Herbert Spencer

by J. Arthur Thomson

Problems of Heredity—Physiological Units—A Digression—The Germ-Cells—Transmission of Acquired Characters—Inconceivability—A Priori Argument—Practical Conclusion

Heredity is the relation of genetic continuity which links generation to generation. An inheritance is all that the organism is or has to start with on its life-journey in virtue of the hereditary relation to parents and ancestors. In all ordinary cases, the inheritance has its initial material basis in the egg-cell and the sperm-cell which unite in fertilisation at the beginning of a new life, and these two kinds of germ-cells, which bear the maternal and the paternal contributions, have their peculiar virtue of reproducing like from like, just because they are the unchanged or very slightly changed cell-descendants of the fertilised ova from which the parents arose. A bud or a cutting separated off from a living creature—tiger-lily or potato, polyp or worm—reproduces an entire organism like the parent, if the appropriate nurture-conditions are available; and it can do so because it is a fair sample of the parental organisation. Similarly a germ-cell or two germ-cells in conjunction can develop into a creature like the parent or parents, in virtue of being the condensed essence of the parental organisation. And the germ-cell is this because of its direct continuity through undifferentiating cell-divisions with the original germ-cell from which the parental body developed.

Even in ancient times men pondered over the resemblances and differences between children and their parents—for like only tends to beget like—and wondered as to the nature of the bond which links generation to generation. But although the problems are old, the precise study of them is altogether modern. The first great step towards clearness was the formulation of the cell-theory by Schwann and Schleiden (1838-9), by Goodsir and Virchow, which made it clear that all but the simplest organisms are built up of cells or modifications of cells, and that the individual life usually begins as a fertilised egg-cell which proceeds by division and re-division, by differentiation and integration, to develop a more or less complex "body." It has become gradually clear that while the fertilised egg-cell gives rise to body-cells which become specialised, it also gives rise to unspecialised descendant-cells, which take no share in body-making, but become the germ-cells—the potential starting-points of another generation. A second great step was the accumulation of facts of inheritance showing that all sorts of qualities innate or inborn in the parents, essential and trivial, normal and abnormal, bodily and mental, may be transmitted to the offspring as part of the organic heritage. A third great step was implied in the acceptance which Darwin in particular won for the general idea of descent, for it is hardly too much to say that the scientific study of the problems of heredity began when it was recognised that heredity is a fundamental condition of evolution.

Problems of Heredity.—In regard to Heredity there are three large problems which tower above the crowd of more detailed problems. The first is: In what way are the germ-cells peculiar, how do they differ from ordinary cells, what gives them their unique reproductive power, how do they come to be such marvellous units that their development results in a new organism? Only two answers have been suggested: (1) that the germ-cells become receptacles of representative samples from the different parts of the body (the pangenetic theory), and (2) that the germ-cells owe their unique character to the fact that they are, along lines of undifferentiated cell-lineage, the direct descendants of the fertilised ova of the parents (the theory of germinal continuity). Thanks, largely, to Weismann, the second view has prevailed over the first, for which there is little factual basis.

The second large problem is as to the way in which it may be supposed that the hereditary qualities are represented in the germ-cell. Is the germ-cell an extremely complex chemical mixture without pre-formed architecture, which, as it lives and grows, gradually gives rise to heterogeneous elements, differentiating along diverse lines according to their diverse relations to one another and to their surrounding conditions? Or is it from the first a complex architecture, an intricate organisation of a large number of items representing particular qualities, a mosaic of inheritance-bearers?

The third large problem is as to the modes in which the inheritance, normally bi-parental, and in some sense always a mingling of ancestral contributions, can express itself. Sometimes the expression is one-sided, sometimes it is a blend. The mother may look out of one eye, and the father out of another, or the grandfather may be re-incarnated. By inter-breeding hybrids pure types may be got, or reversions, or "an epidemic of variations." This is the problem of the diverse modes of hereditary transmission, which we know in some cases to be expressible in a formula, such as Mendel's law or Galton's law, and for which we can sometimes hazard a hypothetical physiological interpretation.

Physiological Units.—To each of these three problems Spencer made a contribution. He started with the legitimate and fertile hypothesis of "physiological units"—the ultimate life-bearing elements, intermediate between the chemical molecules and the cell. Just as the same kinds and even the same number of atoms compose by different arrangements numerous quite different chemical molecules, e.g. in the protein-group, so out of similar molecules diversely grouped an immense variety of "physiological units" may be evolved. Out of the same pieces of coloured glass one may get in the kaleidoscope a very large number of distinct patterns, so in the course of nature similar molecules, grouping themselves differently, have formed a very large number of distinct "physiological units." The grouping is not merely positional and static as in the kaleidoscope; it is dynamic and vital. Since Spencer sketched his idea in 1864 many biologists have thought of units intermediate between the chemical molecules and the cell, and the number of different names which have been bestowed upon them is extraordinary, each voyager re-naming his discovery, ignorant of or ignoring those who had previously sailed the same seas. This recognition of "physiological units" was a natural step in analysis as soon as it began to be recognised that the cell was a little world in itself, a "firm" with many partners. While we cannot agree with Delage that "Spencer est le vrai père de la conception initiale," since Brücke expressed the same idea in 1861, Spencer's exposition in 1864 was quite independent, and it has not found the recognition it deserved.

It should be noted that the "gemmules" which Darwin assumed in his provisional hypothesis of pangenesis to be given off by the various cells of the body, were supposed to be of innumerable unlike kinds, whereas in Spencer's argument "the implication everywhere is that the physiological units are all of one kind."

It is admitted that the molecules of a crystallisable substance have more or less mysterious relations to one another—"polarities" as we call them—which result in definite crystalline forms appearing in definite conditions, with a certain amount of diversity as everyone may see in snow-crystals, and as is more precisely known in the case of certain substances which have several forms of crystallisation. But just as chemical molecules have in virtue of their organisation (always dynamic as well as static) certain prescribed modes of relating themselves to others like themselves, and building up a beautiful integrate, a crystal, so, as Spencer pointed out, the "physiological units" have their "polarities," i.e. their inherent constitutional tendencies to build up forms along with their fellows. Here we have two useful suggestions, (1) that development is like an elaborate organic crystallisation, only much more energetically dynamic, and (2) that the big fact of heredity—that like tends to beget like—has its parallel in the way in which a minute fragment of a crystal can in the appropriate environment of a solution of the same substance build up a crystal like the original form from which it was separated. Germ-cells are potential samples of the organisation which is expressed in the parent, but Spencer did not advance to the more distinctively modern position which recognises that they are separated off rather from the fertilised ovum which gave rise to the parent's body than from that body itself. The parental body is the trustee rather than the producer of the germ-cells.