CONDITIONS OF EXISTENCE 21 



contrary, are specialists, and do not appear in many environments. It 

 is entirely compatible with this fact that such species may occur 

 everywhere on the earth where their habitat requirements are found, 

 like the salt-resistant crustaceans of the genus Artemia in strongly 

 salt inland waters. 



The law of the minimum. — The basic habitat requirements to- 

 gether condition the distribution of animals, but the deciding factors 

 are those which are most subject to variation. Light and oxygen are 

 wanting at relatively few places, but temperature, humidity, and food 

 and water supply vary in a much greater degree and these factors 

 accordingly are the most important causes of the variation in animal 

 distribution. Liebig found that in the growth of plants the food ele- 

 ment which is least plentiful in proportion to the plants' needs, limits 

 their growth. This is Liebig's "Law of the Minimum." This rule may 

 be extended in a similar sense to cover the effect of the environment 

 upon animals since that factor for which a species has the narrowest 

 range of adaptability limits its existence. In other words, the selection 

 of the animals in a given environment is determined by that habitat 

 factor which most approaches the minimum. The closer even a single 

 factor approaches a limiting value, the fewer is the number of species 

 in the situation in question. There may be sufficient oxygen, favorable 

 temperature, and abundant food in a salt pond, but the high salt con- 

 tent permits the existence of only a few euryhaline animals. A pol- 

 luted body of water may have a superabundance of food and optimum 

 temperature, but the low oxygen content limits the animals to a few 

 forms such as some Protozoa and the oligochaete, Tubifex. 



There is another application of ecological valence to the distribu- 

 tion of animals, an extension of the law of the minimum. The con- 

 tinued presence of an animal in an environment depends on that de- 

 velopmental stage in which it has the least adaptability. For example, 

 the lobster (Astacus ho?narus) does not pass the latitude of the 

 Lofoten Islands on the Norwegian coast because the post-embryonic 

 development of the larvae demands an average temperature of 15- 

 16°, which is not reached by the Arctic waters; neither the eggs, the 

 segmentation stages, nor the adults are injured by low temperature. 34 



BIBLIOGRAPHY 



1) Berger, 1907, Arch. ges. Physiol., 118, p. 607-611.— 2) Adolf, 1930, Quart. 

 Rev. Biol., 5, p. 51; Babcock, 1912, Univ. Wise. Agric. Exp. Sta. Res. Bull., 22, 

 p. 17-24; Hall, 1922, Biol. Bull., 42, p. 31; Schmidt, P., 1918, J. Exp. Zool., 27, 

 p. 57-73; Shelford & Deere, 1913, Biol. Bull., 25, p. 79.-3) Shelford, 1929, 

 Laboratory and Field Ecology.— 4) Murray, 1908, Int. Rev. Hydrob, 1, p. 10 ff. 



