THE FACTOKS OF OKGANIC EVOLUTION. 59 



to those of the fundamental tissue&quot; which they clothe,* while 

 the cuticular covering is relatively thin ; whereas in stems 

 the epidermis (often further differentiated) is composed of 

 layers of cells which are smaller and thicker- walled : a 

 stronger contrast of structure corresponding to a stronger 

 contrast of conditions. By way of meeting the suggestion 

 that these respective differences are wholly due to the 

 natural selection of favourable variations, it will suffice if 

 I draw attention to the unlikeness between imbedded roots 

 and exposed roots. While in darkness, and surrounded by 

 moist earth, the outermost protective coats, even of large 

 roots, are comparatively thin ; but when the accidents of 

 growth entail permanent exposure to light and air, roots 

 acquire coverings allied in character to the coverings of 

 branches. That the action of the medium causes these 

 and converse changes, cannot be doubted when we find, on 

 the one hand, that &quot; roots can become directly transformed 

 into leaf-bearing shoots,&quot; and, on the other hand, that in 

 some plants certain &quot; apparent roots are only underground 

 shoots,&quot; and that nevertheless &quot;they are similar to true 

 roots in function and in the formation of tissue, but have 

 no root-cap, and, when they come to the light above 

 ground, continue to grow in the manner of ordinary leaf- 

 shoots, &quot;t If, then, in highly developed plants inheriting 

 pronounced structures, this differentiating influence of the 

 medium is so marked, it must have been all-important at 

 the outset while types were undetermined. 



As with plants so with animals, we find good reason for 

 inferring that while the specialities of the tegumentary 

 parts must be ascribed to the natural selection of favourable 

 variations, their most general traits are due to the direct 

 action of surrounding agencies. Here we come upon the 

 border of those changes which are ascribable to use and 

 disuse. But from this class of changes we may fitly 

 exclude those in which the parts concerned are wholly or 

 * Sachs, p. 83. f Ibid. p. 147. 



