INFLUENCE OF LIGHT. 329 



During the semi-pupa state of butterflies, before the 

 chrysalis is fully formed, the surface is photographically 

 sensitive to the color of the surroundings, and the gay 

 hues of such jjupse are due to exjiosure to the surround- 

 ings. Thus Poulton found that where the pupas trans- 

 formed in boxes lined with black jiaper they became dark, 

 while white light produced jiale ones, many of the last 

 being brilliantly golden; this suggested gilt surroundings, 

 which were far more efficient than white in producing 

 chrysalids of a distinctly golden color, and even of a deejier 

 hue than often occurs in nature. 



The effects of a life in total darkness are seen in cave 

 animals, which are either eyeless, or have lost the ojitic 

 nerves, and are pale and colorless, the pigment-cells being 

 atrojihied. Certain deep-sea Crustacea and fishes are cither 

 eyeless, or if the eyes are present they are much enlarged. 

 In the mole the eyes are much reduced, while the optic 

 nerves are either partially or totally atrophied. 



Changes of Temperature. — These, with changes of food, 

 are the most important agencies in bringing about variation. 

 Temperature is the main factor in geographical distribution, 

 especially in the sea. Variations due to changes of tem- 

 perature and of food are usually not fortuitous, but in 

 definite directions. 



A map, prepared by Merriam, illustrating the zones of 

 life in the United States, shows how dependent the distribu- 

 tion of animals is on temperature. The southern limit of 

 the boreal zone (arctic, Hudsonian, and Canadian) is marked 

 by the isotherm of 40° F.; of the transition (Alleghenian, a 

 Western arid area, and a Pacitic coast humid area) about 

 48° F. ; of the upper Austral (Carolinian and Upper 

 Mexican) 60° F.; of the lower Austral (Austroriparian and 

 Lower Mexican) 72° F. 



There is for each individual, and hence for each species, an 

 optimum tejuperature which is most favorable to its welfare, 

 and most favors nutrition, and hence growth and multipli- 

 cation. On the other hand extremes of cold (minimum) 



