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Show CHAPTER XII TI-m GEOGRAPHICAL DISTRIBUTION OF ORGAN! 'MS The facts to be cxplaiueu-'l'hc conditions wltich have determined distribution- The permauencc of oceans-Oceanic allll continental areas -Madacrascar and New Zealand-The teachings of the thousandfath om (')line- The distribution of marsupials- The distribution or tapir. - Powers of ui. pcrsal as illustrated by insular orgauisms- Birtls and insects at sea- Insects at great altitmlcs- Th eli ·pcrsal of plants -Dispersal of seeds by the wiml-Mincral matter arriecl by the wind - Objections to the theory of wind-dispersal aus11·crcu-Explanalion of north temperate plants in the southern hemisphere- No proof of glaciation in th' tropies-L·1wcr temperature not needed to explain the facts- Concluding remarks. TnE theory which we may now take as established-that n-11 the exi. ting forms of life have been derived from other form~ by a natural process of de. cent with modification, and that this same proce s has been in action during past geological time-should enable us to give a rational account not only of the peculiarities of form and structure presented by animal:-; and plant. , but also of their grouping together in ccrtn,in areas, and their general distribution over the earth's snrfa,ce. In the absence of any exact knowledge of the fad:; of distribution, a student of the theory of evolution might naturally anticipate that all groups of allied organisms wonld he found in the same region, and that, as he tnwellcd farLher and farth er from any given centre, the forms of life would differ more and more from those which prevailed at the starting-point, till, in the remotest regions to which he conld penetrate, he would find an entirely new a sembln.gc of animals and plants, altogether unlike those with which he was cu. XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 339 familiar. He would also anticipate that diversities of climate would always be associated with a corresponding diversity in the forms of life. Now these anticipations arc to a considerable extent justifie~. Rcmo.tene~s o~ the earth's surface is usually an indicatJOn of d1_vers1ty m the fauna and flora, while strongly contrasted climates are always accompanied by a considerable contrast in the forms of l_ife. But this correspondence is by no means exa_ct or proport10nate, and the converse propositions arc often qmte untrue. Countries which are ncar to each othc~· often di_ffer ~-a~ica_lly in th_eir animal and vegetable productJOns; wh1le s1m1lanty of chmate, together with moderate g?ogr~p_hic~l proximit;y,_ are often accompanied by marked diVcrs1t1es m t~e preva1lmg forms of life. Again, while many groups of ammals-gcnera, families, and sometimes even orders-are confined to limited regions, most of the families, many genera, and even son:e species arc found in every part of the earth. An enumeratiOn of a few of these anomalies will better illustrate the nature of the problem we have to solve. As_ exampl~s _of extreme di vcrsity, notwithstanding geographiCal_ proximity, we may adduce Madagascar and Africa, whose ammal and vegetable productions are far less alike than are those of Great Britain and Japan at the remotest extremities of the ~reat northern continent; while an equal, or perhaps even a st1ll greater, diversity exists between Australia and New Zealand. On the other hand, Northern Africa and South Europe, though separated by the Mediterranean Sea have faunas and floras which do not differ from each othe; more ~h~n ~o the v~rious countries of Europe. As a proof that s1m1lanty of chmate and general adaptability have had but a small part in determining the forms of life in each country, we have the fact of the enormous increase of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the common sparrow in North America, though in none of these cases are the animals natives of the countries in which they thrive so well. And lastly, in illustration of the fact that allied forms are not. always found in adjacent regions, we have the tapirs, whiCh arc found only on opposite sides of the globe, in tropical America and the Malayan Islands ; the camels of |