12 SENESCENCE AND REJUVENESCENCE 



they grow at the expense of nutritive material, they reproduce 

 their kind. In other words, the problems of development, growth, 

 reproduction, and inheritance exist for each of them and the assump- 

 tion of their existence brings us not a step nearer the solution of 

 any of these problems. These theories are nothing more nor less 

 than translations of the phenomena of life as we know them into 

 terms of the activity of multitudes of invisible hypothetical or- 

 ganisms, and therefore contribute nothing in the way of real ad- 

 vance. No valid evidence for the existence of these units exists, 

 but if their existence were to be demonstrated we might well 

 despair of gaining any actual knowledge of life. 



But these theories possess another fundamental defect in that 

 they do not provide any adequate mechanism for the control 

 and co-ordination or dominance and subordination of the activity 

 of the ultimate units. It is absolutely inconceivable that a mul- 

 titude of these units, such as is assumed to constitute the basis 

 of the cell or the organism, should always in a given species 

 arrange themselves in a perfectly definite manner so as to pro- 

 duce always essentially the same total result. In other words, 

 these theories do not account satisfactorily for the peculiarly con- 

 stant course and character of development and morphogenesis. 

 If we follow them to their logical conclusion, which their authors 

 have not done, we find ourselves forced to assume the existence of 

 some sort of controlling and co-ordinating principle outside the 

 units themselves, and superior to them. If the units constitute the 

 physico-chemical basis of life, as their authors main tarn, then this 

 controlling principle, since it is an essential feature of life, must of 

 necessity be something which is not physico-chemical in nature. 

 In short, these theories lead us in the final analysis to the same 

 conclusion as that reached by the neo-vitalists. If we are not 

 content to accept this conclusion we must reject the theories..^ 



The development within recent years'-oL the '"experimental 

 method of investigation~and the^consequent* approach' "of mor- 

 phology and physiology toward a common ground have accom- 

 plished much in inducing biologists to turn their attention in other 

 directions for interpretation and synthesis of the facts. But the 

 Weismannian germ plasm as an entity distinct from the soma and 



