CHAPTER III

Heredity and Atavism

Heredity, like other biologic factors, starts with the cell. As elsewhere pointed out, reproduction is first unicellular in type and involves an expenditure of nutritive force antagonistic to the growth of the cell. As Geddes remarks,[66] no one can dispute that the nutritive, vegetative, or self-regarding processes within the plant or animal are as opposed to the reproductive, multiplying, species-regarding processes, as income to expenditure or as building up to breaking down. But within the ordinary nutritive or vegetative functions of the body there is necessarily a continuous antithesis between two tissue-changing sets of processes, constructive and destructive metabolism. The contrast between these two processes is seen throughout nature, whether in the alternating phases of cell-life or in the great antithesis of growth and reproduction.

The starfish, deprived of an arm, replaces this by a fresh growth; crabs can renew the great claws which they have lost in fighting; even as high up as the lizard the loss of a leg or a tail can be made good. In a great variety of cases a kind of physiological forgiveness is shown in the reparation of even serious injuries. Now this “regeneration,” as it is called, is a process of reproduction. By continuous growth the cells of a persistent stump are able to reproduce the entire number. A sponge, a hydra or a sea-anemone may be cut into pieces with the result that each fragment grows into a new organism. The same is done with many plants; and though the division is artificial the result shows how very far from unique is the process spoken of as reproduction, which is but more or less discontinuous growth. This is well shown in the evolution onward insensibly from cases of continuous budding, as in sponge or rosebush, to discontinuous budding in hydra zoophyte and tiger-lily, where the offspring vegetatively produced are sooner or later set free.

The enormous expenditure of force required for unicellular reproduction is lessened by conjugation with another cell through satisfaction of cell hunger; and this, by making two cells do the work of one, lessens the amount of nutritive force expended by each. Evolution in fertilisation has the following steps:—

I.Formation of plasmodia.
II.Multiple conjugation.
III.Conjugation of the two similar cells.
IV.Union of incipiently dimorphic (different) cells.
V.Fertilisation of differentiated sex elements.

As Maupas has shown, by the time conjugation of two similar cells is reached, the paranucleus in both is incipiently hermaphroditic. The impelling force leading to conjugation is, as Rolph has shown, cell hunger. Conjugation, he remarks, is a necessity for satisfaction, a gnawing hunger which drives the animal to engulf its neighbour, to isophagy (self-eating). The process of conjugation is only a special form of nutrition which occurs because of a reduction of the nutritive income or an increase of the nutritive needs. It is an “isophagy” which occurs in place of “heterophagy” (eating of others). “The less nutritive, and therefore smaller, hungrier, and more mobile organism is the male, the more nutritive and usually relatively more quiescent organism the female. Therefore, too, is it that the small starving male seeks out the large, well-nourished female for purpose of conjugation, to which the latter, the larger and better nourished, is on its own motive less inclined.” The unicellular type of reproduction long remains after sex differentiation has occurred and assumes the form of parthenogenesis (virgin generation). The phenomena of this demonstrate that the female element is the highest in evolution. Spitzka[67] has shown that the ovum possesses an inherent activity independently of fructification. How far this may extend in the direction of more mature development is shown by what is known about parthenogenesis. This is the development of living beings without a father. Bees, some butterflies, ants and wasps notoriously multiply their kind without sexual congress. As a rule the parthenogenetic offspring are themselves incapable of further procreating their kind. But to this there are remarkable exceptions. The aphides multiply for many generations without the intervention of a male. Weigenbergh has shown that the silk-moth can be propagated as long as the male element is permitted to act at every fourth generation. The Artemsia salina, a minute crustacean living in saline springs, reproduces its kind for years without a male being present, males being produced at definite intervals only (Von Siebold). Among the vertebrata parthenogenetic development has also been observed, though rarely reaching maturity. Thus, segmentation occurs in unfertilised ova of the chicken (Oellacher), of the fish (Burnett and Agassiz), and of frogs (Moquia-Tanden). Spitzka has seen a blastoderm form in unfertilised ova of the toad-fish (Batrachus tau). Hensen isolated the oviducts of a rabbit, thus rendering the admission of semen impossible, while the ova, discharged at heat, were compelled to remain in these oviducts. Three years later he killed the animal and found the ova had developed into twisted, club-shaped, hollow sacs. The development in the female ovary (also, though very rarely, in the male testicle and parotid gland, which show such a remarkable metastatic sympathy in mumps), of dermoid cysts (containing bones recognisable as maxillaries with teeth, hair, and skin, rudimentary bowel, gland, and brain traces), even in undoubted virgins, proves that even the human ovum is capable of parthenogenetic development. While such development, so far as known to science, is always abortive, and while, as Washington Irving remarks, the ingenious maiden who to-day would attribute conception to any other cause than sexual congress would find it difficult to overcome the prejudice of scientists, yet embryology, while declaring immaculate generation improbable, does not pronounce it impossible. A worker bee may be an offspring of an unimpregnated queen bee. What is a regular occurrence in one class of animals is sometimes observed as an exceptional one in another class. If the startling and apparently miraculous nature of a virgin generation of a living child be regarded as the sole objection to receiving such a fact, its defender might urge that the virgin generation of a dermoid cyst with all the traces, however aborted, of vertebrate organisation, is only a shade less startling and miraculous.

This power of parthenogenesis, however, cannot continue indefinitely without extinction. This has been shown by the careful experiments of Maupas, who had observed 215 generations of an infusorian without sexual union. He found that then the family became extinct. Powers of nutrition, division, and conjugation with unrelated forms come to a standstill. The first symptom of this senile degeneration is decrease in size, which may go on till the individuals only measure a quarter of their normal proportions. Various internal structures then follow suit “until at last formless abortions occur, incapable of living and reproducing themselves.” The nuclear changes are no less momentous. The important paranucleus is fatally sterile. The larger nucleus may also become affected, “the chromatin gradually disappears altogether.” Physiologically, too, the organisms become manifestly weaker, though there is excessive sexual excitation. Such senile decay of the individuals and of the isolated family inevitably ends in death. Sexual union in those infusorians, dangerous perhaps for the individual life, a loss of time so far as immediate multiplication is concerned, is, in a new sense, necessary for the species. The life runs in cycles of a sexual division which are strictly limited. Conjugation with unrelated forms must occur else the whole life ebbs. Without it the protozoa, which some have called “immortal,” die a natural death. Conjugation is the necessary condition of their eternal youth and immortality.