xviii Preface to the English Edition. 



calling into existence the chief types and sub- 

 types of the animal and vegetable kingdoms ? 



At the time these " Studies " first appeared 

 (1875) they had been preceded by a whole series 

 of attempts to introduce into science such an un- 

 known power. The botanists, Nageli and Askenasy, 

 had designated it the " perfecting principle " or the 

 " fixed direction of variation;" Kolliker as the "law 

 of creation;" the philosophers, Von Hartmann and 

 Huber, as the " law of organic development," and 

 also " the universal principle of organic nature." 



It was thus not entirely superfluous to test the 

 capabilities of the known factors of transformation. 

 We had here before us a question of the highest 

 importance a question which entered deeply into 

 all our general notions, not only of the organic 

 world, but of the universe as a whole. 



This question does there exist a special 

 11 developmental force " ? obviously cannot be 

 decided by mere speculation ; it must also be 

 attempted to approach it by the inductive 

 method. 



The five essays in this volume are attempts to 

 arrive, from various sides, somewhat nearer at a 

 solution of the problem indicated. 



The first essay on the " Seasonal Dimorphism 

 of Butterflies " is certainly but indirectly connected 

 with the question ; it is therein attempted to dis- 

 cover the causes of this remarkable dimorphism, 

 and by this means to indicate at the same time 



