AND OF LIGHT. 249 



Of these twenty different species of grasses and 

 plants (collected in Madagascar), we see that the ex- 

 posed specimens had a reduced leaf length ratio in 1G 

 cases, whilst in only one was the length actually in- 

 creased. The average reduction for the whole series 

 amounts to 21.0 per cent., or is very considerable. 

 Similar results to these have also been obtained by 

 Sorauer.* 



Upon members of the Animal Kingdom the direct 

 effect of light is not nearly so considerable. Yung f 

 found that tadpoles exposed to daylight during the first 

 25 to 60 days of development were about 16 per cent, 

 larger than those kept in absolute darkness. He found 

 also that eggs of the sea-trout, if reared in the light, 

 hatched a day earlier than if reared in the dark, whilst 

 pond snails (Lymrwea stagnalis) hatched in 27 days in 

 the light, as against 33 days when in the dark.t It is 

 possible, however, that these effects were due rather to 

 the presence or absence of heat rays than those of 

 light. 



The most important influence of light in the produc- 

 tion of variations in animals lies in its connection with 

 the phenomena of pigmentation. Absence of light 

 leads to diminution or even total abolition of pigmenta- 

 tion, whilst its presence leads to an increase in some 

 degree proportionate to the intensity of the light. 

 This, at least, is the more or less direct action of light. 

 The indirect action, through the intermediation of the 

 nervous system, is, as a rule, exactly the reverse. A 



* Wollny's " Forschungen a. d. Geb. Agricultur," Bd. ix. 



f Arch. Zool. Exper. et Gen., vii. p. 251. 



\ Davenport's " Experimental Morphology," p. 426. 



