These several statements yield no adequate idea of the extent to whichliving organic substance is thus distinguished from other substances havinglike sensible forms of aggregation. But some approximation to such an ideamay be obtained by contrasting the bulk occupied by this substance, withthe bulk which its constituents would occupy if uncombined. An accurate comparisoncannot be made in the present state of science. What expansion would occurif the constituents of the nitrogenous compounds could be divorced withoutadding motion from without, is too complex a question to be answered. Butrespecting the constituents of that which forms four-fifths of the weightof an ordinary animal -- its water -- a tolerably definite answer can begiven. Were the oxygen and hydrogen of water to lose their affinities, andwere no molecular motion supplied to them beyond that contained in waterat blood-heat, they would assume a volume twenty times that of the water.*<*I am indebted for this result to Dr. [afterwards Sir] Edward Frankland, whohas been good enough to have the calculation made for me.> Whether proteinunder like conditions would expand in a greater or a less degree, must remainan open question; but remembering the gaseous nature of three out of itsfour chief components, remembering the above-named peculiarity of nitrogenouscompounds, remembering the high multiples and the colloidal form, we mayconclude that the expansion would be great. We shall not be wrong, therefore,in saying that the elements of the human body if suddenly disengaged fromone another, would occupy far more than a score times the space they do: the movements of their molecules would compel this wide diffusion. Thus theessential characteristic of living organic matter, is that it unites thislarge quantity of contained motion with a degree of cohesion which permitstemporary fixity of arrangement. §104. Besides seeing that organic aggregates differ from other aggregates,alike in the quantity of motion they contain and the amount of re-arrangementof parts which accompanies the progressive integration; we shall see thatamong organic aggregates themselves, differences in the quantities of containedmotion are accompanied by differences in the amounts of re-distribution.
The contrasts among organisms in chemical composition yield us the firstillustration. Animals are distinguished from plants by their far greateramOunts of structure, as well as by far greater rapidity with which changesgo on in them; and, in comparison with plants, animals contain immenselylarger proportions of those nitrogenous molecules in which so much motionis locked up. So, too, is it with the contrasts between the different partsof each animal. Though certain nitrogenous parts, as cartilage, are stableand inert, yet the parts in which secondary re-distributions have gone on,and are ever going on, most actively, are those mainly formed of highly-compoundednitrogenous molecules; while parts which, like deposits of fat, consist ofrelatively-simple molecules, that are non-nitrogenous, are seats of but littlestructure and but little change.
We find proof, too, that the continuance of the secondary re-distributionsby which organic aggregates are distinguished depends on the presence ofthat locked-up motion which gives mobility to the water diffused throughthem; and that, other things equal, there is a direct relation between theamount of re-distribution and the amount of contained water. The evidencesmay be put in three groups. There is the familiar fact that a plant has itsformative changes arrested by cutting off the supply of water: the primaryredistribution continues -- it withers and shrinks or becomes more integrated-- but the secondary re-distributions cease. There is the less familiar factthat the like result occurs in animals -- occurs, indeed, after a relativelysmaller diminution of water. Certain of the lower animals furnish additionalproofs. The Rotifera may be rendered apparently lifeless by desiccation,and will yet revive if wetted. When the African rivers it inhabits are driedup the Lepidosiren remains torpid in the hardened mud until return of therainy season brings water. Humboldt states that during the summer drought,the alligators of the Pampas lie buried in a state of suspended animationbeneath the parched surface, and struggle up out of the earth as soon asit becomes humid. The history of each organism teaches the same thing. Theyoung plant, just putting its head above the soil, is more succulent thanthe adult plant; and the amount of transformation going on in it is relativelygreater. In that portion of an egg which displays the formative processesduring the early stages of incubation, the changes of arrangement are morerapid than those which an equal portion of the body of a hatched chick undergoes.
As may be inferred from their respective powers to acquire habits and aptitudes,the structural modifiability of a child is greater than that of an adult;and the structural modifiability of a young man is greater than that of anold man: contrasts which are associated with contrasts in the densities ofthe tissues; since the ratio of water to solid matter diminishes with advancingage. And then we have this relation repeated in the contrasts between partsof the same organism. In a tree, structural changes go on rapidly at theends of shoots, where the ratio of water to solid matter is very great; whilethe changes are very slow in the dense and almost dry substance of the trunk.
Similarly in animals, we have the contrast between the high rate of changegoing on in a soft tissue like the brain, and the low rate of change goingon in dry non-vascular tissues -- hairs, nails, horns, etc.
