328 Causes and Course of Organic Evolution 



a double source of energy, the oxidation of sulphur and the 

 absorption of sunlight or infra-red light, a great advantage 

 to them in the struggle for existence, inasmuch as they would 

 be able to subsist in places where, from want of sulphuretted 

 hydrogen, the colorless Thiobacteria would perish." 



Gradually then a yellowish or etiolin and later a greenish 

 or chlorophyllin pigment may have been elaborated, as increas- 

 ingly perfected screens for absorbing the sun's rays and trans- 

 forming these into biotic energy. If such be true, the suc- 

 cession of color growths in many thermal springs would be 

 a living picture still left to us of the graded steps in color- 

 evolution from colorless autotrophic organisms in hot springs, 

 through pink to pink-yellow, yellowish green or yello\^'ish 

 brow^l, on to emerald-green in cooler waters as already stated. 



So, from primitive colorless cells or cell-threads, cyanophy- 

 ceous organisms with colored peripheral chromatophore may 

 well have originated. During such changes the plant organ- 

 isms would less and less depend on sulphur and iron com- 

 pounds as sources of energy, and would more and more depend 

 on absorbed and condensed sunlight as the source of energy. 

 But the continued need still of iron, in order to the formation 

 or activation of chlorophyll, or even in complex union as a 

 conductor and condenser of energy, may possibly be explained 

 as a hereditary chemical relation that is still retained. 



Whatever the primitive origin and mode of sustenance of 

 plants may have been, however, it seems undoubtedly true 

 that they evolved in fresh- water, not in marine, areas. For, 

 even if it be sho^Mi that Drew's marine chalk-precipitating 

 bacteria ("Science" N. S. XXXV (1912) 441) derive energy 

 or food-materials from the ocean, this could quite be a migrant 

 marine derivative from earlier autotrophic fresh-water organ- 

 isms. Tlie more recent studies of Vaughan seem to emphasize 

 this. 



It might well be argued liowever that the Cyanophycese 

 are the primitive plant organisms, and that the pink and color- 

 less ones are degraded derivatives from these. From our 

 fairly intimate knowledge now of parasitic plants that are 



