earlier the epoch at which we examine them. No two ani mals can be more unlike than an adult Medusa (Fig. 31 and an adult Campanularia, (Fig. 143;) they even seem to belong to different classes of the Animal Kingdom, the former being considered as an Acaleph, the latter as a Polyp, On the other hand, if we compare them when first hatched from the egg, they appear so much alike that it is with the greatest difficulty they can be distinguished. They are then little Infusoria, without any very distinct shape, and moving with the greatest freedom. The larvæ of certain intestinal worms, though they belong to a different department, have nearly the same form, at one period of their life. Farther still, this resemblance extends to plants. The spores of certain sea-weeds have nearly the same appearance as the young Polyp, or the young Medusa; and what is yet more remarkable, they are also furnished with cilia, and move about in a similar manner. But this is only a transient state. Like the young Campanularia and the young Medusa, the spore of the sea-weed is free for only a short time; soon it becomes fixed, and from that moment the resemblance ceases.

360. Are we to conclude, then, from this resemblance of the different types of animals at the outset of life, that there is no real difference between them; or that the two King doms, the Animal and the Vegetable, actually blend, because their germs are similar? On the contrary, we think nothing is better calculated to strengthen the idea of the original separation of the various groups, as distinct and independent types, than the study of their different phases. In fact, a difference so wide as that between the adult Medusa and the adult Campanularia must have existed even in the young; only it does not show itself in a manner appreciable by our senses; the character by which they subsequently differ so much being not yet developed

To

deny the reamy of natural groups, because of these early resemblances, would be to take the semblance for the reality. It would be the same as saying that the frog and the fish are one, because at one stage of embryonic life it is impossible, with the means at our command, to distinguish them.

361. The account we have above given of the developmer.t, the metamorphoses, and the alternate reproduction of the lower animals, is sufficient to undermine the old theory of Spontaneous Generation, which was proposed to account for the presence of worms in the bodies of animals, for the sudden appearance of myriads of animalcules in stagnant water, and under other circumstances rendering their occurrence mysterious. We need only to recollect how the Cercaria insinuates itself into the skin and the viscera of mollusks, (339, 342,) to understand how admission may be gained to the most inaccessible parts. Such beings occur even in the eye of many animals, especially of fishes; they are numerous in the eye of the common fresh-water perch of Europe. To the naked eye they seem like little white spots, (Fig. 145;) but when magnified, they have the form of Fig. 146.

Fig. 145. Fig. 146.

a

362. As to the larger intestinal worms found in other animals, the mystery of their origin has been entirely solved by recent researches. A single instance will illustrate their history. At certain periods of the year, the Sculpins of the Baltic are infested by a particular species of Tænia or tapeworm, from which they are free at other seasons. Mr. Eschricht found that, at certain seasons, the worms lose a great portion of the long chain of rings of which they are coinposed. On a careful examination, he found that each ring

contained several hundred eggs, which, on being freed from their envelop, float in the water. As these eggs are innumerable, it is not astonishing that the Sculpins should occasionally swallow some of them with their prey. The eggs, being thus introduced into the stomach of the fish, find conditions favorable to their development; and thus the species is propagated, and at the same time transmitted from one generation of the fish to another. The eggs which are not swallowed are probably lost.

363. All animals swallow, in the same manner, with their food, and in the water they drink, numerous eggs of such parasites, any one of which, finding in the intestine of the animal favorable conditions, may be hatched. It is probable that each animal affords the proper conditions for some particular species of worm; and thus we may explain how it is that most animals have parasites peculiar to themselves.

364. As respects the Infusoria, we also know that most of them, the Rotifera especially, lay eggs. These eggs, which are extremely minute, (some of them only 1200ʊ of an inch in diameter,) are scattered every where in great profusion, in water, in the air, in mist, and even in snow. Assiduous observers have not only seen the eggs laid, but moreover, have followed their development, and have seen the young animal forming in the egg, then escaping from it, increasing in size, and, in its turn, laying eggs. They have been able, in some instances, to follow them even to the fifth and sixth generation.

365. This being the case, it is much more natural to suppose that the Infusoria * are products of like germs, than

* In this connection, it ought to be remembered that a large proportion of the so-called Infusoria are not independent animals, but immature germs, belonging to different classes of the Animal Kingdom, and that many must be referred to the Vegetable Kingdom.

o assign to them a spontaneous origin altogether incompati ple with what we know of organic development. Their rapid appearance is not at all astonishing, when we reflect hat some mushrooms attain a considerable size in a few hours, but yet pass through all the phases of regular growth; and, indeed, since we have ascertained the different modes of generation among the lower animals, no substantial difficul. ties to the axiom "omne vivum ex ovo," (275,) any longer exist.

15*

CHAPTER TWELFTH.

METAMORPHOSES OF ANIMALS.

366. UNDER the name of metamorphoses are included those changes which the body of an animal undergoes after its birth, and which are modifications, in various degrees, of its organization, form, and its mode of life. Such changes are not peculiar to certain classes, as has been so long supposed, but are common to all animals, without exception.

367. Vegetables also undergo metamorphoses, but with this essential difference, that in vegetables the process consists in an addition of new parts to the old ones. A succession of leaves, differing from those which preceded them, comes on each season; new branches and roots are added to the old stem, and woody layers to the trunk. In animals, the whole body is transformed, in such a manner that all the existing parts contribute to the formation of the modified body. The chrysalis becomes a butterfly; the frog, after having been herbivorous during its tadpole state, becomes carnivorous, and its stomach is adapted to this new mode of life; at the same time, instead of breathing by gills, it be comes an air-breathing animal; its tail and the gills disappear; lungs and legs are being developed, and, finally, it is to live and move on land.

368 The nature, the duration, and importance of metamorphoses, as also the epoch at which they take place, are infinitely varied. The most striking changes which naturally present themselves to the mind when we speak of metamo