328 SCIENCE PROGRESS. 



views as to the nature of alternation which have been ex- 

 pressed since 1878, the results of a number of investigations, 

 which have a more or less direct bearing on the question, 

 may be first considered. 



INVESTIGATIONS INTO THE NORMAL LIFE HISTORIES OF 

 THALLOPHYTES AND ARCHEGONIATES. 



The life histories of the main groups of Bryophyta and 

 Pteridophyta, and of the Thallophytes which present the 

 closest resemblance to them in this respect, were already 

 so fully known that comparatively few facts of this kind 

 bearing directly on the nature of alternation have been 

 recorded since 1878. The important work of Klebs, 1 

 however, an account of which has already been given in 

 Science Progress,- has gone far to place the question of 

 the nature of alternation in Thallophyta on an experimental 

 basis. Klebs has shown that the alternation of the free 

 living generations in a number of Algae can be con 

 trolled by suitable modifications in the conditions of 

 cultivation, and that, in CEdogonium, Hydrodictyon, and 

 Vatuheria for example, the development of sexual or 

 asexual organs of reproduction can be determined by the 

 investigator. This places beyond any doubt the homology 

 of the sexual and the free neutral generations in these 

 Algae, but it leaves untouched the all-important question 

 whether or not the first neutral generation (to use Prings- 

 heim's terminology) is or is not homologous with the other 

 generations in the life cycle. To establish this homology, 

 it will be necessary to show that the zygote of CEdogonium 

 or Coleochcete can be induced to develop directly into the 

 corresponding thallus, bearing sexual or asexual reproduc- 

 tive organs. The work of Klebs as yet published, although 

 it does not settle this question, is of the highest importance, 

 since it suggests the possibility of direct experimental evi- 

 dence being obtained upon it in the Algae at least. In this 

 connection the work of Goebel, 3 Dodel Port, 4 and Klebs 5 



1 Klebs (2). 2 Ward. 3 Goebel (3), (4). 



4 Dodel Port. 5 Klebs (1). 



