ttTO PHYSIOLOGICAL DEVELOPMENT. 



result that follows when the current of aerated blood is 

 interrupted. 



Here, indeed, it becomes obvious both that certain physio, 

 logical differentiations make possible certain physiological 

 integrations ; and that, conversely, these integrations make 

 possible other differentiations. Besides the waste products 

 that escape through the lungs, there are waste products that 

 escape through the skin, the kidneys, the liver. The blood 

 has separated from it in each of these structures, the par- 

 ticular product which this structure has become adapted to 

 separate ; leaving the other products to be separated by the 

 other adapted structures. How have these special adaptations 

 ueen made possible? By union of the organs as recipients of 

 one circulating mass of blood. While there is no efficient 

 apparatus for transfer of materials through the bodv, the 

 waste products of each part have to make their escape locallv; 

 and the local channels of escape must be competent to take 

 off indifferently all the waste products. But it becomes prac- 

 ticable and advantageous for the differently- localized ex- 

 creting structures, to become fitted to separate different waste 

 products, as soon as the common circulation through them 

 grows so efficient that the product left unexcreted by one is 

 quickly carried to another better fitted to excrete it. So that 

 the integration of them through a common vascular system, 

 is the condition under which only they can become differen- 

 tiated. How the specialization of each is rendered possible 

 only by its connexion with others that have become similarly 

 specialized, we indirectly see in such a fact as that in chronic 

 jaundice secondary disease of the kidneys is apt to arise in 

 consequence of the biliverdine accumulated in the system 

 being partly excreted through them : the implication being 

 that a structure peculiarly fitted to excrete urea can exist 

 only when it is functionally united with another structure 

 peculiarly fitted to excrete biliverdine. Perhaps the 



clearest idea of the way in which differentiation leads to 

 integration, and how, again, increased integration makes 



