The Principles of Biology, Volume 1 (of 2)

by Herbert Spencer

We conclude, then, that the products of a fertilized germ go on accumulating by simple growth, so long as the forces whence growth results are greatly in excess of the antagonist forces; but that when diminution of the one set of forces or increase of the other, causes a considerable decline in this excess and an approach towards equilibrium, fertilized germs are again produced. Whether the germ-product be organized round one axis or round the many axes that arise by agamogenesis, matters not. Whether, as in the higher animals, this approach to equilibrium results from that disproportionate increase of expenditure entailed by increase of size; or whether, as in most plants and many inferior animals, it results from absolute or relative decline of nutrition; matters not. In any case the recurrence of gamogenesis is associated with a decrease in the excess of tissue-producing power. We cannot say, indeed, that this decrease always results in gamogenesis: some organisms multiply for an indefinite period by agamogenesis only. The Weeping Willow, which has been propagated throughout Europe, does not seed in Europe; and yet, as the Weeping Willow, by its large size and the multiplication of generation upon generation of lateral axes, presents the same causes of local innutrition as other trees, we cannot ascribe the absence of sexual axes to the continued predominance of nutrition. Among animals, too, the anomalous case of the Tineidæ, a group of moths in which parthenogenetic multiplication goes on for generation after generation, seems to imply that gamogenesis does not necessarily result from an approximate balance of assimilation by expenditure. What we must say is 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 gamogenesis; but that there appear to be other conditions, in the absence of which approach to equilibrium is not followed by gamogenesis.

§ 79. The above induction is an approximate answer to the question—When does gamogenesis recur? but not to the question which was propounded—Why does gamogenesis recur?—Why cannot multiplication be carried on in all cases, as it is in many cases, by agamogenesis? As already said, biologic science is not yet advanced enough to reply. Meanwhile, the evidence above brought together suggests a certain hypothetical answer.

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. The several arguments which support this view, cannot be satisfactorily set forth until after the topics of Heredity and Variation have been dealt with. Leaving it for the present, I propose hereafter to re-consider it in connexion with sundry others raised by the phenomena of Genesis.

But before ending the chapter, it may be well to note the relations between these different modes of multiplication, and the conditions of existence under which they are respectively habitual. While the explanation of the teleologist is untrue, it is often an obverse to the truth; for though, on the hypothesis of Evolution, it is clear that things are not arranged thus or thus for the securing of special ends, it is also clear that arrangements which do secure these special ends tend to establish themselves—are established by their fulfilment of these ends. Besides insuring a structural fitness between each kind of organism and its circumstances, the working of "natural selection" also insures a fitness between the mode and rate of multiplication of each kind of organism and its circumstances. We may, therefore, without any teleological implication, consider the fitness of homogenesis and heterogenesis to the needs of the different classes of organisms which exhibit them.

Heterogenesis prevails among organisms of which the food, though abundant compared with their expenditure, is dispersed in such a way that it cannot be appropriated in a wholesale manner. Protophyta, subsisting on diffused gases and decaying organic matter in a state of minute subdivision, and Protozoa, to which food comes in the shape of extremely small floating particles, are enabled, by their rapid agamogenetic multiplication, to obtain materials for growth better than they would do did they not thus continually divide and disperse in pursuit of it. The higher plants, having for nutriment the carbonic acid of the air and certain mineral components of the soil, show us modes of multiplication adapted to the fullest utilization of these substances. A herb with but little power of forming the woody fibre requisite to make a stem that can support wide-spreading branches, after producing a few sexless axes produces sexual ones; and maintains its race better, by the consequent early dispersion of seeds, than by a further production of sexless axes. But a tree, able to lift its successive generations of sexless axes high into the air, where each gets carbonic acid and light almost as freely as if it grew by itself, may with advantage go on budding-out sexless axes year after year; since it thereby increases its subsequent power of budding-out sexual axes. Meanwhile it may advantageously transform into seed-bearers those axes which, in consequence of their less direct access to materials absorbed by the roots, are failing in their nutrition; for it thus throws off from a point at which sustenance is deficient, a migrating group of germs that may find sustenance elsewhere. The heterogenesis displayed by animals of the Cœlenterate type has evidently a like utility. A polype, feeding on minute annelids and crustaceans which, flitting through the water, come in contact with its tentacles, and limited to that quantity of prey which chance brings within its grasp, buds out young polypes which, either as a colony or as dispersed individuals, spread their tentacles through a larger space of water than the parent alone can; and by producing them, the parent better insures the continuance of its species than it would do if it went on slowly growing until its nutrition was nearly balanced by its waste, and then multiplied by gamogenesis. Similarly with the Aphis. Living on sap sucked from tender shoots and leaves, and able thus to take in but a very small quantity in a given time, this creature's race is more likely to be preserved by a rapid asexual propagation of small individuals, which disperse themselves over a wide area of nutrition, than it would be did the individual growth continue so as to produce large individuals multiplying sexually. And then when autumnal cold and diminishing supply of sap put a check to growth, the recurrence of gamogenesis, or production of fertilized ova which remain dormant through the winter, is more favourable to the preservation of the race than would be a further continuance of agamogenesis. On the other hand, among the higher animals living on food which, though dispersed, is more or less aggregated into large masses, this alternation of gamic and agamic reproduction ceases to be useful. The development of the germ-product into a single organism of considerable bulk, is in many cases a condition without which these large masses of nutriment could not be appropriated; and here the formation of many individuals instead of one would be fatal. But we still see the beneficial results of the general law—the postponement of gamogenesis 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 accumulation; 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.

