Ö PREFACE TO THE 



species and genera are seen to be stages in tlie develop- 

 ment of one and tlie same animal, &c. All these must 

 be considered as secondary results, — as facts which are 

 only adduced, so far as they serve to illustrate the fact of 

 the " alternation of generations/' 



In order to prevent my being further misunderstood, 

 as I have repeatedly been in my oral exposition of this 

 subject, and in the lately published Danish version of the 

 work, I must remark that the phenomena, upon which 

 my view of the alternation of generations rests, are, as 

 every naturalist knows, new only in part, but that they 

 have in this work received another, and, in my opinion, a 

 natural explanation. They have generally been looked 

 upon as instances of metamorphoses or transformation, 

 the essential objection being overlooked, that a metamor- 

 phosis can only imply changes which occur in the same 

 individual ; but when from it other individuals originate, 

 something more than a metamorphosis is concerned. Thus, 

 it is quite erroneous to term a Scyphistoma the larval 

 condition of Medusa aurita, since a Scyphistoma never 

 becomes a medusa, but is the quasi mother of some scores 

 of them. Sars and Loven have taken a more correct 

 view of the relation in which these creatm-es stand with 

 regard to each other, seeing in the development of the 

 Medusce and Campanularia a series oii generations under- 

 going metamorphosis. 



It is of the more essential importance that the distinc- 

 tion between an alternation of generation and a meta- 

 morphosis should be understood, because a metamor- 

 phosis may readily occur in one or other of the alternating 

 generations themselves, as we see, for example, in the 

 Distomata and Aphides. There is no transition from a 

 metamorphosis to an alternation of generations, and a 

 metamorphosis once commenced, cannot be continued 

 beyond the generation, nor from the living or dead indi- 

 vidual to another individual. I am not aware that any 

 one has published concerning the alternation of genera- 



