CHARACTERISTICS: RELATION TO ENVIRONMENT 249 



ence, sex-linked characters; they are typical hereditary con- 

 ditions. 



But it is found further that the number of facets in most 

 of these conditions depends also on the temperature to 

 which the developing animals are subjected. The numbers 

 given above are those observed by investigators working 

 with fruit-flies cultivated at ordinary, not precisely con- 

 trolled temperatures. If different sets of flies are kept at 

 different temperatures, it is found that high temperatures 

 reduce the number of facets in the individuals that carry 

 the Bar genes, though not in the normal individuals. Krafka 

 found the following mean numbers of facets in the various 

 gene conditions, at different temperatures: 



Thus the number of eye facets depends both on the genes 

 and on the temperature. The number 61 may be produced 

 either by a double Bar Male (BB-Y) at 15 degrees or by a 

 Bar Female (B-B) at between 25 and 30 degrees. Chang- 

 ing either the genes or the temperature will change the num- 

 ber of facets. 



Similar situations occur in vertebrates; one of these may 

 be described briefly. The spotted salamander, Salamandra 

 maculosa, is largely black in color, with spots or stripes of 

 yellow.^ The amount of yellow differs in different cases, so 

 that on the whole some specimens are lighter, some darker. 

 In the lighter individuals the yellow spots tend to run to- 



