On the Seasonal Dimorphism of Butterflies. 8 7 



In this cycle newly-acquired changes, however 

 minute in character at first, are only transmitted 

 to a later, and not to the succeeding generation, 

 appearing only in the one corresponding, i.e. in 

 that generation which exists under similar trans- 

 forming influences. Nothing can more clearly 

 show the extreme importance which the conditions 

 of life must have upon the formation and further 

 development of species than this fact. At the 

 same time nothing shows better that the action of 

 these conditions is not suddenly and violently 

 exerted, but that it rather takes place by small and 

 slow operations. In these cases the long-con- 

 tinued .accumulation of imperceptibly small varia- 

 tions proves to be the magic means by which the 

 forms of the organic world are so powerfully 

 moulded. By the application of even the greatest 

 warmth nobody would be able to change the winter 

 form of A. Levana into the summer form ; never- 

 theless, the summer warmth, acting regularly on 

 the second and third generations of the year, has, 

 in the course of a lengthened period, stamped 

 these two generations with a new form without the 

 first generation being thereby changed. In the 

 same region two different climatic varieties have 

 been produced (just as in the majority of cases 

 climatic varieties occur only in separate regions) 

 which alternate with each other, and thus give rise 

 to a cycle of which each generation propagates 

 itself sexually. 



