ENVIRONMENTAL SELECTION 27 



of rotifers in Galicia, 100 may be found in bog waters. Admixture of 

 hydrogen sulphide, as in Ritom Lake, Canton Tessin, reduces the 

 number of species. Lakes very rich in iron, in New Caledonia, are 

 inhabited by a fauna dwarfed by the severe habitat conditions; tiny 

 crustaceans, small snails, and worms. 9 



Environmental selection of land animals. — Terrestrial animal 

 life is also impoverished as compared with marine life, not in number 

 of species or genera, and certainly not in the numbers of individuals 

 of special forms, but in types of organization. The number of ter- 

 restrial species greatly exceeds those living in water. The great major- 

 ity of insects are terrestrial, and these alone compose three-fourths of 

 the known species of animals (600,000 out of a total of 800,000 de- 

 scribed forms*). The myriapods, arachnids, land snails, land crus- 

 taceans, and the reptiles, birds, and mammals, in addition, raise the 

 number of land animals probably to at least four-fifths of the total. 

 In spite of this fact, terrestrial animal life exhibits an impoverishment 

 in wealth and variety of structural types. Not one of the animal 

 phyla is absent in the ocean (unless the Gastrotricha be regarded as 

 of this rank), while protozoans, sponges, coelenterates, ctenophores, 

 nemertians, bryozoans, and echinoderms, and many subphyla, are 

 entirely absent among the air-breathers. A still greater number of 

 classes are absent, whereas the only prominent classes entirely absent 

 in the sea are the Onychophora (Peripatus), myriapods, and amphib- 

 ians. Comparing groups for numbers of species, and disregarding 

 rank, the differences between the 175,250 or more species of beetles, 

 or the more than 60,000 species of Lepidoptera, are almost negligible 

 in comparison with the astonishing morphological differentiation of 

 the 4000 species of coelenterates or echinoderms. Even the entire class 

 of insects, with 600,000 species, does not exhibit such great differences 

 in structure and appearance as the coelenterates with their hydroid 

 polyps, medusae, sea anemones, corals, etc. 



It is the general rule for air-breathers that their development is 

 favored by sufficient humidity, relatively constant temperature, and 

 abundance of light and food. These conditions are found in optimum 

 combination in the openings in tropical forests like those of the 

 Amazon, the Congo, or of New Guinea. Decrease of moisture and tem- 

 perature, and especially great fluctuations of these conditions, con- 

 stantly demand adaptation. As in aquatic habitats, animal life is least 

 abundant on land where the habitat conditions approach the limiting 



* Independent estimates by L. O. Howard and A. E. Emerson; see also Pratt, 

 1935. 10 



