The class, Vertebrates, would seem to represent an adaptation to environment typically Male; earlier than and contrary in trend to that of the Mammalia, whereof the impulse was obviously Female.
Increasing vertebration was characterised by such a progressive differentiation of Male from Female traits as progressively segregated these in opposite sides of the body; with spinal column and spinal cord for, respectively, physical and nervous central lines of demarcation. Thus the Male traits were enabled more and more to detach themselves at will from Female inhibition, and thereby increasingly to specialise and exercise those powers of force and fierceness and activity by way of which species became ever more individuated; aggressive, intelligent, efficient, in terms of Fitness for the struggle for survival.
Until that later evolution of female adaptation to Unfitness, in the sacrificial function of Lactation, inhibiting and tempering the earlier male trend, engendered the yet higher order of Mammalia.
(With that intuitive illumination inspiring speech, men and races lacking in virility are contemptuously described as being "invertebrate.")
According to this hypothesis, the paternal (and male) inherences of any mother may be said to be transmitted to the grandson in the direct male line of her heredity—an unbroken line of Maleness reaching back to its amœbic origin. While the maternal (and female) inherences of any father are transmitted, in the direct female line, to the grand-daughter—a similar line of continuity. The Woman-sex and traits of the grandmother remain thus for a generation dormant, or Recessive, in the father; "skipping a generation," as the phrase is. Then, in the third generation, they re-appear in the grand-daughter; by power of a maternal contribution in which the female inherence is prepotent. While the male-sex and traits of the grandfather remain dormant, or potential, in the mother; likewise "skipping a generation." Then they emerge in the grandson, by power of a male gamete evoking the inherent male in them.
VI
The attributes of the one sex invested thus in the other, although normally submerged, form nevertheless a valuable endowment; supplying supplementary and complementary factors to counterpoise, to energise, and fructify the powers proper to the sex of the individual.
Man bears throughout life the Woman-potential his mother transmitted to him. But it is not his to realise. He bears it in trust for his daughters. He transmits it to his daughters, and in them this potential, recovering its woman-impulse, evolves to a further degree of woman-power. The like with mothers and sons.
All of which is supported by the Mendelian doctrine that the mother transmits "Femaleness" as a Dominant factor to her daughters and as a Recessive factor to her sons.