The cases are analogous to some before named, in which more abundant food simultaneously aggrandizes the individual and adds to the production of new individuals: the difference between the cases being, that instead of a better external supply of materials there is a better internal utilization of materials. Creatures of the same species notoriously differ in goodness of constitution. Here there is some visceral defect, showing itself in feebleness of all the functions; while here some peculiarity of organic balance, some high quality of tissue, some abundance or potency of the digestive juices, gives to the system a perpetual high tide of rich blood, which serves at once to enhance the vital activities and to raise the power of propagation. Such variations, however, are independent of changes in the proportion between Individuation and Genesis. This remains the same, while both are increased or decreased by the increase or decrease of the common stock of materials.
An illustration will best clear up any perplexity. Let us say that the fuel burnt in the furnace of a locomotive steam-engine, answers to the food which a man consumes. Let us say that the produced steam expended in working the engine, corresponds to that portion of absorbed nutriment which carries on the man’s functions and activities. And let us say that the steam blowing off at the safety-valve, answers to that portion of the absorbed nutriment which goes to the propagation of the race. Such being the conditions of the case, several kinds of variations are possible. All other circumstances remaining the same, there may be changes of proportion between the steam used for working the engine and the steam that escapes by the safety-valve. There may be a structural or organic change of proportion. By enlarging the safety-valve or weakening its spring, while the cylinders are reduced in size, there may be established a constitutionally-small power of locomotion and a constitutionally-large amount of escape-steam; and inverse variations so produced, will answer to the inverse variations between Individuation and Genesis which different types of organisms show us. Again, there may be a functional change of proportion. If the engine has to draw a considerable load, the abstraction of steam by the cylinders greatly reduces the discharge by the safety-valve; and if a high velocity is kept up, the discharge from the safety-valve entirely ceases. Conversely, if the velocity is low, the escape-steam bears a large ratio to the steam consumed by the motor apparatus; and if the engine becomes stationary the whole of the steam escapes by the safety-valve. This inverse variation answers to that which we have traced between Expenditure and Genesis, as displayed in the contrasts between species of the same type but unlike activities, and in the contrasts between active and inactive individuals of the same species. But now beyond these inverse variations between the quantities of consumed steam and escape-steam, which are structurally and functionally caused, there are coincident variations, producible in both by changes in the quantity of steam supplied—changes which may be caused in several ways. In the first place, the fuel thrown into the furnace may be increased or made better. Other things equal, there will result a more active locomotion as well as a greater escape; and this will answer to that simultaneous addition to its individual vigour and its reproductive activity, caused in an animal by a larger quantity, or a superior quality, of food. In the second place, the steam generated may be economized. Loss by radiation from the boiler may be lessened by a covering of non-conducting substances; and part of the steam thus prevented from condensing, will go to increase the working power of the engine, while part will be added to the quantity blowing off. This variation corresponds to that simultaneous addition to bodily vigour and propagative power, which results in animals that have to expend less in keeping up their temperatures. In the third place, by improvement of the steam-generating apparatus, more steam may be obtained from a given weight of fuel. A better-formed evaporating surface, or boiler tubes which conduct more rapidly, or an increased number of them may cause a larger absorption of heat from the burning mass or the hot gases it gives off; and the extra steam generated by this extra heat will, as before, augment both the motive force and the emission through the safety-valve. And this last case of coincident variation, is parallel to the case with which we are here concerned—the augmentation of individual expenditure and of reproductive energy, that may be caused by a superiority of some organ on which the utilizing or economizing of materials depends.
Manifestly, therefore, an increased expenditure for Genesis, or an increased expenditure for Individuation, may arise in one of two quite different ways—either by diminution of the antagonistic expenditure, or by addition to the store which supplies both expenditures; and confusion results from not distinguishing between these. Given the ratio 4 to 20, as expressive of the relative costs of Genesis and Individuation; then the expenditure for Genesis may be raised to 5 while the expenditure for Individuation is raised to 25, without any alteration of type, merely by favourable circumstances or superiority of constitution. On the other hand, circumstances remaining the same, the expenditure for Genesis may be raised from 4 to 5, by lowering the expenditure for Individuation from 20 to 19: which change of ratio may be either functional and temporary, or structural and permanent. And only when it is the last does it illustrate that inverse variation between degree of evolution and degree of procreative dissolution, which we have everywhere seen.
§ 370. There is no reason to suppose, then, that the laws of multiplication which hold of other beings, do not hold of the human being. On the contrary, there are special facts which unite with general implications to show that these laws do hold of the human being. The absence of direct evidence in some cases where it might be looked for, we find fully explained when all the factors are taken into account. And certain seemingly-adverse facts prove, on examination, to be facts belonging to a different category from that in which they are placed, and harmonize with the rest when rightly interpreted.
The conformity of human fertility to the laws of multiplication in general, being granted, it remains to inquire what effects must be caused by permanent changes in men’s natures and circumstances. Thus far we have observed how, by their exceptionally-high evolution and exceptionally-low fertility, mankind display the inverse variation between Individuation and Genesis, in one of its extremes. And we have also observed how mankind, like other kinds, are functionally changed in their rates of multiplication by changes of conditions. But we have not observed how alteration of structure in Man entails alteration of fertility. The influence of this factor is so entangled with the influences of other factors which are for the present more potent, that we cannot recognize it. Here, if we proceed at all, we must proceed deductively.
[Note.—From among the publications of the American Academy of Political and Social Science, there was sent to me some years ago an essay entitled “The Significance of a Decreasing Birth Rate” by (Miss) J. L. Brownell, Fellow in Political Science, Bryn Mawr College. This essay contains a number of elaborate comparisons drawn from the vital statistics of the tenth United States Census. The results of these comparisons are thus summed up:—
“1. Whether or not it be true that the means spoken of by Dr. Billings, M. Dumont, M. Levasseur, and Dr. Edson has become an important factor in the diminishing birth-rate of civilized countries, it is evident that it is not the only factor, and that, quite apart from voluntary prevention, there is a distinct problem to be investigated. This is shown by the fact that the white and the colored birth-rate vary together.
“2. Mr. Spencer’s generalization that the birth-rate diminishes as the rate of individual evolution increases is confirmed by a comparison of the birth-rates with the death-rates from nervous diseases, and also with the density of population, the values of agricultural and manufactured products, and the mortgage indebtedness.”
Of course multitudinous differences of race, class, mode of living, occupation, locality, make it difficult to draw positive inferences from the data; but the inferences above drawn are held to remain outstanding after allowing for all the qualifying conditions.]