SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 381 



faces : especially in those of them which, like Snakes, ex- 

 pose these surfaces to the most diverse actions. Even in 

 Birds and Mammals which usually, by raising the under 

 surface considerably above the ground, greatly diminish the 

 contrast between its conditions and the conditions to which 

 the upper surface is subject, there still remains some unlike- 

 ness of clothing answering to the remaining unlikeness be- 

 tween the conditions. Thus, without 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 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 that 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 Phtenogams, 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 



