scopic animalcules, commonly united under the name of infusory animals, (Infusoria,) from their being found specially abundant in water infused with vegetable matter. These minute beings do not, however, constitute a natural group in the Animal Kingdom. Indeed, a great many that were formerly supposed to be animals are now found to be vegetables. Others are ascertained to be crustaceans, mollusks, worms of microscopic size, or the earliest stages of development of larger species. In general, however, they are exceedingly minute, and exhibit the simplest forms of animal life, and are now grouped together, under the title of Protozoa. But, as they are still very imperfectly understood, notwithstanding the beautiful researches already published on this subject, and as many of them are likely to be finally distributed among vegetables, and the legitimate classes in the Animal Kingdom to which they belong, we have not assigned any special place for them.

PHYSIOLOGICAL ZOOLOGY.

CHAPTER FIRST.

THE SPHERE AND FUNDAMENTAL PRINCIPLES OF ZOÖLOGY.

1. ZOOLOGY is that department of Natural History which relates to animals.

2. To enumerate and name the animals which are found on the globe, to describe their forms, and investigate their habits and modes of life, are the principal, but by no means the only objects of this science. Animals are worthy of our regard, not merely when considered as to the variety and elegance of their forms, or their adaptation to the supply of our wants; but the Animal Kingdom, as a whole, has a still higher signification. It is the exhibition of the divine thought, as carried out in one department of that grand whole which we call Nature; and considered as such, it teaches us most important lessons.

3. Man, in virtue of his twofold constitution, the spiritual and the material, is qualified to comprehend Nature.

Being made in the spiritual image of God, he is competent to rise to the conception of His plan and purpose in the works of Creation. Having also a material body, like that of other animals, he is also in a condition to understand the mechanism of organs, and to appreciate the necessities of matter, as well as the influence which it exerts over the intellectual element throughout the domain of Nature.

4. The spirit and preparation we bring to the study of Nature, is a matter of no little consequence. When we would study with profit a work of literature, we first endeavor to make ourselves acquainted with the genius of the author; and in order to know what end he had in view, we must have regard to his previous labors, and to the circumstances under which the work was executed. Without this, although we may perhaps enjoy its perfection as a whole, and admire the beauty of its details, yet the spirit which pervades it will escape us, and many passages may even remain unintelligible.

5. So, in the study of Nature, we may be astonished at the infinite variety of her products; we may even study some portion of her works with enthusiasm, and nevertheless remain strangers to the spirit of the whole, ignorant of the plan on which it is based, and fail to acquire a proper conception of the varied affinities which combine beings together, so as to make of them that vast picture in which each animal, each plant, each group, each class, has its place, and from which nothing could be removed without destroying the proper meaning of the whole.

6. Besides the beings which inhabit the earth at the pres ent time, this picture also embraces the extinct races which are now known to us by their fossil remains only. And these are of the greatest importance, since they furnish us with the means of ascertaining the changes and modifications which the Animal Kingdom has undergone in the suc

cessive creations, since the first appearance of living beings.

7. It is but a short time since it was not difficult for a man to possess himself of the whole domain of positive knowledge in Zoology. A century ago, the number of known animals did not exceed 8000; that is to say, from the whole Animal Kingdom, fewer species were then known than are now contained in many private collections of certain families of insects merely. At the present day, the number of living species which have been satisfactorily made out and described, is more than 50,000.* The fossils already described exceed 6000 species; and if we

*The number of vertebrate animals may be estimated at 20,000. About 1500 species of mammals are pretty precisely known, and the number may probably be carried to about 2000.

The number of Birds well known is 4 or 5000 species, and the probable number is 6000.

The Reptiles number about the same as the Mammals, 1500 described species, and they will probably reach the number of 2000.

The Fishes are more numerous: there are from 5 to 6000 species in the museums of Europe, and the number may probably amount to 8 or 10,000:

The number of Mollusks already in collections probably reaches 8 or 10,000. There are collections of marine shells, bivalve and univalve, which amount to 5 or 6000; and collections of land and fluviatile shells, which count as many as 2000. The total number of mollusks would, therefore, probably exceed 15,000 species.

Among the articulated animals it is difficult to estimate the number of species. There are collections of coleopterous insects which number 20 to 25,000 species; and it is quite probable, that by uniting the principal collections of insects, 60 or 80,000 species might now be counted; for the whole department of articulata, comprising the crustacea, the cirrhipeda, the insects, the red-blooded worms, the intestinal worms, and the infusoria so far as they belong to this department, the number would already amount to 100,000; and we might safely compute the probable number of species actually existing at double that sum.

Add to these about 10,000 for radiata, including echini, star-fishes, medusæ, and polypi, and we have about 250,000 species of living animals; and supposing the number of fossil species only to equal them, we have, at a very moderate computation, half a million of species.

conside: that wherever any one stratum of the earth has been well explored, the number of species discovered has not fallen below that of the living species which now inhabit any particular locality of equal extent, and then bear in mind that there is a great number of geological strata, we may anticipate the day when the ascertained fossil species will far exceed the living species.*

8. These numbers, far from discouraging, should, on the contrary, encourage those who study Natural History. Each new species is, in some respects, a radiating point which throws additional light on all around it; so that, as the picture is enlarged, it at the same time becomes more intelligible to those who are competent to seize its prominent traits.

9. To give a detailed account of each and all of these animals, and to show their relations to each other, is the task of the Naturalist. The number and extent of the volumes already published upon the various departments of Natural History show, that only a mere outline of a domain so vast could be fully sketched in an elementary work, and that none but those who make it their special study can be expected to survey its individual parts.

10. Every well-educated person, however, is expected to have a general acquaintance with the great natural phenomena constantly displayed before his eyes. There is a general knowledge of man and the subordinate animals, embracing their structure, races, habits, distribution, mutual relations, &c., which is not only calculated to conduce es

*In a separate work, entitled "Nomenclator Zoologicus," by L. AGASSIZ, the principles of nomenclature are discussed, and a list of the names of genera and families proposed by authors is given. To this work those are referred who may desire to become more familiar with nomenclature, and to know in detail the genera and families in each class of the Animal Kingdom.