Consider for a moment one of the simplest of these cases. Let us take the minute seed of one of the great tropical fig-trees, and another seed of a strawberry, or of garden cress. Both will be about the same size and shape, and the most acute microscopist would not find any difference in the internal structure that could intelligibly account for the different results when these little grains of protoplasm are exposed to identical conditions. For, even if planted near each other, and exposed to the same amount of heat and moisture, to the very same atmosphere, and the same kind of water, as well as identically the same soil, yet invariably the one will grow into a large tree, the other into a small herb, and in the course of time, still with no change whatever of the physical conditions to which both are exposed, each will produce its peculiar foliage, and flowers, and fruit, very different in all their characters from those of the other. Were this result not so common as to seem to us “natural,” we should call it a miracle; and it is really and essentially as inexplicable as many things which are termed miracles only because they are unfamiliar and inexplicable. Now, this wonderful substance, the physical base of all life--and as it is the only base that exists, or has ever existed, on the earth, we may fairly assume that no other is possible--can only maintain itself and perform its functions under certain very definite conditions, which conditions are now maintained on our earth’s surface, and must have been maintained throughout the long geological periods during which life has been slowly developing. What these conditions are we will now proceed to show. [Sidenote: The First Essential for Life]