IOO TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 



Indeed such confusion has arisen in our text books as to many named 

 forms, that any intermediate forms can be indifferently assigned to two 

 totally different type forms created by their authors. 



This would therefore force upon us the conclusion that forms are 

 constantly and rapidly changing and that they pass from one species to 

 another by infinitesimal transitions. 



But this opinion cannot be maintained. I have previously stated that 

 it may be assumed that our many existing forms sprang from one or from 

 several primitive forms. These primordial forms have given birth to secondary, 

 tertiary forms, etc., which were differentiated in certain directions and 

 which have continued to evolve more or less in their turn in those 

 directions. 



But this evolution, we believe, has only been brought about very gradually, 

 and probably also under the influence of special circumstances, of which we 

 know but little, but some of which we may at least suspect, such as the 

 presence of a greater or less quantity of silica or salt in the water, the 

 temperature and the amount of illumination, etc. 



The assumption of primitive and derived forms (species, varieties, etc.) 

 is all the more easy to admit from the fact that diatoms can, in certain 

 instances, if the experiments of Dr. Miquel can be trusted, reproduce 

 themselves, almost indefinitely, by subdivision and without the interven- 

 tion of any act of fecundation. 



The diatomist has at his disposal in fossil forms, which came into 

 existence ages immeasurably distant, materials so perfectly preserved that he 

 might almost believe they dated from yesterday. These materials shew us that 

 a diatom form can be maintained so quasi-indefmitely, that it is scarcely 

 modified at all so long as it remains under the same conditions of life. 



Van Hcurckia rhomboides, which is found in so many different fossil 

 deposits, is identically the same as that which is found living to-day in 

 numberless localities ; Arachnoidiscus Etirenbergii, which is still found 

 inhabiting the Sea of Japan and occurs in all respects the same in the 

 deposits of Hungary, which date back to the remote tertiary period when a 

 tropical sea covered Central Europe, again confirms this assertion. 



It is therefore more logical to admit that the apparent transitions recorded 

 arise from the fact that authors have created different " species " at the 

 expense of varieties or of races of the same specific type form, and that true 

 species are in reality much less numerous than has been hitherto imagined. 



It cannot happen therefore, till research has been much further pro- 

 longed, that the many living or fossil forms, as yet unknown to us, can be 

 compared and connected with one another ; that studies can be guided by 



