On the Seasonal Dimorphism of Butterflies. 107 



forms must always maintain the tendency of the 

 latter to produce the former. 



That the above conclusion is correct that a 

 secondary species, when exposed to the external 

 conditions under the influence of which the 

 primary form originated, does not again revert toi 

 the latter is proved by experience with plants. 

 Botanists 3 assure us "that cultivated races which 

 become wild, and are thus brought back to their 

 former conditions of life, do not become changed 

 into the original wild form, but into some new 



one." 



A second point which appears to me to be 

 elucidated by seasonal dimorphism, is the origin 

 of variability. It has already been prominently 

 shown that secondary forms are for the most part 

 considerably more variable than primary forms. 

 From this it follows that similar external in- 

 fluences either induce different changes in the 

 different individuals of a species, or else change all 

 individuals in the same manner, variability arising 

 only from the unequal time in which the indi- 

 viduals are exposed to the external influence. 

 The latter is undoubtedly the case, as appears 

 from the differences which are shown by the 

 various individuals of a secondary form. These 



8 Nageli, " Entstehung und Begriff der naturhistorischen 

 Art," Munich, 1865, p. 25. The author interprets the facts 

 above quoted in a quite opposite sense, but this is obviously - 

 erroneous. 



