Tallness, being an impulse toward extension, can never be Dwarfness, which is an impulse toward contraction. Black can never be White. Square can never be Round. Yet two opposite traits, both influencing development, may come to a mean, or poise, in an individual organism; as is seen in the grey offspring of a black rabbit mated with a white rabbit. But it is a _counterpoise_ merely of contrary factors. The traits of Blackness and Whiteness remain absolute and unalterable. If now, the reader has grasped these leading principles of Plant-biology, he is in a position to follow the new application of them to Human Biology which I now venture to present. Without going into details of physiology, it may be stated that the principles of reproduction are so identical in plants and living creatures as wholly to justify argument from one to the other. The only differences are in degrees of structural complexity as organisms rise higher in the scale of development, and demand, accordingly, more complex organs and functions for the more perfect manifestation of their characteristics; as also for the transmission of these to offspring. It may be repeated, however, that Mendelian law is found to hold good in humans, both in the hereditary transmission of normal characteristics and in the hereditary transmission of the abnormal traits of disease and degeneracy. Increasing complexities, structural and functional, are indispensable to the presentment of the attributes of the higher species, Man. But such complexities are, nevertheless, continuous with and have sprung out of the simplicities of lower and rudimentary organisms, precisely as the branches and leaves and flowers of a plant are continuous with and have sprung out of its roots. A vital and important biological detail (to be considered later) is that plants are not, as living creatures are, differentiated into a right and a left-side, identical in construction. Another is that plants are self-fertilising. With the lower animals, plural births are the rule. And in these, the still crude and imperfect differentiations of the Contrasting Traits allow of piebald and other modes of chequered colour and amorphous construction. The higher the organism, the more complex are the biological requirements for its pre-natal development, as for its post-natal nurture. The functions of Parenthood, both physiological and psychological, are always evolving to higher and more complex issues, therefore, as the species to be reproduced and nurtured becomes more complex. In human births, single offspring is the normal. Twin births are comparatively rare. And that these are abnormal is shown by twins being below the average always in health or in faculty; usually in both. IV