THE CHAIN OF SPECIES. 467 



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 tlie 

 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 mo- 

 tion and volition, seemed to be present or absent, now readily take 

 their places. For instance, the aethalium — to show the impossibility 

 of a definite boundary — is sometimes observed as a flying vibrio, then 

 a crawling amoeba-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 ulvaceae ; or even baculate types, as confer- 

 vse. This is our biological surface. 



Fold it down upon itself, the two surfaces coming in contact soon 

 adhere, and then wx 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 foetal 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 Avhole creature and change the type. To this simple fact 

 of the adhesion of organic surfaces all the innumerable morphologi- 

 cal 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 an^ sizes. 

 Adhesion by their surfaces causes many wonders, among tliem the 

 evolution of all carpels, seeds, and fruits. Sometimes, also, there is 



