384 PHYSIOLOGICAL DEVELOPMENT. 



cause of physiological differentiations, not only in its neigh- 

 bours from which it abstracts sap and presently 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 

 materials, 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 

 evolution at large, the multiplication of effects has been 

 a factor constantly at work, and working more actively 

 as the development 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 differentiating the organism and, b} r 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 



