as the facts of heredity are concerned, there is no reason why germ-plasm should not frequently have had its hereditary qualities modified by some greater or less degree of commerce with somatic-tissues, and yet never have reproduced in progeny the identical acquired characters which caused the modification of germ-plasm in the parents: some other and totally different characters might with equal—or even more—likelihood have been the result, as we shall see more clearly a few pages further on. Why, then, does Weismann so insist upon this continuity of germ-plasm as perpetual “since the origin of life”? It appears to me that his only reason for doing so is to provide a basis, not for his theory of heredity, but for his additional theory of evolution. It is of no consequence to the former that germ-plasm should be regarded as thus perpetual, while it is of high importance to the latter that the fundamental postulate of continuity should be supplemented by this further postulate of the continuity as thus perpetual.

Similarly as regards the postulate of the stability of germ-plasm as absolute. It is enough for all the requirements of Weismann’s theory of heredity that the material basis of heredity should present a merely high degree of stability, such as the facts of atavism, degeneration, &c. abundantly prove that it possesses. For his sequent theory of evolution, however, it is necessary to postulate this stability as absolute “since the first origin of sexual reproduction.” Otherwise there would be no foundation for any of the distinctive doctrines which go to constitute this theory.

It may not be immediately apparent that Weismann’s theory of heredity is not per se concerned with either of these two additional postulates of the continuity of germ-plasm as perpetual, and the stability of germ-plasm as absolute; while both are logically necessary to his further theory of evolution. On this account, and also for the sake of clearness in all that is to follow, we had best begin by comparing his theory of heredity with those of his principal predecessors—Darwin and Galton.

For the purposes of this comparison we may start by again alluding to the fact, that even in the multicellular organisms reproduction is not confined to the sexual methods. Many kinds of invertebrated animals will reproduce entire organisms from the fragments into which a single organism has been chopped: plants of various kinds can be propagated indefinitely by cuttings, grafts, and buds, or even by leaves, as we have already observed in Chapter I. Now, when the whole organism is thus reproduced from a severed portion of somatic-tissue, it reproduces its sexual elements. Whence, then, in such cases are these elements derived? Obviously they are not derived immediately from the sexual organs—or even from the sexual cells—of their parents: they are derived from the somatic-cells of a single parent, if we choose to retain this term; and therefore, as Strasburger pointed out soon after Weismann’s theory was published, it seems as if such facts are in themselves destructive of the theory. How, then, does Weismann meet them? As we have already seen in Chapter II, he meets them in the only way they can be met on the lines of his theory—viz., by those newer amendments of his theory which suppose that in all these cases the germ-plasm is not confined to the specially sexual cells, but occurs also in the nuclear substance of those somatic-cells which thus prove themselves capable of developing into entire organisms. In other words, the sexual elements which develop during what I have previously called this “somatic reproduction” of multicellular organism, are supposed to be derived from the sexual cells of ancestors, not indeed immediately (for this they plainly are not), but mediately through the somatic-tissues of their a-sexual parent. Now, in view of this extension, the theory of germ-plasm becomes somewhat closely allied to that of pangenesis. For example, when the fragment of a leaf of Begonia is laid upon moist soil, there strikes root, and grows a new Begonia plant capable of sexual reproduction, Darwin supposes the explanation to be that what he calls “formative material” occurs in all cells of the leaf, while Weismann supposes the explanation to be that what he calls “germ-plasm “ occurs in all—or at any rate in most—of the cells of the leaf. So that, except as regards the terms employed, the two theories are identical in their mode of viewing this particular class of phenomena.

Moreover by thus allowing, in his second essay on Heredity, that germ-plasm need not be restricted to the specially sexual cells, but in some cases, at any rate[12], may occur distributed in full measure of reproductive efficiency throughout the general tissues of the organism, Weismann cannot refrain from taking the further step of supposing that the germ-plasm, like the gemmules of Darwin, is capable of any amount of multiplication in the general cellular tissues of plants—seeing that plants can be propagated by cuttings, buds, &c., indefinitely. And this, as we have seen, Professor Weismann, in his second essay, does not shrink from doing. Moreover, although I cannot remember that he has anywhere expressly said so, it is obvious that the allied phenomena of regeneration and repair admit of explanation by his hypothesis of “ontogenetic grades,” after the manner already stated in Chapter II. Indeed, it is evident that in no other way can these phenomena be brought within the range of his theory. But from this it follows that not only in the case of organisms which are capable of somatic reproduction is the formative nucleo-plasm (idio-plasm-B) diffused throughout the somatic-tissues: on the contrary, it must be universally diffused throughout all the somatic-cells of all living organisms; and whether as it there occurs it is capable of reproducing entire organisms, single organs, single tissues, or a mere cicatrix, depends only on the “ontogenetic grade” of differentiation which this diffused nucleo-plasm has (or has not) previously undergone. Moreover, as we have already seen, at whatever ontogenetic grade of differentiation it may be present in a given somatic-tissue, it must there be capable of indefinite self-multiplication. Therefore, in all these respects this “formative nucleo-plasm” (or idio-plasm-B) of Weismann precisely resembles the “formative material” (or gemmules) of Darwin.

Lastly, as De Vries has pointed out[13], there must be at least as many divisions and subdivisions in the substance of germ-plasm, as there are differences between the somatic organs, tissues, and even cells, to which germ-plasm eventually gives rise—no matter through how many ontogenetic grades of idio-plasm it may first have to pass. Or, in other words, we must accept, as the material basis of heredity, ultimate particles[14] of germ-plasm, which are already differentiated into as many diverse categories as there are differences between all the constituent parts of the resulting soma; for, as shown in the Appendix, no change in the facts of the case has been shown by simply changing the original term “germ-plasm” into “idio-plasm,” wherever the phenomena of ontogeny are concerned. It may be convenient, for the sake of presenting newer additions to the theory, to restrict the term “germ-plasm” to “idio-plasm of the first ontogenetic stage”; but as idio-plasms of all subsequent ontogenetic stages are supposed to be ultimately derived from this idio-plasm of the first stage, it is evident that the particulate differences in question must already have been present in the so-called “undifferentiated idio-plasm of the first ontogenetic stage.” Unless we are to have a mere juggling with words, we cannot put into our successive idio-plasms any particles of kinds differing from those which are contained in the original germ-plasm. Therefore I say that, notwithstanding this change of terminology, Weismann must continue to assume, as the material basis of heredity, ultimate particles of germ-plasm which are already differentiated into as many diverse categories as there are differences between the parts of the resulting soma—although, of course, these ultimate particles need not be nearly so numerous in each of their categories as they afterwards become by self-multiplication while forming each of the resulting tissues.

But this is precisely what the theory of pangenesis supposes; so that I see no reason why these ultimate particles of germ-plasm should not be regarded as “gemmules,” so far as their size, number, and function are concerned. In point of fact, they differ from gemmules only in respect to their origin: they are not particles derived from somatic-cells of the preceding generation, but particles derived from germ-plasm of the preceding generation. Or, to state the difference in another form, if we regard the sexual elements as constituting the physiological centre of the organism, then the theory of germ-plasm supposes these ultimate carriers of heredity to originate at this centre, and then to travel centrifugally; while the theory of pangenesis supposes them to originate at the periphery, and then to travel centripetally.