It is a general physiological truth that while the building-up of the individual is going on rapidly, the reproductive organs remain imperfectly developed and inactive; and that the commencement of reproduction at once indicates a declining rate of growth and becomes a cause of arrest in growth.
It has now to be noticed how complexity of organisation is hindered by reproductive activity and conversely. The hydra's power to produce young ones from nearly all parts of its body is due to the comparative homogeneity of its body, while it is not improbable that the smallness of human fertility, compared with the fertility of large feline animals, is due to the greater complexity of the human organisation—more especially the organisation of the nervous system.
Of the inverse variation between activity and genesis we have examples in the contrast between the fertility of birds and the fertility of mammals. Comparing the large with the large and the small with the small, we see that creatures which continually go through the muscular exertion of sustaining themselves in the air and propelling themselves rapidly through it are less prolific than creatures of equal weights which go through the smaller exertion of moving about over solid surfaces. The extreme infertility of the bat is most striking when compared with the structurally similar but very prolific mouse; a difference in the rate of multiplication which may fairly be ascribed to the difference in the rate of expenditure.
Interpretation and Qualification
Derived as the self-sustaining and waste-sustaining forces are from a common stock of force, it necessarily happens that, other things being equal, increase of the one involves decrease of the other. It may therefore be set down as a law that every higher degree of organic evolution has for its concomitant a lower degree of the peculiar organic dissolution which is seen in the production of new organisms.
How is the ratio between individuation and genesis established in each case? All specialties of the reproductive process are due to the natural selection of favourable variations. Given a certain surplus available for race preservation, and it is clear that by indirect equilibration only can there be established that peculiar distribution of this surplus which is seen in each case.
Here a qualification must be made. Recognising the truth that every increase of evolution which is appropriate to the circumstances of an organism brings an advantage somewhat in excess of its cost, the general law, more strictly stated, is that genesis decreases not quite so fast as individuation increases. The result of greater individuation—whether it takes the form of greater strength or higher speed, facilitates some habitual movement or utilises better the absorbed aliment—is a greater surplus of vital capital; part of which goes to the aggrandisement of the individual and part to the formation of new individuals. Hence every type that is best adapted to its conditions has a rate of multiplication that insures a tendency to predominate. Survival of the fittest, acting alone, is ever replacing inferior species by superior species. But beyond the longer survival, and therefore greater chance of leaving offspring, which superiority gives, we see here another way in which the spread of the superior is insured. Though the more evolved organism is the less fertile absolutely, it is the more fertile relatively.
Multiplication of the Human Race
What causes increase or decrease of genesis in other creatures causes increase or decrease of genesis in man. It is true that, even more than hitherto, our reasonings are here beset with difficulties. So numerous are the inequalities in the conditions that but few unobjectionable comparisons can be made. The human races differ not only in their sizes and foods, and in the climates they inhabit, but also their expenditures in bodily and mental action are extremely unequal.