ded 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 pla