BY G. I. PLAYFAIR. 485 



sides divergent, (3) semicells hexagonal. Simple and reasonable 

 as this classification looks, when only a few forms are known, it 

 is entirely artificial and broke down at once as soon as proper 

 research was made into the life of the species. First, a large 

 number of forms appeared, both in quantity and as formce 

 mixtce, which had rounded outlines and yet were too closely con- 

 nected with more typical forms to be separated from them and 

 ■ classed by themselves. Secondly, I discovered that between the 

 extremes of the type, on the one hand, and var. sexangulare on 

 the other, there might be found every degree of slope in the 

 lower sides. This goes far to prove the connection of them all 

 in one species. 



St. monticulosum Breb., and St. forficulatum Lund. 



When a small isolated pool or patch of swampy ground is 

 selected, and the Desmids it contains subjected to close and 

 systematic investigation extending over several years, two things 

 -ipay be expected. Firstly, we may be simply overwhelmed with 

 polymorphic forms, all plainly connected together in life and 

 growth (among which will probably be many well-known 

 "species"), and all anastomosing togethei', through transition- 

 forms, into a perfect maze. Secondly, a series of forms may be 

 obtained, showing every stage in the development of any given 

 Desmid; and, by patient search, the difference between any two 

 terms of the series may be made as small as desired. 



There might be some justification for the theory that a certain 

 similarity in outline is merely the record of evolutionary relation- 

 ship, provided we were able to retain a reasonable gap between 

 the forms. But when two such evolutionary relations become con- 

 nected by a series of forms exhibiting no greater diff'erence among 

 themselves than may be found in the figures of a kinematograph 

 film, we are justified in considering that it is the record of one 

 -continuous action. Such a series is figured in PI. xii. , f. 15-20. 



The following are the specifications of the various forms : — 



St. orbiculare var. MUTicuM(Breb.). (PI. xii., f.l5). 

 jLong.SOj lat.24; isth,8/x. 



