Page:Popular Science Monthly Volume 4.djvu/712

692 Now, what is the meaning of these relations between organs and between animals? For that they have a meaning must be clear to all, and it is fair to presume that it is one that can be discovered by investigation.

The fact that two or many different animals are constructed on the same plan seems to indicate some kind of connection between the animals themselves, and it is the work of the zoologist to find what it is that thus connects them.

Two theories have been proposed, each of which seems to meet most of the points to be explained, but each seems to fail in some respects. One of these is, that the connection between different groups of animals is to be found only in the mind of their Creator; the other is, that there is a direct genetic connection or relationship between them.

Each of these theories is conceivable and worthy of consideration, for we can find examples of the building up of systems some-what resembling the animal kingdom in each of these ways. The various kinds of steam-engines, for instance, are adapted each to its special work, with an accuracy rivaling that of Nature, yet all of them can be shown to be constructed on substantially the same plan.

If we trace the history of any form, such as the steep-grade locomotive, we find, as we go backward, that it loses, one by one, all of its special adaptations, until at last it is only a common locomotive at up-hill work. Tracing the history of the locomotive in the same way, we find that its special adaptations disappear, until it is nothing