Here, too, may fitly be pointed out the fact that, by "natural selection," there will in every case be produced the most advantageous proportion of males and females. If the conditions of life render numerical inequality of the sexes beneficial to the species, in respect either of the number of the offspring or the character of the offspring; then, those varieties of the species which approach more than other varieties towards this beneficial degree of inequality, will be apt to supplant other varieties. And conversely, where equality in the number of males and females is beneficial, the equilibrium will be maintained by the dying out of such varieties as produce offspring among which the sexes are not balanced.

Note.—Such alterations of statement in this chapter as have been made necessary by the advance of biological knowledge since 1864 have not, I think, tended to invalidate its main theses, but have tended to verify them. Some explanations to be here added may remove remaining difficulties.

Certain types, which are transitional between Protozoa and Metazoa, exhibit under its simplest form the relation between self-maintenance and race-maintenance—the integration primarily effecting the one and the disintegration primarily effecting the other. Among the Mycetozoa a number of amœba-like individuals aggregate into what is called a plasmodium; and while, in some orders, they become fused into a mass of protoplasm through which their nuclei are dispersed, in other orders (Sorophora) they retain their individualities and simply form a coherent aggregate. These last, presumably the earliest in order of evolution, remain united so long as the plasmodium, having a small power of locomotion, furthers the general nutrition; but when this is impeded by drought or cold, there arise spores. Each spore contains an amœboid individual; and this, escaping when favourable conditions return, establishes by fission and by union with others like itself a new colony or plasmodium. Reduced to its lowest terms, we here see the antagonism between that growth of the coherent mass of units which accompanies its physical prosperity, and that incoherence and dispersion of the units which follows unfavourable conditions and arrest of growth, and which presently initiates new plasmodia.

This antagonism, seen in these incipient Metazoa which show us none of that organization characterizing the Metazoa in general, is everywhere in more or less disguised forms exhibited by them—must necessarily be so if growth of the individual is a process of integration while formation of new individuals is a process of disintegration. And, primarily, it is an implication that whatever furthers the one impedes the other.

But now while recognizing the truth that nutrition and innutrition (using these words to cover not supply of nutriment only but the presence of other influences favourable or unfavourable to the vital processes) primarily determine the alternations of these; we have also to recognize the truth that from the beginning survival of the fittest has been shaping the forms and effects of their antagonism. By inheritance a physiological habit which modifies the form of the antagonism in a way favourable to the species, will become established. Especially will this be the case where the lives of the individuals have become relatively definite and where special organs have been evolved for casting off reproductive centres. The resulting physiological rhythm may in such cases become so pronounced as greatly to obscure the primitive relation. Among plants we see this in the fact that those which have been transferred from one habitat to another having widely different seasons, long continue their original time of flowering, though it is inappropriate to the new circumstances—the reproductive periodicity has become organic. Similarly in each species of higher animal, development of the reproductive organs and maturation of reproductive cells take place at a settled age, whether the conditions have been favourable or unfavourable to physical prosperity. The established constitutional tendency, adapted to the needs of the species, over-rides the constitutional needs of the individual.