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 animals 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 significance.  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