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But without drawing this inference, let us set out with the unquestionablefact that the compounds which can exist at the highest temperatures, andwhich must therefore have been the first formed as the Earth cooled, arethose of the simplest constitutions. The protoxides (including under thathead the alkalies, earths, etc.) are, as a class,the most stable compoundsknown -- the least changeable by heat. These, consisting severally of oneatom of each component element, are but one degree less homogeneous thanthe elements themselves. More heterogeneous than these, more decomposableby heat, and therefore later in the Earth's history, are the deutoxides,tritoxides, peroxides, etc.; in which two, three, four, or more atoms ofoxygen are united with one atom of metal or other base. Still less able toresist heat are the salts, which present us with compound atoms each madeup of five, six, seven, eight, ten, twelve, or more atoms, of three or morekinds. Then there are the hydrated salts of a yet greater heterogeneity,which undergo partial decomposition at much lower temperatures. After themcome the further-complicated supersalts and double salts, having a stabilityagain decreased; and so throughout. After making a few unimportant qualificationsdemanded by peculiar affinities, it may be asserted as a general law of theseinorganic combinations that, other things equal, the stability decreasesas the complexity increases. When we pass to the compounds which make uporganic bodes, we find this general law further exemplified; we find muchgreater complexity and much less stability. A molecule of albumen, for instance,consists of more than two hundred ultimate units of five different kinds.

According to the latest analyses it contains in each molecule, 72 of carbon,18 of nitrogen, 1 of sulphur, 112 of hydrogen, and 22 of oxygen -- in all,225 atoms; or, more strictly speaking, equivalents. And this substance isso unstable as to decompose at quite moderate temperatures; as that to whichthe outside of a joint of roasting meat is exposed. Possibly it will be objectedthat some inorganic compounds, as phosphuretted hydrogen, chloride of nitrogen,and the nitrogen-explosives in general, are more decomposable than most organiccompounds. This is true. But the admission may be made without damage tothe argument. The proposition is not that all simple combinations are morestable than all complex ones. To establish our inference it is necessaryonly to show that, as an average fact, the simple combinations can existat a higher temperature than the complex ones. And this is beyond question.

Thus it is manifest that the present chemical heterogeneity of the Earth'ssurface, and of the bodies upon it, has arisen by degrees as the decreaseof heat has permitted; and that it has shown itself in three forms: -- first,in the multiplication of chemical compounds; second, in the greater numberof different elements contained in the more modern of these compounds; andthird, in the higher and more varied multiples in which these more numerouselements combine.

Without specifying them, it will suffice just to name the meteorologicprocesses eventually set up in the Earth's atmosphere, as further illustratingthe alleged law. They equally display that destruction of a homogeneous statewhich results from unequal exposure to incident forces. §152. Take a mass of unorganized but organizable matter -- eitherthe body of one of the lowest living forms, or the germ of one of the higher: both comparatively homogeneous. Consider its circumstances. Either it isimmersed in water or air or is contained within a parent organism. Whereverplaced, however, its outer and inner parts stand differently related to surroundingagencies -- nutriment, oxygen, and the various stimuli. But this is not all.

Whether it lies quiescent at the bottom of a pool or on the leaf of a plant;whether it moves through the water preserving some definite attitude; orwhether it is in the inside of an adult; it equally happens that certainparts of its surface are more exposed to surrounding agencies than otherparts -- in some cases more exposed to light, heat, or oxygen, and in othercases to the maternal tissues and their contents. Hence must follow the lossof its original equilibrium. This may take place in one of two ways. Eitherthe disturbing forces may be such as to over-balance the affinities of theorganic elements, and there results decomposition; or, as ordinarily occurs,such changes are induced as do not destroy the organic compounds but onlymodify them: the parts most exposed to the modifying forces being most modified.

To elucidate this a few cases are required.

Observe first what appear to be exceptions. Certain minute animal formspresent either no appreciable differentiations or differentiations so obscureas to be made out with great difficulty. Concerning these forms, however,note the fact that in all cases (some say in nearly all) the presence ofa nucleus shows conformity to the general law, since it implies a contrastbetween the innermost protoplasm and the protoplasm surrounding it. But letus pass on to the seemingly exceptional fact that the surrounding protoplasmdoes not exhibit the kind of differentiation between inner and outer abovealleged. To this objection, there immediately presents itself the answerthat this homogeneous body-substance does not become heterogeneous becauseits parts are not subject to any permanent heterogeneity of conditions: ithas no fixed surface. In all members of the lowest group, Proteomyxa, theprotoplasm continually protrudes itself, now in thicker now in thinner processes-- pseudopodia; proved to have no limiting membranes by often coalescing.

These, when they touch fragments of nutriment, contract and draw them intothe mass of the body; so that what was just before external now becomes internal.