SUMMARY OF PHYSIOLOtilCAL DEVELOPMENT. 379 



of exterior and interior. If we turn to adult or- 



ganisms, vegetal or animal, we see that whether they do or 

 do not display other contrasts of parts, they always display 

 this contrast. Though otherwise almost homogeneous, such 

 Fungi as the Puff-ball, or, among Algce, all which have a 

 thallus of any thickness, present marked differences between 

 those of their cells which are in immediate contact with the 

 environment and those which are not. Such differences they 

 present in common with every higher plant ; which, 

 here in the shape of bark and there in the shape of 

 cuticle, has an envelope inclosing it even up to its petals : 

 the only parts not so inclosed, being those short-lived 

 terminations of the fructifying organs, from which the dis- 

 integrated tissue is being cast off to form the germs of new 

 individuals. In like manner among animals, there is always 

 either a true skin or an outer coat analogous to one. Wher- 

 ever aggregates of the first order have united into ag- 

 gregates of the second and third orders — wherever they 

 have become the morphological units of such higher aggre- 

 gates — the outermost of them have grown unlike those lying 

 within. Even thr: Sponge is not without a layer that may 

 by analogy be called dermal. 



This lapse of the relatively homogeneous into the rela- 

 tively heterogeneous, first showing itself, as on the hypothesis 

 of evolution it must do, by the rise of an unlikeness between 

 outside and inside, goes on next to show itself, as we infer 

 that it must do, by the establishment of secondary contrasts 

 among the outer parts answering to secondary contrasts 

 among the forces falling on them. So long as the whole sur- 

 face of a plant remains similarly related to the environment, 

 ns in a Protococcus or a Volvox, it remains uniform ; but when 

 there come to be an attached surface and a free surface, 

 these, being subject to unlike actions, are rendered unlike. 

 This is visible even in a unicellular Alga when it becomes 

 fixed ; it is shown in the distinction between the under 

 and upper parts of ordinary Fungi; and we see it in 



