156 BIOLOGY. [Book ii. 



for us, in order to complete the descriptioti of the adjuvant 

 means of nutrition, to examine the organs of motility and 

 innervation, which, while being more specially consecrated to the 

 life of relation, are, in one sense, not less indispensable to the 

 exercise of the great functions mentioned before. For, in 

 complex organisms, each anatomical element, while having its 

 own existence, and a certain degree of- nutritive and functional 

 independence, nevertheless does not cease to be closely and 

 wholly united to the other histological citizfens of the confedera- 

 tion. There is unity in diversity. 



Befoi-e commencing the exposition of the great physiological 

 functions, we have yet a general remark to formulate. In 

 organised beings, from the lowest to the highest, the most 

 differentiated, there is a, graduated hierarchy. From the physio- 

 logical confusion which exists at the lowest step of the ladder, 

 we. pass, step by step, through a series of organic models, better 

 and better finished, to the most perfect specialisation. Nothing 

 is more interesting than this seriation of organs, especially from 

 the point of view of thei great doctrine of evolution, still so much 

 discussed, but which more and more vivifies all the branches of 

 natural history. Now, however succinct may be the general 

 picture of life which we have undertaken to draw in this book, 

 the reader will nevertheless find there all the great features, the 

 principal outlines of the animated world. We shall try then 

 to describe briefly, but clearly, the various degrees, the successive 

 advances of each great function in the animal world, the only 

 one in which organic specialisation has attained a high degree of 

 perfection. Apart from all application to the doctrine of 

 evolution, the result of this mode of proceeding will be to give 

 a more complete and exact idea of each function. 



