There are two facts prominent on this subject which afford a full answer to such questions. One is, that these generations of larvæ always begin with the spermatozoon and the ovum of parents; the other is, that the series always closes, if allowed to run its natural course, in individuals with sex, exactly identical with those that started it; so that the species always remains entire. The whole process is simply one of the infinitely varied modes which nature employs to preserve and perfect the species. The process never stops with any of the larvæ intervening between the fertile parents at the beginning, and the fertile individuals at the end of the series. Professor Owen supposes—certainly with much plausibility—that some of the original germ-cells, not wanted for the production of the first larva, pass on to form the successive generations, till the series is complete; so that, after all, the case is not an exception to the general law of reproduction by parental agency; and instead of sustaining, it certainly goes against, the notion of spontaneous generation and of transmutation of species; because it shows how far parental influence may reach, and how tenacious nature is of specific distinctions. For the same reasons, the case affords a presumption against other alleged cases of equivocal generation and metamorphoses of species.[18]
Appeal has also been made to the vegetable kingdom for examples of the production of organic beings, viz., plants without seeds. Who has not observed, for instance, how the clearing up and burning over of a piece of land will often cause an entirely new tribe of plants to spring up and flourish? Whence came the seeds? We have seen, for instance, (in Richmond, Virginia,) a thick growth of pines upon a spot where from six to ten feet of soil had been removed a few years previously.
It is very possible, in some cases of this kind, that the soil, having been produced by aqueous agencies, may contain seeds to a considerable depth, and that their vitality may have been preserved for centuries; for we know that seeds three thousand years old, taken from Egyptian catacombs, have germinated, in favorable circumstances. In most cases of this sort, however, the winds have probably supplied the seed, it may be, long before. We were one day wandering over Mount Holyoke, where a spot recently cleared was covered with the fire-weed, a species of senecio; and as we were musing upon its origin, a strong blast of wind swept over the plants, just ready to throw off their seeds. Sustained by their light egrets, they floated away on the air in numbers sufficient to cover half the mountain with the plant, when it should be cleared and burnt over. Yet their existence would never be suspected till those circumstances should be developed. At least, until we can prove that the soil contains no seeds by the most careful examination, it will be premature to infer the equivocal production of the plants growing upon it.
Vegetable physiology furnishes another fact, which seems to me to look still more favorable to this law hypothesis than the preceding, although it has not been noticed, so far as I know, by the advocates of that hypothesis. Speaking of the matter of which certain flowerless plants are composed, Dr. Lindlay says, “It is even uncertain whether this matter will produce its like, and whether it is not a mere representation of the vital principle of vegetation, capable of being called into action, either as a fungus, or algæ, or lichen, according to the particular conditions of heat, light, and moisture, and the medium in which it is placed; producing fungi upon dead or putrid organic beings, lichens upon living vegetables, earth, or stones, and algæ where water is the medium in which it is developed.” Again, in speaking of that green slime which often covers the soil, rocks, walls, and glass in damp places, he says, “The slime resembles a layer of albumen, spread with a brush; it exfoliates in drying, and finally becomes visible by the manner in which it colors green or deep brown. One might call it a provisional creation, waiting to be organized, and then assuming different forms according to the nature of the corpuscles which penetrate it, or develop among it. It may further be said to be the origin of two very distinct existences, the one certainly animal, the other purely vegetable.”—Natural System, pp. 326, 328, 334.
Now, admitting all the facts that have been detailed respecting the production of infusoria, entozoa, acari, and cryptogamian plants to be true, although most of them are far from being proved, it seems to me that they do not show us how vitality is produced by mere law, without the special agency of the Deity. Writers on the subject seem to overlook the distinction between organization and life. The first may be present in its highest perfection without the latter, as it is in animals and plants recently killed. The organization is merely a preparation to receive the mysterious principles which we call life and intellect. Light, heat, and electricity may be the essential agents in producing the organization, but they do not explain the nature, or account for the presence, of life. That must, so far as we know, come from some other and a higher source. Galvanism may bring gelatinous matter into the form of an insect, or infusoria, or entozoa; but there is no evidence that it can impart life, however exquisite the organization. It may be, and we have reason to suppose it is, the divine will to bestow life whenever a certain organization exists; but this does not show that his special agency is not concerned in it. He may will that the peculiar life of a lichen shall be given to the same elementary matter which, in another situation, he constitutes an alga, or a fungus, or even an animal. But this would not prove that natural law alone could produce life. There is nowhere any evidence that sensibility, contractility, and especially intellect and volition, are the result of any natural operations. In their properties they are so entirely diverse from all known physical effects, that we must impute them to some other than a natural cause. We must call in the power of a supreme intelligent Being. The laws of affinity, light, heat, and electricity, of endosmose and exosmose, may prepare the organization, but their power ends there; and hence true philosophy requires us to impute the phenomena of life and intellect to an extraneous and infinitely higher cause.
