mal, 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 Cœlenterata, 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 or cavities of the body have walls of their own, distinct from the body-wall; whereas in the former they 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 prime division. But the structural plan of this type is a cylinder 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, being, in fact, an articulated cylinder with variously combined rings and more or less complicated tubular appendages.

Branches or types, then, are natural groups of the animal kingdom, founded on plans of structure or structural ideas. What now are classes? Are they lesser divisions, differing only in extent, or are they founded on special characters? I believe the latter view to be the true one, and that class characters have a significance quite different from that of their mere range or extent. These divisions are founded on certain categories of structure; and were there but one animal of a class in the world, if it had those characters on which a class is founded, it would be as distinct from all other classes as if its kind were counted by thousands.

Baer approached the idea of the classes when he discriminated between plan of structure or type and the degree of perfection in the structure. But while he understands the distinction between a plan and its execution, his ideas respecting the different features of structure are not quite so precise. He does not, for instance, distinguish between the complication of a given structure and the mode of execution of a plan, both of which are combined in what he calls degrees of perfection. And yet, without this

distinction, the difference between classes and orders cannot be understood; for classes and orders rest upon a just appreciation of these two categories, which are quite distinct from each other, and have by no means the same signifi

cance.

Again, quite distinct from both of these is the character of form, not to be confounded either with complication of structure, on which orders are based, or with the execution of the plan, on which classes rest. An example will show that form is no guide for the determination of classes or orders. Take, for instance, a Beche-de-Mer, a member of the highest class of Radiates, and compare it with a Worm. They are both long cylindrical bodies; but one has parallel divisions along the length of the body, the other has the body divided by transverse rings. Though in external form they resemble each other, the one is a worm-like Radiate, the other is a worm-like Articulate, each having the structure of its own type; so that they do not even belong to the same great division of the animal kingdom, much less to the same class. We have a similar instance in the Whales and Fishes, the Whales having been for a long time considered as Fishes, on account of their form, while their structural complication shows them to be a low order of the class of Mammalia, to which we ourselves belong,

that class being founded upon a particular mode of execution of the plan characteristic of the Vertebrates, while the order to which the Whales belong depends upon their complication of structure, as compared with other members of the same class.

We may therefore say that neither form nor complication of structure distinguishes classes, but simply the mode of execution of a plan. In Vertebrates, for instance, how do we distinguish the class of Mammalia from the other classes of the type? By the peculiar development of the brain, by their breathing through lungs, by their double circulation, by their bringing forth living young and nursing them with milk. In this class the beasts of prey form a distinct order, superior to the Whales or the herbivorous animals, on account of the higher complication of their structure; and for the same reason we place the Monkeys above them all. But among the beasts of prey we distinguish the Bears, as a family, from the family of Dogs, Wolves, and Cats, on account of their different form, which does not imply a difference either in the complication of their structure or in the mode of execution of their plan.