Nutrition and Reproduction.—Spencer's doctrine of the antithesis between Nutrition and Reproduction is of great importance in biology, and we must dwell on it a little longer.
The life of organisms is rhythmic. Plants have their long period of vegetative growth, and then suddenly burst into flower. Animals in their young stages grow rapidly, and as the growth ceases reproduction normally begins; or again, just as perennial plants are strictly vegetative through a great part of the year or for many successive years, but have their periodic recurrence of flowers and fruit, so it is with many animals which after remaining virtually asexual for prolonged periods, exhibit periodic returns of a reproductive or sexual tide. Foliage and fruiting, periods of nutrition and crises of reproduction, hunger and love, must be interpreted as life-tides, punctuated by the seasons and other circumstances through the agency of Natural Selection, but none the less as expressions of the fundamental organic rhythm between rest and work, upbuilding and expenditure, repair and waste, which on the protoplasmic plane are known as anabolism and katabolism.[8]
[8] P. Geddes and J. Arthur Thomson, The Evolution of Sex, revised edition, 1901, p. 238.
Anabolism and katabolism are the two sides of protoplasmic life, and the major rhythms of the respective preponderance of these give the antitheses of growth and multiplication, asexual and sexual reproduction. The contrasts of metabolism represent the swings of the organic see-saw; the periodic contrasts correspond to alternate weightings or lightenings of the two sides.
Spencer's induction that "an approach towards equilibrium between the forces which cause growth and the forces which oppose growth, is the chief condition to the recurrence of sexual reproduction," is an approximate answer to the question—When does sexual reproduction recur? But there remains, he says, the more difficult question—Why does sexual reproduction recur? Why cannot multiplication be carried on in all cases, as it is in many cases, by asexual reproduction?
As yet, he says, biology is not advanced enough to give a reply, but a certain hypothetical answer may be suggested. "Seeing, on the one hand, that gamogenesis recurs only in individuals which are approaching a state of organic equilibrium; and seeing, on the other hand, that the sperm-cells and germ-cells thrown off by such individuals are cells in which developmental changes have ended in quiescence, but in which, after their union, there arises a process of active cell-formation; we may suspect that the approach towards a state of general equilibrium in such gamogenetic individuals is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell and germ-cell is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ—a result probably effected by mixing the slightly different physiological units of slightly different individuals."
Now, while Spencer was probably right in saying that fertilisation promotes change, we cannot think that he succeeded in finding what he was seeking, namely a primary physiological reason why sexual reproduction should occur. It may be pointed out that it is only in a limited sense that sperm-cells or egg-cells can be spoken of as in a state of "quiescence," and that it is only in a limited sense that the organism which has finished growing and is beginning to be sexual can be spoken of as in a state of general or molecular equilibrium. An egg-cell is quiescent, as a seed lying in the ground is quiescent, awaiting its stimulus of warmth and moisture; a sperm-cell is quiescent, as a spore floating in the air is quiescent, awaiting its appropriate soil. The egg-cells and sperm-cells cannot be very quiescent since they do so much when they unite. Moreover, we have simply to recall the facts of natural parthenogenesis on the one hand or of artificial parthenogenesis on the other, to see that the quiescence of the egg is a secondary restriction adapted to secure amphimixis. Moreover, the familiar external and internal changes which occur in the bodies of organisms when they are approaching sexual maturity suggest the very opposite of general or molecular equilibrium.
It may be pointed out that although asexual multiplication persists in many organisms both large and small, and is sometimes the only method of multiplication, yet it is apt to be a somewhat expensive process and would be difficult to arrange for in highly differentiated animals. On the other hand, asexual multiplication succeeds admirably in many cases; it does not imply degeneration; it is not inconsistent with the occurrence of variations; and it is conceivable that it might have been arranged for even in the highest animals. What other reason can there be why the circuitous process of sexual reproduction has been preferred? It may be said that the arrangement by which multiplication is secured through special germ-cells, more or less apart from the cells which build up the body, may be justified as an arrangement which prevents or tends to prevent the transmission of bodily modifications, many of which are detrimental. But as this cuts both ways, preventing or tending to prevent the transmission of useful modifications, there must be some other reason why the circuitous process of sexual reproduction has been preferred. We believe the answer to be that sexual reproduction is an adaptive process securing the benefits of amphimixis, for in amphimixis and in the changes preparatory to it, there is an important source of variation. In one of his essays Weismann wrote as follows:—
"Sexual reproduction is well known to consist in the fusion of two contrasted reproductive cells, or perhaps even in the fusion of their nuclei alone. These reproductive cells contain the germinal material or germ-plasm, and this again, in its specific molecular structure, is the bearer of the hereditary tendencies of the organisms from which the reproductive cells originate. Thus in sexual reproduction two hereditary tendencies are in a sense intermingled. In this mingling, I see the cause of the hereditary individual characteristics; and in the production of these characters, the task of sexual reproduction. It has to supply the material for the individual differences from which selection produces new species."
When we inquire into the reasons for the occurrence of a process such as sexual reproduction, there are four different questions which may be put: (1) We may inquire into the historical evolution of the process, so far as that can be legitimately imagined or inferred from still persistent grades. (2) We may try to discover what factors may have operated in the course of evolution in raising the process from one step of differentiation to another. (3) We may also try to show how the process is justified by its advantages either self-regarding or species-maintaining. (4) We may inquire into the physiological sequences in the internal economy of the individual organism which lead up to the process in question. There is no doubt always an immediate necessity for the occurrence of an organic process, but we are in many cases quite unable at present to do more than describe the series of events without understanding their causal nexus. The reason for this is apparent, since the organism is much more than a detached inanimate engine; it is a system which has summed up in it the long results of time, the history of ages. Its rhythms and periodicities and crises puzzle us because they originated under conditions which obtained untold millennia ago. Thus some processes in higher animals may have had originally a reference to tides from the reach of which their present possessors are far withdrawn.
We have entered on this digression partly for clearness sake, and partly to explain why Spencer had, as we think, very limited success in his answer to the question: Why does sexual reproduction occur? The curious reader may be referred to the discussion of these problems in The Evolution of Sex, Contemporary Science Series, Revised Edition, 1901.
The Germ-Cells.—But we cannot leave the interesting chapter on genesis without referring to another of Spencer's conclusions, which does not seem to us to be quite consistent with facts.