PhyUtic Parallelism in Mctamorphic Species. 461 



The more commonly occurring case in which 

 one stage has a greater form-divergence than the 

 other, is explicable by the one stage having changed 

 more frequently or more strongly than the other. 



The explanation of the phenomena thus far 

 lies on the surface, and it is scarcely possible to 

 advance any other; but why should one stage 

 become changed more frequently or to a greater 

 extent than the other ? why should one portion 

 be induced to change more frequently or more 

 strongly than another ? whence come these in- 

 ducements to change ? These questions bring us; 

 to the main point of inquiry : Are the causesj 

 which give rise to these changes internal or ex-j 

 ternal ? Are the latter the result of a phyleti 

 vital force, or are they only due to the action o 

 the external conditions of life ? 



Although an answer to this question will be 

 found in the preceding essay, I will not support 

 myself on the results there obtained, but will en- 

 deavour to give another solution of the problem 

 on fresh grounds. The answer will indeed be the 

 same as before : A phyletic force must be dis- 

 countenanced, since in the first place it does not 

 explain the phenomena, and in the second place 

 the phenomena can be well explained without its 

 assumption. 



The admission of a phyletic vital force does not 

 explain the phenomena. The assumption that 

 there is a transforming power innate in the organ- 



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