642 Studies in the Theory of Descent. 



indeed still contain many gaps." He points out 

 how very far we are still removed from being able 

 to reduce to physical causes, the processes through 

 which the fertilized yelk of an egg becomes de- 

 veloped into a chicken. 



How comes it therefore that we all have a con- 

 viction that such a complete reduction will in time 

 become possible, or if not this, that the develop- 

 ment of the individual depends entirely upon the 

 same forces which are in operation without the 

 organism ? For what reason have we rejected the 

 "vital force"? 



Simply because we see no reason for assuming 

 that known forces are insufficient for explaining 

 the phenomena, and because we are not justified 

 in admitting directive forces as long as we have 

 any hope of one day furnishing a mechanical 

 explanation. 



But if it is not only permissible, but even neces- 

 sary, to explain the ontogenetic vital power by 

 known forces, and to commence to indicate the 

 mechanism which produces the individual life, 

 why should it not be equally necessary to abandon 

 that assumption of a phyletic vital force which 

 stifles any deeper inquiry, and to attempt to point 

 out that here also the co-operation of mechanical 

 forces has brought about the multitudinous and 

 wonderful phenomena of the organic world ? 



The renunciation of the old vital force was 

 certainly an immediate consequence of the acqui- 



