PART II.
EVIDENCES OF THE TRUTH OF EVOLUTION.
CHAPTER I.
GENERAL EVIDENCES OF EVOLUTION AS A UNIVERSAL LAW.
Let us again remind the reader that evolution means, first of all, continuity. The law of evolution, although it doubtless means much more, means, first of all, a law of continuity, or causal relation throughout Nature. It means that, alike in every department of Nature, each state or condition grew naturally out of the immediately preceding. In a word, it means that, in the course of Nature, nothing appears suddenly and without natural cause, but, on the contrary, everything is the natural and usually the gradual outcome of a previous condition. This is now admitted by every one in regard to nearly everything: evolutionists apply it to the whole course of Nature. I said this is now admitted by every one in regard to nearly everything; but this has not always been so. The world has come to its present position on this subject only by a very gradual process. Let us then trace rapidly the history of the gradual change, for it will prepare us for much that follows.
There was a time (and that not many decades ago) when all things, the origin of which transcends our ordinary experience, were supposed to have originated suddenly and without natural process—to have been made at once, out of hand. There was a time when, for example, mountains were supposed to have been made at once, with all their diversified forms, of beetling cliffs and thundering waterfalls, or gentle slopes and smiling valleys, just as we now find them. But now we know that they have become so only by a very gradual process, and are still changing under our very eyes. In a word, they have been formed by a process of evolution. We know now the date of mountain-births; we trace their growth, maturity, decay, and death; and find even, as it were, the fossil bones of extinct mountains in the crumpled strata of their former places. There was a time when continents and seas, gulfs, bays, and rivers, were supposed to have originated at once, substantially as we now see them. Now, we know that they have been changing throughout all geological time, and are still changing. Not, however, change back and forth in any direction indifferently and without goal, but gradual change from less perfect to more perfect condition, with more and more complex inter-relations—i. e., by a process of evolution. We are able now, though still imperfectly, to trace some of the stages of this evolution. There was a time when rocks and soils were supposed to have been always rocks and soils; when soils were regarded as an original clothing made on purpose to hide the rocky nakedness of the new-born earth. God clothed the earth so, and there an end. Now we know that rocks rot down to soils; soils are carried down and deposited as sediments; and sediments re-consolidate as rocks—the same materials being worked over and over again, passing through all these stages many times in the history of the earth. In a word, there was a time when it was thought that the earth with substantially its present form, configuration, and climate, was made at once out of hand, as a fit habitation for man and animals. Now we know that it has been changing, preparing, becoming what it is by a slow process, through a lapse of time so vast that the mind sinks exhausted in the attempt to grasp it. It has become what it now is by a process of evolution. The same change of view has taken place concerning the origin of all the heavenly bodies. We may, therefore, confidently generalize—we may assert without fear of contradiction that all inorganic forms, without exception, have originated by a process of evolution.
The proof of all this we owe to geology—a science born of the present century. This science establishes the law of universal continuity of events, through infinite time, as astronomy does that of universal inter-relation of objects through infinite space. How great the change these two sciences have made in the realm of human thought! Until the birth of modern astronomy the intellectual space-horizon of the human mind was bounded substantially by the dimensions of our earth; sun, moon, and stars, being but inconsiderable bodies circulating at a little distance about the earth, and for our behoof. Astronomy was then but the geometry of the curious lines traced by these wandering fires on the concave blackboard of heaven. With the first glance through a telescope the phases of Venus and the satellites of Jupiter, revealed clearly to the mind the existence of other worlds besides and like our own. In that moment the idea of infinite space, full of worlds like our own, was for the first time completely realized, and became thenceforward the heritage of man. In that moment the intellectual horizon of man was infinitely extended. So also until the birth of geology, about the beginning of the present century, the intellectual time-horizon of the human mind was bounded by six thousand years. The discovery about that time of vertebrate remains, all wholly different from those now inhabiting the earth, revealed the existence of other time-faunas, besides our own and the idea of infinite time, of which the life of humanity is but an epoch, was born in the mind of man; and again the intellectual horizon of man was infinitely extended. These two are the grandest ideas, and their introduction the grandest epochs, in the intellectual history of man. We have long ago accepted and readjusted our mental furniture to the requirements of the one, but the necessary readjustment to the other is not yet complete.
All inorganic forms, then, it is admitted, have come by evolution. But how is it with organic or living forms? Let us see.