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.