Long and closely biologists have pondered these many and involved problems. How is it, they inquire, that an embryo bred of two parents of opposite s.e.x develops the s.e.x of one only of these? How is it that the mother, who belongs to one s.e.x only, produces--and produces in about equal number--offspring of both? The phenomenon is expressed, biologically, in the term, "s.e.x-limited factor"--an incalculable something in the embryo which limits its s.e.x to the s.e.x of one only of its parents. But the "something," and the method of this s.e.x-limitation have remained enigmas.
s.e.x is regarded by the new Mendelian school of biologists as that which is known as a "Mendelian factor." And to follow the argument to its conclusions, a few simple words about the Mendelian theory of Heredity are essential to those unacquainted therewith.
About forty years ago, a German monk, Mendel by name, was struck by the facts that in his bed of edible peas certain plants grew tall, while others remained dwarf; that the blossoms of certain plants were white always, while those of others were always coloured. He made a number of experiments in crossing the plants, with a view to discovering the law of inheritance by way of its operation in hybrid varieties. Briefly, the results of his experiments--which have since been repeated and confirmed by many later observers--were as follows:
There are plants that are tall and can transmit only Tallness to offspring. There are plants that are dwarf and can transmit only Dwarfness to offspring. So too, there are plants of white blossom or of coloured blossom that can transmit, respectively, only White or Coloured blossoming to offspring.
When a Tall is crossed with a Dwarf plant, however, or a Coloured with a White plant, strange to say, the hybrid offspring of this cross shows _one_ only of these opposite traits, to the exclusion of the other. No intermediate, or mixed, forms are produced.
Thus, a Tall crossed with a Dwarf produces only Talls. Plants of Coloured flower crossed with those of White flower give only Coloured flowering varieties. A yellow and a green-seeded cross produce only yellow-seeded plants.
In the cross between plants of opposite traits, _one_ set of traits appears thus, exclusively, in the hybrid offspring. These traits--because they _dominate_ growth and development--Mendel styled "Dominant." While those traits which are dominated by the other and opposite traits and do not appear in offspring, he styled "Recessive."
On further breeding, a new and stranger thing happens, however. Because when such hybrids--plants bred of parents that had borne, respectively, "Dominant" and "Recessive" characteristics, but with the parental Dominant traits so overpowering the Recessive traits of the other parent that these latter are submerged and concealed--When these hybrids are crossed with other hybrids like themselves, both the Dominant and the Recessive traits of the original parents reappear in offspring. The tall hybrids resulting from the cross between Tall and Dwarf plants, when crossed with other tall hybrids of similar origin, produce both Tall and Dwarf plants. So with Colour, and with the other so-called "Contrasted Traits."
It becomes evident, therefore, that although the Dominant traits of Tallness and Colour overpower in the growth and development of the second generation of plants, the Recessive traits of Dwarfness and Whiteness, these latter traits are _submerged_ only, and are neither impaired in their values, nor destroyed. In the third generation, under different conditions of mating, the original Recessive, and submerged, traits re-appear, and reveal themselves in offspring-plants as the Dwarfness or the Whiteness that had characterised their grandparents.
Mendel a.s.sumed that such hybrid plants--offspring of a Dominant and of a Recessive parent--produce two varieties of s.e.x-cells, or gametes, and that one order of cells contain the Dominant traits of the Dominant parent, while the other order contain the Recessive traits of the Recessive parent.
But any individual s.e.x-cell, or gamete, cannot (according to his view) bear both Dominant and Recessive traits. The Dominant traits and the Recessive traits of the respective parents he regarded as being segregated, absolutely, in one or in the other set of s.e.x-cells produced by hybrid varieties. And of these, the cells bearing Dominant traits are able to transmit Dominant traits only to offspring; while the cells bearing Recessive traits transmit Recessive traits only to offspring.
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 const.i.tution, produce, nevertheless, two sorts of s.e.x-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 a.s.sertive Dominant traits) is able to develop its Recessive characteristics.
These interesting and significant laws of plant-heredity and const.i.tution, 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 a.s.sociations 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 const.i.tution and the opposite attraction of The Poles. But in order to produce her countless progressive variations of form and attribute, physical and vital, she a.s.sembles 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 a.s.sociations 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 a.s.sociations 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 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, s.e.x 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 const.i.tution, the duality of the Human s.e.xes, with their respective orders of Contrasting characteristics, suggests itself as being a.n.a.logous.
Human attributes, physical and mental, are seen, like those of plants, to group themselves into two distinct categories, the Male and the Female s.e.x-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 s.e.xes.
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 s.e.x-traits differentiating one s.e.x from the other.
And at once a solution of the many baffling presentments and problems of s.e.x 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 a.n.a.logy 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 a.s.sociation 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 s.e.x from the other. In such lower organisms, s.e.x-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, s.e.x 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 s.e.x are but faintly defined. The very lowly forms are bi-s.e.xual and self-fertilising. While the first and simplest mode of reproduction is by cell-division merely; the principle of s.e.x, with its dual factors, functioning, but not yet differentiated into dual forms.
The evolution of Species and the evolution of s.e.x have been so absolutely co-incident in biological progress, indeed, that we are forced to perceive them as cause and effect; or, rather, as one and the same thing. And the evolution of s.e.x has meant, of course, the ever further divergence and the more complex specialisation, in form and in function, of the characteristics of the one s.e.x from those of the other.
V
On still closer consideration, it appears, moreover, that the evolution of s.e.x has meant pre-eminently the evolution of the _female_ s.e.x--the slow and gradual emergence and development, in species, of female characteristics, as, in course of Evolution, these have freed themselves and have risen ever further into evidence from long subjection by the stronger, fiercer, more a.s.sertive--in a word, the Dominant--traits of the male.