82 



ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



The starfishes of the genus Pentaceraster, ranging through the 

 entire Indian and Pacific oceans, present a great variety of form, but 

 so many transitional and intermediate types are present that from a 

 conservative standpoint they all belong to a single species, P. mami- 

 latus. 31 The same phenomenon is true for some sponges, whose vagility 

 is slight and for the most part confined to a short larval period. Thus 

 thirty-two supposed species of Spirastrella, a widespread genus in the 

 Atlantic, Pacific, and Indian oceans, have been united into a single 

 protean species by the demonstration of transitional forms. 32 Reef 



Fig. 5. — Outline, average size, and shell thickness (heavy contour = thick 

 shell) of five forms of Anodonta cygnea at their centers of distribution: a, forma 

 typica; b, var. cellensis; c, var. piscinalis; d, var. anatina; e, var. lacustnna. After 

 Buchner. 



corals are famous for their diversification and adaptability, and their 

 environmental conditions have such great variation that each indi- 

 vidual branch may be subject to special conditions, and there are 

 almost as many modifications as branches. 33 



Geographic isolation in inland waters. — Geographic isolation of 

 small areas is especially characteristic of inland waters. A lake, a 

 pond, a swamp, or a pool is surrounded by land as an island is by 

 water. The great majority of inland lakes are small; less than 30 

 exceed 1000 sq. km. in extent, and many are very small. Rivers are 

 separated by land at their sources, by the sea at their mouths, equally 

 effective barriers for their fresh-water inhabitants. The environmental 

 conditions, especially in standing waters, are extremely variable: the 

 daily and seasonal temperature range; the dissolved substances, or- 

 ganic and inorganic; the kind of bottom; the vegetation of the shores; 

 all combine to give an individual character to almost every body of 

 water, in sharp contrast with the uniformity of marine conditions. 



