Page:Popular Science Monthly Volume 29.djvu/374

360 had been the same as in low latitudes, became wholly changed to new species after the Tertiary, while those in low latitudes remain as they were in general character, and, in some cases, the identical species yet survive.

In our Southern States, for example, the flora is closely allied to, and as to some species identical with, those of the Miocene of arctic regions. From the Miocene back, the geological record tells of life conditions—environments—the same all over the world.

So far, therefore, as geology is concerned, the evidence appears to be all one way, and I think I am justified in saying that the conclusion to which it points would be readily adopted, were it not for reasons derived from another science.

Astronomers say that a permanent change in the inclination of the earth's axis by means of any force known to science is impossible. We know, however, very little of the means by which our system was brought into its present state.

The only theory that attempts to explain, on purely mechanical principles, the existence of the solar system is the nebular hypothesis in some of its forms, although even that requires a self-existent entity back of it. According to this hypothesis, the earth and moon were once one body, which revolved, of course, on one axis. At some remote time a separation occurred. But no force of avulsion, whether the moon was merely left behind as the mass contracted, or whether, as Mr. Darwin thinks, it was thrown off after the earth had become solid, and pushed back to its present distance, could affect the plane of rotation, or the direction of the axes. On mechanical principles, the moon when it left the earth must have moved in the plane of the earth's equator, and the three axes—that of the earth, that of the lunar orbit, and that of the moon itself—must have been parallel to each other. But such is not the case now. The axis of the moon is inclined about 1° 30', that of its orbit 5° 9', and that of the earth 23°. It is evident that at some time the axes, or some of them, have undergone a change of direction. On purely mechanical principles, the change did not occur before the earth and moon separated, nor at the moment of separation, nor, in fact, at any time.

Astronomy, therefore, proves too much! It proves that the present condition is not eternal; that the earth was not created with its present oblique axes—in fact, that normally it was perpendicular to the ecliptic; and that, once in any position, it was impossible for it to become more oblique "by any force known to science." To all of which those of us who are not astronomers can only answer: "What you say may all be true, but, nevertheless, the earth's axis is inclined, and, if we can not show the cause—an inability which extends to a great many other things—our business is to discover, if we can, the time when it became inclined. It is not a question of possibilities, but of chronology."