The early part of this century saw the establishment of most of the fundamental principles of the science of physics, especially as applied to astronomy. A few decades later saw the science of chemistry emerge from the empirical and enter the philosophical stage. It has been reserved for the second half of the century to witness the discovery of the facts and principles of the history of life on the earth. The public mind is gradually awakening to the fact that the grandest truths of creation are being placed within their reach by the researches of contemporary science, and that the knowledge of the manner of the origin of the human race is no longer withheld from us. The study of the fossil remains of animals has revealed an immense number of forms of life which in former ages have peopled the world. The study of geology has shown that the history of our planet is marked by successive deposits in water, which have become beds of rocks. The relation of these beds to each other gives us the relations of the animals and plants whose fossil remains they contain. Thus we have obtained a consecutive history of life from its early appearance to the present day. Before the doctrine of evolution was understood, the successive populations that filled the successive periods were supposed to have been the products of special creations. Now it is believed, with the best of reason, that all forms of life have been produced by changes of structure which arose in the course of descent, the one species coming from the other; and that interruptions in the series of species from older to later periods are simply due to the absence of the means of preserving their remains at certain times during the course of the history of the world. These interruptions indicate periods of dry land, since fossils are not preserved unless they are excluded from the air by a covering of water or of mud.

This book deals with the evolution of human and organic forms. "That men, or thinking beings akin to man, exist only on that minute fragment of the universe we call the earth is a conception so highly improbable, in view of the vast multitude of planets which we may logically conceive to exist, that it seems as if no reasoning being could entertain it. It is true, indeed, that in our own solar system perhaps only two or three of the planets, perhaps only the earth, are in a condition suitable for human habitation, and that the earth has been so for a comparatively brief period. It may well be, therefore, that only a very small percentage of the planets of space are in a similar condition. But in view of the vast multitude of planets that presumably exist, the number of those that possess reasoning beings is probably great. If we deal with this question from the point of view of actual evidence, the fact that the only planet whose conditions we know is inhabited by man is a strong argument in favor of his wide-spread existence. On the other hand, the fact that man's existence upon the earth is dependent upon a certain limited range of temperature, of brief duration in the earth's total history, is an argument on the opposite side, and goes far to narrow the possible domain of life in the universe. Yet if we extend our view to embrace the past and the future as well as the present, we cannot avoid the conclusion that the realm of life and thought in the universe is an immense one..."