Page:Popular Science Monthly Volume 2.djvu/124

 by Prof. Leconte that there are no less than three hundred and seventy facts concerning the distribution, form, and motions, of the sun and planets, which are the simple consequences of the nebular hypothesis, and can be accounted for in no other way. The nebular hypothesis is the doctrine of to-day, in its application to the most perfect of the sciences, and it is nothing less or other than an hypothesis of astronomic evolution. Are we to be told that it is but an a priori speculation? On the contrary, has it not replaced an a priori cosmogony that swayed the human mind for thousands of years before the solar system was discovered?

As regards the earth, it has been studied by the method of science for more than a century, and the result is, a vast mass of facts and inductions which make up our knowledge of geology. All these go to establish one proposition, viz., that our planet is not what it was millions of years ago, but has undergone a series of developing changes resulting in the present order of things. Our eminent geologist, Prof. Dana, in his manual, says: "This law of specialization—the general being before the special—is the law of all development. The egg is at first a simple unit, and, gradually, part after part of the new structure is evolved, that which is most fundamental appearing earliest, until the being is complete in all its outer and minor details. The principle is exhibited in the physical history of the globe—which was first a featureless globe of fire, then had its oceans and dry land, in course of time received mountains and rivers, and finally all those diversities of surface which now characterize it. Again, the climates began with universal tropics; and at last the diversities of the present day." Is this to be accounted a high-flown a priori speculation, or a vast and valid induction from a hundred years' study of the facts of Nature? Let it be remembered that, according to the highest authorities, inductive geology was put back two centuries by the enslavement of the human mind to an old a priori speculation in regard to the age of the world.

The study of the course of life upon the earth shows that it conforms to the same great plan. The life of the globe a few millions of years ago was a very different thing from what it is now. Different races of plants and animals have appeared and disappeared in slow succession, and their remains are found entombed in successive rock-formations. The facts are a part of geology, and have been arrived at by the same processes of observation and induction that have revealed the order and history of the stratified systems. The course of life upon the earth has conformed to a method, and that method is universally described as a progress and a development. It shows an advance from the simpler to the more complex, from the general to the special, from the lower to the higher; in short, it is an evolution in the strictest sense. There was, first, a period of no life—the azoic age; then appeared the lower forms of life, vegetable and animal; then higher and higher kinds, until man, the highest of all, appeared last. The progress evinces continuity, harmony, and gradation. As remarked by Mr. Dana, "the beginning of an age will be in the midst of a preceding age; and the marks of the future coming out to view are to be regarded as prophetic of that future. The age of mammals was foreshadowed by the appearance of mammals long before in the course of the reptilian age, and the age of reptiles was prophesied in types that lived in the earlier Carboniferous age." The lower forms that perish do not reappear, and, as Mr. Wallace observes, "no group or species has come into existence twice," but "every species has come into existence coincident, both in space and time, with a preexisting, closely-allied species."