246 



CAUSES OF VARIATION 



than in light, and a few, prominent among which are the poas, 

 germinate more readily in the light, as do also the spores of ferns 

 and the seeds of the mistletoe. 1 



Evidence as to whether sunlight influences the growth of ani- 

 mals is inconclusive. Experiments have been conducted with 

 tadpoles, snails, the eggs of certain fish, and with the young of 

 higher animals, but while slightly better growth is reported 



Red Green White Blue 



FIG. 27. Effect of light upon rate of growth : sensitive plants grown for three 

 and a half months in red, green, white, and blue light. After C. B. Daven- 

 port, from Flammarion 



during daylight, it is not certain that other conditions did not 

 contribute to the results. 2 Experiments in feeding fattening ani- 

 mals in darkness and in light fail to establish special differences. 

 All considerations point to the conclusion that light exerts a 

 strongly modifying influence upon living matter in general, but 

 that the higher animals are substantially free from its direct 

 effect except to some extent in the matter of color. 



1 C. B. Davenport, Experimental Morphology, Part II, pp. 419 and 424. 



2 Ibid. pp. 425-426. 



