But although Nature has thus been governed by strict rules in the construction of the organs of primary importance, there is some considerable latitude with regard to those that are of secondary. Thus in the dog, notwithstanding the great differences exhibited by the several varieties of this animal in color, shape, size, quantity of hair, expression of countenance, and other external characteristics, anatomists do not detect any corresponding differences of internal structure. It is not unlikely that even these varieties may be actually owing to the operation of some definite laws of Nature; but, if so, we have not yet attained to a knowledge of them.

It is upon considerations derived from the study of the animal kingdom in these relations, that systems of classification have been founded; and, although it is no part of the plan of this work to teach systematic zoology, yet is it necessary to have some such system before us in order to the most perspicuous treatment of our subject.

In a system of classification, animals are arranged in certain divisions, and these divisions are distinguished by characteristic marks. The divisions universally recognized as founded in Nature, are those of genera and species. Each distinct kind of animal constitutes a species, and there is usually supposed for each a distinct creation. The dog, the horse, the cat, are distinct species, and in them the essential characteristics of structure do not vary. Still there are many differences in subordinate particulars, especially with regard to external peculiarities, which give rise to what are called varieties. These varieties are most numerous among domesticated animals. Thus in the horse, the Canadian pony is an example of variet, in size, the racehorse and cart-horse of variety in shape and proportion, - black, bay, and white, in color. Other domestic animals, the cow, sheep, fowl, dog, exhibit varieties as great. But examine the feet, the teeth, the heart, the lungs, the stomach, the intestines, of the individuals that vary so much externally, and the difference vanishes.

When a number of species resemble each other so closely in some of the secondary peculiarities of anatomical structure as to distinguish them from all others and give them a kind of family

likeness themselves, they constitute a GENUS. Thus the among genus Felis includes all those of the cat kind; and these animals, although differing one from another very much in size and color, have yet a close resemblance in their general form, figure, character, and habits of life. The genus Canis includes those of the dog kind; the wolf, the fox, the jackal, and the domestic dog, of which the same remark may be made. Thus, too, the horse, the ass, and the zebra are of the same genus, Equus, on account of their obvious general similarity.

The genus then is made up of a number of species, there being as many species as there are sorts of animals. Thus the cat, the tiger, the lion, leopard, jaguar, and catamount are all separate species; but taken together with others, they constitute the genus Felis. Thus, too, the genus Canis contains the dog, the wolf, the jackal, the fox, &c., which are all so many distinct species. The genus Sciurus contains the gray, red, striped, and many other squirrels. In treating of any particular animal, naturalists are accustomed to designate it by a name derived from its genus and species. This name is composed of two words; the first being the name of its genus; and the second, the name of the species, being altogether arbitrary, or else expressing some circumstance relating to the color, size, or residence of the animal, which serves in a degree to distinguish it from others. The first is called its generic, the second its trivial or specific name, and they correspond very closely to the names of human individuals; the generic term answering to the Surname, which designates the family to which any one belongs, and the trivial to the Christian name, which designates the particular individual.

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To give an example: the different species of the genus Felis, above mentioned, are distinguished one from another in the following manner. The lion is called Felis leo; the tiger, Felis tigris; the leopard, Felis leopardus; the jaguar, Felis onca; the lynx, Felis lynx; the serval, Felis serval. In the genus Canis, the dog is called Canis domesticus; the wolf, Canis lupus; the black wolf, Canis lycaon; the fox, Canis vulpes. In this way, each animal is capable of being clearly and accurately desigчated, by a name less liable to mistake and confusion than its

common one, which is sometimes applied to several different species. This is called the scientific or systematic name.

But beside this natural and obvious division into genera and species, many others have been constituted, arranging the genera into a variety of groups, according to principles of classification

which have been founded more or less on anatomical considerations, and have varied from time to time with the progress of knowledge.

In the late work of Professor Agassiz upon the Zoölogy of the United States, a different view from that commonly entertained is presented, and copiously illustrated. According to him, none of the divisions of natural objects are arbitrary, but strictly founded in nature, and are parts of the plan of the creation as it existed in the mind of its Designer.

The following are the divisions according to which he would thus arrange all the subjects of the animal kingdom.

1. BRANCH; founded upon differences in the plan upon which animals are constructed; of which there are four, Vertebrata, Articulata, Mollusca, Radiata.

2. CLASS. Each Branch is divided into classes, according to the mode in which the plan is carried out. Thus the general plan of vertebrated animals is the same in all, but is carried out in different ways in Mammalia, Birds, Reptiles, and Fish.

3. ORDER. This is constituted according to the degrees of complication in the execution of the plan.

4. FAMILY. By the form of animals, as far as determined by structure.

5. GENUS. By the details of the execution in special parts.

6. SPECIES. By the relations of individuals to one another, and to the world in which they live, as well as by the proportions of their parts, their ornamentation, &c.

CHAPTER V.

GENERAL SUBDIVISIONS OF THE ANIMAL KINGDOM.

In order to convey to the student an intelligible view of the great plan according to which the Creator has distributed animal life over the surface of our earth, it will be necessary to present such a sketch of the subdivisions of the animal kingdom as the amount of information of the general reader will enable him to comprehend.

In the first place, the whole animal kingdom is arranged under four grand divisions, branches, or sub-kingdoms, according to the PLAN upon which different animals are constructed. Thus the monkey is constructed upon one plan; the bee another plan; the oyster upon another; and the starfish upon another. There are, then, the following grand divisions:

1. Vertebrata.

2. Articulata.

These are each, again, subdivided into classes, according to the mode in which the plan is carried out. Thus among the Vertebrata; in quadrupeds, the plan is carried out in one way; in another way in Birds; in another in Reptiles; and in another in Fishes.

Each class is again subdivided into orders, families, genera, and species, in the arrangement of which regard is had to more particular details of structure and form.

I. THE VERTEBRATA, Vertebral animals, of which man is the representative and the most

upon

3. Mollusca.

4. Radiata.

Fig. 6.

Mammalia. Chimpanzee.

perfect specimen, are formed after his model, varied according to the conditions of existence for which they are intended. They have all a vertebral column which is the basis of an internal bony skeleton; a nervous system comprising a brain contained in a cranium and a spinal nerve inclosed in the vertebral column, from which the nerves are distributed to the whole body; five senses; four extremities, or parts corresponding to them; two jaws moving vertically; and a heart circulating red blood. Among them are terrestrial, aërial, aquatic, and amphibious animals. Some of them breathe only air; and some breathe it through the medium of water. Man, quadrupeds, birds, reptiles, fishes, are representatives of this branch.

II. The ARTICULATA, Articulated animals, so called because their bodies are composed of a succession of segments, rings, or joints, which are movable upon each other. They have no internal skeleton, but, instead, an external case or envelope, firm and solid as bone in some, soft and yielding in others. To the internal surface of this the muscles are attached, and it thus forms the basis of the motions of the animal, and takes the place of the bones of the Vertebrata. They have a head, but no proper brain. The nervous system consists of a knotted cord, differently developed in different cases. They have the senses of sight, touch, and taste; and many of them probably those of hearing and smelling, though their organs are not obvious. Some of them have no legs, but, if any, never less than six, and often many more. Their bodies are symmetrically arranged, i. e. the two sides correspond to each other. Some of them are winged insects; others inhabit the water, as the leech; and others crawl in or upon the earth, as the centipede and worms.

Fig. 7.

Articulata. Centipede.

III. The MOLLUSCA, soft-bodied animals, derive their name from the peculiar structure of their bodies, which are destitute