Deep and broad rivers or straits of the sea may interrupt the spread of many species- The Iltolson seems never to have been crossed by the opossum, which enuld neither get over nor around it. The :\lississippi put 211 (9111 to the eastward progress of the jaguar along the (lull region, and nun+ smaller ricers might slop many other creatures, such as monkeys (which are unable to swim) : yet none would prove a harrier 1.1 flying amphibians animals, and many scree 10 ex tend hundreds of miles the range of aquatic and semi-aquatic ones.
Frequently. however, neither the nor various ether 'barriers' that might be mentioned scent to have prevented the growth of species which nevertheless are scant in numhers and extreme ly restricted in area of range. The blaullok (q.v.) of South .\ frica was unknown beyond a single valley, and has almost become extinct. This is a sharp example of many puzzling eases where probably the restraining milli .nee has been competition. Each locality sup plies food. water, and shelter for only a cer tain quota of animals. The ablest and best fitted in each class for that region will get the most out of it and will increase and spread. The trngglc for existence' is everywhere a real daily light for space and subsistence, and the weakest will gradually succumb or become prisoners within so peculiarly favorable to them that there they are able to hold their own. The powerful and gregarious cattle long ago forced the sheep to 1:eep to the mountain pastures. Active enemies must also be considered. certain animals not being able to exist in the same ter ritory. A river infested with crocodiles might forever prove uncrossahle for small quadrupeds that otherwise might soon pass over it. The faunal distinctions between forest and prairie, mountain and plain, diurnal and nocturnal methods, are effects of enmity as well as of •ompet it ion.
ANn AS FAc"rotis. Of cli mate as a determinant in geographical distribu tion, perhaps too much has been made in the past.
Climate, within its extremes, seeins influential upon animal life mainly as cold or prevailing alrects the higher forms of vegetation (see OF PLANTS ) and consequently the food of herbivorous and insectivorous animals, Snell specie, as can lake 41 Varied fare, and can by migration, storage of food, or hibernation, escape or provide against storm and scarcity. defy climatic bounds. The big eats and bears range front tot-rid jungles to snowy mountain tops ;Ind sultan:tic latitudes. Transplanted spe cies frequently flourish in climates the opposite of that to which they are native. In it general nevertheless, similar animals are found in similar and within the larger re gions lesser faunal divisions often conform closely to isothermal belts, a fact more noticeable in North America than anywhere else. Here, 100, in the so-called distribution of life observable in high ithroutains, where various altitudes exhibit an animal and plant life iden tical with or similar to that of northern lati tudes, where a corresponding average of tem perature prevails. Thus ranges lying north and south carry far toward the equator along their cool ridges species which exist near sea-level only in high latitudes. Our bighorn, once numerous along the Boekies south to New 'Mexico, is all example of this feature; and the guanaco of Patagonia. following the Andes north to the equator in Ecuador, is another. Terrestrial life is everywhere most plentiful near sea-level and in warm regions, and beeotnes more scant}' as j1011.- are approached or mountains are ascended. Mountain-tops, however, frequently form refuges for animals elsewhere k !WWII only in subarctic regions, which were left on their lofty and now isolated homes from the time when semi-glacial conditions prevailed over the whole distinct.