The lay reader need not perplex himself with the problems and phenomena of Mendelism.
All he requires to remember are its three leading principles. Firstly, that in the world of Life, plant and animal, living attributes are divided into two contrasting orders. Secondly, that of these two orders of so-called "Contrasted Traits" ("Contrasting Traits" would be a fitter phrase), the two groups are as absolute and opposite in character and in significance as are the plus and the minus signs of Algebra, the Positive and the Negative potentials of Electricity, the conditions of Light and Darkness, of Blackness and Whiteness, of Heat and Cold. Thirdly, that the Dominant order of traits are paramount over and extinguish the Recessive order of traits.
To sustain her equilibrium by a counterpoise of dual and contrary factors, physical and vital, Nature must preserve these factors absolute and unchangeable as the constitution and the opposite attraction of The Poles. But in order to produce her countless progressive variations of form and attribute, physical and vital, she assembles these contrary factors in countless progressively complex combinations, co-operations and correlations.
It is conceivable, therefore, that the infinite gradations and variations of form and attribute found in the world of living creatures are, as in the world of plants, phenomena of the ever further differentiation and more complex combination, in the hybrid offspring of two parents, of two orders of Contrasting Traits, transmitted by the respective parents.
In all their multiple associations and diverse developments, however, the two Sets of Traits remain unchanged, precisely as do the individual elements of chemical combinations. Variations in species result, accordingly, not from change in the essential traits, but from changes in the modes and the degrees of the commingling of these in organisms; and in the modes and degrees of their ever more complex associations in such.
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.