GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 48 1 



3. Galapagos. — These islands, off the west coast of 

 South America, strongly attracted the attention of Dar- 

 win during his voyage on the Beagle on account of the 

 singularity of their fauna. They have been visited by 

 many other naturalists for the same reason. There are 

 no mammals or amphibians, and the species of other 

 groups are all peculiar. Among them are found many 

 very large lizards and several species of gigantic land 

 tortoises. These islands are separated from South Amer- 

 ica by deep water. 



4. River Mussels. — We might multiply examples in- 

 definitely. We take one more. In the Altamaha River, 

 Georgia, among other shells found there is one, Unio 

 spinosus, with needlelike spines an inch and a half long. 

 This species is not found elsewhere on the face d£ the 

 earth. How did it get there ? 



In all these cases it would seem as if the groups of 

 animals and plants had been made and put there where 

 we find them, and are not found elsewhere because they 

 could not get away. 



* Marine Faunas. — Thus far we have spoken only 

 of land organisms, but the same laws are true in less 

 degree for marine species. 



Temperature Regions.— Here, also, of course, we 

 have temperature regions, and consequent gradations by 

 change of species as we go north or south. Here, also, 

 at certain places we may have more abrupt changes. 

 For example, on the east coast of the United States we 

 have two abrupt changes of coast fauna, one at Cape 

 Cod and the other at Cape Hatteras. Scarcely a single 

 species passes from north to south of these points, or 

 vice versa. The cause is found in the marine currents off 

 the coast. The warm Gulf waters coming through the 

 straits of Florida hug the coast and warm the shore 

 waters as far as Cape Hatteras (a, Fig. 336), and then 



