During the greater portion of his life, the author of this book believed, in common with everybody else, that the problem of the future life is out of our reach, and that true wisdom consists in not troubling our minds about it. But, one dreadful day, a thunderbolt fell in his path. He lost the son in whom centred all the hope and ambition of his life. Then, in the bitterness of his grief he reflected deeply on the new life which must open for each of us, above the tomb. After long dwelling on this idea in solitary meditation, he asked of the exact sciences what positive information, on this question, they could furnish him with, and subsequently, he interrogated ignorant and simple people, peasants in their villages, and unlettered men in towns, an ever precious source of aid in re-ascending towards the true principles of nature, for it is not perverted by the progress of education, or by the routine of a commonplace philosophy. Thus the author of this book succeeded in constructing for himself an entire system of ideas concerning the new life of man, which is to follow his terrestrial existence. But his system is all contained in nature. Each organized being is attached to another which precedes, and another which follows it, in the chain of the living creation. The plant and the animal, the animal and the man, are linked, soldered to one another; the moral and physical order meet and mingle. It results from this, that any one who believes himself to have discovered the explanation of any one fact concerning this organization, is speedily led to extend this explanation to all living beings, to reconstruct, link by link, the great chain of nature. Thus it was with the author of this book. After having sought out the destination of man, when dismissed from his terrestrial life, he was led to apply his views to all other living beings, to animals, and then to plants. The power of logic forced him to study those beings, impossible to be seen by our organs of vision, by which he holds the planets, the suns, and all the innumerable stars dispersed over the vast extent of the heavens, to be inhabited. So that you will find in this book, not only an attempt at the solution of the problem of the future life by science, but also the statement of a complete theory of nature, of a true philosophy of the universe. It may be that I am deceiving myself; it may be that I am taking the dreams of my imagination for serious views; I may lose myself in that dark region through which I am trying to grope my way; but at least I write with absolute sincerity, and that is my excuse for writing this book at all. I hope that others may be induced by my example to attempt similar efforts, to apply the exact sciences to the study of the great question of the destinies of man after this life. A series of works undertaken in this branch of learning, would be the greatest service which could be rendered to natural philosophy, and also to the progress of humanity.