234 ANCESTRY AND CLASSIFICATION. 



has produced the extraordinary metamorphoses 

 of the butterfly. 



Let me here call attention to the various paths 

 through which new forms of butterflies may 

 spring from previously existing species. For be- 

 sides those which butterflies follow in common 

 with all animals, there are many, which, if not 

 peculiar to them, are, at least so far as known, 

 more conspicuous in this group. Ordinary varia- 

 tion, due to unknown or diverse causes, as well as 

 that which springs from latitudinal range and 

 distinct climatic influence, appears in butterflies 

 as in other creatures. In these cases we suppose 

 advantageous variations to be perpetuated and 

 intensified by the survival of the fittest, through 

 the law of inheritance. In this way, by slow 

 accretions, a species multiplies into varieties, each 

 departing from the other and from the original 

 type, until all become firmly fixed as species, 

 again to undergo division. Now just as the 

 climatic influences of latitude appear to be an 

 important factor in the development of new 

 forms, so the difference of the seasons may work 

 similar alterations in double-brooded butterflies ; 

 we have merely to suppose the Zebra Swallow- 

 tail to hibernate exclusively in the imago state, 

 to fix the variety Marcellus as the only form that 

 will survive ; on the other hand, let the insect 

 hibernate as now in the chrysalis and be only 



