We may distinguish two principal kinds of sexual reproduction, namely: unisexual reproduction and bisexual reproduction. When a single gamete such as an unfertilized egg gives rise (with, or without, chromosomal reduction) to a new organism, we have unisexual reproduction or parthenogenesis. Parthenogenesis from a reduced egg gives rise to an organism having only the haploid number of chromosomes, as is the case with the drone or male bee, but unreduced eggs give rise to organisms having the diploid number of chromosomes. Parthenogenesis, as we shall see presently, can, in some cases, be induced by artificial means. When reproduction takes place from a zygote or diploid germ cell formed by the union of two gametes, we have what is known as bisexual reproduction or syngamy. It is, perhaps, permissible to distinguish a third or intermediate kind of sexual reproduction, for which we might coin the term autosexual. What we refer to as autosexual reproduction is usually known as autogamy, and occurs when a diploid nucleus is formed in a germ cell by the union (or, we might say, reunion) of two daughter-nuclei derived from the same mother-nucleus. Autogamy occurs not only among the protists (e. g. Amœba albida), but also among the metists, as is the case with the brine shrimp, Artemia salina, in which the diploid number of chromosomes is restored after reduction by a reunion of the nucleus of the second polar body with the reduced nucleus of the egg. Autogamy is somewhat akin to kleistogamy, which occurs among hermaphroditic metists of both the plant and animal kingdoms. The violet is a well-known example. In kleistogamy or self-fertilization, the zygote is formed by the union of two gametes derived from the same parent organism. Strictly speaking, however, kleistogamy is not autogamy, but syngamy, and must, therefore, be classed as bisexual reproduction. It is, of course, necessarily confined to hermaphrodites.
Loeb’s experiments in artificial parthenogenesis have been sensationally misinterpreted by some as an artificial production of life. What Jacques Loeb really did was to initiate development in an unfertilized egg by the use of chemical and physical excitants. The writer has repeated these experiments with the unfertilized eggs of the common sea urchin, Arbacia punctulata, using very dilute butyric acid and hypertonic sea water as stimulants. Cleavage had started within an hour and a half after the completion of the aforesaid treatment, and the eggs were in the gastrula stage by the following morning (9 hours later). In three days, good specimens of the larval stage known as the pluteus could be found swimming in the normal sea water to which the eggs had been transferred from the hypertonic solution. Since mature sea urchin eggs undergo reduction before insemination takes place, the larval sea urchins arising from these artificially activated eggs had the reduced or haploid number of chromosomes instead of the diploid number possessed by normal larvæ arising from eggs activated by the sperm. For, in fertilization, the sperm not only activates the egg, but is also the means of securing biparental inheritance, by contributing its quota of chromosomes to the zygotic complex. Hence, it is only in the former function, i. e. of initiating cleavage in the egg, that a chemical excitant can replace the sperm. In any case, it is evident that these experiments do not constitute an exception to the law of genetic cellular continuity. The artificially activated egg comes from the ovaries of a living female sea urchin, and in this there is small consolation for the exponent of abiogenesis. The terse comment of an old Irish Jesuit sizes up the situation very aptly: “The Blue Flame Factory,” he said, “has announced another discovery of the secret of life. A scientist made an egg and hatched an egg. The only unfortunate thing was that the egg he hatched was not the egg he made.” How an experiment of this sort could be interpreted as an artificial production of life is a mystery. The only plausible explanation is that given by Professor Wilson, who traces it to the popular superstition that the egg is a lifeless substrate, which is animated by the sperm. The idea owes its origin to the spermists of the 17th century, who defended this doctrine against the older school of preformationists known as ovists. It is now, however, an embryological commonplace that egg and sperm are both equally cellular, equally protoplasmic, and equally vital.
The phenomena of the life-cycle in organisms find their explanation in what, perhaps, is inherent in all living matter, namely, a tendency to involution and senescence. This tendency, in the absence of a remedial process of rejuvenation, leads inevitably to death. Living matter seems to “run down” like a clock, and to stand in similar need of a periodic “rewinding.” This reinvigoration of protoplasm is accomplished by means of several different types of nuclear reorganization. Since no nuclear reorganization occurs in somatogenic reproduction, there seem to be limits to this type of propagation. Plants, like the potato and the apple, cannot be propagated indefinitely by means of tubers, shoots, stems, etc. The stock plays out in time, and, ever and anon, recourse must be had to seedlings. Hence a process of nuclear reorganization seems, in most cases, at least, to be essential for the restoration of vitality and the continuance of life. Whether this need of periodic renewal is absolutely universal, we cannot say. The banana has been propagated for over a century by the somatogenic method, and there are a few other instances in which there appears to be no limit to this type of reproduction. Nevertheless, the tendency to decline is so common among living beings that the rare exceptions serve only to confirm (if they do not follow) the general rule.
