APPENDIX D. 



On the Variability of the Lower Forms of Living Matter. 



Lichens. Within the last few years the notion has been 

 gradually growing up that many of the so-called unicellular 

 Algae and their allies are but stages in the growth of the 

 gonidia of certain Lichens. This view was originally started 

 by the valuable observations of Itzigsohn \ With regard to 

 Chlorococcus, Dr. Braxton Hicks says^ he has thoroughly 

 satisfied himself that such a relationship occurs between this 

 so-called Alga and the Lichen known as Parmelia parietinus. 

 After describing the changes that take place in the vegetative 

 multiplication of Chlorococcus, and comparing them with those 

 occurring in the similar growth from the gonidia of Lichens, 

 he summarizes the results of his observations thus : — ' I think, 

 then, from the above remarks, that there can be no doubt but 

 that what has been called " Chlorococcus " is nothing more 

 than the gonidia of some Lichens, which, having been con- 

 veyed by the movements of the atmosphere, had been de- 

 posited on a favourable surface, where they soon begin 

 to increase by various modes of segmentation which con- 

 tinue for an unlimited period. But under suitable conditions, 

 chiefly drought and warmth, the gonidium throws out from 



* 'Botan. Zeitung,' Jan. 5, 1855. 

 ^ ' Journ. of Microsc. Science,' i860. 



