INFLUENCE OF EXTENT OF RANGE 131 



nated in Ceylon by man. It becomes evident that only small animals 

 have been able to maintain themselves as relicts of the ice age or of 

 the steppe period in central Europe, since the available areas are too 

 small to support larger forms, like the musk ox, snow hare, and 

 ptarmigan. The relicts range from the size of the hamster down, and 

 are for the most part much smaller. Thus trout, the turbellarians 

 Planaria alpina and Polycelis cornuta, and the spring snail Bithynella 

 are relicts of the ice age, and some xeric snails and grasshoppers are 

 relicts of the steppes. Engelhardt 31 explains the absence of coastal 

 species of sharks from the coasts of the small Pacific islands, the 

 Hawaiian Islands, for example, on this ground, i.e., that the available 

 space is not sufficient to maintain them. The introduction and self- 

 extermination of rabbits on Laysan and Lisiansky Islands, described 

 by Wetmore, 32 is an instructive phenomenon in this connection. 



The extent of the space available has a direct effect upon the con- 

 stitution of its inhabitants. Experiments have shown that the individ- 

 uals of a species reach different sizes varying with the amount of 

 space given them. In the original experiments of Semper, 33 made with 

 Limnaea stagnalis, the specimens grown in 100, 200, 600, and 2000 cc. 

 of water attained a shell length of respectively 5, 9, 12, and 18 mm., 

 in the same length of time, and if larger numbers of specimens were 

 kept in the same container, their size remained smaller than when 

 there were fewer. The results of numerous recent researches in this 

 field which confirm and extend those of Semper have been comprehen- 

 sively reviewed by Allee, 34 who discusses at length the different causal 

 factors involved. 



Although the causes at the bottom of this relation between size of 

 animal and range are not yet clear, a large number of facts may be 

 brought together from this standpoint. Thus the size of the pelagic 

 copepods of the genus Diaptomus depends on the size of the bodies of 

 water they inhabit. 35 Fresh-water mussels {Unio, Anodonta) have 

 smaller individuals in brooks than the same species in rivers. 36 Trout 

 from the Aar averaged 240 grams, those from its affluents 132 grams. 37 

 The whitefish {Coregonus) from the small lakes in Switzerland are 

 dwarfs as compared with those from the Lake of Zurich. 38 The smaller 

 size of most of the animals of the Baltic and Mediterranean compared 

 with those of the Atlantic probably does not fall into this category, 

 the physical conditions in these basins probably being the determining 

 factors. It is probably true, however, that the frequent presence of 

 dwarfed forms of animals on small islands depends on space relation 

 acting primarily through reduced food (cf. Chapter XXVI). Dwarfing 

 in consequence of reduced range may also be seen in the numerous 



