microscopic spot; if it were evolved at more than one spot, and under slightly varying conditions, there would necessarily have arisen in these earliest formations the _initial_ diversities which afterwards determined the essential independence and difference of organisms. 129. Let us for a moment glance at the resemblances and diversities observable in all organisms. All have a _common basis_, all being constructed out of the same fundamental elements: carbon, hydrogen, nitrogen, and oxygen; these (the organogens, as they are named), with varying additions of some other elements, make up what we know as Organic Substance, vegetal and animal. Another peculiarity all organisms have in common, namely, that their matter is neither solid nor liquid, but viscid. Beside this community of _Substance_ we must now place a community of _History_. All organisms grow and multiply by the same process; all pass through metamorphic stages ending in death; all, except the very simplest, differentiate parts of their substance for special uses, and these parts (cilia, membranes, tubes, glands, muscles, nerves) have similar characters in whatever organism they appear, and their development is always similar, so that the muscles or nerves of an intestinal worm, a lobster, or a man, are in structure and history fundamentally alike. When, therefore, we see that there is no biological character of fundamental importance which is not universal throughout the organic world, when we see that in Structure and History all organisms have a community pervading every variety, it is difficult not to draw the conclusion that some hidden link connects all organisms into one; and when, further, it is seen that the most divergent forms may be so arranged by the help of intermediate forms only slightly varying one from the other, that the extreme ends--the monad and the man--may be connected, and a genealogical tree constructed, which will group all forms as modified descendants from a single form, the hypothesis that kinship is the bidden link of which we are in search becomes more and more cogent. 130. But now let the other aspect be considered. If there is an unmistakable uniformity, there is also a diversity no less unmistakable. The chemical composition of organic substances is various. Unlike inorganic substances, the composition of which is rigorously definite, organic substances are, within narrow limits, variable in composition (§ 45).