30 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



animal to flourish under changing conditions which are unfavorable for 

 unadapted animals. Low temperatures are necessary for stenothermal 

 cold-tolerant animals like Planaria alpina or the trout. There are also 

 broader adaptations resulting from increased bodily resistance, such 

 as eurythermy and euryhalinity. Nereis diversicolor persists not only 

 in very saline waters, but also in waters almost fresh, and Limnaea 

 truncatula occurs both in glacial streams and in hot springs. 



Number of species and of individuals.— However the adaptation 

 to adverse conditions may have been reached, the number of com- 

 petitors for the food supply will be reduced in the new environment, 

 and when a sufficient food supply is available the numbers of indi- 

 viduals of the few adapted species may become enormous. Thus in 

 regions equally well supplied with food, of which one has other 

 environmental conditions favorable and the other unfavorable, the 

 numbers of individuals of the species will be in inverse proportion to 

 the number of the species present. The brackish-water fauna is char- 

 acterized by the presence of few species, often of reduced size, but 

 with enormous numbers of individuals. 15 An intensively and scien- 

 tifically managed fish pond, which is annually drained, plowed, 

 manured, and freed of large plants, is unproductive for a collector, 

 for whom a neglected pond is a rich source of supply of invertebrate 

 forms. For the fish culturist, the unproductiveness in species is more 

 than compensated by the enormous number of individuals of the few 

 flagellates, rotifers, cladocerans, and cyclops which do persist. In salt 

 seas inhabited by Artemia salina, this crustacean is often the only ani- 

 mal of considerable size, perhaps accompanied only by a few dipterous 

 larvae. The number of individuals in such waters may be so great 

 that the water appears to be a thick broth of Artemia. In the tropics 

 the favorable conditions (combined with the intensity of the competi- 

 tive struggle) permit almost unlimited speciation, but the individuals 

 of each species are for the most part not abundant, Wallace 16 col- 

 lected 158 specimens of moths in one night in Borneo, which belonged 

 to 120 different species. Koningsberger 17 states that in Java, collecting 

 on the flowers and shrubs of cultivated districts, 100 species of spiders 

 with one specimen each are more easily secured than 100 specimens of 

 a single species. Whitehead 18 never saw the well-known birds of Borneo 

 in numbers, and secured only one specimen of many species, but the 

 number of Bornean bird species, 580, is almost equal to that of Europe, 

 13 times as large (658 species) . The countless numbers of bison which 

 dominated the plains of North America may be compared with the 

 great numbers of species of antelopes on the savannas of South Africa, 

 each of which is represented by smaller numbers of individuals. 



