16 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



near the threshold temperature of development. Many experiments 

 show that such relationships hold for medial temperatures but that 

 the rate of development is greater than that called for by this formula 

 at low temperatures near the threshold of development and less near 

 the maximum temperature tolerated. 



Within medial temperatures thermal constants of development can 

 be calculated by multiplying the number of degrees above the 

 threshold by the number of days, or hours, required for the develop- 

 ment of the stage in question. The life zones of Merriam, widely 

 used in biogeographic studies in America, are based upon the number 

 of day-degrees available in different altitudinal or latitudinal zones. 

 The temperature relations of terrestrial animals are affected markedly 

 by the accompanying humidity, and the physiological action of both 

 is affected by the rate of air movement. 20 



The relations between temperature and rainfall or temperature 

 and humidity for a given area can be shown by plotting the monthly 

 mean temperatures against mean monthly rainfall or against mean 

 monthly relative humidity. Figure 1 shows two such temperature- 

 rainfall charts in which the vertical axis gives temperatures in degrees 

 F. or C. and the horizontal axis shows rainfall in inches and in centi- 

 meters. The graph at the top of the figure extends along the rainfall 

 axis with only slight temperature variations. This is based on data 

 from Barro Colorado Island in the Canal Zone in Panama. The lower 

 graph extends along the temperature axis with only slight monthly 

 changes in rainfall; it summarizes these two elements of the climate 

 at Chicago. 



Light. — In contrast with green plants, animals are not directly 

 dependent on light for their food. Many, such as cave dwellers or 

 animals of the oceanic depths, spend their entire life in darkness. 

 At a depth in the sea no greater than 1700 meters, a photographic 

 plate remains unchanged after hours of exposure. Arthropods and 

 mollusks, fishes and amphibians, as well as simpler forms of life, 

 are found among the dark-tolerant, photonegative animals. Horses 

 in mines, maintained for years in their underground stalls, show that 

 even the highest forms, which usually live in sunlight, can dispense 

 with light. This is by no means the general rule. Frog's eggs do not 

 develop normally if sunlight is excluded. Salmon eggs hatch more 

 quickly and the salmon minnows are more active in the light than in 

 darkness, but they grow more slowly. 21 Mytilus grows more rapidly 

 in darkness, 22 and light is definitely more injurious to marine plankton 

 from considerable depths than to that from the surface. 23 Absence of 

 light slows up the development of insect larvae that normally live in 



