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FOURTH WEEK-TUESDAY.

REPRODUCTION AMONG THE LOWER ORDERS OF ANIMALS.

SPRING is peculiarly the season of reproduction; and the diversified means by which the various species of animals as well as plants are thus preserved, furnish one of the appropriate subjects of the season, and perhaps the most interesting of all. The same character of similarity in the plan, and difference in the details, which belongs to the rest of the Creator's works, appears strikingly in this. A difference of sexes, which we have seen as one of the features in the reproduction of plants, is exhibited with not less uniformity in all the higher species of animated beings; in whom, however, this difference exists constantly in distinct individuals, and never, as frequently takes place among vegetables, in the same individual. But among the lowest orders of the animated creation, this uniformity seems to be departed from, and we have instances of reproduction altogether peculiar and anomalous. According to our plan of beginning at the lowest links of the scale, these shall form the subject of discussion in the present paper.

Fissiparous generation, as it is called, that is, the spontaneous division of the parent into two or more parts, is the simplest of all the modes of reproduction. Of such a mode, frequent instances occur among infusory animalcules. Many species of globular monads multiply in this way. At a certain period of their development a slight circular depression appears round the centre of these living balls, which, by degrees, becomes deeper, changing their form to the resemblance of an hour-glass, till at last two globes are formed out of one, attached, like the Siamese Twins, by a single point. These twin existences are now seen swimming irregularly in the fluid which they inhabit, as if animated by two different wills. They

struggle to get disunited, and for this purpose laboriously drag each other, first one way and then another, or, by a simultaneous movement, dart through the thickest crowd of the surrounding animalcules. The moment this slender filament is broken, they are observed moving away, without apparent recognition, and each beginning its own independent existence.

These globular animalcules are exceedingly small; and as the infusory tribes increase in size, they take a different shape, many of them being comparatively thick at one end, and tapering to a point at the other, something in the form of the tadpole. Such of these, as follow the same law of reproduction with the monads we have mentioned, separate from each other lengthwise, forming the division by an incision beginning at the thicker end, and gradually becoming larger and larger, till, after a struggle similar to that of their globular congeners, they finally disunite at the hair-like extremity of the tail. Each animalcule thus formed, soon grows to the size which again determines a further spontaneous subdivision; and thus the same process goes on to an indefinite extent.

"The most singular circumstance attending this mode of multiplication is, that it is impossible to pronounce which of the two new individuals, thus formed out of a single one, should be regarded as the parent, and which as the offspring, for they are both of equal size. Unless, therefore, we consider the separation of the parts of the parent animal to constitute the close of its individual existence, we must recognise an unbroken continuity in the vitality of the animal, thus transmitted from the original stem, throughout all succeeding generations. This, however, is one of those metaphysical subtleties, for which the subject of reproduction affords abundant scope.'

"

In the vegetable kingdom we have no instance of this kind of spontaneous division of an organized being into complete separate existences; but, by artificial means, * Roget's Bridgewater Treatise, vol. ii. pp. 583-5.

this mode of reproduction may be effected. Thus a tree may be divided horizontally into slips, which may continue to grow, and may by-and-by produce a tree similar in all respects to that from which it originated. Even from a small fragment of a plant, under favourable circumstances, a complete plant may be formed, by roots shooting out from the one end, and a stem from the other. These facts show the reproductive power of the vegetable creation under the culture of man, but do not occur in the ordinary course of things.

Of this latter mode of multiplication, we have numerous examples in the lower departments of the animal kingdom. The Hydra, or fresh-water polype, is capable of indefinite multiplication by division, although this is not the natural mode of its reproduction. If it be cut asunder transversely, the part containing the head soon supplies itself with a tail, and the detached tail shoots forth a new head, with a new set of tentacula; and if the whole animal be divided into a great number of pieces, each separate fragment acquires, in a short time, all the parts which are wanting to render it an entire individual. The same phenomena are observed, and nearly to the same extent, in the Plantaria. The Asteria, the Actinia, and some of the lower species of Annelida, are also capable of being multiplied by artificial divisions, each segment having the power of producing others, and containing within itself a kind of separate vitality.

Something analogous to this, but by no means extending to the capability of reproducing a complete being, exists, in a certain degree, among the higher orders of animals. The claws, the feet, the antenna, and the entire limbs of some of the inferior species of these orders, are restored when lost, by a fresh growth of the organs. The crab renews its limbs when torn off. If the head of a snail be amputated, the whole of that part of the animal, including its eyes and other organs of sense, will be reproduced. The tails of newts, and of some species of lizards, will grow again, if lost; and even the eyes of

these animals, with all their complex apparatus of coats and humours, will, if removed, be replaced by the growth of new eyes. Among the highest class of all, too, similar powers exist, though much restricted. The principle which, in the human frame, closes a wound, repairs a broken bone, or alters the course of the blood when the ordinary channels are closed, is of the same nature, and cannot be viewed by a rightly constituted mind, without gratitude to the Creator for the beneficent provisions.

The less perfect orders of that low class of animated nature, called Zoophytes, produce the species in a manner analogous to the buds of plants. At the earliest period at which the young of the Hydra, for example, is visible, it appears like a small tubercle or bud rising from the surface of the parent. It grows in this situation, and remains attached for a considerable time, at first deriving its nourishment from the parent; then occasionally stretching forth its tentacula, and learning the art of catching and swallowing its natural prey. At length the tube through which it received parental nourishment closes, the attaching filaments become more slender and break, and the young hydra moves away, and provides for its own subsistence.

Another plan of reproduction is that in which the germs are developed in the interior of the animal, assuming, in the first stages of animation, the form of the parent. In the Volvox, a spherical revolving animalcule, of the infusory order, this mode is exemplified. The germs of this animal appear, by aid of the microscope, in great numbers, in its interior, through its transparent covering; and while these are yet retained within the parent's body, other still minuter globules are developed within them, constituting a third generation. This progeny continues to swell till the parent bursts, and thus, in expiring, opens a passage for them to the element where they are in their turn to undergo the same destiny.* In the case of the Actinia, the young, or gemmules,

* Roget's Bridgewater Treatise, vol. ii. p. 586.

as such productions are called, force their way through the sides of the body, which readily open to give them passage, and quickly heal; but in most instances of such kind of spontaneous 'evolution, channels are provided, through which they find their way to a separate existence.

These are some of the extraordinary ways by which the species is preserved while the individual perishes, affording us a new and striking proof, that, though the analogies of nature are preserved with remarkable regularity in all the higher species of existences, it is not because the Creator fails in resources and expedients, but because some wise plan of his government has induced him to restrict his creative power, as regards these classes, to certain forms and types, while, in the less complicated organizations, he has found it expedient to take more latitude. Nothing can be more curious and instructive than the view which the microscope has opened up to us, of that busy world of living beings, which lies beyond the sphere of our unassisted vision, so incalculable in their numbers, so diversified in their forms, so opposite from our experience in their modes of propagation, and their means of subsistence; and yet so wonderfully harmonizing, amidst all their vast and anomalous varieties, with the more general analogies with which our senses have made us conversant, as to indicate most unequivocally the contriving mind and plastic hand of One Infinite Intelligence.

FOURTH WEEK-WEDNESDAY.

REPRODUCTION AMONG THE HIGHER ORDERS OF ANIMALS.

IN ascending from the lower and less complicated forms of animal existence to higher orders, we find the mode of reproduction more uniform, or at least more strictly confined to a single type, though still various