The key to the solution was found somewhat in the same way as led to the Darwinian theory concerning the Origin of Species. When the question touching the causes of variation and persistence in the natural kinds of plants and animals seemed so complex as to baffle all attempts at an answer, Darwin found what seemed a clue likely to lead to a theory of descent. The methods adopted in order to keep up, or to vary, a species under domestication were open to anybody's inspection: and those principles, which were consciously pursued in artificial selection by the breeder, suggested a theory of similar selection in free nature. In studying the phenomena of thought, of which the species or types were no less numerous and interesting than those in organic nature, it was perhaps impossible to survey the whole history of humanity. But it was comparatively easy to observe the process of thought in those cases where its growth had been fostered consciously and distinctly. The history of philosophy records the steps in the conscious and artificial manipulation of what for the far greater part is transacted in the silent workshops of nature. Philosophy, in short, is to the general growth of intelligence what artificial breeding is to the variation of species under natural conditions. In the successive systems of philosophy, the order and concatenation of ideas was, as it were, clarified out of the perturbed medium of real life, and expressed in its bare equivalents in terms of thought, and thus first really acquired. Half of his task was already performed for the logician, and there remained the work, certainly no slight one--of showing the unity and organic development which marked the conscious reasoning, and of connecting it with the general movement of human thought. The logician had to break down the rigid lines which separated one system of philosophy from another,--to see what was really involved in the contradiction of one system by its successor,--and to show that the negation thus given to an antecedent principle was a definite negation, ending not in mere zero or vacuity, but in a distinct result, and making an advance upon the previous height of intelligence. To say this was to give a new value to the history of philosophy. For it followed that each system was no _mere_ opinion or personal view, but was in the main a genuine attempt of the thinker to give expression to the tacit or struggling consciousness of his age. Behind the individual--who is often unduly regardless of his contemporaries and predecessors, and who writes or thinks with little knowledge or sympathy for them, there is the general bearing and interest of the age, its powerful solidarity of purpose and conception. The philosopher is the prophet, because he is in a large part the product of his age. He is an organ of the mind of his age and nation; and both he and it play a part in the general work of humanity.