62 THE FACTORS OP ORGANIC EVOLUTION. 



stems and brandies. Naturally, too, we need not expect 

 them in plants having modes of growth which early produce 

 an outer practically dead part, that effectually shields the 

 inner actively living part of the stem from the influence 

 of the medium long-lived acrogens such as tree-ferns and 

 long-lived endogens such as palms. But in the highest 

 plants, exogens, which have the actively living part of 

 their stems within reach of environing agencies, we find 

 this part, the cambium layer, is one from which there 

 is a growth inwards forming wood, and a growth outwards 

 forming bark : there is an increasingly thick covering (where 

 it does not scale off) of tissue changed by the medium, 

 and inside this a film of highest vitality. In so far as 

 concerns the present argument, it is the same with the 

 Metazoa, or at least all of them which have developed 

 organizations. The outer skin grows up from a limiting 

 plane, or layer, a little distance below the surface a place 

 of predominant vital activity. Here perpetually arise new 

 cells, which, as they develop, are thrust outwards and 

 form the epidermis : flattening and drying up as they 

 approach the surface, whence, having for a time served 

 to shield the parts below, they finally scale off and leave 

 younger ones to take their places. This still undifferentia- 

 ted tissue forming the base of the epidermis, and existing 

 also as a source of renewal in internal organs, is the 

 essentially living substance ; and facts above given imply 

 that it was the action of the medium on this essentially 

 living substance, which, during early stages in the organiza 

 tion of the Metazoa, initiated that protective envelope which 

 presently became an inherited structure a structure which, 

 though now mainly inherited, still continues to be modifi 

 able by its initiator. 



Fully to perceive the way in which these evidences 

 compel us to recognize the influence of the medium as a 

 primordial factor, we need but conceive them as interpreted 

 without it. Suppose, for instance, we say that the structure 



