This section is from the book "The London Medical Dictionary", by Bartholomew Parr. Also available from Amazon: London Medical Dictionary.
(From the same). Sec Axthereon. Generatio, (from genero, to beget). Generation. This peculiarly curious and interesting subject has employed the ingenuity and sagacity of physiologists of every age, though with little success. They have approached only the sacred fane; destroyed many ill-founded fabrics; exploded many ridiculous . and established the question at least on a secure basis.
Every animal propagates its like; and each being proceeds from an egg. In the lowest classes of animals, however, nature providently guards against the destruction of the species, by an impregnation continued through several generations: and, in some instances, has accumulated individuals in an apparently single body. To take, then, an accurate survey of the whole subject, we must commence at the earliest stage of animated existence.
Naturalists have, at last, agreed, that plants have distinct sexes; and the common experiment in the east on the date tree is admitted to be a general example of what takes place in the animation of the seed, styled its fecundation. We find this animation so perfect, that time scarcely destroys it if the access of the air is prevented. The ground, which has been covered for ages with buildings, will, if turned up, produce the peculiar plants of the soil, and those only. Where gardens have once been an exotic will spring among indigenous plants, claiming the distinction of a denizen. The impregnation of the ovum of an animal will be occasionally, in similar circumstances, lasting; for the tanks in India, though dried for months, will, after the first rains, swarm with eels similar to its former inhabitants. Where the sexes are separated by the force of winds, or other accidents, the former impregnation is continued to many, sometimes even six successive generations. In all these instances, the unvaried form and properties of the species show that the succession is not fortuitous; that the generation is not equivocal.
There is, however, a vicarious mode of reproduction, or rather a mode of increase by buds, where, as we have said, numerous individuals are collected in a single body. This mode is well known in the vegetable kingdom; but it is also found in the lower classes of the animal. The polypus is an instance of this kind. Some as possess a similar power of sprouting from buds, and can reproduce a considerable portion after mutilation: many animals of a higher order can reproduce a lost limb. In these cases, some appearances of successive impregnation may be suspected; but as we advance in the scale the power is less. When the operations of nature are more perfect, common resources no longer an effect; the power of the bud, which can produce a new polypus, is not able to form another man, nor even reproduce the smallest portion.
We find also life more profusely bestowed in proportion to the simplicity of the structure. The production of mites mocks calculation: the elephant seldom produces more than two. Millions are contained in the spawn of a herring; but the human being products one only. This simplicity of structure does not, however, depend on that of the muscles, for in a caterpillar Lyonnet enumerated some thousands, but on the construction and functions of the nervous system. Two or three years bring the greater number of animals to their perfection: man requires at least twenty. Yet impregnation is equally the work of an instant; and, in these years, man, by the exertions of his own constitution, by his own efforts, brings forward his body and mind, till the result is a Locke, a Newton, or a Leibnitz. Life then, as by the touch of a Promethean torch, is the illumination of the moment: the constitution of the speck that is animated completes the work. When we survey in this atom the future being, its minuteness surpasses the conception, and its supposed increase appears an impossibility. It is a trite remark, that every thing is great or little by comparison; but it is of more consequence to observe, that our idea of" little" is regulated by our organs. What we can scarcely discern is very minute: what the greatest aid of glasses discovers appears to our minds the limits of existence. But we know it is not so. Light, for instance, has an almost insensible momentum; and we know its velocity is incapable of being measured, and is estimated at an immense rate. What then must be the body? It must be as far beneath the smallest atom that our glasses can discover, as that is to the column of a cathedral. When, then, we reach the smallest point which our organs, with the aid of lenses, can convey to the mind, it is our conceptions only that have attained their limits, in consequence of the imperfection of those channels by which ideas are conveyed. The world below us is apparently as extensive as that above; and we know not but that it may sink to atoms as minute, compared to the smallest we can perceive, as the whole solar system is vast and superior to it.
These reflections will not, we trust, appear misplaced, as they will facilitate our progress in the present consideration, and be applicable in many future disquisitions. We are now prepared then to repeat, with more confidence, that generation consists only in animation; and that the growth is the progressive evolution of organized parts, by the interposition of inorganic matter. To suppose that in the first created animals were contained the germs of every future generation (the Swiss hypothesis styled emboitement,') is apparently too extravagant, even with the assistance of our former reflections. It is probable that so"wonderful a piece of work as man" must be for ages forming by the concurrence of second causes. Such is the profusion with which the Creator seems to have bestowed life, that though we would reject the molecules or-ganiques of Buffon in their immediate operation, according to his system, we think it highly probable that in the successive arrangements of the component parts of the more perfect animals they may have some share. The philosopher will perceive the tendency and end of these reflections, which it is unnecessary at present to pursue farther, as inapplicable to the present subject.