SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 391 



eiilly turns from leaf-bearers into fruit-bearers, but also in 

 the remoter parts. 



That among animals physiological development is fur- 

 thered by the multiplication of effects, we have lately seen 

 proved by the many changes in other organs, which the 

 growth or modification of each excreting and secreting organ 

 initiates. By the abstracted as well as by the added mate- 

 rials, it alters the quality of the blood passing through all 

 members of the body; or by the liquid it pours into the 

 alimentary canal, it acts on the food, and through it on the 

 blood, and through it on the system as a whole: an addi- 

 tional differentiation in one part thus setting up additional 

 differentiations in many other parts; from each of which, 

 again, secondary differentiating forces reverberate through 

 the organism. Or, to take an influence of another order, we 

 have seen how the modified mechanical action of any member 

 not only modifies that member, but becomes, by its reactions, 

 a cause of secondary modifications — how, for example, the 

 burrowing habits of the common mole, leading to an almost 

 exclusive use of the fore limbs, have entailed a dwindling 

 of the hind limbs, and a concomitant dwindling of the 

 pelvis, which, becoming too small for the passage of the 

 young, has initiated still more anomalous modifications. 



So that throughout physiological development, as in evo- 

 lution at large, the multiplication of effects has been a factor 

 constantly at work, and working more actively as the develop- 

 ment has advanced. The secondary changes wrought by 

 each primary change, have necessarily become more numerous 

 in proportion as organisms have become more complex. And 

 every increased multiplication of effects, further differen- 

 tiating the organism and, by consequence, further integrating 

 it, has prepared the way for still higher differentiations and 

 integrations similarly caused. 



§ 313. The general truth next to be resumed, is that these 

 processes have for their limit a state of equilibrium — proxi- 



