610 DYNAMICAL GEOLOGY. 



America, owing to its extent in latitude, from the Arctic circle to the 

 hot tropics, is remarkable for its very wide extremes. The severe 

 cold of winter passes over the land to the far south, destroying what- 

 ever cannot stand its power ; and the summer's intense heat sweeps 

 back again, with a similar effect ; so that the continent cannot grow 

 as many kinds of terrestrial plants or animals as that on the opposite 

 side of the Atlantic. 



2. Continental idiosyncrasies, or peculiarities that cannot be referred 

 to climate. Each continent has its characteristic types of plants and 

 animals. The Marsupials, in Australia, and Edentates or Sloth tribe, 

 in South America, are examples ; the sedate Platyrrhine Monkeys, 

 in South America, and the nimble frolicsome Catarrhines, in Africa, 

 are others ; so also the abundance of Humming-birds in the Occident, 

 and their absence in the Orient. Examples might be mentioned in- 

 definitely. Moreover, the range of animal life, or that of vegetable 

 life, has often a continental feature. 



3. Diversities of soil. — Some plants require wet soil, others mode- 

 rately dry, others arid ; some rich, others sandy, others a surface of 

 rock ; some the presence of limestone, others of rocks containing 

 silica, etc. ; some the presence of salt, or a salt marsh. 



B. The distribution of aquatic species is determined — 1. By the 

 character of the water, whether fresh, brackish, or salt, pure or impure 

 from mixed sediment ; and but few species adapted for one condition 

 survive in the other. Hence, changing a salt lake to a fresh one, or 

 even making an addition of fresh waters which exceeds much the 

 amount lost by evaporation (and the reverse), will dwindle or destroy 

 the living species. Most reef-forming corals grow in the purest 

 ocean waters, where sediments make no encroachments ; a few, in- 

 cluding some of the Porites, survive where there is much sediment. 



The Aral and Caspian probably made formerly one great salt sea: owing to the 

 rivers that enter them, the living species are few. The shells are now of but twelve 

 species, and mainly of the Cardium family, with Mytilus edulis and a Dreissena 

 (Mytilus family) ; and only two are quoted from the Aral, — Cardium edule and Adacna 

 (Cardium) vitrea. The Cardium and Mytilus families are hence capable of enduring 

 very wide extremes in the saline condition of the waters. It is interesting to note that 

 the earliest of American bivalves (Acephals) were of the Cardium family (genus Cono- 

 cardium); and the Mytilus family was but little later in introduction. 



Certain species are confined to excessively saline waters. Artemice (Crustaceans) are 

 found in the salt and alkaline lakes of all the continents. The larves under several 

 genera of Dipterous insects are other examples. 



2. By temperature. — The reef-forming corals grow in the warmer 

 ocean-waters, in which the mean temperature for the coldest month 

 does not fall below 68° F. The limit in depth also appears to depend 

 mainly on temperature. 



The currents of the ocean distribute temperature through it ; and, 



