290 PHYSIOLOGICAL DEVELOPMENT. 



contents squeezed out longitudinally and laterally with in 

 creasing force; and, consequently, amid them are formed 

 new sap-channels, from which there is the most active local 

 exudation, producing the greatest deposit of dense matter. 



Thus fusing together, as it were, the individualities of 

 successive generations of plants, and recognizing as all- 

 important that facilitation of the process which natural 

 selection has all along given, we are enabled to interpret the 

 chief internal differentiations of plants as consequent on an 

 equilibration between inner and outer forces. Here, indeed, 

 we see illustrated in a way more than usually easy to follow, 

 the eventual balancing of outer actions by inner reactions. 

 The relation between the demand for liquid and the formation 

 of channels that supply liquid, as well as that between the 

 incidence of strains and the deposit of substance which resists 

 strains, are among the clearest special examples of the general 

 truth that the moving equilibrium of an organism, if not 

 overthrown by an incident force, must eventually be adjusted 

 to it. 



The processes here traced out are, of course, not to be 

 taken as the only differentiating processes to which the inner 

 tissues of plants have been subject. Besides the chief changes 

 we have considered, various less conspicuous changes have 

 taken place. These must be passed over as arising in ways 

 too involved to admit of specific interpretations; even sup 

 posing them to have been produced by causes of the kind 

 assigned. But the probability, or rather indeed the certainty, 

 is that some of them have not been so produced. Here, as 

 in nearly all other cases, indirect equilibration has worked in 

 aid of direct equilibration; and in many cases indirect equili 

 bration has been the sole agency. Besides ascribing to 

 natural selection the rise of various internal modifications 

 of other classes than those above treated, we must ascribe 

 some even of these to natural selection. It is so with the 

 dense deposits which form thorns and the shells of nuts : 

 these cannot have resulted from any inner reactions imme- 



