LIFE. 607 



perpetual snow. Again, the amount of moisture for which a spe- 

 cies is made determines its position in either a moist or an arid 

 region. 



Each continent has its own characteristic climate, arising mainly 

 out of its special combination of these two elements, temperature 

 and moisture ; and this is one source of the great diversity of life 

 among the continents. Another point in which the climate of 

 continents differs is the limit of extreme heat and cold. For ex- 

 ample, North America, owing to the extent of its range from the 

 Arctic circle to the hot tropics, is remarkable for its very wide ex- 

 tremes. The severe cold of winter passes over the land to the far 

 south, destroying whatever cannot stand its power, and the sum- 

 mer'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 Mon- 

 keys (p. 422) in South America, and the nimble frolicsome Catar- 

 rhines in Africa, are others ; so also the abundance of Humming- 

 birds in the Occident and their absence in the Orient. Examples 

 might be mentioned indefinitely. 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 salt or a salt marsh. 



The distribution of water-species is determined — First, by the cha- 

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

 from mixed sediment ; and but few species adapted for one condi- 

 tion 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 dwin- 

 dle or destroy the living species. 



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 waters. 

 It is interesting to note that the earliest of American bivalves (Acephals) was 

 of the Cardium family (genus Conocardium), and the Mytilus family was but 

 little later in introduction. 



