Phylctic Parallelism in Metamorphic Species. 5 1 1 



with the degree of difference in the conditions of 

 life in the two stages. 



With respect to the question as to the final 

 cause of transformation this result is certainly of 

 the greatest importance. 



The interdependence of structure and function 

 has often been insisted upon, but so long as this 

 has reference only to the agreement of each par- 

 ticular form with some special mode of life, this 

 harmony could still be regarded as the result of a 

 directive power ; but when in metamorphic forms 

 we not only see a double agreement between struc- 

 ture and function, but also that the transformation 

 of the form occurs in the two chief developmental 

 stages in successive steps at unequal rates and with 

 unequal strength and rhythm, we must at least 

 so it appears to me abandon the idea of an in- 

 herent transforming force ; and this becomes the 

 more necessary when, by means of the opposite and 

 extremely simple assumption that transformations 

 result entirely from the response of the organism 

 to the actions of the environment, all the phe- 

 nomena so far as our knowledge of facts at 

 present extends can be satisfactorily explained. 

 A power compelling transformation, i.e. aphyletic 

 vital force, must be abandoned, on the double 

 ground that it is incapable of explaining the 

 phenomena (incongruence and unequal phyletic 

 lopment), and further because it is super- 

 fluous. 



