P/iylctii Parallelism in Mctaviorphic Species. 515 



especially those connected in their physiological 

 action, i.e. the system of organs do not keep pace 

 \vith reference to the modifications which the 

 species undergoes in the course of time ; at one 

 period one system and at another period some 

 other system of organs advances while the others 

 remain behind. 



This corresponds exactly with the result already 

 deduced from the unparallel development of the 

 independent ontogenetic stages. If the inequality 

 in the phyletic development is more sharply pro- 

 nounced in this than in the last class of cases, this 

 can be explained by the greater degree of corre- 

 lation which exists between the individual systems 

 of organs in any single organism as compared 

 with that existing between the ontogenetic stages, 

 which, although developed from one another, are 

 nevertheless almost completely independent. We 

 should have expected a priori that a strong corre- 

 lation would have here existed, but as a matter of 

 fact this is not the case, or is so only in a very 

 small degree. 



Just as in the stages of metamorphosis the in- 

 equality of phyletic development becomes the 

 more obliterated the more distant and comprehen- 

 sive, or, in other words, the greater the period of 

 existence of the groups which we compare, so does 

 the unequal divergence of the systems of organs 

 become obliterated as we bring into comparison 

 larger and larger systematic groups. 



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