464 Studies in the Theory of Descent. 



can also be shown to be absurd. How can the 

 fact be explained that larval and imaginal families 

 by no means always coincide ; or that the larvae 

 can only be formed into families whilst the 

 imagines partly form sharply defined groups of a 

 higher order ? How can an internal directive 

 force within the same organism urge in two quite 

 distinct directions ? If the evolution of a definite 

 system were designed, and the admission of such 

 a continually acting power rendered necessary, 

 why such an incomplete, uncertain, and confused 

 performance ? 



I must leave others to answer these questions ; 

 to me a vital force appears to be inadmissible, not 

 only because we cannot understand the phenomena 

 by its aid, but above all because it is superfluous 

 for their explanation. In accordance with general 

 principles the assumption of an unknown force 

 can, however, only be made when it is indispensa- 

 ble to the comprehension of the phenomena. 



I believe that the phenomena can be quite well 

 understood without any such assumption both 

 the phenomena of congruence and incongruence, 

 in their two forms of unequal divergence and 

 unequal group-formation. 



Let us in the first place admit that there is no 

 directive force in the organism inciting periodic 

 change, but that every change is always the con- 

 sequence of external conditions, being ultimately 

 nothing but the reaction the response of the 



