I have endeavored thus far to show how scientific facts have been systematized so as to form a classification that daily grows more true to Nature, in proportion as its errors are corrected by a more intimate acquaintance with the facts; but I will now attempt a more difficult task, and try to give some idea of the mental process by which facts are transformed into scientific truth. I fear that the subject may seem very dry to my readers, and I would again ask their indulgence for details absolutely essential to my purpose, but which would indeed be very wearisome, did they not lead us up to an intelligent and most significant interpretation of their meaning. I should be glad to remove the idea that science is the mere amassing of facts. It is true that scientific results grow out of facts, but not till they have been fertilized by thought The facts must be collected, but their mere accumulation will never advance the sum of human knowledge by one step;--it is the comparison of facts and their transformation into ideas that lead to a deeper insight into the significance of Nature. Stringing words together in incoherent succession does not make an intelligible sentence; facts are the words of God, and we may heap them together endlessly, but they will teach us little or nothing till we place them in their true relations and recognize the thought that binds them together as a consistent whole. I have spoken of the plans that lie at the foundation of all the variety of the Animal Kingdom as so many structural ideas which must have had an intellectual existence in the Creative Conception independently of any special material expression of them. Difficult though it be to present these plans as pure abstract formulae, distinct from the animals that represent them, I would nevertheless attempt to do it, in order to show how the countless forms of animal life have been generalized into the few grand, but simple intellectual conceptions on which all the past populations of the earth as well as the present creation are founded. In such attempts to divest the thought of its material expression, especially when that expression is multiplied in such thousand-fold variety of form and color, our familiarity with living animals is almost an obstacle to our success. For I shall hardly be able to allude to the formula of the Radiates, for instance,--the abstract idea that includes all the structural possibilities of that division of the Animal Kingdom,--without recalling to my readers a Polyp or a Jelly-Fish, a Sea-Urchin or a Star-Fish. Neither can I present the structural elements of the Mollusk plan, without reminding them of an Oyster or a Clam, a Snail or a Cuttle-Fish,--or of the Articulate plan, without calling up at once the form of a Worm, a Lobster, or an Insect,--or of the Vertebrate plan, without giving it the special character of Fish, Reptile, Bird, or Mammal. Yet I insist that all living beings are but the different modes