DISTRIBUTION OF ANIMALS. 310 DISTRIBUTION OF ANIMALS. consoqueiioe of which its lowly or<»:iiiizocl fnuna of Edeiitiitfs were enabled to nttain -iueli a re- markahle development in the later geological IH'riods. ri)n>*iilt Wallace, Island Life (Xew York. 1S80). See Isolation. "The above instances [and their converse, ex- emplified by the identity of insular and conti- nental faunas where a land union is known to have existed lately] arc sullicient to show how largely the prcs<'nt distribution of manunalian Hie is boind up with the past his- tory of our globe. We nuist, however, not omit to mention another very important agency of past times which has likewise had great influ- ence on the present distribution of the various faunas of the Xorthern Hemisphere. This is the so-called Glacial EjKicli, which took place im- mediately before the establishment of the present state of things, and appears to have been tlie cause of the extinction of many of the larger manimaiian types which formerly inhabited Europe." l!ut while many species have had a far wider distribution in past ages than now (and in a general way a widely distributed form may be regarded as an old form), many species seem never to have expanded much, some being limited to a short stretch of coast, or a single river- valley, or to a certain mountain or island. Deserts, Moi".nt.ins, etc., as Barriers. Other barriers than the sea. therefore, have been ef- fective in impeding the spread of crescent varieties, species, and groui)s. Most prominent among these is a desert region. The Sahara cuts ofT almost completely the fauna of Europe from that of Africa; and the continuation of the arid treeless area northeastward across Asia divides the northern Asiatic animals from the southern in a most emphatic way. Similarly, the fama of Xorth .merica stops and that of the southern continent begins where the ]ilains and tablelands of Arizona and northern Jlexico interpose a waterless, scantily planted space of .semi-desert and sterile hills, uninviting to either set of animals as a whole, yet invaded by both. Long and lofty mountain ranges stand next in importance, probably, as physical barriers, but this is more marked in the warmer than in the cooler parts of the world, since animals accus- tomed to the torrid conditions of tropical plains cannot or will not endure the cold and lack of customary food encountered even on the passes, and so do not cross over the highlands. The moiuilains of Xorth .merica seem to make less diHercnce with the distrilnition of our ani- mals, as between the .tlantic and Pacific sloi>c<. than does the central-plains area; and that of Europe has been little afTected by the presence of moimtains. The .Atlas range is more marked as a boimdary, but that is l)eoause of the neigh- boring desert: and similarly the great Central- Asian ranges are part of the lofty, cold, and arid region which as a whole forms probably the most efTective inland harrier in the world. Deep and broad rivers or straits of the sea may interrupt the siiread of many species. The Hudson seems nivir to have been crossed by the opossum, which could neither get over nor aroimd it. The Mi^si'.isippi put an end to the eastward progress of the jainiar alonu' the r!ilf region, and much smaller rivers might stop many other creatures, such as mcmkevs (which are unable to swim) : yet none would prove a barrier to flying j:>r amphibious animals, and many serve to" ex- tend hundreds of miles the range of aquatic and semi-aquatic ones. Frequently, however, neither these nor various other 'barriers' that might be mentioned seem to have prevented the growth of species which nevertheless are scant in numbers and extreme- ly restricted in area of range. The blaubok (q.v. ) of South .frica was unknown beyond a single valley, and has almost become cMinct. This is a sharp cxami)le of many i)uzzling cases where probably the restraining influ- ence has been competition. Each locality sup- plies food, water, and shelter for only "a cer- tain quota of aninuils. The ablot ami best- litted in each class for that region will get the most out of it and will increase and spread. The 'struggle for existence' is everywhere a real daily fight for space and subsistence, and the weakest will gradually sviccumb or become prisoners within areas so jieculiarly favorable to them that there they are able to hold their own. The powerful and gregarious cattle long ago forced the sheep to keep 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 uncrossablc for small qua(lru])eds that otherwise might soon pass over it. The faunal distinctions between forest and prairie, mountain and plain, diurnal and nocturnal methods, are efi'ects of enmity as well ns of competition. Climate .v.nd Altitude as Factors. Of cli- mate as a determinant in geographical distribu- tion, perhaps too nuich has been made in the past. Climate, within its extremes, seems inthicntial upon animal life mainly as cold or prevailing drought afTccts the higlier forms of vegetation (see DlsTRiniTiON OF Plants), and con.sequcnily the food of herbivorous and insectivorous animals. Such species as c.in (ake a varied fare, and can by migration, storage of food, or hibernation, escape or provide against storm and .scarcity, defy climatic bounds. The big i^ats and bears range from torrid jun.ffles to snowy mountain- tops and subarctic latitudes. Transplanted spe- cies frequently llourish in climates the opposite of that to which the.v are native. In a general way, nevertheless, similar animals are fouml in similar climates, and within the larger re- gions lesser faimal divisions often conform closely to isothermal belts, a fact more noticeable in Xorth America than anywhere else. Here. too. comes in the so-calleil 'vertical' distribution of life observable in high mountains, 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 mountain ranges Iving north and south carrv far toward the equator along their cool ridges spwies which exist near sea-level imlv in high latitudes. Our bighorn, once numerous along (be Kockies south to Xew Mexico, is an example of this feature: and the guanaco of Patagonia, following the . dcs north to the equator in Ecuador, is another. Terrestrial life is everywhere most plentiful near sealevel and in warm regions, and becomes more scanty as the i)olcs are approached or mountains are ascended. Mountain-tops, however, frequently
