On the Seasonal Dimorphism of Butterflies. 1 09 



species to a certain extent balances the fluctuations 

 of form. Both facts taken together confirm the 

 law formerly enunciated by me, 4 that in every 

 species a period of variability alternates with one 

 of (relative) constancy the latter indicating the 

 culmination, and the former the beginning or end, 

 of its development. I here call to mind this law, 

 because the facts which I advanced at that time, 

 viz., Hilgendorf's history of the phyletic develop- 

 ment of the Steinheim fossil shells, having since 

 become somewhat doubtful, one might easily be 

 inclined to go too far in mistrusting them and refuse 

 to give them any weight at all. 5 



In the essay just indicated I traced the origin 

 of a certain class of local forms to local isolation. 

 I attempted to show that when a species finds 

 itself in an isolated district in a condition (period) 

 of variability, it must there necessarily acquire 

 somewhat deviating characters by being prevented 

 from crossing with the individuals of other regions,* 

 or, what comes to the same thing, a local form 

 must originate. This production of local forms 



4 See my essay, " Uber den Einfluss der Isolirung auf die 

 Artbildung." Leipzig, 1872. 



8 [Eng. ed. In the summer of 1877, Dr. Hilgendorf again 

 investigated the Steinheim fossil shells, and found his former 

 statements to be completely confirmed. At the meeting of the 

 German Naturalists and Physicists at Munich, in 1877, he ex- 

 hibited numerous preparations, which left no doubt that the 

 chief results of his first research were correct, and that there 

 have been deposited a series of successively derived species 

 together with their connecting intermediate forms.] 



