206 THE BIOLOGY OF THE SEASONS 



in the Tropics, is perhaps explained by one of Professor 

 Loeb's experiments, which showed that lowering thetempera- 

 ture greatly increases the length of life of small crusta- 

 ceans. So in the cold waters there are more generations 

 living at the same time than there are in the Tropics. 



As in other cases, it is difficult to distinguish the 

 direct physiological effect of a change of temperature from 

 the indirect effect due to the liberation of what is latent. 

 Lowering the temperature of the caterpillar box may be 

 followed by curious aberrations of colour in the moths and 

 butterflies, especially in the direction of darkening (melan- 

 ism). If the vessel containing some sea-urchin's eggs in 

 sea-water be placed near the stove for a short time, a large 

 proportion of them may form twins ! 



Light has subtle influences of many kinds, often illus- 

 trated by changes in coloration. Professor Poulton has 

 proved that the surrounding colour influences the colour 

 of caterpillars and pupae, apparently operating indirectly 

 through the nervous system. He showed, for instance, 

 that the caterpillars of the small tortoise-shell are for a 

 short time so sensitive, that those in a white or gilded box 

 have light or golden pupae, while those from a dark box 

 have dark pupae. 



In the case of young flat fishes, such as soles and flounders, 

 light is the necessary stimulus for pigment-formation, and 

 the down-turned shaded side remains unpigmented. Mr. 

 J. T. Cunningham made the beautiful experiment of 

 placing a mirror on the floor of a tank where young flounders 

 were undergoing their gradual metamorphosis from sym- 

 metrical into asymmetrical forms. Out of thirteen young 

 flounders whose under-sides were thus illumined by a 

 mirror for about four months, only three failed to develop 

 black and yellow colour-cells on the skin of the down- 

 turned side, which normally remains white. 



