LIFE WITHOUT OXYGEN 133 



as soon as magmatic extrusions were exposed to atmospheric 

 action. 



These considerations establish with a fair degree of proba- 

 bility, first that the oxygen in the present atmosphere has been 

 derived largely if not exclusively from plant action ; secondly 

 that life began in the presence of very little oxygen, if the latter 

 was not absolutely lacking. This conclusion derives further 

 support from the history of the mechanism by which, in the 

 above view, the oxygen we breathe has been set free. 



It is well known that the characteristic plant function, the 

 splitting up of carbon dioxide accompanied by the liberation of 

 oxygen, is due to the presence of a complex substance, chloro- 

 phyll, whose function apparently is to absorb the lower rays of 

 the spectrum and with this energy to dissociate the molecules of 

 carbon dioxide and water; 1 and these according to the prevailing 

 theory give rise to formaldehyde, CH 2 0, oxygen being liber- 

 ated. 2 These " colour bodies " or chloroplasts present one of the 

 most interesting instances of transition from highly organised to 

 living substance. There is no evidence, as yet, that they can 

 live by themselves ; on the other hand they divide and reproduce 

 in exactly the same manner as unicellular animals and plants. 3 



We seem to have here the clearest evidence of a symbiosis ; 

 community-life, we know, extends very far down in the scale 4 

 even to the green algae and their like. A step lower in the 

 scale and this association with the complicated chlorophyll 

 mechanism is lost. We have then the series of bacteria and 

 moulds which, apparently, do not utilise carbonic acid and 

 often give off hydrogen instead of oxygen. 



It is to be noted further that in the metaphyta and most 

 strikingly in the higher plants, the chlorophyll function does 

 not exist obviously in the seed and does not appear until long 

 after germination has begun. We should infer from this that 

 the chlorophyll " habit " was of relatively late appearance in the 

 plant world and this conclusion is precisely that to which we 

 are led from consideration of the non-chlorophyllian autotropic 



1 G. Ciamician, La Chifnica Organica negli Organising p. 14 (Bologna, 1908). 



2 Cf. Euler, Pflanzenchemie, 109 ff. ; Maze, LJ Evolution du Carbone et de P Azote 

 (Paris, 1899) ; Bokorny, Natunv. Woch. 361, 8, 1909. 



3 O. Hertwig, Allg. Biol. 3te auf. p. 95 (Jena, 1909) ; J. R. Green, Man. ofBot. 

 3rd ed. p. 277 (London, 1904). 



1 P. Kammerer, Archiv /. Rassenbiol. 585, 6, 1909. 



