SECTION III.

REPRODUCTION.

CHAPTER I.

ON THE NATURE OF REPRODUCTION, AND THE ORIGIN OF PLANTS AND ANIMALS.

THE process of reproduction is the most characteristic, and in many respects the most interesting, of all the phenomena presented by organized bodies. It includes the whole history of the changes taking place in the organs and functions of the individual at successive periods of life, as well as the production, growth, and development of the new germs which make their appearance by generation.

For all organized bodies pass through certain well defined epochs or phases of development, by which their structure and functions undergo successive alterations. We have already seen that the living animal or plant is distinguished from inanimate substances by the incessant changes of nutrition and growth which take place in its tissues. The muscles and the mucous membranes, the osseous and cartilaginous tissues, the secreting and circulatory organs, all incessantly absorb oxygen and nutritious material from without, and assimilate their molecules; while new substances, produced by a retrogressive alteration and decomposition, are at the same time excreted and discharged. These nutritive changes correspond in rapidity with the activity of the other vital phenomena; since the production of these phenomena, and the very existence of the vital functions, depend upon the regular and normal continuance of the nutritive process. Thus the organs and tissues, which are always the seat of this double change of renovation and decay, retain nevertheless their original constitution, and continue to be capable of exhibiting the vital phenomena.

The above changes, however, are not in reality the only ones which take place. For although the structure of the body and the composition of its constituent parts appear to be maintained in an unaltered condition, by the nutritive process, from one moment to another, or from day to day, yet a comparative examination of them at greater intervals of time will show that this is not precisely the case; but that the changes of nutrition are, in point of fact, progressive as well as momentary. The composition and properties of the skeleton, for example, are not the same at the age of twenty-five that they were at fifteen. At the latter period it contains more calcareous and less organic matter than before; and its solidity is accordingly increased, while its elasticity is diminished. Even the anatomy of the bones alters in an equally gradual manner; the medullary cavities enlarging with the progress of growth, and the cancellated tissue becoming more open and spongy in texture. We have already noticed the difference in the quantity of oxygen and carbonic acid inspired and exhaled at different ages. The muscles, also, if examined after the lapse of some years, are found to be less irritable than formerly, owing to a slow, but steady and permanent deviation in their intimate constitution.

The vital properties of the organs, therefore, change with their varying structure; and a time comes at last when they are perceptibly less capable of performing their original functions than before. This alteration being dependent on the varying activity of the nutritive process, continues necessarily to increase. The very exercise of the vital powers is inseparably connected with the subsequent alteration of the organs employed in them; and the functions of life, therefore, instead of remaining indefinitely the same, pass through a series of successive changes, which finally terminate in their complete cessation.

The history of a living animal or plant is, therefore, a history of successive epochs or phases of existence, in each of which the structure and functions of the body differ more or less from those in every other. Every living being has a definite term of life, through which he passes by the operation of an invariable law, and which, at some regularly appointed time, comes to an end. The plant germinates, grows, blossoms, bears fruit, withers, and decays. The animal is born, nourished and brought to maturity, after which he retrogrades and dies. The very commencement of existence, by leading through its successive intermediate stages, conducts at last necessarily to its own termination.

But while individual organisms are thus constantly perishing and disappearing from the stage, the particular kind, or species, remains in existence, apparently without any important change in the character or appearance of the organized forms belonging to it. The horse and the ox, the pine and the palm-tree, the different kinds of wild and domesticated animals, even the different races of man himself, have remained without any essential alteration ever since the earliest historical epochs. Yet during this period innumerable individuals, belonging to each species or race, must have lived through their natural term and successively passed out of existence. A species may therefore be regarded as a type or class of organized beings, in which the particular forms or structures composing it die off constantly and disappear, but which nevertheless repeats itself from year to year, and maintains its ranks constantly full by the regular accession of new individuals. This process, by which new organisms make their appearance, to take the place of those which are destroyed, is known as the process of reproduction or generation. Let us now see in what manner it is accomplished.

It has always been known that, as a general rule in the process of generation, the young animals or plants are produced directly from the bodies of the elder. The relation between the two is that of parents and progeny; and the new organisms, thus generated, become in turn the parents of others who succeed them. For this reason wherever such plants or animals exist, they indicate the previous existence of others belonging to the same species; and if by any accident the whole species should be destroyed in any particular locality, no new individuals could be produced there, unless by the previous importation of others of the same kind.

The commonest observation shows this to be true in regard to those animals and plants with whose history we are more familiarly acquainted. An opinion, however, has sometimes been maintained that there are exceptions to this rule; and that living beings may, under certain circumstances, be produced from inanimate substances, without any similar plants or animals having preceded them; presenting, accordingly, the singular phenomenon of a progeny without parents. Such a production of organized bodies is known by the name of spontaneous generation. It is believed by the large majority of physiologists at the present day that no such spontaneous generation ever takes place; but that plants and animals are always derived, by direct reproduction, from previously existing parents of the same species. As this, however, is a question of some im

portance, and one which has been frequently discussed in works on physiology, we shall proceed to pass in review the facts which have been adduced in favor of the occurrence of spontaneous generation, as well as those which would lead to its disproval and rejection.

It is evident, in the first place, that many apparent instances of spontaneous generation are found to be of a very different character so soon as they are subjected to a critical examination. Thus grasshoppers and beetles, earthworms and crayfish, the swarms of minute insects that fill the air over the surface of stagnant pools, and even frogs, moles, and lizards, have been supposed in former times to be generated directly from the earth or the atmosphere; and it was only by investigating carefully the natural history of these animals that they were ascertained to be produced in the ordinary manner by generation from parents, and were found to continue the reproduction of their species in the same way. A still more striking instance is furnished by the production of maggots in putrefying meat, vegetables, flour paste, fermenting dung, &c. If a piece of meat be exposed, for example, and allowed to undergo the process of putrefaction, at the end of a few days it will be found to contain a multitude of living maggots, which feed upon the decomposing flesh. Now these maggots are always produced under the same conditions of warmth, moisture and exposure, and at the same stage of the putrefactive process. They are never to be found in fresh meat, nor, in fact, in any other situation than the one just mentioned. They appear, consequently, without any similar individuals having existed in the same locality; and considering the regularity of their appearance under the given conditions, and their absence elsewhere, it has been believed that they were spontaneously generated, under the influence of warmth, moisture, and the atmosphere, from the decaying organic substances.

A little examination, however, discovers a very simple solution of the foregoing difficulty. On watching the exposed animal or vegetable substances during the earlier periods of their decomposition, it is found that flies and other insects, attracted by the odor of the decaying material, hover round it and deposit their eggs upon its surface or in its interior. These eggs, hatched by the warmth to which they are exposed, produce the maggots; which are simply the young of the winged insects, and which after a time become transformed, by the natural progress of development, into perfect insects similar to their parents. The difficulty of accounting for the presence of the maggots by generation, therefore, de