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
As already mentioned, Sex is regarded by the large and ever-increasing order of the adherents of Mendel as a "Mendelian factor." But in applying Mendelian truth to humans, I venture to think the applications have not been carried to their ultimate and most momentous conclusions.
Because, given the keynote to the Principle of Duality in the phenomenon of the Contrasting Traits found manifesting in plant-heredity and constitution, the duality of the Human Sexes, with their respective orders of Contrasting characteristics, suggests itself as being analogous.
Human attributes, physical and mental, are seen, like those of plants, to group themselves into two distinct categories, the Male and the Female sex-characteristics, primary and secondary. And these, though wholly contrary in nature and in trend, are found—precisely as occurs in plants—linked together in the hybrid offspring of the two parents from whom they were, respectively, derived; blending in a temporal unity, but remaining, nevertheless, unchanged in their essential differences; coming to means and counterpoises in individual organisations, yet nevertheless preserved distinct and unalloyed in these, as is shown by their emergence, unaltered, in offspring of opposite sexes.
As a hybrid plant is the product of two parents characterised by opposite traits—Tallness and Dwarfness, for example—so, I submit, a human creature is the hybrid offspring of two parents characterised by opposite traits—Maleness and Femaleness, with the Sex-traits differentiating one sex from the other.
And at once a solution of the many baffling presentments and problems of Sex presents itself—of the enigma of man with Woman potential in him, of woman with Man potential in her; a key to the mysterious Duality of human biology and psychology, with its conflict of battling impulses, its harmonies of blending attributes, its innumerable and diverse developments in proportions, in means, in extremes; in normalities, eccentricities, deviations and reversions. And the analogy between the two orders of Traits—in Plant-life at the lower end of the scale of species, and in Human life and psychology at the higher end—suggests that the ever-increasing complexity of organisation and faculty which has characterised Evolutionary Progress, has had for aim, as it has had for method, the ever further differentiation and more perfect segregation, but, nevertheless, the ever closer and more intricate association of the contrary factors of Maleness and Femaleness.
In the lower organisms—plant and animal—the two groups of Traits are but crudely differentiated as characteristics distinguishing one sex from the other. In such lower organisms, Sex-development is merely rudimentary; the first foreshadowings in Life of two intrinsic orders of Essential Attribute, the progressive evolution whereof reveals two contrary trends in physiological and psychical inherences.
Like Light and Darkness, Heat and Cold, Sex is a phenomenon of Dual states which manifest by way of relativity. Without Maleness, Femaleness has no significance—no existence, in fact. And the converse. And in the lower and rudimentary forms of existence, in proportion to their degrees of undevelopment, the dual states of Sex are but faintly defined. The very lowly forms are bi-sexual and self-fertilising. While the first and simplest mode of reproduction is by cell-division merely; the principle of Sex, with its dual factors, functioning, but not yet differentiated into dual forms.