BY G. I. PLAYFAIR. 463 



gression of the cell from one so-called " species " to another. 

 The specimens of the latter series all occurred in profusion in the 

 same gathering(]Sro.60, N.H.S.). 



In the following notes, I have made some attempt to exemplify 

 the real conditions of life and development among the Desmid- 

 iacece, the result of fifteen years continued observations of the 

 same "species" from the same localities, at different times and 

 under a variety of circumstances. This kind of study does not, 

 indeed, cause one to become acquainted with a very large number 

 of true Desmid-species, but on the other hand it affords excellent 

 opportunities for gaining an insight into the life, development, 

 and connections of such as are to be found locally. The truth 

 turns out to be exactly what Rev. W. Archer so cautiously 

 suggested in Quart. Journ. Micr. Sci. (New Ser., Vol. ii., 1862). 

 "It is not proved," he says, " that some other form, whioh in the 

 {present state of knowledge we are constrained to suppose a 

 distinct species, may not in truth be only a phase of variation or 

 of development, or an 'alternation of generation' of the actual 

 species, whose extremes of variation, or whose life-history, are as 

 yet unknown." 



The Desmidiacece are essentially plants that require to be 

 studied on the spot, by comparison of the contents of repeated 

 -gatherings from the same habitat; no reliable determinations can 

 possibly be made from the contents of isolated samples, and this 

 is true also not only of the so-called Unicellular Algse, but 

 -equally of the Diatoms, Peridiniete, Flagellata, and Infusoria as 

 Avell. In all these realms of life something like ninety per cent, 

 of the " species " are polymorphic forms of the other ten; and it 

 is only by tracing out their life-histories through the observation 

 of transition-forms, that the specific connection of their innumer- 

 able variations can be established. 



I am not aware that this aspect of the subject has received 

 ;niuch consideration hitherto. Three short papers, however, are 

 mentioned by Prof. 0. Nordstedt in the Bibliography of his 

 invaluable Index Desmidiacearum, viz.: — F. B. Carter, "Desmids, 

 ;their life-history and classification," Amer. Month. Micr. Journ., 



