In the final, the positive state, the mind has given over the vain search after absolute notions, the origin and destination of the universe, and the causes of phenomena, and applies itself to the study of their laws--that is, their invariable relations of succession and resemblance. Reasoning and observation, duly combined, are the means of this knowledge. What is now understood when we speak of an explanation of facts is simply the establishment of a connection between single phenomena and some general facts the number of which continually diminishes with the progress of science. There is no science which, having attained the positive stage, does not bear the marks of having passed through the others. Some time since it was (whatever it might be now) composed, as we can now perceive, of metaphysical abstractions: and, further back in the course of time, it took its form from theological conceptions. Our most advanced sciences still bear very evident marks of the two earlier periods through which they passed. The progress of the individual mind is not only an illustration, but an indirect evidence of that of the general mind. The point of departure of the individual and the race being the same, the phases of the mind of men correspond to the epochs of the mind of the race. How each of us is aware, if he looks back upon his own history, that he was a theologian in his childhood, a metaphysician in his youth and a natural philosopher in his manhood. All men who are up to their age can verify this for themselves. According to the scientific classification, there are only three kingdoms or states of life, the mineral, the vegetable and the animal. The life of the vegetable kingdom has arisen out of the life of the mineral kingdom and is sustained by it. The distinguished scientist, Professor Lowell, says, "there is now no more reason to doubt that plants grew out of chemical affinity than to doubt that stones did," and nearly all outstanding zoologists would say as much of animals. Sir J. Burdon Sanderson, one of the most eminent among biologists, insists that "in physiology the word life is understood to mean the chemical and physical activities of the parts of which the organism consists." The renowned Sir Ray Lankester strenuously holds that "zoology is the science which seeks to arrange and discuss the phenomena of animal life and form, as the outcome of the operation of the laws of physics and chemistry," and goes so far as to say that he knows of no leading biologist who is of a different opinion. The prince of biologists, the late Professor Haeckel, occupied this position and impregnably fortified it in several great books, especially in his "Riddle of the Universe."