f 



4io Studies in the Theory of Descent. 



stages of one and the same species, must therefore 

 find their explanation elsewhere than in the type 

 of the marking itself. The explanation must be 

 found in the circumstance that each stage changes 

 independently of the others, and at different 

 periods can enter a new phase of variability. 



We are here led in anticipation to the main 

 question : Are changes produced by internal or 

 external causes ? is it the physical nature of the 

 organism which is compelled to become re- 

 moulded spontaneously after the lapse of a certain 

 period of time ? or does such modification only 

 occur when produced directly or indirectly by the 

 external conditions of life ? 



In the cases before us the facts undoubtedly 

 indicate a complete dependence of the transforma- 

 tions upon external conditions of life. 



The independent appearance of variability in 

 the separate stages of the metamorphosis might, 

 however, be regarded as only apparent. It might 

 still be attempted to attribute the changes to a 

 purely inherent cause, i.e., to a phyletic vital force, 

 by assuming that the latter acts periodically in 

 such a manner that at first one and then the fol- 

 lowing stage ^becomes variable, until finally the 

 entire species isVtransformed. 



There is but Irttle to be said in reply to this if 

 we once take refuge in entirely unknown forces, 

 the operation of which can be arbitrarily con- 

 ceived to be either constant or periodic. 



But granting that such a transforming power 



