On the Mechanical Conception of Nature. 691 



briefly summarized as the reaction of organized 

 living matter to influences of the environment. 



Our opponents either cannot boast of such 

 harmony in their conception of nature, or else 

 they must, together with the phyletic vital force, 

 re-admit into their theory the old ontogenetic 

 vital force. I know not indeed why they should 

 not do so. Whoever inclines to the view that 

 organic nature is governed not merely by causal, 

 but at the same time by teleological, forces, may 

 admit that the latter are as effective as inciting 

 causes of individual, as they are of phyletic, de- 

 velopment. According to my idea they are even 

 bound to admit this, since it cannot be perceived 

 why the adaptations of the ontogeny should not 

 depend upon the same metaphysical principle 

 assumed for each individual, as the adaptations 

 of the phylogeny ; the latter are indeed only 

 brought about by the former. I believe therefore 

 that the vital force (ontogenetic) of the ancients 

 stands or falls with the modern (phyletic) vital 

 force. We must admit both or neither, since 

 they both rest on the same basis, and are sup- 

 ported or opposed by the same arguments. 

 Whoever feels justified in setting up a meta- 

 physical principle where complete proof that 

 known forces are sufficient for the explanation of 

 the phenomena has not yet been adduced, must 

 do the same with respect to individual, as he does 

 to phyletic, development, since this proof is in 



