20 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



the omnivorous raven ranges almost from pole to equator. The dis- 

 tribution of the green sea urchin coincides with that of the hydroids 

 which constitute its principal food. 31 The euphorbia sphinx would have 

 a much wider distribution if its caterpillar were not so strictly limited 

 to a single genus of plants ; it was unknown at Gottingen until Euphor- 

 bia was planted in the Botanical Garden, when it appeared at once. 32 

 Because the snail, Helix aspersa, is euryphagous it has been able to 

 accompany man over the greater part of the earth. The wide dis- 

 tribution of most predatory mammals may be attributed to their 

 euryphagy. 



Since most animals are unable to build up organic substances 

 directly from inorganic materials, they are primarily dependent upon 

 plants for food. Some plants are protected against being consumed by 

 animals. The mosses and ferns are little eaten; a few snails and insects 

 feed upon them, and a few birds may be driven to eat them in times 

 of famine. 33 Islands with a flora primarily of ferns accordingly have a 

 strikingly poor fauna. Most plants, however, are eaten by animals, 

 and they form the foundation of their food supply, in water as well 

 as on land. The flesh-eaters are thus indirectly dependent upon the 

 plant world, the lion that eats a calf equally with the fly that sucks 

 the calf's blood. 



Animals are not restricted in their distribution to the areas where 

 green plants are found. It is sufficient if organic matter reaches their 

 habitat; they may thus occur in caves, in the earth, in ground waters, 

 and in the depths of the sea. If sufficient organic matter (always, in 

 the last analysis, of plant origin) reaches these situations, animals 

 may be abundant in them. 



Ecological valence. — Animal life is not tied hard and fast to un- 

 alterable values of the conditioning factors. Each factor has a specific 

 range, lying between an upper and a lower limiting value. The ampli- 

 tude of the range of the conditions of life, within which an animal is 

 able to exist, may be designated as the ecological valence of the ani- 

 mal. When the limits for the greatest possible number of single factors 

 are widely separated, the species can live in various habitats; when 

 close together, it will be limited to one or a few types of environment. 

 The former have a large, the latter a small, ecological valence. The 

 snail, Limnaea truncatula, which is both eurythermal and euryhaline, 

 or the tiger, which is eurythermal and euryphagous, are such adaptable 

 forms. In contrast with these the oleander sphinx, which is stenother- 

 mal and stenophagous, is limited in habitat. Species which can adapt 

 themselves to varied environments can naturally be widely distributed. 

 A few extreme cases are ubiquitous. Habitat-limited species, on the 



