Including under individuation all those processes completing and maintaining individual life, and under genesis all those aiding the formation and perfecting of new individuals, the two are necessarily antagonistic. Every higher degree of individual evolution is followed by a lower degree of race multiplication, and _vice versâ_. Progress in bulk, complexity or activity involves retrogress in fertility; and progress in fertility involves retrogress in bulk, complexity, or activity. The same quantity of matter may be divided into many small wholes or few large wholes; but number negatives largeness, and largeness negatives number. 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.