368 THE EVOLUTION THEORY 



separated to determine in different ways the various appendages. 

 But that is just another way of saying that this single determinant 

 actually includes within itself several different determinants. For 

 a determinant means nothing more than an element of the germ- 

 substance by whose presence in the germ the specific development of 

 a particular part of the body is conditioned. If we could remove the 

 determinants of a particular appendage from the germ-plasm this 

 appendage would not develop ; if we could cause it to vary the 

 appendage also would turn out differently. 



In this general sense the determinants of the germ-plasm are not 

 hypothetical, but actual ; just as surely as if we had seen them with 

 our eyes, and followed their development. Hypothesis begins when 

 we attempt to make creatures of flesh and blood out of these mere 

 symbols, and to say how they are constituted. But even here there 

 are some things which may be maintained with certainty; for 

 instance, that they are not miniature models, in Bonnet's sense, of the 

 parts which they determine ; and, further, that they are not lifeless 

 material, mere substances, but living parts, vital units. If this were not 

 so they would not remain as they are throughout the course of 

 development, but would be displaced and destroyed by the meta- 

 bolism, instead of dominating it as living matter alone can do — doubt- 

 less undergoing oxidation, but at the same time assimilating material 

 from without, and thereby growing. There cannot be lifeless deter- 

 minants ; they must be living units capable of nutrition, growth, and 

 multiplication by division. 



And now we have arrived at the point at which a discussion of 

 the organization of the living substance in general can best be inter- 

 polated. 



The Viennese physiologist, Ernst Briicke, forty years ago promul- 

 gated the theory that living matter could not be a mere mixture of 

 chemical molecules of any kind whatever ; it must be ' organized,' that 

 is, it must be composed of small, invisible, vital units. If, as we must 

 certainly assume, the mechanical theory of life is correct, if there is 

 no vital force in the sense of the ' Natur-Philosophie,' Brlicke's 

 pronouncement is undoubtedly true ; for a fortuitous mixture of 

 molecules could no more produce the phenomena of life than a single 

 molecule could, because, as far as our experience goes, molecules do 

 not live ; they neither assimilate, nor grow, nor multiply. Life can 

 therefore arise only through a particular combination of diverse 

 molecules, and all living substance must consist of such definite groups 

 of molecules. Shortly after Briicke, Herbert Spencer likewise, 

 assumed the reality of such vital ' units,' and the same assumption! 



