Consequently, we arrive at this curious result. No matter how many generations of organisms there may have been, and therefore no matter how many combinations of germ-plasm may have taken place to give rise to an existing population, each existing unit of germ-plasm must have remained of the same essential nature or constitution as when it was first started in its immortal career millions of years ago. Or, reverting to our illustration of sand and clay, the particles of each must always remain the same, no matter how many admixtures they may undergo with particles of other materials, such as chalk, slate, &c. Now, inasmuch as it is an essential—because a logically necessary—part of Weismann’s theory to assume such absolute stability or unchangeableness on the part of germ-plasm, the question arises, and has to be met, What was the origin of those differences of character in the different germ-plasms of multicellular organisms which first gave rise, and still continue to give rise, to congenital variations by their mixture one with another? This important question Weismann answers by supposing that these differences originally arose out of the differences in the unicellular organisms, which were the ancestors of the primitive multicellular organisms. Now, as before stated, different forms of unicellular organisms are supposed to have originated as so many results of differences in the direct action of the environment. Consequently, according to the theory, all congenital variations which now occur in multicellular organisms, are really the distant results of variations that were aboriginally induced in their unicellular ancestors by the direct action of surrounding conditions of life.
I think it will be well to conclude by briefly summarising the main features of this elaborate theory.
Living material is essentially, or of its own nature, imperishable; and it still continues to be so in the case of unicellular organisms which propagate by fission or gemmation. But as soon as these primitive methods of propagation became, from whatever cause, superseded by sexual, it ceased to be for the benefit of species that their constituent individuals should be immortal; seeing that, if they continued to be so, all species of sexually-reproducing organisms would sooner or later have come to be composed of broken-down and decrepit individuals. Consequently, in all sexually-reproducing or multicellular organisms, natural selection set to work to reduce the term of individual lifetimes within the narrowest limits that in the case of each species were compatible with the procreation and the rearing of progeny. Nevertheless, in all these sexually-reproducing organisms the primitive endowment of immortality has been retained with respect to their germ-plasm, which has thus been continuous, through numberless generations of perishing organisms, from the first origin of sexual reproduction till the present time. Now, it is the union of germ-plasms which is required to reproduce new individuals of multicellular organisms that determines congenital variations on the part of such organisms, and thus furnishes natural selection with the material for its work in the way of organic evolution—work, therefore, which is impossible in the case of unicellular organisms, where variation can never be congenital, but always determined by the direct action of surrounding conditions of life. Again, as the germ-plasm of multicellular organisms is continuous from generation to generation, and at each impregnation gives rise to a more or less novel set of congenital characters, natural selection, in picking out of each generation the congenital characters which are of most service to the organisms presenting them, is really or fundamentally at work upon those variations of the germ-plasm which in turn give origin to these variations of organisms that we recognize as congenital. Therefore, natural selection has always to wait and to watch for such variations of germ-plasm as will eventually prove beneficial to the individuals developed therefrom, who will then transmit this peculiar quality of germ-plasm to their progeny, and so on. Therefore also—and this is most important to remember—natural selection as thus working becomes the one and only cause of organic evolution in all the multicellular organisms, just as the direct action of the environment is the one and only cause of it in the case of all the unicellular organisms. But inasmuch as the multicellular organisms were all in the first instance derived from the unicellular, and inasmuch as their germ-plasm is of so stable a nature that it can never be altered by any agencies internal or external to the organisms presenting it, it follows that all congenital variations are the remote consequences of aboriginal differences on the part of unicellular ancestors. And, lastly, it follows also that these congenital variations—although now so entirely independent of external conditions of life, and even of activities internal to organisms themselves—were originally and exclusively due to the direct action of such conditions on the lives of their unicellular ancestors; while even at the present day no one congenital variation can arise which is not ultimately due to differences impressed upon the protoplasmic substance of the germinal elements, when the parts of which these are now composed constituted integral parts of the protozoa, which were directly and differentially affected by their converse with their several environments.
Again, if for the sake of distinctness we neglect all these far-reaching deductions from his theory of heredity whereby Weismann constructs this elaborate theory of organic evolution, and fasten our attention only upon the former, we may briefly summarize the fundamental difference between his theory of heredity and Darwin’s theory of heredity thus.
Darwin’s theory of heredity is the theory of Pangenesis: it supposes that all parts of the organism generate anew in every individual the formative material which, when collected together in the germ-cells, constitutes the potentiality of a new organism; and that this new organism, when developed, resembles its parents simply because all the formative material in each of the parents has been thus generated by, and collected from, all parts of their respective bodies. Weismann’s theory of heredity, on the other hand, is the theory of the Continuity of Germ-plasm: it supposes that no part of the parent organism generates any of the formative material which is to constitute the new organism; but that, on the contrary, this material stands to all the rest of the body in much the same relation as a parasite to its host, showing a life independent of the body, save in so far as the body supplies to it appropriate lodgement and nutrition; that in each generation a small portion of this substance is told off to develop a new body to lodge and nourish the ever-growing and never-dying germ-plasm—this new body, therefore, resembling its so-called parent body simply because it has been developed from one and the same mass of formative material; and, lastly, that this formative material, or germ-plasm, has been continuous through all generations of successively perishing bodies, which therefore stand to it in much the same relation as annual shoots to a perennial stem: the shoots resemble one another simply because they are all grown from one and the same stock.
CHAPTER II.
Later Additions to Weismann’s System up to the year 1892.