wards. Therefore a classification, to be true and natural must accord with the succession of organs in the embryonic development. This coincidence, while it corroborates the anatomical principles of Cuvier's classification of the Animal Kingdom, furnishes us with new proof that there is a general plan displayed in every kind of development.

323. Combining these two points of view, that of Embry ology with that of Anatomy, the four divisions of the Animal Kingdom may be represented by the four figures which are to be found, at the centre of the diagram, at the beginning of the volume.

324. The type of Vertebrates, having two cavities, one above the other, the former destined to receive the nervous system, and the latter, which is of a larger size, for the intestines, is represented by a double crescent united at the centre, and closing above, as well as below.

325. The type of Articulata, having but one cavity, growing from below upwards, and the nervous system forming a series of ganglions, placed below the intestine, is represented by a single crescent, with the horns directed upwards.

326. The type of Mollusks having also but one cavity, the nervous system being a simple ring around the œsophagus, with ganglions above and below, from which threads go off to all parts, is represented by a single crescent with the hoins turned downwards.

327. Finally, the type of Radiata, the radiating form of which is seen even in the youngest individuals, is represented by a star.

CHAPTER ELEVENTH.

PECULIAR MODES OF REPRODUCTION.

SECTION I.

GEM MIPAROUS AND FISSIPAROUS REPRODUCTION.

328. We have shown in the preceding chapter, that ovulation, and the development of embryos from eggs, is cominon to all classes of animals, and must be considered as the great process for the reproduction of species. Two other modes of propagation, applying, however, to only a limited number of animals, remain to be mentioned, namely, gemmiparous reproduction, or multiplication by means of buds, and fissiparous reproduction, or propagation by division; and also some still more extraordinary modifications yet involved in much obscurity.

329. Reproduction by buds occurs among the polyps, medusæ, and some of the infusoria. On the stalk, or even on the body of the Hydra, (Fig. 132,) and of many infusoria, there are formed buds, like those of plants. On close examination they are found to be young animals at first very imperfectly formed, and commu nicating at the base with the parent body, from which they derive their nourishment. By degrees, the animal is developed; in most cases, the tube by which it is connected with the parent

Fig. 132.

withers away, and the animal is thus detached and becomes Independent. Others remain through life united to the parent stalk, and, in this respect, present a more striking analogy to the buds of plants. But in the polyps, as in trees, budding is only an accessory mode of reproduction, which presupposes a trunk already existing, originally the product of ovulation.

с

330. Reproduction by division, or fissiparous reproduction is still more extraordinary; it takes place only in polyps and some infusoria. A cleft or fissure at some part of the body takes place, very slight at first, but constantly increasing in depth, so as to become a deep furrow, like that observed in the yolk, at the beginning of embryonic development; at the same time the contained organs are di

a b

Fig. 133.

vided and become double, and thus two individuals are formed of one, so similar to each other that it is impossible to say which is the parent and which the offspring. The division takes place sometimes vertically, as, for example, in Vorticella, (Fig. 133,) and in some Polyps, (Fig. 134,) and sometimes trans

d

Fig. 134.

versely. In some Infusoria, the Paramecia, for instance, this division occurs as often as three or four times in a day.

331. In consequence of this same faculty, many animals are able to reproduce various parts of their bodies when accidentally lost. It is well known that crabs and spiders, on losing a limb, acquire a new one. The same happens with the arms of the star-fishes. The tail of a lizard is also

teadily reproduced. Salamanders even possess the facu‍ty of reproducing parts of the head, including the eye with all its complicated structure. Something similar takes place in our own bodies, when a new skin is formed over a wound or when a broken bone is reunited.

332. In some of the lower animals, this power of repara· tion is carried much farther, and applies to the whole body, so as closely to imitate fissiparous reproduction. If an earthworm, or a fresh-water polyp, be divided into several pieces, the injury is soon repaired, each fragment speedily becoming a perfect animal. Something like this reparative faculty is seen in the vegetable kingdom, as well as the animal. A willow branch, planted in a moist soil, throws out roots below and branches above; and thus, after a time, assumes the shape of a perfect tree.

333. These various modes of reproduction do not exclude each other. All animals which propagate by gemmiparous or fissiparous reproduction also lay eggs. Thus the freshwater polyps (Hydra) propagate both by eggs and by buds. In Vorticella, according to Ehrenberg, all three modes are found; it is propagated by eggs, by buds, and by division. Ovulation, however, is the most common mode of reproduction, the other modes, and also alternate reproduction, are only additional means employed by Nature to secure the perpetuation of the species.

SECTION II.

ALTERNATE AND EQUIVOCAL REPRODUCTION.

334. It is a matter of common observation, that individuals of the same species have the same general appearance, by which their peculiar organization is indicated. The trans

mission of these characteristics, from one generation to the next, is justly considered as one of the great laws of the Animal and Vegetable Kingdoms. It is, indeed, one of the points on which the definition of species is generally founded. We would, however, unhesitatingly adopt the new definition of Dr. S. G. Morton, who defines species to be "primordial organic forms."

335. But it does not follow that animals must resemble their parents in every condition, and at every epoch of their existence. On the contrary, as we have seen, this resem blance is very faint, in most species, at birth; and some, such as the caterpillar and the tadpole, undergo complete metamorphoses before attaining their final shape as the butterfly and frog. Nevertheless, we do not hesitate to refer the tadpole and the frog to the same species; and so with the caterpillar and the butterfly; because we know that there is the same individual observed in different stages of development.

336. There is, also, another series of cases, in which the offspring not only do not resemble the parent at birth, but, moreover, remain different during their whole life, so that their relationship is not apparent until a succeeding generation. The son does not resemble the father, but the grandfather ; and in some cases the resemblance reappears only at the fourth or fifth generation, and even later. This singular mode of reproduction has received the name of alternate generation. The phenomena attending it have been of late the object of numerous scientific researches, which are the more deserving of our attention, as they furnish a solution to several problems alike interesting in a zoölogical and in a philosophical point of view.

337. Alternate generation was first observed among the Salpæ. These are marine mollusks, without shells, belong. ing to the family Tunicata. They are distinguished by the curious peculiarity of being united together in considerable numbers so as to form long chains, which float in the sca