But while the principles noted above may be said to be known laws of the apparition of new forms of life, they do not reach to the secondary efficient causes of the introduction of new species. What these may ultimately prove to be, to what extent they can be known by us, and to what extent they may include processes of derivation, it is impossible now to say. At present we must recognise in the prevailing theories on the subject merely the natural tendency of the human mind to grasp the whole mass of the unknown under some grand general hypothesis, which, though perhaps little else than a figure of speech, satisfies for the moment. We are dealing with the origin of species precisely as the alchemists did with chemistry, and as the Plutonists and Neptunists did with geology; but the hypotheses of to-day may be the parents of investigations which will become real science to-morrow. In the meantime it is safe to affirm that whatever amount of truth there may be in the several hypotheses which have engaged our attention, there is a creative force above and beyond them, and to the threshold of which we shall inevitably be brought, after all their capabilities have been exhausted by rigid investigation of facts. It is also consolatory to know that species, in so far as the Modern period, or any one past geological period may be concerned, are so fixed that for all practical purposes they may be regarded as unchanging. They are to us what the planets in their orbits are to the astronomer, and speculations as to the origin of species are merely our nebular hypotheses as to the possible origin of worlds and systems. References:--Address as Vice-President of American Association at Detroit, 1875. "The Chain of Life in Geological Time," London, 1879. Addresses to Natural History Society of Montreal, published in _Canadian Naturalist_, "Apparition of Animal Forms," _Princeton Review_. _THE GENESIS AND MIGRATIONS OF PLANTS._ DEDICATED TO THE MEMORY OF DR. OSWALD HEER, The Able and Successful Student of the later Floras of the Northern Hemisphere. Geological Periods as Related to Plants--Arctic Origin of Floras--The Devonian Flora--Arctic Climates of the Past--History of Some Modern Forms--Laws of the Succession [Illustration: Vegetation of the Middle Devonian or Erian, restored from actual specimens (p. 202).] CHAPTER VIII. _THE GENESIS AND MIGRATIONS OF PLANTS._ If, for convenience of reference, we divide the whole history of the earth, from the time when a solid crust first formed on its surface and began to be ridged up into islands or mountains in the primeval ocean, into four great periods, we shall find that each can be characterized by some features in relation to the world of plants.