In cytogenic reproduction three kinds of rejuvenation by means of nuclear reorganization are known: (1) amphimixis or syngamy; (2) automixis or autogamy; (3) endomixis. In amphimixis or syngamy, two gametic (haploid) nuclei of different parental lineage are commingled to form the diploid nucleus of the zygote, which is consequently of biparental origin. In automixis or autogamy, two reduced or haploid nuclei of the same parental lineage unite to form a diploid nucleus, the uniting nuclei being daughter-nuclei derived from a common parent nucleus. In endomixis, the nucleus of the exhausted cell disintegrates and fuses with the cytoplasm, out of which it is reformed or reconstructed as the germinal nucleus of a rejuvenated cellular series. Endomixis occurs as a periodic phenomenon among the protists, and it appears to be homologous with parthenogenesis among metists. In certain ciliates, like the Paramœcium, endomixis and syngamy are facultative methods of rejuvenation. This has been proved most conclusively by Professor Calkins’ work on Uroleptus mobilis, an organism in which both endomixis and conjugation are amenable to experimental control. Nonsexual reproduction in this protozoan (by binary fission) is attended with a gradual weakening of metabolic activity, which increases with each successive generation. The initial rate of division and metabolic energy can, however, be restored either by conjugation (of two individuals), or by endomixis, which takes place (in a single individual) during encystment. The race, however, inevitably dies out, if both encystment and conjugation are prevented. Even in such protists as do not exhibit the phenomenon of nuclear reorganization through sexual reproduction, Kofoid points to the phenomenon of alternating periods of rest and rapid cell-division as evidence that some process of periodically-recurrent nuclear organization must exist in the organisms, which do not conjugate. This process of nuclear reorganization manifested by periodic spurts of renewed divisional energy is, according to Kofoid, a more primitive mode of rejuvenation than endomixis. “The phenomenon of endomixis,” he says, “appears to be somewhat more like that of parthenogenesis than a more primitive form of nuclear reorganization.” (Science, April 6, 1923, p. 403.) At all events, it seems safe to conclude that the tendency to senescence is pretty general among living organisms, and that this tendency, unless counteracted by a periodic reorganization of the nuclear genes, results inevitably in the deterioration and final extinction of the race.
In this inexhaustible power of self-renewal inherent in all forms of organic life, the mechanist and the upholder of abiogenesis encounter an insuperable difficulty. In inorganic nature, where the perpetual-motion device is a chimera, and the law of entropy reigns in unchallenged supremacy, nothing analogous to it can be found. The activity of all non-living units of nature, from the hydrogen atom to the protein multimolecule, is rigidly determined by the principle of the degradation of energy. The inorganic unit cannot operate otherwise than by externalizing and dissipating irreparably its own energy-content. Nor is its reconstruction and replenishment with energy ever again possible except through the wasteful expenditure of energy borrowed from some more richly endowed inorganic unit. In order to pay Paul a little, Peter must be robbed of much. Wheresoever atoms are built up into complex endothermic molecules, the constructive process is rigidly dependent upon the administration thereto of external energy, which in the process of absorption must of necessity fall from a higher level of intensity. And when the energy thus absorbed by the complex molecule is again set free by combustion, it is degraded to a still lower potential, from which, without external intervention, it can never rise again to its former plane of intensity. The phenomena of radioactivity tell the same tale. All the heavier atoms, at least, are constantly disintegrating with a concomitant discharge of energy. There is no compensating process, however, enabling such an atom to re-integrate and recharge itself at stated intervals; and, once it has broken down into its component protons and electrons, “not all the king’s horses nor all the king’s men can ever put Humpty-Dumpty together again.” In a word, none of the inorganic units of the mineral world exhibits that wonderful power of autonomous recuperation which a unicellular ciliate manifests when it rejuvenates itself by means of endomixis. The inorganic world knows of no constructive process comparable to this. It is only in living beings that we find what James Ward describes as the “tendency to disturb existing equilibria, to reverse the dissipative processes which prevail throughout the inanimate world, to store and build up where they are ever scattering and pulling down, the tendency to conserve individual existence against antagonistic forces, to grow and to progress, not inertly taking the easier way but seemingly striving for the best, retaining every vantage secured, and working for new ones.” (“On the Conservation of Energy,” I, p. 285.)
