the time that Linnæus showed us the necessity of a scientific system as a framework for the arrangement of scientific facts in Natural History, the number of divisions adopted by zoölogists and botanists increased steadily. Not only were families, orders, and classes added to genera and species, but these were further multiplied by subdivisions of the different groups. But as the number of divisions increased, they lost in precise meaning, and it became more and more doubtful how far they were true to Nature. Moreover, these divisions were not taken in the same sense by all naturalists: what were called families by some were called orders by others, while the orders of some were the classes of others, till it began to be doubted whether these scientific systems had any foundation in Nature, or signified anything more than that it had pleased Linnæus, for instance, to call certain groups of animals by one name, while Cuvier had chosen to call them by another.
These divisions are, first, the most comprehensive groups, the primary divisions, called branches by some, types by others, and divided by some naturalists into so-called sub-types, meaning only a more limited circumscription of the same kind of group; next we have classes, and these also have been divided into sub-classes, then orders and sub-orders, families, sub-families, and tribes; then genera, species, and varieties. With reference to the question, whether these groups really exist in Nature or are merely the expression of individual theories and opinions, it is worth while to study the works of the early naturalists, in order to trace the natural process by which scientific classification has been reached; for in this, as in other departments of learning, practice has always preceded theory. We do the thing before we understand why we do it: speech precedes grammar, reason precedes logic; and so a division of animals into groups, upon an instinctive perception of their differences, has preceded all our scientific creeds and doctrines. Let us, therefore, proceed to examine the meaning of these names as adopted by naturalists.
When Cuvier proposed his four primary divisions of the animal kingdom, he added his argument for their adoption,—because, he said, they are constructed on four different plans. All the progress in our science since his time confirms this result; and I shall attempt to show that there are really four, and only four, such structural ideas at the foundation of the animal kingdom, and that all animals are included under one or another of them. But it does not follow, that, because we have arrived at a sound principle, we are therefore unerring in our practice. From ignorance we may misplace animals, and include them under the wrong division. This is a mistake, however, which a better insight into their organization rectifies; and experience constantly proves, that, whenever the structure of an animal is perfectly understood, there is no hesitation as to the head under which it belongs. We may consequently test the merits of these four primary groups on the evidence furnished by investigation. It has already been seen that these plans may be presented in the most abstract manner without any reference to special animals. Radiation expresses in one word the idea on which the lowest of these types is based. In Radiates we have no prominent bilateral symmetry, as in all other animals, but an all-sided symmetry, in which there is no right and left, no anterior and posterior extremity, no above and below. They are spheroidal bodies; yet, though many of them remind us of a sphere, they are by no means to be compared to a mathematical sphere, but rather to an organic sphere, so loaded with life, as it were, as to produce an infinite variety of radiate symmetry. The whole organization is arranged around a centre toward which all the parts converge, or, in a reverse sense, from which all the parts radiate. In Mollusks there is a longitudinal axis and a bilateral symmetry; but the longitudinal axis in these soft concentrated bodies is not very prominent; and though the two ends of this axis are distinct from each other, the difference is not so marked that we can say at once, for all of them, which is the anterior and which the posterior extremity. In this type, right and left have the preponderance over the other diameters of the body. The sides are the prominent parts,—they are charged with the important organs, loaded with those peculiarities of the structure that give it character. The Oyster is a good instance of this, with its double valve, so swollen on one side, so flat on the other. There is an unconscious recognition of this in the arrangement of all collections of Mollusks; for, though the collectors do not put up their specimens with any intention of illustrating this peculiarity, they instinctively give them the position best calculated to display their distinctive characteristics, and to accomplish this they necessarily place them in such a manner as to show the sides. In Articulates there is also a longitudinal axis of the body and a bilateral symmetry in the arrangement of parts; the head and tail are marked, and the right and left sides are distinct. But the prominent tendency in this type is the development of the dorsal and ventral region; here above and below prevail over right and left. It is the back and the lower side that have the preponderance over any other part of the structure in Articulates. The body is divided from end to end by a succession of transverse constrictions, forming movable rings; but the character of the animal, its striking features, are always above or below, and especially developed on the back. Any collection of Insects or Crustacea is an evidence of this; being always instinctively arranged in such a manner as to show the predominant features, they uniformly exhibit the back of the animal. The profile view of an Articulate has no significance; whereas in a Mollusk, on the contrary, the profile view is the most illustrative of the structural character. In the highest division, the Vertebrates, so characteristically called by Baer the Doubly Symmetrical type, a solid column runs through the body with an arch above and an arch below, thus forming a double internal cavity. In this type, the head is the prominent feature; it is, as it were, the loaded end of the longitudinal axis, so charged with vitality as to form an intelligent brain, and rising in man to such predominance as to command and control the whole organism. The structure is arranged above and below this axis, the upper cavity containing all the sensitive organs, and the lower cavity containing all those by which life is maintained.
While Cuvier and his followers traced these four distinct plans, as shown in the adult animal, Baer opened to us a new field of investigation in the embryology of the four types, showing that for each there was a special mode of growth in the egg. Looking at them from this point of view, we shall see that these four types, with their four modes of growth, seem to fill out completely the plan or outline of the animal kingdom, and leave no reason to expect any further development or any other plan of animal life within these limits. The eggs of all animals are spheres, such as I have described them; but in the Radiate the whole periphery is transformed into the germ, so that it becomes, by the liquefying of the yolk, a hollow sphere. In the Mollusks, the germ lies above the yolk, absorbing its whole substance through the under side, thus forming a massive close body instead of a hollow one. In the Articulate, the germ is turned in a position exactly opposite to that of the Mollusk, and absorbs the yolk upon the back. In the Vertebrate, the germ divides in two folds, one turning upward, the other turning downward, above and below the central backbone. These four modes of development seem to exhaust the possibilities of the primitive sphere, which is the foundation of all animal life, and therefore I believe that Cuvier and Baer were right in saying that the whole animal kingdom is included under these four structural ideas.
Leuckart proposed to subdivide the Radiates into two groups: the Coelenterata, including Polyps and Acalephs or Jelly-Fishes,—and Echinoderms, including Star-Fishes, Sea-Urchins, and Holothurians. His reason for this distinction is the fact that in the latter the organs are inclosed within walls of their own, distinct from the body-wall; whereas in the former the organs are formed by internal folds of the outer wall of the body, as in the Polyps, or are hollowed out of the substance of the body, as in Jelly-Fishes. This implies no difference in the plan, but merely a difference in the execution of the plan. Both are equally radiate in their structure; and when Leuckart separated them as distinct primary types, he mistook a difference in the material expression of the plan for a difference in the plan itself. So some naturalists have distinguished Worms from the other Articulates as a separate division. But the structural plan of this type is a body divided by transverse constrictions or joints; and whether those joints are uniformly arranged from one end of the body to the other, as in the Worms, or whether the front joints are soldered together so as to form two regions of the body, as in Crustacea, or divided so as to form three regions of the body, as in winged Insects, does not in the least affect the typical character of the structure, which remains the same in all. Branches or types, then, are natural groups of the animal kingdom,
