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Show 30 THE PAST CONDI'riON plant obtained the materials constituting it.s subst~nce by a peculiar combination of matters belong1ng entirely to the inorganic world; that, then, the animal was constantly appropriating the nitrogenous matters of the plant to its own nourishment, and returning them back to the inorganic world, in what we spoke of as its waste; and that, finally, when the animal ceased to exist, the constituents of its body were dissolved and transmitted to that inorganic world whence they had been at first abstracted. Thus we saw in both the blade of grass and the horse but the same elements differently combined and · arranged. We discovered a continual circulation going on,-the plant drawing in the elements of inorganic nature and combining them into food for the animal creation; the animal borrowing from the plant the matter for its own support, giving off during its life products which returned immediately to the inorganic world; and that, eventually, the constituent materials of the whole structure of both animals and plants were thus returned to their original source: there was a constant passage from one state of existence to ~:mother, and a returning back again. Lastly, when we endeavoured to form some notion of the nature of the forces exercised by living beings, we discovered that they-if not capable of being subjected to the same minute analysis as the constituents of those beings themselves-that they were correlative withthat they were the equivalents of the forces of inorganic nature-that they were, in the sense in which the term is now used, convertible with them. That was our general result. And now, leaving the Present, I must endeavour OF ORGANIC NATURE. 31 in the same manner to put before you the facts that are to be discovered in the Past history of the living world, in the past conditions of organic nature. We have, to-night, to deal with the facts of that history -a history involving periods of time before which our mere human records sink into utter insignificance-a history the variety and physical magnitude of whose events cannot even be foreshadowed by the history of human life and human phenomena-a history of the most varied and complex character. We must deal with the history, then, in the first place, as we should deal with all other histories. The historical student knows that his first business should be to inquire into the validity of his evidence, and the nature of the record in which the evidence is contained, that he 1nay be able to form a proper estimate of the correctness of the conclusions which have been drawn from that evidence. So, here, we must pass, in the first place, to the consideration of a matter which may seem foreign to the question under discussion. We must dwell upon the nature of the records, and the credibility of the evidence they contain; we must look to the completeness or incompleteness of those records themselves, before we turn to that which they contain and reveal. The question of the credibility of the history, happily for us, will not require much consideration, for, in this history, unlike those of human origin, there can be no cavilling, no differences as to the reality and truth of the facts of which it is made up; the facts state themselves, and are laid out clearly before us. But, although one of the greatest difficulties of |