8 INTRODUCTION. 



of the alternation of generations. Other observations which 

 still remain problematical, the results of which also will not 

 accord with the fundamental laws of the history of reproduction 

 as at present known, will probably, as we must hope, be here- 

 after lighted up by the rays of the alternation of generations. 

 Nevertheless from my own investigations I have also come to 

 the conviction, that on the other hand we must not expect too 

 much from the alternation of generations^ as when we wish and 

 imagine it, we do not always obtain an explanation from it. I 

 must give especial warning against following an investigation 

 too far with the preconceived notion that we have to do with an 

 alternation of generations, as otherwise we may be led widely 

 astray upon a false course, and never find the right way. 



Not to deviate too far from the object which I have set before 

 me in these pages, I will only here give prominence to that in 

 the history of Insects which people have been induced to regard 

 as a peculiarity of the alternation of generations, — I mean the 

 remarkable reproduction of the Aphides ; this, after standing so 

 long as something quite abnormal and inexplicable, has now 

 found its complete explanation in the nature of the alternation 

 of generations. It is well known that in the Aphides, a sexual 

 generation, represented by separate males and females, is followed 

 by a series of generations, only including a single form, which 

 proceed from each other in manifold repetition without any pre- 

 vious copulation, until after about seven to eleven such genera- 

 tions, a generation of males and females again makes its appear- 

 ance. Steenstrup* regarded these forms of Aphides, which are 

 capable of reproduction without the influence of the male gene- 

 important error of supposing that the Cercarice became developed into 

 Distoma by casting off their tails. Ehrenberg at the same time refers to his 

 exposition, description and figure of the Cercaria Ephemera, given in the year 

 1828 (Symbolce Physica, Phytozoa Entozoa), which to his regret has not 

 been referred to by Steenstrup, and which would probably have preserved him 

 from some errors. In this description, however, Ehrenberg has declared the 

 excretory organ representing a primordial kidney to be ovaries, and its coarsely 

 granular contents, eggs, to which I have long since called attention (see my 

 Lehrbuch der vergleichenden Anatomie der Wirbellosen, p. 139). Diesing, no 

 less consistently, holds fast by the belief that the Cercarice are an independent 

 and closed group of animals (see his Revision der Cercarien, in the number for 

 March 1855 of the Sitzungsberichte der Akad. der Wiss. in Wien). 



* Op. cit. supra, p. 121. 



