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Show 352 DARWINISM CHAP. remained almost com1 lotely isolated ; and, being free from tho competition of higher forms, they have developed into tho groat variety of types we now behold thoro. These occupy tho place, and have to some extent acquired the form and strncturc of eli tinct orders of the higher mammals-the rodents, tho insectivora, and the carnivora,-while still proscrvi11g tho cs ential characteristics and lowly organisation of the marsupials. At a mnch later period-proba.bly in late Tertiary times-the ancestors of the various species of rats and mice which now a.bound in Australia, and which, with the aerial bats, constitute its only forms of placental mammals, entered tho country from some of the adjacent islands. For this purpose a land connection was not necessary, as the e small creatmc:-; might easily be conveyed among the branches or in the crevice:-; of trees uprooted by floods and carried down to the sea, and then floated to a shore many miles distant. That no actuallawl connection with, or very close approximation to, an A:i:tti<' island has occurred in recent times, is sufficiently proved h) the fact that no squirrel, pig, civet, or other wide. proad mammal of the Eastern hemisphere has been able to reach tho Australian continent. The Distt·ibution of 1'api1's. T?es~ cu~·ious a~1imals form one of tho puzzles of gcogmphical dJstrJbutwn, bemg now confined to two very remote rc()·ions of the globe-the Malay Peninsula and adjacent islands of Suma~ra and Borneo, inhabited by· one species, and tropic:tl Ame:ICa, where there are three or four species, ranging from Brazil to Ecuador and Guatemala. If we considered thc::;e living forms only, we should be obliged to specubte on enormous changes of land and sea in order that these tropic:d animals might have passed from one country to the other. But geological di coveries have rendered all such hypotheti cal changes unneccs ary. During Miocene and Pliocene times tapir~ abom~ded over the whole of Europe and Asia, their rem.ams ha.vmg been found in the tertiary deposits of Fnmec, India, Burmah, and China. In both North and So11th America fossil remains of tapirs . occur only in caves mHl deposits of Post-Pliocene age, showing that they are comp<U'<.tr tively recent immigrants into that continent. They perhaps xu GEOGRAPHICAL DI~TRIBUTION OF ORGANISMS 353 ·Cr~torod by the route of Kamchatka and Alaska, where the dtmato, even now so much milder and more equable than on the nor.th-oast ~f America, might have been warm enough in ht? Phocone tim?s to have allowed the migration of these ammal~.. In Asm they wore driven southwards by the competitiOn of numerous higher and more powerful forms hut have found a last resting-place in the swampy forests of the Malay region. !Vhat these Facts P1'ove. N.ow these. two cases, of the marsupials and the tapirs, are m th~ highest degree instructive, because they show _us that, without any hypothetical bridO'ing of deep oceans and with only such changes of sea a;d land as arc indi~ -cated b~ the extent of the comparatively shallow seas snrroundmg and connecting the existing continents, we are able to account for the anomaly of allied forms occurrinO' only in ren:ote and ~idely separated areas. These example~ really co~st1tute crucml tests, because, of all classes of animals, mamma~Ia are least able to surmount physical barriers. They are obviOusly unable to pass over wide arms of the sea while the necessity for constant supplies of food and watc; renders sandy deserts or snow-clad plains equally impassable. Then, again, the peculiar kinds of food on which alone many of them can sub ist, and their liability to the at~ack~ of other animaL , put a further chock upon their migratiOns. In these respects almost all other organisms have great advantages over mammals. Birds can often fly long di~tances, and ~an thus cross arms of the sea, deserts, or mou.ntam rang~s; msects not only fly, but arc frequently earned great distances by gales of wind, as shown by the numerous cases of their visits to ships hundreds of miles from la~. Reptiles, th?ugh slow of movement, have advantages in their greater capacity for cndurin()' hnncrer or thirst their !)Ower f . . b b ' ·O resistmg cold or drought in a state of torpidity, and they have ~lso some f~cilities for migration across the ·oa by means Qf then· eggs, wh1eh may be conveyed in crevices of timber or among masses of floating vegotn l1le matter. And ·when we come .to the. vegetable kingdom, the means of transport are at thmr maximum, nnmbers of seeds having special adaptations 2 A |