SUMMARY OF PHYSIOLOGICAL DEVELOPMEJs'T. 383 



ject, Is shown by the universal rise of the primar}^ differentia- 

 tion between the parts that are universally most contrasted 

 in their circumstances, and by the rise of secondary differen- 

 tiations obviously related in their order to secondary contrasts 

 of conditions. 



§ 312. How physiological development has all along been 

 aided b}^ 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 b}' progressive increase 

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

 groups of axes, sets up unlikenesses of action among the in- 

 tegrated units, followed by unlikenesses of minute structure. 

 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 un- 

 equal 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, initiating new differences 

 internally. Externally, too, new differences are initiated. 

 Shaded by the leaf -bearing outer stratum of shoots, the inner 

 structures cease to bear leaves, or to put out shoots that 

 bear leaves ; and instead of that green covering 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 cir- 

 cumstanccfl, flourishes more than its neighbours, becomes a 



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