l894-] NEW-YORK MICROSCOPICAL SOCIETY. 69 



life, and whatever form they make take, whether foliaceous, fru- 

 ticose, or crustaceous, they are always actuated by a principle 

 new to their nature — the exposure of the chlorophyll to the light 

 and atmosphere as much as possible. How important a force, 

 then, is this commensalism ! What part may it not have taken, 

 fostered by natural selection, in the separation of forms? The 

 behavior of the protonema of mosses and fungi ; the transforma- 

 tion by algas of some of the simpler animals ' to forms widely differ- 

 ent and independent of organic food absorption; the easy passage 

 of the algae to purely histological factors in the anatomy of the 

 lichens — all suggest that this force of commensalism may have 

 played a more important role and been a more potent factor in 

 the determination of species than is ascribed to it. I am greatly 

 indebted to Mr. A. E. Anderson for the greater part of the draw- 

 ings. I also wish to acknowledge the valuable advice and assist- 

 ance of Prof. N. L. Britton in the prosecution of the research 

 work. 



1. De Bary: Ueber die Erscheinung der Symbiose. 



2. ScHWENDENER : Ueber den Bau und das Wachsthum des Flechten- 

 Thallus, etc. 



3. Famintzin UND Baranetzki: Zur Entwickelungsgeschichte d. Gonidien 

 und Zoosporenbildung der Flechten. 



4. Bornet : Recherches sur les Gonidies des Lichens. 



5. Rees : Ueber die Natur der Flechten. 



6. Stahl : Beitrage zur Entwickelungsgeschichte der Flechten. 



7. BOMNIER : Germination des Lichens sur les Protonemas des Mousses. 



8. KoRBER : Zur Abwehr der Schwendener-Bornet'schen Flechtentheorie. 

 g. Brandt : Ueber das Zusammenleben von Thieren und Algen. 



10. Treub : Lichencultur. 



