sentially to our happiness, but which it would be quite inexcusable to neglect. This general view of Zoology, it is the purpose of this work to afford.

11. A sketch of this nature should render prominent the more general features of animal life, and delineate the arrangement of the species according to their most natural relations and their rank in the scale of being; thus giving a panorama, as it were, of the entire Animal Kingdom. To accomplish this, we are at once involved in the question, What is it that gives an animal precedence in rank?

12. In one sense, all animals are equally perfect. Each species has its definite sphere of action, whether more or less extended, its own peculiar office in the economy of nature; and a complete adaptation to fulfil all the purposes of its creation, beyond the possibility of improvement. In this sense, every animal is perfect. But there is a wide difference among them, in respect to their organization. In some it is very simple, and very limited in its operation; in others, extremely complicated, and capable of exercising a great variety of functions.

13. In this physiological point of view, an animal may be said to be more perfect in proportion as its relations with the external world are more varied; in other words, the more numerous its functions are. Thus, an animal, like a quadruped, or a bird, which has the five senses fully developed, and which has, moreover, the faculty of readily transporting itself from place to place, is more perfect than a snail, whose senses are very obtuse, and whose motion is very sluggish.

14. In like manner, each of the organs, when separately considered, is found to have every degree of complication, and, consequently, every degree of nicety in the performance of its function. Thus, the eye-spots of the star-fish and jelly-fish are probably endowed with merely the fac

ulty of perceiving light, without the power of distinguishing objects. The keen eye of the bird, on the contrary, discerns minute objects at a great distance, and when compared with the eye of a fly, is found to be not only more perfect, but constructed on an entirely different plan. It is the same with

every other organ.

15. We understand the faculties of animals, and appreciate their value, just in proportion as we become acquainted with the instruments which execute them. The study of the functions or uses of organs, therefore, requires an examination of their structure; they must never be disjoined, and must precede the systematic distribution of animals into classes, families, genera, and species.

16. In this general view of organization, we must ever bear in mind the necessity of carefully distinguishing between affinities and analogies, a fundamental principle recognized even by Aristotle, the founder of scientific Zoology. Affinity or homology is the relation between organs or parts of the body which are constructed on the same plan, however much they vary in form, or even serve for very different uses. Analogy, on the contrary, indicates the similarity of purposes or functions performed by organs of different structure.

17. Thus, there is an analogy between the wing of a bird and that of a butterfly, since both of them serve for flight. But there is no affinity between them, since, as we shall hereafter see, they differ totally in their anatomical relations. On the other hand, there is an affinity between the bird's wing and the hand of a monkey; since, although they serve for different purposes, the one for flight, and the other for climbing, they are both constructed on the same plan. Accordingly, the bird is more nearly allied to the monkey than to the butterfly, though they both have in common the faculty of flight. Affinities, and not analogies, therefore, must guide us in the arrangement of animals.

18. Our investigations should not be limited to adult animals, but should also include the changes which they undergo during the whole course of their development. Otherwise, we shall be liable to exaggerate the importance of certain peculiarities of structure which have a predominant character in the full-grown animal, but which are shaded off, and vanish, as we revert to the earlier periods of life.

19. Thus, for example, by regarding only adult individuals, we might be induced to divide all animals into two groups, according to their mode of respiration; uniting, on the one hand, all those which breathe by gills, and, on the other, those which breathe by lungs. But this distinction loses its importance, when we consider that various animals, for example, frogs, which respire by lungs in the adult state, have only gills when young. It is thence evident that the respiratory organs cannot be taken as a satisfactory basis of our fundamental classification. They are, as we shall see, subordinate to a more important system, namely, the nervous system.

20. Again, we have a means of appreciating the relative grade of animals by the comparative study of their development. It is evident that the caterpillar, in becoming a butterfly, passes from a lower to a higher state. Clearly, therefore, animals resembling the caterpillar, the worms, for instance, must occupy a lower rank than those approaching the butterfly, like most insects. There is no animal which does not undergo a series of changes similar to those of the caterpillar or the chicken; only, in many of them, the most important ones occur before birth, during what is called the embryonic period.

21. The life of the chicken has not just commenced when issues from the egg; for if we break the egg some days previous to the time of hatching, we find in it a living animal, which, although imperfect, is nevertheless a chicken :

it has been developed from a hen's egg, and we know that, should it continue to live, it would infallibly display all the characteristics of the parent bird. Now, if there existed in Nature an adult bird as imperfectly organized as the chicken on the day, or the day before it was hatched, we should assign to it an inferior rank.

22. In studying the embryonic states of the mollusks or worms, we observe in them points of resemblance to many animals of a lower grade, to which they at length become entirely dissimilar. For example, the myriads of minute aquatic animals embraced under the name of Infusoria, generally very simple in their organization, remind us of the embryonic forms of other animals. We shall have occasion to show that the Infusoria are not to be considered as a distinct class of animals, but that among them are found members of all the lower classes of animals, mollusks, crustaceans, worms, &c.; and many of them are even found to belong to the Vegetable Kingdom.

23. Not less striking are the relations that exist between animals and the regions they inhabit. Every animal has its home. Animals of the cold regions are not the same as those of temperate climates; and these latter, in their turn, differ from those of tropical regions. Certainly, no one will maintain it to be the effect of accident that the monkeys, the most perfect of all brute animals, are found only in hot countries; or that by chance merely the white bear and reindeer inhabit only cold regions.

24. Nor is it by chance that most of the largest animals, of every class, the whales, the aquatic birds, the sea-turtles, the crocodiles, dwell in the water rather than on the land. And while the water affords freedom of motion to the largest, it is also the home of the smallest of living beings, allowing a degree of liberty to their motion, which they could not enjoy elsewhere.

25. Nor are our researches to be limited to the animals now living. There are buried in the crust of the earth the remains of a great number of animals belonging to species which do not exist at the present day. Many of these remains present forms so extraordinary that it is almost impossible to trace their alliance with any animal now living. In general, they bear a striking analogy to the embryonic forms of existing species. For example, the curious fossils known under the name of Trilobites (Fig. 156) have a shape so singular that it might well be doubted to what group of articulated animals they belong. But if we compare them with the embryo crab, we find so remarkable a resemblance that we do not hesitate to refer them to the crustaceans. We shall also see that some of the Fishes of ancient epochs present shapes altogether peculiar to themselves, (Fig. 157,) but resembling, in a striking manner, the embryonic forms of our common fishes. A determination of the successive appearance of animals in the order of time is, therefore, of much importance in assisting to decide the relative rank of animals.

26. Besides the distinctions to be derived from the varied structure of organs, there are others less subject to rigid analysis, but no less decisive, to be drawn from the immaterial principle with which every animal is endowed. It is this which determines the constancy of species from generation to generation, and which is the source of all the varied exhibitions of instinct and intelligence which we see displayed, from the simple impulse to receive the food which is brought within their reach, as observed in the polyps, through the higher manifestations, in the cunning fox, the sagacious elephant, the faithful dog, to the exalted intellect of man. which is capable of indefinite expansion.

27. Such are some of the general aspects in which we are to contemplate the animal creation. Two points of