ECOLOGICAL FACTORS 395 



yellow pigments) undergo an increase at higher temperatures and a 

 reduction at lower temperatures, just as they are affected by increased 

 and decreased humidity. The yellow lipochromes Izooxanthines) are 

 somewhat increased by heat and diminished by extreme cold; the reds 

 (zooerythrins) are not affected by heat. The dependence of colors 

 on external conditions offers an explanation for the phenomenon that 

 non-migratory birds are much more inclined to the development of 

 geographical forms than migratory birds, and the latter in turn the 

 less so, the shorter the time they spend in their nesting ground, that 

 is, the more pronounced their migratory nature. Thus, among the 

 birds listed by Hartert there are on the average 9.6 geographical forms 

 for every species of permanent residence, 6.1 for every migrating 

 species arriving as early as March, 3.1 for the ones arriving in April, 

 and 2.0 such forms for one arriving as late as May. It must be ad- 

 mitted that in most instances many other environmental causes can 

 be influential besides temperature, among which may be mentioned 

 humidity, light, and food. 



Light. — Light has a much stronger effect upon terrestrial than 

 upon aquatic animals, since in water the intensity of the light rays 

 diminishes rapidly with depth and the coloring of animals is closely 

 related to light intensity. Cave dwellers are almost always colorless; 

 but Proteus, even if exposed only to weak daylight in an aquarium, 

 immediately becomes dark brown, whereas this does not occur among 

 the true albinos, like the albinistic axolotl. That the internal organs 

 are protected by the pigmentation of the skin from the influence of 

 strong light, especially from the chemically active short-wave rays, is 

 shown by a series of observations on the domesticated animals. Mot- 

 tled cattle, fed on buckwheat, when allowed to graze freely in the 

 direct sunlight, develop a skin rash on the white spots, a disease that 

 increases to the point of desquamation of those skin regions, fever and 

 symptoms of cerebral irritation, finally ending in death. White mice 

 get along well on buckwheat in a dark room, but in the bright light 

 they quickly die. Materials derived from buckwheat apparently act as 

 a sensitizer 00 of protoplasm to the short-wave rays. Buckwheat is a 

 common winter food for man in the United States, but is avoided in 

 summer. In Florida, white domestic pigs die if they eat Lachnanthes 

 growing in the pastures, but black ones are immune to its effect. 61 

 Decomposition in blood and the formation of haematoporphyrin 

 through the action of tropical sunlight may be the reason for the diffi- 

 culty with which the blond Nordics establish permanent colonies in 

 the tropics. The protection that the skin pigment provides against the 

 penetration of rays into the inner parts of the body is often further 



