15*5 How is this Ge7ieratio7t-Cycle to be explained? 



was a habit which was acquired by degrees, but at what 

 time it became the rule, would be now as difficult to 

 determine, as whether originally there were one or two 

 generations produced in the year. It is probable that 

 at first there was only one generation developed 

 annually, as is still the case with the purely agamous 

 species. 



In any case, I consider it certain that parthenogenesis 

 is the primitive mode, and that sexual reproduction is 

 subordinate to it. 



It is of the greatest importance to the proper 

 comprehension of the whole subject of alternating 

 generations, to be able definitely to distinguish one 

 generation as the primary one. The important differences 

 existing between the generations of our time un- 

 doubtedly demand long periods for their development, 

 while any certain means for estimating such periods are 

 wanting. If gall-flies were found among fossil insects, 

 we should have a definite starting-point, but no such 

 discovery has been made. We only know that in 

 earlier epochs a totally different climate to the present 

 existed ; under the powerful influences of a constantly 

 though gradually changing climate this remarkable 

 alternation of generations has been developed, while 

 the adaptation to new conditions of life has left 

 the general organization of the species more or less 

 changed. 



A glance at these totally diff'erent generations as they 

 now occur shows above all how difficult is the problem 

 to seek 



'in der Erscheinungen Flucht den ruhenden Pol.' 