Summing up, then, we have seen that the reproductive process, whereby the metists or multicellular organism originate, resolves itself ultimately into a process of cell-division. The same is true of the protists or unicellular organisms. For all cells, whether they be protists, germ cells, or somatic cells, originate in but one way, and that is, from a preëxistent living cell by means of cell-division. Neither experimentation nor observation has succeeded in revealing so much as a single exception to the universal law of genetic cellular continuity, and the hypothesis of spontogenesis is outlawed, in consequence, by the logic of scientific induction. Even the hope that future research may bring about an amelioration of its present status is entirely unwarranted in view of the manifest dynamic superiority of the living organism as compared with any of the inert units of the inorganic world. “Whatever position we take on this question,” says Edmund B. Wilson, in the conclusion of his work on the Cell, “the same difficulty is encountered; namely, the origin of that coördinated fitness, that power of active adjustment between internal and external relations, which, as so many eminent biological thinkers have insisted, overshadows every manifestation of life. The nature and origin of this power is the fundamental problem of biology. When, after removing the lens of the eye in the larval salamander, we see it restored in perfect and typical form by regeneration from the posterior layer of the iris, we behold an adaptive response to changed conditions of which the organism can have no antecedent experience either ontogenetic or phylogenetic, and one of so marvelous a character that we are made to realize, as by a flash how far we still are from a solution of this problem.” Then, after discussing the attempt of evolutionists to bridge the enormous gap that separates living, from lifeless nature, he continues: “But when all these admissions are made, and when the conserving action (sic) of natural selection is in the fullest degree recognized, we cannot close our eyes to two facts: first, that we are utterly ignorant of the manner in which the idioplasm of the germ cell can so respond to the influence of the environment as to call forth an adaptive variation; and second, that the study of the cell has on the whole seemed to widen rather than to narrow the enormous gap that separates even the lowest forms of life from the inorganic world.” (“The Cell,” 2nd edit., pp. 433, 434.)
§ 5. A “New” Theory of Abiogenesis
Since true science is out of sympathy with baseless conjectures and gratuitous assumptions, one would scarcely expect to find scientists opposing the inductive trend of the known facts by preferring mere possibilities (if they are even such) to solid actualities. As a matter of fact, however, there are not a few who obstinately refuse to abandon preconceptions for which they can find no factual justification. The bio-chemist, Benjamin Moore, while conceding the bankruptcy of the old theory of spontaneous generation, which looked for a de novo origin of living cells in sterilized cultures, has, nevertheless, the hardihood to propose what he is pleased to term a new one. Impressed by the credulity of Charlton Bastian and the autocratic tone of Schäfer, he sets out to defend as plausible the hypothesis that the origination of life from inert matter may be a contemporaneous, perhaps, daily, phenomenon, going on continually, but invisible to us, because its initial stages take place in the submicroscopic world. By the time life has emerged into the visible world, it has already reached the stage at which the law of genetic continuity prevails, but at stages of organization, which lie below the limit of the microscope, it is not impossible, he thinks, that abiogenesis may occur. To plausibleize this conjecture, he notes that the cell is a natural unit composed of molecules as a molecule is a natural unit composed of atoms. He further notes, that, in addition to the cell, there is in nature another unit higher than the monomolecule, namely, the multimolecule occurring in both crystalloids and colloids. The monomolecule consists of atoms held together by atomic valence, whereas the multimolecule consists of molecules whose atomic valence is completely saturated, and which are, consequently, held together by what is now known as molecular or residual valence. Moore cites the crystal units of sodium bromide and sodium iodide as instances of multimolecules. The crystal unit of ordinary salt, sodium chloride, is an ordinary monomolecule, with the formula NaCl. In the case of the former salts the crystal units consist of multimolecules of the formula NaB·(H2O)2 and NaI·(H2O)2, the water of crystallization not being mechanically confined in the crystals, but combined with the respective salt in the exact ratio of two molecules of water to one of the salt. Judged by all chemical tests, such as heat of formation, the law of combination in fixed ratios, the manifestation of selective affinity, etc., the multimolecule is quite as much entitled to be considered a natural unit as is the monomolecule.
But it is not in the crystalloidal multimolecule, but in the larger and more complex multimolecule of colloids (viscid substances like gum arabic, gelatine, agar-agar, white of egg, etc.), that Moore professes to see a sort of intermediate between the cell and inorganic units. Such colloids form with a dispersing medium (like water) an emulsion, in which the dispersed particles, known as ultramicrons or “solution aggregates,” are larger than monomolecules. It is among these multimolecules of colloids that Moore would have us search for a transitional link connecting the cell with the inorganic world. Borrowing Herbert Spencer’s dogma of the complication of homogeneity into heterogeneity, he asserts that such colloidal multimolecules would tend to become more and more complex, and consequently more and more instable, so that their instability would gradually approach the chronic instability or constant state of metabolic fluxion manifest in living organisms. The end-result would be a living unit more simply organized than the cell, and evolution seizing upon this submicroscopic unit would, in due time, transform it into cellular life of every variety and kind. Ce n’est que le premier pas qui coûte!
It should be noted that this so-called law is a mere vague formula like the “law” of natural selection and the “law” of evolution. The facts which it is alleged to express are not cited, and its terms are far from being quantitative. It is certainly not a law in the sense of Arrhénius, who says: “Quantitative formulation, that is, the establishing of a connection, expressed by a formula, between different quantitatively measurable magnitudes, is the peculiar feature of a law.” (“Theories of Chemistry,” Price’s translation, p. 3.) Now, chemistry, as an exact science, has no lack of laws of this kind, but no branch of chemistry, whether physical, organic, or inorganic, knows of any law of complexity, that can be stated in either quantitative, or descriptive, terms. We will, however, let Moore speak for himself: