LINNEAK SOCIETY OF LONDON. 57 



The exhibited forms come from two European and two 

 American localities. They are all freshwater. 8ome are firmly 

 established species, others intermediate forms, wliich may be given 

 so-called hyphenated nominations, stating the initial species from 

 which they seem to start and the final species to which they seem 

 to tend. 



The lantern-slides were shown in the following sequence, 

 corresponding to the line of their evolutionary descent : — 



Navicula monmonthiana forma genuina. American fossil. 



Navicula nionmoutJiiana, withjirst sliglit variation towards the 

 Stodderi form. American fossil. 



Navicida monmouthiana-Stodderi. An American living hyphe- 

 nated form. 



Gymbella Stodderi. American living. 



Gymhella Stodderi-angustata. American living. 



Cymbella cequalis. European living. 



Cymbella angustata. European living. 



Encyonema gracilis. American fossil. 



Encyonema scotica. European living. 



Cymbella delicatula. European living. Also living in 

 America. 



Cymbella gracilis-Ceaatii. American living. A hyphenated 

 form. 



Cymbella Cesatii. European living. 



Cymbella microcephala. European living. 



The detailed examination of the above evolutionary ladder 

 suggests the hypothesis that the large, simple, and homogeneous 

 ancestral form Navicula monmonthiana was a primordial species 

 adapted to the more uniform conditions of life on the planet 

 during the pre-GIaeial epochs, and that the Cymbellce which 

 subsequently evolved from it are smaller, more complex hetero- 

 geneous forms, gradually derived from Navicula monmonthiana 

 under the influence of quite different and more varied conditions 

 of life and climate, which established themselves on the earth 

 after the Glacial epochs, at least under the latitudes between 

 40 and 60 degrees North. 



A similar trend of changes from larger and less varied forms to 

 smaller heterogeneous ones has affected the whole of organic life 

 after the Glacial extensions towards the South. No wonder that 

 ttie same trend has wrought similar changes also in Diatoms. 



►Several authors admit that Cymbella; are degenerated Navicula. 

 This seems plausible, only the term " degenerated " can hardly be 

 used as we cannot know the direction of Evolution as a whole. 

 Tiie most we can say is, that the immediate result of evolutionary 

 changes consists in a better adaptation, in the struggle for 

 existence, to changed conditions of life. 



LINN. SOC. PROCEEBINGS. — SESSION 1917-1918. /" 



