BIOTOPES AND BIOCOENOSES 145 



other forms; as the unfavorable conditions of this habitat prevent 

 pursuit, they may be present in such numbers as to color broad areas 

 of the bottom in shallow water a deep red. Other examples are cited 

 above. 



The available food accordingly determines the population density, 

 but whether the fauna will be uniform or varied depends upon other 

 factors. Optimum conditions favor speciation, as abundant food favors 

 variation among the domestic animals. Newly developed forms will be 

 more likely to be preserved under those conditions than where the 

 selection is more severe. Variety in the plant world also favors variety 

 of animal life. 



In the temperate and colder regions active life is concentrated into 

 a certain season, which may be short. The development of organisms 

 is slow, and often, as in insects, may extend over several years. The 

 resting stages, necessitated by the winter, interrupt the active life of 

 the great majority of animals, and probably increase the longevity 

 of the individual. The European may beetle (Melolontha vulgaris) 

 has a three-year generation south of the Main, a four-year period in 

 north Germany, and in east Prussia its development takes five years. 11 

 It is quite otherwise in the tropics. In favorable regions development 

 is scarcely or not at all interrupted. Generation follows generation; the 

 high temperatures hasten the development of the poikilothermal forms, 

 and in birds and mammals as well as in other vertebrates the breeding 

 period is often not restricted to any season of the year. Here life 

 pulsates with much more rapid beats, and even if death comes earlier, 

 as in an insect whose life is closed with the conclusion of egg laying, 

 those that die only make room for the new generation which follows. 



BIBLIOGRAPHY 



1) Ratzel, 1901, Der Lebensraum — 2) Gams, 1918, Vierteljschr. natf. Ges. 

 Zurich, 63, p. 293-493.-3) Mobius, 1877, Die Auster, etc.; Dahl, 1909, Zool. Anz., 

 33, p. 349-353; Thienemann, 1918, Natw. Wschr., (NF.) 17, p. 282 & 297.-4) 

 Kjellens, 1914, Die Grossmachte der Erde, 3 ed., p. 9.-5) Doflein, 1910, in 

 Hesse & Doflein, Tierbau und Tierleben, 2, p. 20.— 6) Shackleford, 1929, Ecology, 

 10, p. 124-166.— 7) Seitz, 1890, Zool. Jb., Syst., 5, p. 301.— 8) Mobius, 1877, 

 op. cit., p. 83. — 9) Michaelsen, 1903. Geogr. Verbr. Oligochaeten, p. 5 ff. — 

 10) Ehrenbaum, Seemuscheln als Nahrungsmittel, p. 2. — 11) Judeich & 

 Nitsche, 1895, Forstinsektenkunde, 1, p. 298. 



