420 SIR NICHOLAS YERMOLOFF ON SOME INTERMEDIATE FORMS'- 



Along the patli of the evolution we actually note the following; 

 gradual changes : 



(1) The ancestral fossil form, N. Monmouthiana, is large^ 

 symmetrical, showing no rostration. Its axial area is broad and 

 regular. The striation is dense and punctate. From a geometii— 

 cal point of view the form is, so to say, perfect. 



(2) Then the striation becomes less and less pronounced and 

 dense, and less and less visibly punctate or lineolate. The areas 

 appear less and less regular and become narrower. 



(3) Although none of the forms show any pronounced asym- 

 metry, yet they become more and more asymmetrical, actuaL 

 Naviculae transforming themselves into Cymbellae. 



(4) The forms seem to gradually acquire first rostrate, theiir 

 even capitate ends. 



(5) The forms gradually become reduced in size. 



(6) At the end of the series, Cymhella microcephala shows no 

 hyaline area at all. It is of very small size, has rostrate-capitate- 

 ends and very faint striae. 



Such seems to have been the march of evolution and of changes 

 in the forms. The naviculoid form seems to have lost its perfect- 

 outline and become a Cymbella, losing symmetry and size and 

 acquiring, perhaps at the expense of striation, dilferences in the 

 shapes of the ends. I am therefore strongly inclined to think 

 that indeed the whole process has been a sort of " degeneration " 

 from a simple and perfect form to varied and complex forms. I 

 again repeat that I do not like to use the term " degeneration," 

 because we cannot know the trend of evolution. I therefore 

 use this term only for convenience sake. 



After all, the above trend of changes seems to be natural enough^ 

 being perhaps a logical and inevitable result of the more complex 

 conditions of life appearing on our planet and establishing them- 

 selves on the globe after the Glacial periods. After all, the same 

 trend of changes, from large, uniform and simpler forms to smaller, 

 more varied and more complex ones, seems to have affected the- 

 whole of organic life. No wonder therefore that the same trend 

 has wrought similar changes in Diatoms. It has actually beea. 

 a transition from homogeneous to heterogeneous forms. 



Thus a minute study of systematics seems to have helped us 

 on towards regions of generalisation. I therefore consider that 

 systematics are certainly not the dry and useless work they seenii 



