77ie Origin of the Markings of Caterpillar:. 357 



again could be scarcely otherwise explained than 

 by a " fixed direction of variation." We have here 

 one of that class of cases which the supporters 

 of a phyletic vital force have already so often made 

 use of in support of their view. 



The explanation of such a case i.e. its refer- 

 ence to known causes of species transformation 

 is never easy, and is indeed impossible without a 

 precise knowledge of the ontogeny of many species, 

 as well as of the original significance of the charac- 

 ters in question. In the case of the Deilephila 

 larvae, however, such knowledge is still wanting. 

 It is true that they present us with parallel develop- 

 mental series, but these do not depend on an 

 unknown phyletic force the parallelism can be 

 referred to the action of the imperfectly known 

 laws of growth innate in segmented organisms. 

 Because the characters of one segment have a 

 tendency to repeat themselves on the others, from 

 one parent-form possessing ring-spots on one seg- 

 ment only, there may have proceeded several 

 developmental series, all of which developed rows 

 of such spots independently of each other. 



From these considerations we may venture to 

 construct the following genealogical tree : 



