We may therefore feel it to be an almost certain conclusion that--the elements being the same, the laws which act upon, and combine, and modify those elements being the same--organised living beings wherever they may exist in this universe must be, fundamentally, and in essential nature, the same also. The outward forms of life, if they exist elsewhere, may vary almost infinitely, as they do vary on the earth; but, throughout all this variety of form--from fungus or moss to rose-bush, palm or oak; from mollusc, worm, or butterfly to humming-bird, elephant, or man--the biologist recognises a fundamental unity of substance and of structure, dependent on the absolute requirements of the growing, moving, developing, living organism, built up of the same elements, combined in the same proportions, and subject to the same laws. We do not say that organic life _could_ not exist under altogether diverse conditions from those which we know or can conceive, conditions which may prevail in other universes constructed quite differently from ours, where other substances replace the matter and ether of our universe, and where other laws prevail. But, _within_ the universe we know, there is not the slightest reason to suppose organic life to be possible, except under the same general conditions and laws which prevail here. We will, therefore, now proceed to describe, very generally, what are the conditions essential to the existence and the continuous development of vegetable and animal life. FOOTNOTE: [10] _The Astrophysical Journal_, vol. xiv., July 1901, p. 17. CHAPTER X THE ESSENTIAL CHARACTERS OF THE LIVING ORGANISM Before trying to comprehend the physical conditions on any planet which are essential for the development and maintenance of a varied and complex system of organic life comparable to that of our earth, we must obtain some knowledge of what life is, and of the fundamental nature and properties of the living organism.