28 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



values, as in high mountains, toward the poles, in steppes, or in 

 deserts. The number of species of insects in India is 29,700 ; n Green- 

 land (half as large) has only 437. 1 - The number of species of animals 

 decreases steadily with increasing altitude on the mountains. The size 

 of species which range from the lowlands to the tops of mountains 

 often decreases with altitude. In the tropics and subtropics the average 

 size of terrestrial animals is larger than in the temperate and cold 

 zones (warm-blooded animals form an exception, cf . Chapter XX) . 

 This relation applies to ancient as well as to modern groups, whether 

 of insects, myriapods, arachnids, snails, amphibians, or reptiles. 13 

 The extinct insect fauna of the Pennsylvanian coal, in a tropical 

 climate, was composed of giants. 



Caves, on account of the absence of light and plant food, and 

 their relatively low temperature, support only a restricted fauna, poor 

 in species and individuals which are reduced in size. They live by 

 eating flesh or fungi, or matter originating outside the caves. 



In some places the selective action of the environment leaves 

 species with special powers of adaptation unchanged in their new 

 habitat, or changes them only to a slight degree, into dwarfed forms 

 or varieties. For the most part, however, the inhabitants of unfavor- 

 able environments are so changed by their adaptations, both physio- 

 logically and structurally, that they are recognized as new species or 

 even new genera and families. The influence of the environment upon 

 the transformation of species is an extremely effective one. This 

 factor does not seem to have been the cause of the development of 

 the primary divisions of the animal kingdom which apparently devel- 

 oped in the same rather uniform environment, i.e., in the sea. The 

 three principal groups of terrestrial animals, the mollusks, arthropods, 

 and vertebrates, all have representatives in the sea. 



The number of species, however, seems to depend on the variety 

 of habitat conditions and on the adaptations required by them. The 

 herring of the coast, which is subjected to diverse habitat conditions, 

 is much more variable than the oceanic herring which keeps to the 

 open sea except for spawning. The fauna of Africa south of the Sahara, 

 on account of the relative uniformity of the physical conditions, is 

 much less varied than would be expected for so large an area. By way 

 of contrast, Formosa, with its north-south axial mountain range, 4000 

 meters in height, with its wooded mountain slopes and ravines, and 

 its coastal plains, is remarkable for the range of its climatic conditions 

 and for a fauna rich in number of species. According to Wallace 14 it 

 has almost as many species of birds (128:165) and mammals (35:40) 

 as Japan. 



