8o Studies in the Theory of Descent. 



V. 



ON ALTERNATION OF GENERATIONS. 



SEASONAL dimorphism has already been designated 

 by Wallace as alternation of generation, 1 a term 

 which cannot be disputed so long as it is confined 

 to a regular alternation of dissimilar generations. 

 But little is gained by this definition, however, 

 unless it can be proved that both phenomena are 

 due to similar causes, and that they are conse- 

 quently brought about by analogous processes. 

 The causes of alternation of generation have, until 

 the present time, been scarcely investigated, owing 

 to the want of material. Haeckel alone has quite 

 recently subjected these complicated phenomena 

 generally to a searching investigation, and has 

 arrived at the conclusion that the various forms of 

 metagenesis can be arranged in two series. He 

 distinguishes a progressive and a retrogressive 

 series, comprising under the former those species 

 " which, to a certain extent, are still in a transition 

 stage from monogenesis to amphigenesis (asexual 

 to sexual propagation), and the early progenitors 



1 [Trans. Linn. Soc., vol. xxv. 1865, p. 9. R.M.] 