The case, then, stands thus: In ninety-nine cases out of a hundred, we are certain that organization requires the previous existence and agency of a being similarly organized, which we call the parent. But suppose that, in a very few cases, the laws of nature can produce the organization. It still demands another and a higher power—not a blind impulse, but an intelligent cause—to bestow life and intellect. To prove the existence of a natural cause for the arrangement of the atoms into an organic structure, does by no means prove the same for those higher and mysterious principles that make that structure a living, thinking being.
Such, however, are the strongest arguments by which the advocates of the law hypothesis sustain their views of the origin of organism, life, and intellect. The next step in their reasoning is to show how animals and plants may be transmuted from one species, or genus, or family, to another; so that the existing vast variety can be traced to a few original germs. They maintain that these developments of the more from the less perfect have proceeded along certain parallel lines; one series of developments, for instance, taking the line of the fishes, another of the reptiles, another of the birds, another of quadrupeds, and so on.
To prove these developments or transmutations, they appeal first to the physiological history of the mammalian embryo. In its earliest stages, it can hardly be distinguished, except in size, from the unborn polygastric infusoria. The brain of a human embryo appears at first like that of an invertebrate animal; next like that of a fish; then successively like that of a reptile, a bird, a rodent mammal, a ruminant, and a monkey. So the heart, at an early stage, looks like that of an insect; then it has two chambers, like that of a fish; then it becomes three chambered, like that of a reptile; and finally, four chambered, as in the mammalia. The inference which these theorists would draw from such facts is, that man actually begins his existence as an animalcule, and passes successively through the mould or condition of other animals, before he reaches the highest. And the reasons why he does become a man, rather than an echinoderm, or a fish, or a monkey, is only some slightly modifying circumstance, as, for instance, a longer gestation. It appears to me, however, that the inferences sound philosophy should derive from such facts are, first, that, while there is a seeming resemblance between the human embryo and that of lower animals, there is, in fact, a real and a wide diversity; so that the one infallibly becomes an inferior animal, and the other a man. Could a single example be produced in which a human embryo stopped at and became an insect, or a fish, or a monkey, there might be some plausibility in the supposition. But it is as certain to become a man as the sun is to rise and set; and, therefore, the human condition results from laws as fixed as those that regulate the movements of the heavenly bodies. That is a very superficial philosophy which infers identity of nature from mere external resemblance.
The phenomena of hybridity furnish another ground of argument in favor of the transmutation of species, and of course in favor of the law hypothesis; for that hybrids are sometimes the result of the union of different species will not be denied. There is, however, a natural repugnance to union between different species; and in a state of nature this can very rarely be overcome. But domestication changes and almost obliterates many natural instincts, and hence hybridity is far more common among domesticated animals and plants. As a general fact, also, the hybrid offspring is incapable of propagating its own race, without union with one of the original species by which it was produced; and this inability to continue this mixed race has been generally regarded among naturalists as the best characteristic of species. Some, however, attempt to show that some hybrid races do continue from generation to generation to propagate their kind. But in most cases the hybrid race ere long runs out, and there is always a strong tendency to revert to the original stock; and were it not for the influence of man, probably such a thing as hybridity would scarcely ever have been heard of. Nature seems to have established strong barriers around species, so that an identity should be preserved; and even if we admit the possibility of their coalescence in some cases, yet we have evidence that almost always they are preserved distinct from century to century; and the same is true even of the more prominent varieties, for we find not only the same species, but the same varieties of animals and plants, preserved some three thousand years in the Egyptian catacombs, that are now alive in the same country. How idle, then, to suppose that the laws of hybridity will account for such radical and entire transmutations as this hypothesis supposes! To accomplish this, it would need as strong a tendency in nature to a union of species, genera, and families, as now exists against it.
But a special appeal has been made on this subject to geology. The history of organic remains, it is thought, corresponds to what we might expect, if the hypothesis of development is true. In the oldest rocks we find chiefly the more simple invertebrate animals, and the vertebrated tribes appear at first in the form of fish, then of reptiles, then of birds, then of mammals, and last of all of man. What better confirmation could we wish than this gradually expanding series? True, all the great classes of organic beings, vegetable and animal, are found nearly at the earliest epoch, and continue through the entire series of rocks. But we have only to suppose a distinct stirps for each of the classes, and that the developments took place along parallel lines, in order to harmonize the facts with the hypothesis.