Such sterility in the offspring of creatures of different species is weighty evidence that the Evolution of Type, or Species, and the Evolution of the Individual are wholly independent phenomena; occurring upon wholly different planes, and involving wholly different principles and sets of processes. In the mating of alien species, the two sex-cells, although of dissimilar species-inherence, unite nevertheless and develop in the maternal environment into a living entity of mongrel order. But the Germ-plasm contained in the gamete of one species will not germinate in the alien environment of an organism of alien species. No potential, either Vital or of Differentiation, is engendered, therefore, for production of offspring. Hence sterility results. The potential of a living individual is seen thus to belong to a wholly different plane of phenomena from the potential of Stock. Conditions which do not annul the powers of life and of function in the one, quench life and function in the other with the seal of sterility.
VII
Possible explanation of "Sports"
Mr. Regnart says: "We often meet with Sports. Second- and third-rate parents may produce an exceptionally fine individual, but such animals are always failures to breed from. The law of Filial Regression comes into operation. Our aim is to find families that have produced a large number of fine animals—we know then that we are on safe ground."
In these cases, it would seem that the "fine individual" results from so singularly harmonious and successful a complementing and fructifying of the parental halves in offspring as conduce to develop the best points of both; doubtless, too, to eliminate or to annul weak or faulty factors of either parental strain. Neither of such inferior-grade parents transmitting a fine lineal potential, however, the exceptional fineness of the individual is not inherent in the Germ-plasm he or she transmits to offspring. The fine characteristics of such "Sports" are not transmissible, therefore, to descendants.
Proof again of two planes of Life and Evolution, that of Species and that of the Individual. Moral, too, of the importance of fine selection in mating, since the harmonious mating of second- or third-rate parents may produce finer offspring than are born of ill-assorted matings of two finer breeds of parent.
The case is recorded of a pony about the size of a Shetland pony, which was the offspring of pedigree Shire-parents on both sides, both parents being over 17 hands. The most striking feature about the animal was that there was nothing of the horse-type about him—he was a perfect example of pony.
Shire horses are typical examples of Vigour, or developmental power, expressed in terms of stature, muscle and nervous energy. And for so long as the breeding for these characteristics was supplemented in terms of vital organs and vital functioning, by an equivalent maternal complement of Vital potential, to sustain the constitutional expenditure involved in stature, muscular equipment, and nervous energy, the breed improved in these particulars. Pushed beyond this limit, by introducing into stock further strains of Vigour, or developmental initiative, without simultaneously providing the indispensable equivalents of these in increasing Vital potentials, all at once the balance toppled, and reversion to inferior type resulted.
An excessive proportion of the Vital power of these two Pedigree Shires of great stature and great strength had been expended in the achievement of such great stature and great strength, and in the equipment of digestive and assimilative organs required to sustain these. But little had remained, accordingly, for Reproductive investments. Hence reversion in the de-vitalised stock.
One conceives of the counterpoise in Stock, of Male and Female complements, as being akin to that of the opposite and complementary curves of an arch. So long as equipoise is sustained by the perfect balance of the contrary curves, so long each re-inforces the other to support a heavy superstructure of development. Lopsidedness of either curve leads to collapse.