Iviii THE BEGINNINGS OF LIFE. 



on the Gleocapsa mode of growth, and become enclosed in 

 mucous envelopes. In this condition the masses differ some- 

 what in appearance from the Gleocapsa of Cladonia origin, 

 and afterwards the differences may become even better 

 marked. They may branch off into different modes of develop- 

 ment, and in one of them ' the whole mass, both cell-con- 

 tents and the mucous layer, are of a dark purple colour ; 

 each cell undergoing binary division is surrounded by its 

 own mucous layer, the whole being included in a common 

 one. After a while, the purple coating becomes colourless 

 and fused into one ; while the cell-contents become green, 

 and the divisions separate. As segmentation proceeds, the 

 resulting cells assume a linear tendency, till at last a number 

 of moniliferous filaments are formed, having here and there 

 the vesicular cells (heterocysts) found in the Nostochacece.' 

 The passage of Collema into Nostoc may also, according to 

 Dr. Hicks, take place after different modes ; and, moreover, 

 developmental changes of a precisely similar nature may be 

 undergone by the gonidia of Nostoc itself. He has also 

 described the various steps by which masses of Nostoc revert 

 to, or are changed into, Collema. Thus the relation between 

 Nostoc and Collema is shown to be indubitable. But as a 

 matter of direct observation, it is now known that Nostoc 

 may also originate even from some of the higher Gymno- 

 carpous Lichens. The gonidia of one of these has been 

 seen by Dr. Hicks developing first of all into Gleocapsoid 

 masses, and then into veritable Nostoc balls, whilst still be- 

 neath the apothecia and within the crustaceous thallus. Dr. 

 Hicks is also fully disposed to believe that subsequent re- 

 searches will tend to support the opinion of Itzigsohn, 

 that Nostoc may originate from other Lichens; and that, 

 in fact, all the Nostochacea may have such a mode of 

 origin. 



With regard to the variability of Lichens themselves, the 



