BY G. I. PLAYFAIR. 461 



■waters, however, they are quite the rule of li^e, and are the 

 origin of the immense number of degenerate forms connected 

 with a species. Vide Pl.xiv., fig. 11, which is part of one end of 

 a very long chain composed of not less than 24 semicells. 



The remaining figures of Pl.xiv., satisfactoril}^ establish this 

 point. In a gathering of Desmitls lately obtained, fig. 1, Micr. 

 triiitcaia var. decemdenlata was abundant. In the laboratory, 

 the weather being warm, they commenced to divide rapidly. 

 Fig. 2 shows the result of the first division, but before the young 

 .seniicell(b) has completed its growth, a second division has taken 

 place, and fig.3, an entirely distinct type, has been formed. As 

 the result of the fourth division, fig.5 is produced; and, later, I 

 noted complete cells of the type fig.6rt(practically Micr. oscitans). 

 In six rapidly repeated divisions, therefore, we find three distinct 

 degenerate types brought into existence, in addition to the 

 oiiginal form. All these immature forms, which were present 

 in quantity, will develop, under favourable conditions, gradually 

 into that represented by fig.l; and this is not it.self fully 

 developed, as many specimens were observed with the lateral 

 lobules doubling tlie teeth(figs.7-8), and thus passing over into- 

 the type-form, Micr. truncata. In the face of such evidence as 

 this, it is surely quite clear that the word " mature '' has no 

 meaning except when applied to the fully-developed and, there- 

 fore, final form of the species. 



But Dr. West might at least have accepted his own evidence. 

 In "Variation in the Desmidiece''\3 owrn. Linn. Soc. Bot.,Vol.xxxiv., 

 Pl.x., tigs. 14-15) he has himself illustrated, in the case of Cos. 

 Reynesii, what he now denies can take place. He remarks(^.c., 

 p. 388), "Many stages were observed in the division of the cells, 

 and it often happened that a second division of the cells com- 

 menced before the first was completed. This sometimes con- 

 tinued until several immature cells intervened between th^ 

 original adult semicells." Compare, also, W. k. G. S. West's 

 Monog. Brit. Desm.(Vol. iii., Pl.lxviii., f.25). Again, in "Kew 

 and Interesting Brir. Frw. Algae," Journ. E. Micr. Soc, 1896, 

 Part 2, these authors have described(p 159) and figured{Pl.iv., 

 34 



