Page:Popular Science Monthly Volume 5.djvu/483

Rh the same. The same chemical constituents, the same protoplasm, the same basement of cell-growth, and the same compounding of cells, characterize the development of both. In short, there is but one set of organic creatures. They are all animals, or all plants, as you may please to call them.

Before this simple distinction into endothentic and exothentic, all the difficulties, of discriminating in special cases as to which of the two great kingdoms has the best claim to a particular species, vanish. Those organisms of the boundary-line which could never be located, because sometimes apparently one, and sometimes the other, as motion and volition, seemed to be present or absent, now readily take their places. For instance, the aëthalium—to show the impossibility of a definite boundary—is sometimes observed as a flying vibrio, then a crawling amœba-like drop of sarcode; then, in the still condition, a greenish spherical cell like some protococus; even this mysterious creature need no longer be regarded as amphibious. In every stage it is decidedly exothentic.

What, then, is the distinction, and how does the evolution advance? Let us go back a little. We observe that in plants evolution advances by a folding down of the creature upon itself—an involution. A strip of paper may well represent that type which consists of a single tier or layer of cells, as in ulvaceæ; or even baculate types, as confervæ. This is our biological surface.

Fold it down upon itself, the two surfaces coming in contact soon adhere, and then we have, as we have seen, a creature of two tiers of cells. This begins to look like reducing a loose collection, or a mere association of cells, to a consolidated organization.

Fold the sheet again, and you have another solid, a creature of four tiers of cells, with a distinct axis of growth. This is the type of all the higher cryptogams.

Fold it once more, and you have a type, normally, of eight tiers—the type of the phanerogams.

It is to be noted that every folding develops a new axis of growth at right angles to that of the preceding type, the folding being really the mechanism employed for the evolution of the axis. Involutions do not stop here, for no surfaces come together without a tendency to adhere, especially in the fœtal stage of life when all the elements are plastic; and it is at that period that all variations occur. But, as already noticed, no further involutions, except these three, affect the axis of the whole creature and change the type. To this simple fact of the adhesion of organic surfaces all the innumerable morphological variations may be referred. Upon it depends the success of the surgeon's skill, as well as the analysis of the speculative physiologist. Adhesions of the edges of leaves account for their shapes and sizes. Adhesion by their surfaces causes many wonders, among them the evolution of all carpels, seeds, and fruits. Sometimes, also, there is