Herbert Spencer

by J. Arthur Thomson

But two cautions must be emphasised (a) that Spencer, in this discussion, dealt only with those direct adjustments which are referable to the action of use or disuse, or of surrounding influences; and (b) that we have no security in regarding these as being as such transmissible.

By adaptations biologists usually mean permanent adjustments, and there are two theories of the origin of these: (a) by the action of natural selection on inborn variations, or (b) by the inheritance of the directly acquired bodily modifications.

Cell-Life.—In this chapter, interpolated in the revised edition, Spencer summed up the main results of the study of the structural units or cells which build up a body. "Nature everywhere presents us with complexities within complexities, which go on revealing themselves as we investigate smaller and smaller objects." Thus protoplasm itself has a complicated structure; the nucleus of the cell is a little world in itself; and the cell-firm has other partners, such as the centrosome. When a cell divides, the readily stainable bodies or chromosomes, present in definite number within the nucleus, are divided, usually by a most intricate process, in such a manner that equal amounts are bequeathed by the mother-cell to each of the two daughter-cells. Spencer favoured the view that the chromatin, which "consists of an organic acid (nucleic acid) rich in phosphorus, combined with an albuminous substance, probably a combination of various proteids" may be peculiarly unstable and active.

"From the chromatin, units of which are thus ever falling into stabler states, there are ever being diffused waves of molecular motion, setting up molecular changes in the cytoplasm. The chromatin stands towards the other contents of the cell in the same relation that a nerve-element stands to any element of an organism which it excites." "We may infer that cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes, leaving behind the substance excited, now distinguished as cytoplasm."

But the suggestion that chromosomes may be stimulating, change-exciting elements, does not, Spencer goes on to say, conflict with the conclusion that the chromosomes are the vehicles conveying hereditary traits. "While the unstable units of chromatin, ever undergoing changes, diffuse energy around, they may also be units which, under the conditions furnished by fertilisation, gravitate towards the organisation of the species. Possibly it may be that the complex combination of proteids, common to chromatin and cytoplasm, is that part in which constitutional characters inhere; while the phosphorised component, falling from its unstable union and decomposing, evolves the energy which, ordinarily the cause of changes, now excites the more active changes following fertilisation."

From this speculation Spencer passes to a brief consideration of what occurs before and during the fertilisation of the ovum. Before fertilisation is accomplished the nucleus of the ovum normally divides twice in rapid succession, and gives off two abortive cells—known as polar bodies—which come to nothing. The usual result of this "maturation," as it is called, is that the number of chromosomes in the ovum is reduced to a half of the normal number characteristic of the cells of the species to which it belongs. In the history of the male element or spermatozoon, there is an analogous reduction, so that when spermatozoon and ovum unite in fertilisation the normal number is restored. It is now recognised that the maturation-divisions are useful in obviating the doubling of the number of chromosomes which fertilisation would otherwise involve, and it has also been suggested that this continually recurrent elimination of chromosomes may be one of the causes of variation.

Spencer suggested another interpretation. He pointed out the general fact that sexual reproduction (gamogenesis) commonly occurs when asexual reproduction (agamogenesis) is arrested by unfavourable conditions, that failing asexual reproduction initiates sexual reproduction. Now as egg-cells and sperm-cells are the outcome of often long series of cell divisions (asexual multiplication), may not the polar bodies, which are aborted cells, indicate that asexual multiplication can no longer go on, and that the conditions leading to sexual multiplication have set in? "As the cells which become spermatozoa are left with half the number of chromosomes possessed by preceding cells, there is actually that impoverishment and declining vigour here suggested as the antecedent of fertilisation." In short, the germ-cells, separately considered, are cells in which the power of further asexual multiplication is exhausted, as it is known to become exhausted in Infusorians and such body-cells as nerve-cells; there arises a state which initiates a sexual union or amphimixis of the two kinds of germ-cells, and the decrease in the chromatin is an initial cause of that state.

We quote this speculation as a good instance of Spencer's continual endeavour to rationalise puzzling and exceptional facts by showing that there is a general principle underlying them. But the objections to his hypothesis are numerous. Mature ova or spermatozoa will not normally divide if left to themselves, but that is because they are specialised to secure amphimixis, not because their powers are in any way declining or impoverished. A parthenogenetic ovum gives off one polar body—though without reduction in the number of chromosomes—and then proceeds by asexual multiplication or ordinary cell division to build up a body. The spore of a fern or a moss has only half the number of chromosomes that the cells of its producer have, yet it proceeds by asexual multiplication or ordinary cell-division to build up the gametophyte or sexual generation.

Genesis.—Spencer attempted a classification of the various modes of reproduction that occur among organisms—asexual reproduction (agamogenesis) by fission and budding, sexual reproduction (gamogenesis) by specialised germ-cells usually involving fertilisation or amphimixis, and all the complications involved in "alternation of generations" (metagenesis), the development of eggs without fertilisation (parthenogenesis), and so on. But what gives particular importance to the chapter on genesis is not the discussion of the modes of reproduction, but the general conclusion that nutrition and reproduction are antithetic processes—a very fruitful idea in biology.

Where there is alternation of generation, sexual and asexual, we find that asexual reproduction continues as long as the forces which result in growth are greatly in excess of the antagonistic forces. Conversely the recurrence of sexual reproduction occurs when the conditions are no longer so favourable to growth. Similarly, where there is no alternation, "new individuals are usually not formed while the preceding individuals are still rapidly growing—that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure."

In illustration Spencer points to facts like the following: "Uniaxial plants begin to produce their lateral, flowering axes, only after the main axis has developed the great mass of its leaves, and is showing its diminished nutrition by smaller leaves, or shorter internodes, or both"; "root-pruning" and "ringing," which diminish the nutritive supply, promote the formation of flower-shoots; high nutrition in plants prevents or arrests flowering.

Similarly, the aphides or green-flies, hatched from eggs in the spring, multiply by parthenogenesis throughout the summer; with extraordinary rapidity one generation follows on another; but when the weather becomes cold and plants no longer afford abundant sap, males reappear and sexual reproduction sets in. It has been shown that in the artificial summer of a green-house, parthenogenesis may continue for four years. In a large number of cases of ordinary reproduction, e.g. in birds, the connexion between cessation of growth and commencement of reproduction is very distinct.

It is not difficult to see the advantages in the postponement of sexual reproduction until the rate of growth begins to decline. "For so long as the rate of growth continues rapid, there is proof that the organism gets food with facility—that expenditure does not seriously check assimilation; and that the size reached is as yet not disadvantageous: or rather, indeed, that it is advantageous. But when the rate of growth is much decreased by the increase of expenditure—when the excess of assimilative power is diminishing so fast as to indicate its approaching disappearance—it becomes needful, for the maintenance of the species, that this excess shall be turned to the production of new individuals; since, did growth continue until there was a complete balancing of assimilation and expenditure, the production of new individuals would be either impossible or fatal to the parent. And it is clear that 'natural selection' will continually tend to determine the period at which gamogenesis commences, in such a way as most favours the maintenance of the race."

That natural selection punctuates the life to advantage does not imply that it works directly towards such a remote goal as species-maintaining; it means that the arrangements which do secure this end most effectively are those which tend to establish themselves. Those that do not secure this end are eliminated.