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.
Every one knows, because it is within the limits of ordinary experience, that every _individual_ organism _now_ originates and gradually becomes what we see it, by a natural process--that is, by evolution. If, then, there be any exception, it must be only the _first of each kind_. But what kind? There are many kinds of kinds; classes, orders, families, genera, species, varieties. Now, many of these kinds can be shown to have become what we see them by a gradual process similar, at least, to evolution. Take for example, classes. The class of fishes and the class of reptiles are _now_ widely distinct and have little in common except a vertebrate structure; but, as already shown, page 12, this extreme difference has not always existed. On the contrary, the earliest representatives of these two classes so merged into one another that each seemed either. From this common stock the two classes were gradually separated, each going its own way and becoming more and more widely distinct even to the present day. There can be no doubt, therefore, that _these two classes_, as we now know them, _have become_ what they are by a gradual process. Again: In the whole realm of Nature there is not a class more distinctly separate from every other and without intermediate links than birds. But this has not always been so. They have gradually become so. The earliest birds were so reptilian in structure and appearance that if we could see them now we would be in doubt whether we should call them birds or reptiles. Birds have gradually separated themselves from the reptilian stem, becoming more and more bird-like from age to age, until now, at last, the two classes are wholly separated and the intermediate links destroyed. So far as external characters are concerned, birds may be said to have finally and wholly released themselves from entangling alliance with any other class.