own rank and station among its own immediate

kindred.

Some insight into this classification of the animal kingdom is naturally indispensable to any one, who wishes to test and understand its reasons, and to render an account of it is an essential part of our task.

Since Cuvier's reconstruction of Zoology in the early part of this century, our science has been familiarized with the expression "type," or "fundamental form," introduced, long before, by Buffon. Cuvier, by extensive dissections and comparisons, first proved that animals were not, as people were formerly inclined to suppose, made on a last or shaped upon a block; but that they fall into several great divisions, in each of which expression is given to a peculiar constitution, arrangement, and distribution of the organs; in short, to a peculiar style. The sum of these characteristic. peculiarities, as well as the whole of the species united. in it, was termed a "type." Various views, it is true, even now prevail as to the extent of several of these types or families, as we will already term them; but if we disregard the dubious, and in many ways suspicious, existences, generally comprised under the name of primordial animals, there is a general agreement as to the following number, but less as to the sequence of the animal types, than as to those groups, each of which has its peculiar physiognomy and special characteristic structure.

The class Colenterata includes the Polypes and Medusa, and in the closest connection with it stands. the interesting class of the Spongiada, especially instructive as affording direct evidence of the doctrine of Descent. The organs of these animals are nearly always

TYPES AND FAMILIES.

31

arranged radially round an axis, passing through the dorsal and ventral pole. The cavity, which in most other animals-for instance, in man-is termed the abdominal cavity, the space between the intestinal wall and the abdominal parietes, is deficient in them; but, on the other hand, from the stomach proceed in general various kinds of tubes and branchia, which to a certain. extent replace the abdominal cavity. Fig. 2 represents a

FIG. 2.

Medusa, Tiaropsis Diadema, after Agassiz. The darklyshaded organs form the so-called cœlenteric apparatus.

Of the Echinoderms, the reader is probably acquainted, at least with the star-fish (Asterias) and the sea-urchin (Echinus), of which the general form is likewise usually radiate. Besides a peculiar chalky deposit, or greater or less calcification of the skin covering, a system of water-canals forms a characteristic of this family. With these are connected the rows of suckers, which, by protrusion and retraction, serve as organs of locomotion. On account of the radiate structure prevailing among the Echinoderms, Medusa, and Polypes,

Cuvier believed them to be more nearly related, and introduced them altogether, under the name of Radiata. This similarity, however, is only superficial, for whilst, on the one hand, anatomy discloses the great difference of the Cœlenterata and Echinodermata, the history of evolution still more decidedly banishes the Echinoderm from this position, and connects them more closely with the next division.

In this, that of the Vermes, the systematizer of the old school finds his real difficulty; in so many ways do they deviate from each other, so great is the distance between the lower and the higher forms; and after deducting the distinctive marks of orders, so little remains as a common character, so variegated is the host of smaller scattered groups, and even of single species, which demand admittance to the system of the Vermes. If we attempt to describe their typical nature in a few words, it must be something like this: The Vermes are more or less elongated, symmetric animals, which possess no actual legs, but effect their locomotion by means of a muscular system, closely combined with the integuments, which frequently become an actual muscular cylinder. To this we will add, that the perplexities and difficulties in reference to points of classification are transformed into sources of knowledge for the adherent of the doctrine of Descent.

The relations of the previous family with the type of the Articulata is so conspicuous, that the "kinship of the two was never questioned, even by the older zoologists. The very name of one, the highest division of the Vermes, that is, of the Annelids, or segmented worms, indicate this connection. This distinctive mark

GRADATIONS WITHIN THE TYPE.

33

of the Crustacea, Arachnida, Myriopoda, and Insecta, is that their bodies are constructed of sharply-defined rings or segments, the legs, antennæ and mandibles likewise sharing in this segmented character. A faithful expression of this segmentation is afforded by the nervous system, which lies, ladder-like on the ventral side, that is, beneath the intestinal canal, nearly encircling the gullet with its anterior loop. The display of segmentation is favoured by a deposit of horny substance, which gives a skeleton-like stiffness to the integuments.

The direct reverse is shown in the integuments of the Mollusca, our mussels, snails, and cuttle-fish. For although so many are supplied with protecting scales. and shells, these are mere excretions from the actual skin, which remains soft, and characteristically moist and slimy, owing to the secretions of numerous glands contained in it, and has an inclination to lay itself in folds, and form a mantle-like investment to the body. The body therefore remains more or less clumsy; it possesses none of the grace of the Articulata, and especially of the insect; it is destitute of segmentation, and this deficiency is likewise evinced in the nervous system. This consists only of a ring, encircling the oesophagus, and a few smaller ganglia.

We shall most readily come to an understanding as to the Vertebrata, the family with which man is inseparably united. The essential part is the vertebral column, that portion of the internal and persistently bony or cartilaginous skeleton, in which the main portion of the nervous system is contained.

It is thus established that the systematic classification of the animal kingdom is based on certain prominent

characteristics of form and internal structure; and it is very easy to select from every type forms in which the distinctive marks, comprised in the systematic diagnosis, may be displayed in full perfection. But this is immediately succeeded by a further observation, that of gradations within the type. When we previously compared the polype and the bee, and were obliged to assign to each a very different rank, a portion of this difference of grade is certainly due to the difference of the family; but the forms united by family characteristics likewise diverge widely from each other, and the systematist speaks of lower and higher classes within every type, of lower and higher orders within every class.

Reason is compelled to this by the same considerations which forced themselves upon us in the comparison of the polype and the bee. Why does the mussel stand lower than the snail? Because it does not possess a head, because its nervous system is not so concentrated and so voluminous, because its sensory organs are more defective. In one, as in the other, the structural material is present in quantities sufficient for the completion of the type; but in the snail it is more developed, and the single circumstance of the integration of various parts to form the head confers a higher dignity upon the snail. It is needless to illustrate this gradation within the families by further examples; the most superficial comparison of a fish with a bird or a mammal, of one of the parasitic crustacea with a crayfish or an insect, shows, as the older zoology represented it, that in the actual forms the ground plan, or "ideal types," find very diversified expression.

A further result of this descriptive inquiry is the