;oo Studies in tlie Theory of Descent. 



explanation than would be given by the supporters 

 of per saltum development. If we trace cyclical 

 reproduction to the adaptation of different de- 

 velopmental stages or generations to deviating 

 conditions of life, we thus not only explain 

 the exact and often striking agreement between 

 form and mode of life we not only bridge over 

 the gap between metamorphosis and alternation 

 of generation, but we can also understand how, 

 within one and the same family of Hydrozoa, 

 species can occur with or without alternation of 

 gent-ration, and further how other species can 

 exist in which the alternation of generation (the 

 production of free Medusae) is limited to the one 

 sex ; we can understand in general how one con- 

 tinuous series of forms may lead from the simple 

 sexual organ of the Polypes to the independent 

 and free swimming sexual form of the Medusas, 

 and how hand in hand with this the simple repro- 

 duction becomes gradually cyclical. It is just 

 these intermediate steps between the two kinds of 

 reproduction that make quite untenable the idea 

 that the heterogeneous forms in cyclical propa- 

 gation arise through so-called " heterogeneous 

 generation," t. e. through sudden per saltum 

 transformation. It is excusable if philosophers 

 to whom these facts are strange, or who 'have to 

 take the trouble of working them up, should 

 adduce alternation of generation as an in- 

 stance of " heterogeneous generation," but by 



