326 CONCLUSION 



environment. He was right, but only up to a certain point, 

 and this limitation caused his doctrine to be discarded. Darwin 

 admitted that variations were due to all manner of causes, 

 preserved by heredity, and reinforced by natural selection, 

 but there could be no natural selection unless it could exert 

 its influence upon a great number and variety of beings already 

 in existence. Whence did they come ? He does not say. 



Every conception of this kind — we might make a long list 

 of them — can be defended by arguments drawn from facts, 

 though none can cover all the facts. But all must be allowed a 

 part, though only a part, in the explanation of living forms. 

 As a matter of fact, they have all at some time or other con- 

 tributed to the determination of forms ; and not only these, 

 but many others as well. In addition to the external 

 causes of modification, there are powerful internal causes, 

 often intimately connected with them ; for instance, the 

 modifications of muscles and bones by habitual movements 

 provoked by stimuli in the environment, according to the 

 formula of Lamarck. Every structural cell associated in the 

 task of building up an organism, while it contributes to that 

 organism's life, none the less continues to live for its own sake. 

 On the basis of this " independance des elements anatomiques ", 

 Claude Bernard founded his entire physiological doctrine. Even 

 this is inadequate when taken literally. Each cell does, in fact, 

 contribute its quota to the construction of the common founda- 

 tions, in which all share. Thence it draws all the nourishment 

 it requires ; into it it empties in return all the residue of its 

 nutrition and the products of its activity. This residue and these 

 products constitute the internal secretions, to which Brown- 

 Sequard first called attention, but which, far from being the 

 property of certain glands long regarded as functionless, as 

 we have become accustomed to say, are really the work of 

 all the structural cells. Through the medium of this environ- 

 ment, which they are perpetually modifying, and upon which 

 react all the modifications that they themselves undergo, 

 whether these are due to the action of the external environment 

 itself or to other causes, the cells combined in one and the 

 same organism — even those associated temporarily and 

 accidentally — influence one another, however widely separated. 

 An organism, therefore, carries within itself endless causes of 

 modification, which give it sufficient plasticity to enable it 



