THE BEGINNINGS OF LIFE. 455 



ments, even of the commonest of such Cryptogams, 

 are to be found in the atmosphere. 



It seems, however, to be quite certain that a com- 

 munity of nature exists between Algce, Pediastrei?, 

 Desmids, and Diatoms^ since similar vegetal cells may, 

 on the same or on different occasions, grow into forms 

 belonging to either one of these groups ; and, moreover, 

 the forms are strictly convertible with one another until 

 they chance to assume the forms of Diatoms. This latter 

 step in molecular composition, when once it has been 

 entered upon, cannot be retraced. Diatoms constitute 

 the terminal forms of a divergent series. The middle 

 terms of the series, however, viz. Pediastre^ and Desmids, 

 are convertible in both directions, either back into Con- 

 ferva or onwards into the less-vitalized Diatoms. Thus, 

 after having spoken of the latter transformations, to 

 which we have already referred, Dr. Gros says: — 'II 

 peut se faire aussi que les frustules d'Arthrodesmus et 

 de Micrasterias, en tombant sur un sol humide seule- 

 ment, tournent a la vie vegetale, sois qu'ils derivent de 

 Mousses, d^Euglenes, ou de Chlamidomonas.' Whilst 

 elsewhere the same observer speaks of specimens of 

 Arthrodesmus which subsequently gave birth to unmis- 

 takeable Conferva ^ 



Having considered these transformations of Eugienx 



* See loc. cit., pp. 452 and 333. Some of the Pediastreoe found in 

 any experimental flasks were also seen to grow after the manner of a 

 Conferva (see voL i. p. 453). 



