388 PHYSIOLOGICAL DEVELOPMENT. 



greatly diminish the contrast between its conditions and the 

 conditions to which the upper surface is subject, there still 

 remains some unlikeness of clothing answering to the remain- 

 ing unlikeness between the conditions. Thus, with- 

 out by any means saying that all such differentiations are 

 directly caused by differences in the actions of incident forces, 

 which, as before shown ( 294), they cannot be, it is clear 

 that many of them are so caused. It is clear that parts of 

 the surface exposed to very unlike environing agencies, become 

 very unlike ; and this is all that needs to be shown. 



Complex as are the transformations of the inner parts 

 of organisms from the -relatively homogeneous into the rela- 

 tively heterogeneous, we still see among them a conformity 

 to the same general order. In both plants and animals the 

 earlier internal differentiations answer to the stronger con- 

 trasts of conditions. Plants, absorbing all their 

 nutriment through their outer surfaces, are internally modi- 

 fied mainly by the transfer of materials and by mechanical 

 stress. Such of them as do not raise their fronds above the 

 surface, have their inner tissues subject to no marked con- 

 trasts save those caused by currents of sap; and the lines 

 of lengthened and otherwise changed cells which are formed 

 where these currents run, and are most conspicuous where 

 these currents must obviously be the strongest, are the only 

 decided differentiations of the interior. But where, as in 

 the higher Cryptogams and in Phaenogams, the leaves are 

 upheld, and the supporting stem is transversely bent by 

 the wind, the inner tissues, subject to different amounts of 

 mechanical strain, differentiate accordingly: the deposit of 

 dense substance commences in that region where the sap- 

 containing cells and canals suffer the greatest intermittent 

 compressions. Animals, or at least such of them 

 as take food into their interiors, are subject to forces of 

 another class tending to destroy their original homogeneity. 

 Food is a foreign substance which acts on the interior as an 

 environing object which touches it acts on the exterior is 



