130 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



for an elephant, and that a flying insect or an antelope will demand a 

 wider space in which to move than a snail or a sloth respectively of 

 similar size. The carnivores will require a larger area per individual 

 than the herbivores, on account of their dependence upon them; their 

 number must be much fewer than that of their prey or they would die 

 of starvation. Among many species of carnivorous animals the indi- 

 viduals or the pairs of individuals have a special area which they 

 regard as their own and within which they do not tolerate the presence 

 of another animal of the same species. This is true of bears, foxes, 

 moles, eagles, kingfishers (Alcedo) , and dippers (Cinclus), and even 

 the centipede Lithobius is usually found singly under a given stone, 

 in contrast with the herbivorous milliped Julus. A minimum number 

 of pairs, however, is essential to preserve the species from inbreeding 

 and from "accidental" extermination by adverse conditions, and this 

 number, multiplied by the area required by an individual pair, would 

 express the minimum range of the species. 



Effects of space limitation. — It is very likely that other factors 

 affect the problem of minimum range. The smallest bodies of water, 

 springs, ditches, and puddles, harbor only small mollusks, such as 

 the spring snail Bithynella, the small clam Pisidium, etc. Some small 

 mollusks, Limnaea truncatula, Planorbis rotundatus, and Pisidium 

 certainly are more frequently found in small bodies of water than in 

 large. Perhaps the small ponds provide a refuge from larger enemies 

 or competitors for which the area in question would be too small. Thus 

 the Panamanian caiman is driven from the larger bodies of water by 

 the crocodile. Also we may be dealing again with a direct effect of 

 size of habitat upon growth size. On the other hand, the larger fresh- 

 water fishes occur only in the larger bodies of water, such as the North 

 American Aplodinotus grunniens (about 25 kg.) in the Great Lakes 

 and the Mississippi, the arapaima {Arapaima gigas) in the Amazon, 

 and the catfish (Silurus glanis) in the larger rivers and lakes of Europe. 

 It thus appears obvious that larger animals in general require a wider 

 range than do the smaller related animals, although it must be re- 

 membered that the widest known ranges are those of the protozoans. 



Large animals, in consequence, are not to be expected on small 

 islands, though if introduced they may be able to maintain themselves 

 for a time, like the cattle which were landed on New Amsterdam 

 (66 sq. km.). 30 No exact definition of the size of islands which can 

 maintain a given species is possible. The tiger is absent from Ceylon, 

 though present on the mainland opposite and present on the much 

 smaller island of Bali (about 5000 sq. km.). The size of Ceylon cannot 

 therefore be the determining factor; the tiger may have been extermi- 



