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Show 34G GEOGRAPHICAL DISTRIBUTION. CHAP. XI. CHAPTER XI. GEoG RAPHIOAL Drs'l'RinUTION. Present distribution cannot be accounted for by differences in physical conditions- Importance of barriers- Affinity of the productions of the same continent- Centres of creation- Means of dispersal, by changes of eli mate and of ~he level of .the la~d, and by occasional means- Dispersal durmg the Glac1al penod co-extensive with the world. IN considerina the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions. Of late, almost every author who has studied the subj~ct has come to this conclusion. The case of Amenca alone would almost suffice to prove its truth: for if we exclude the northern parts where the cii·cumpolar land is almost continuous, all authors agree tha.t one of the n1ost fundamental divisions in geographical distribution is that between the New and Old Worlds; yet if we travel over the vast American con tin en t, from the central parts of the lJnited States to its extreme southern point, we meet with the most diversified conditions; the most humid districts, arid deserts, lofty mountains, grassy plains, forests, marshes, lakes, an.d great rivers, under almost every temperature. Ther~ 1s hardly a climate or condition in the Old World whwh cannot be paralleled in the New-at least as closely as the same species generally require; for it is a most rare case to find a group of organisms confined to .any small spot, having conditions peculiar in only a shght CHAP. XI. GEOGRAPHICAL DISTRIBUTION. 347 degree; for instance, small areas in the Old World could be pointed out hotter than any in the New World, yet these are not inhabited by a peculiar fauna or flora. Notwithstanding this parallelism in the conditions of the Old and New Worlds, how widely different are their living productions! In the southern he;misphere, if we compare large tracts of land in Australia, South Africa, and western South America, between latitudes 25° and 35°, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or again we may compare the productions of South America south of lat. 35° with those north of 25°, which consequently inhabit a considerably different climate, and they will be found incomparably more closely related to each other, than they are to the productions of Australia or Africa under nearly the same climate. Analogous facts could be given with respect to the inhabitants of the sea. A second great fact which strikes us in our general review is, that barriers of any kind, or obstac.les to free migration, are related in a close and important manner to the differences between the productions of various regions. We see this in the great difference of nearly all the terrestrial productions of the New and Old Worlds, excepting in the northern parts, where the la~d almost joins, and ·where, under a slightly different chmate, there might have been free migration for the northern temperate forms, as there now is for the strictly arctic productions. We see the same fact in the great difference between the inhabitants of Australia, Africa, and South America under the same latitude: for these countries are almost as much isolated from each other as is possible. On each con tin en t, also, we see the same fact ; for on the opposite sides of |