II
Now, Biology shows that plants and living creatures develop from a single microscopic cell, formed by the union of two half-cells, of which each half was contributed by one of the two parents.
Clearly then, a hybrid plant is one that has sprung from the union of two half-cells, one of which bore the Dominant traits of one parent, while the other bore the Recessive traits of the other parent. But because Dominant traits overpower Recessive traits in development, the cross between a tall plant and a dwarf plant produces tall offspring only—Tallness being a Dominant trait which overpowers the Recessive trait of Dwarfness. So too, the cross between a plant bearing coloured and a plant bearing white flowers produces offspring bearing coloured flowers only—Colour being Dominant over the Recessive Trait of Whiteness.
But because the Recessive traits of Dwarfness and of Whiteness were only overpowered in the plant-development, by the Dominant traits of Tallness and Colour, but were neither lost nor impaired in stock, hybrid plants that had shown only Dominant traits in growth and constitution, produce, nevertheless, two sorts of sex-cells for plant-reproduction: cells that bear the Recessive traits of the one parent, and cells that bear the Dominant traits of the other parent. So that in the fertilisation of one another by such hybrids, cells bearing Dominant traits mate with other cells bearing Dominant traits, and produce plants of pure Dominant type—Tall or Coloured, like one of the grandparents. While cells bearing Recessive traits mate with other cells bearing Recessive traits, and produce plants of pure Recessive type—Dwarf or White, like the other grandparent.
It is seen, therefore, that in plants, when a cell bearing Dominant traits mates with one bearing Recessive traits, the Dominant characteristics so overpower the Recessive that these latter lie latent, and concealed, in the resulting plant. But when a cell bearing Recessive traits mates with another cell bearing Recessive traits, the resulting plant (its growth and development not over-ridden now by the more assertive Dominant traits) is able to develop its Recessive characteristics.
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These interesting and significant laws of plant-heredity and constitution, discovered by Mendel in peas, have since been found by many expert observers to hold true as regards other species of plants; as too in poultry, in mice, and in rabbits, and moreover, in the hereditary transmission of human characteristics.
In Heredity and Variation, Dr. Saleeby points out that in the mating of a black with a white rabbit, some of the offspring will be black like one parent, some white like the other, and some grey—a blend of the colours of both parents.
In the last case, the Dominant trait of Blackness, derived from one rabbit-parent, blends in the fur of the rabbit-offspring with the Recessive trait of Whiteness, derived from the other rabbit-parent; a grey rabbit resulting. But that the Contrasted Traits come to no more than a temporary and partial compromise during the life of such a rabbit-individual, without either of the traits losing its intrinsic characteristic—Blackness and Whiteness, respectively—is proved by the fact that these grey rabbit-offspring, on further breeding, produce not grey rabbits, but black rabbits and white rabbits; proving that the Black trait and the White trait in them remained distinct and segregated, neither altering its character in the least degree.
It is as though one should take a spoonful of black pepper and a spoonful of white salt, and thoroughly mix them. A drab "pepper-and-salt" mixture will result. But neither pepper nor salt will have changed its colour or its properties one iota. Could they be separated out again, each would be precisely as it had been before mixing. So it is with the Dominant and the Recessive traits in living organisms. They commingle intimately, but each retains its original and intrinsic quality.
All the diverse and beautiful varieties of vegetation and the loveliness of flowers, in form and colour, result from multiple associations in hybrid-plants, of those which are known as the "Contrasted Traits" of parent-stock.
III
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.
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