EXTERNAL INFLUENCES AS CAUSES OF VARIATION 263 



Here temperature seems to exert a controlling influence upon 

 pigment formation, although Weismann is careful to inform us l 

 that the two patterns " do not correspond " ; that if we were to 

 ''superpose" one upon the other, "it would seem that the 

 black parts in prorsa do not correspond to the yellow ones in 

 levana, and that the white band in the former does not corre- 

 spond to [either] a yellow or [a] black part in the latter. This 

 band is, on the contrary, entirely wanting in levana, and is 

 represented by both black and yellow regions." 1 



Again, Weismann experimented with Polyommatus phlceas? 

 a species "distributed over the whole of the temperate and 

 colder parts of Europe and Asia." Toward the north (in 

 Germany) the upper surface of the wings is of a " beautiful 

 reddish-gold color," hence its popular name, " fire butterfly." 

 But he says, " Farther south the reddish-gold color is more or 

 less thickly dusted with black, and specimens from Sicily, 

 Greece, or Japan often display only a few reddish-gold scales, 

 the general appearance being almost black." 



" In Germany this butterfly is double-brooded, and the two 

 generations are similar, but in certain districts of southern 

 Europe . . . the first generation is reddish-gold, the second, 

 which flies in midsummer and is known as the variety eleus, 

 having the wings well dusted with black." "As in Germany 

 during exceptionally hot summers individuals with a blackish 

 tint have repeatedly been caught together with the ordinary 

 form . . . ," and Weismann observes that it would seem " the 

 butterfly becomes red when exposed to a moderate temperature 

 and black when the heat is greater." 



Attempts to produce these forms at will, however, by regula- 

 tion of temperature only partially succeeded. But the conditions 

 were severe. There was no common ancestor. Weismann under- 

 took to produce the southern form from the northern stock and 

 vice versa. Insects reared from German butterflies but kept in 

 high temperatures were in many instances " dusted with black, 

 but none of them resembled the darkest forms of the southern 

 eleus" Conversely, butterflies raised in cool temperatures from 



1 Weismann, Germ Plasm, p. 379. 



2 Ibid. pp. 399-400. 



