The nebular hypothesis is a part of the foundation on which the doctrine of creation by law rests. And the high scientific reputation of its author, as well as its apparent coincidence with some of the deductions of geology respecting the earliest condition of the earth, have made philosophers look upon it with considerable favor. Yet very few have been ready to give it implicit credence. And of late the most plausible evidence in its favor seems to be fast vanishing away. The ablest mechanicians are unable to see how a rotary motion should be produced in nebulous matter by refrigeration; or, if this be assumed, how the successive portions, detached by superior centrifugal force, should form spherical masses. But a still more formidable objection lies in the fact that, as improvements are made in telescopes, one and another of the nebulæ, on which the hypothesis rests, have been resolved into stars; and the presumption hence arising is very strong that all are resolvable. In the present aspect of the subject, no sagacious philosopher would dare to rest even an hypothesis upon the unresolved nebulæ. If, however, the nebular hypothesis were shown to be true, it would prove nothing in regard to the production of animals and plants by mere law, without the special agency of the Deity.
The essential and inherent vitality of some kinds of matter is another doctrine on which this hypothesis rests. “In vain,” says Bory St. Vincent, “has matter been considered as eminently brute. Many observations prove that, if it is not all active, by its very nature, a part of it is essentially so; and the presence of this, operating according to certain laws, is able to produce life in an agglomeration of the molecules; and since these laws will always be imperfectly known, it will at least be rash to maintain that an infinite intelligence did not impose them; since they are manifested by their results.”—Dictionnaire Classique d’Histoire Naturelle, art. Materie.
The “observations” to which this writer refers to sustain his hypothesis are those which had been made upon certain vegetable infusions, which, in certain circumstances, exhibited minute particles in motion, apparently by vital forces. These were called monads, and were not supposed to be distinct animals, but only atoms, ready to be organized. The more modern and accurate researches of Ehrenberg and others, however, have shown, beyond all doubt, that these monads are true animals, the minutest of all living beings hitherto discovered. Not less than twenty-six species of them have been described and figured by microscopists, the smallest of which never exceeds the twelve thousandth of an inch in diameter.
The vegetable physiologists have described certain peculiar motions in the minute vessels of plants, that might readily be regarded as matter essentially vital. I refer to what they call rotation and cyclosis. But these are never seen save in the living plant; and, therefore, seem dependent on the general life of the vegetable.
There is, however, danger of mistaking certain motions of the particles of matter, by chemical agency, for the effect of vitality. A curious example is thus described by Ehrenberg, which was discovered by Professor Bornsdorff. “If a solution of the chloride of aluminum be dropped into a solution of potassa, by the alternate precipitation and solution of the aluminum, in the excess of the alkali, an appearance will be given to the drop of aluminate matter, by the chemical changes and reactions which take place, as if the Amœba diffluens were actually present, both as to its form and evolutions, and will seem to be alive. Such appearance is considered by its able discoverer as bearing the same relationship to the real animalcule as a doll, or a figure moved by mechanism, does to a living child.”
We see, then, that the supports on which rests the doctrine of the essential vitality of matter, give way before better instruments and more careful research. Another statement, however, of much higher pretensions, has lately been made, and on no mean authority. Able electricians declare that, by passing currents of galvanism through solutions of silicate or ferrocyanate of potassa, or some analogous substance, after a time, sometimes several years, numerous small insects have been developed, belonging to the acari family.
These experiments appear to have been conducted with fairness and skill; and that the insects showed themselves at the pole of the battery, around which the gelatinous silex collected, cannot be doubted. It is true, however, that, when the solution was exposed to the atmosphere, the insects appeared much sooner and more numerous than when care was taken to exclude every thing but oxygen enough to sustain life. This fact leads to the suspicion that the ova of the insect might have been communicated through the air, and that, even when an attempt was made to exclude the atmosphere, some ova were still present. This conclusion is rendered still more probable by some experiments made by Professor Schulz, of Berlin, on the production of infusoria. Having first boiled the vegetable and animal infusions, so as to destroy all germs of organic life, and expelled all the atmosphere, he attached an apparatus in such a manner that, whatever air entered afterwards, must pass through sulphuric acid, or a solution of potash. The result was, that no infusoria or vegetable forms appeared during two months; but in the same infusion, placed in the open air, and exposed to the same light and heat as that enclosed in the glass vessel, numerous animalcula and fungi appeared in a day or two. It will need, therefore, very long and patient experiments to establish the assertion that galvanism alone can produce living animals without the presence of germs.
Not many years since, the equivocal or casual production of animalcula, without any other parentage than law, was thought to be made out by a multitude of facts. For these minute creatures appeared almost every where, and in places where it seemed impossible that their ova should be found. But the researches of Ehrenberg have cleared up the difficulties of their origination in the ordinary modes of reproduction, in nearly every instance, and the advocates of the law hypothesis have been fairly driven from this stronghold of their argument. In describing the various modes of reproduction with which nature has provided the infusoria, Professor Owen says, “Thus each leaves, by the last act of its life, the means of perpetuating and diffusing its species by thousands of fertile germs. When once the thickly tenanted pool is dried up, and its bottom converted into a layer of dust, these inconceivably minute and light ova will be raised with the dust by the first puff of wind, diffused through the atmosphere, and may there remain long suspended; forming, perhaps, their share of the particles which we see flickering in the sunbeam, ready to fall into any collection of water, beaten down by every summer shower into the streams or pools which receive or may be formed by such showers, and, by virtue of their tenacity of life, ready to develop themselves whenever they may find the requisite conditions of their existence. The possibility, or, rather, the high probability, that such is the design of the oviparous generation of the infusoria, and such the common mode of the diffusion of their ova, renders the hypothesis of equivocal generation, which has been so frequently invoked to explain their origin in new-formed natural or artificial infusions, quite gratuitous.”—Lectures on Comp. Anat. vol. ii. p. 31.
No longer able to maintain a foothold among the animalcula, the defenders of this hypothesis have of late attempted to take a stand among animals of a somewhat higher grade, viz., the entozoa, or animals inhabiting other animals. These being considerably larger than the infusoria, their ova could not float in the atmosphere; but they possess a wonderful tenacity of life; some of them exhibiting signs of life after having been in boiling water for an hour; others have revived after having been packed for a long time in ice, and frozen; others have revived after lying in a dried state for six or seven years. Their power of reproduction, in the ordinary modes, is also prodigious, exceeding even that of the infusoria. It will, then, demand very strong evidence to prove that such animals possess also the power of spontaneous production, without parentage, or that their existence within other animals cannot be explained without such a supposition. For, if capable of being produced without parentage, why should such extraordinary care have been taken for their multiplication, in almost all the ordinary modes in which animals are reproduced?
The extraordinary facts that have been discovered by Professors Steenstrup, Owen, and others, within a few years, respecting what they call alternate generation, or parthenogenesis, have been thought favorable to the hypothesis of development. Among the mollusca, the polyparia, the entozoa, and infusoria, it is found that, in some species, the result of sexual union is the production of a larva without sex, and, therefore, incapable of propagating in the usual way. Yet that larva can of itself produce another larva quite different from itself, and this larva another, and so on, sometimes for eight or ten generations, when the spermatic force seems to be exhausted, and a progeny exactly like the original parents that started the series is produced, capable of giving rise to another and a similar series. Here, then, we find a succession of progeny for several generations, and all quite unlike one another, yet without any immediate parental agency. Why is it not an example of spontaneous generation? and why may not new species be produced in this manner?