386 PHYSIOLOGICAL DEVELOPMENT. 



tions of exterior and interior. If we turn to adult 



organisms, 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 and stamens. In 

 like manner among animals, there is always either a true 

 skin or an outer coat analogous to one. Wherever aggregates 

 of the first order have united into aggregates of the second 

 and third orders — wherever they have become the morpho- 

 logical units of such higher aggregates — the outermost of 

 them have grown unlike those lying within. ' Even the 

 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, 

 as in a Protococcus, it remains uniform; but when there come 

 to be an attached surface and a free surface, these, being sub- 

 ject 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 the universal difference 

 between the imbedded ends and the exposed ends of the 

 higher plants. And then among the less marked contrasts 

 of surface answering to the less marked contrasts in the inci- 



