390 PHYSIOLOGICAL DEVELOPMENT. 



tiations obviously related in their order to secondary contrasts 

 of conditions. 



312. How physiological development has all along been 

 aided by the multiplication of effects how each differen- 

 tiation has ever tended to become the parent of new differen- 

 tiations, we have had, incidentally, various illustrations. Let 

 us here review the working of this cause. 



Among plants we see it in the production of progressively- 

 multiplying heterogeneities of tissue by progressive increase 

 of bulk. The integration of fronds into axes and of axes into 

 groups of axes, sets up unlikenesses of action among the 

 integrated units, followed by unlikenesses of minute struc- 

 ture. Each gust transversely strains the various parts of the 

 stem in various degrees, and longitudinally strains in various 

 degrees the roots; and while there is inequality of stress at 

 every place in stem and branch, so, at every place in stem 

 and branch, the outer layers and the successively inner layers 

 are severally extended and compressed to unequal amounts, 

 and have unequal modifications wrought in them. Let the 

 tree add to its periphery another generation of the units 

 composing it, and immediately the mechanical strains on the 

 supporting parts are all changed in different degrees, initiat- 

 ing new differences internally. Externally, too, new differ- 

 ences are initiated. Shaded by the leaf-bearing outer stratum 

 of shoots, the inner structures cease to bear leaves, or to put 

 out shoots which bear leaves ; and instead of that green cover- 

 ing which they originally had, become covered with bark of 

 increasing thickness. Manifestly, then, the larger integration 

 of units that are originally simple and uniform, entails 

 physiological changes of various orders, varying in their 

 degrees at all parts of the aggregate. Each branch which, 

 favourably circumstanced, flourishes more than its neigh- 

 bours, becomes a cause of physiological differentiations, not 

 only in its neighbours from which it abstracts sap and pres- 



