Page - 395 - in The Origin of Species

CHAPTER XII Geographical Distribution Present distribution cannot be accounted for by differences in physi- cal conditions—Importance of barriers—Affinity of the produc- tions of the same continent—Centres of creation—Means of dispersal, by changes of climate and of the level of the land, and by occasional means—Dispersal during the Glacial period—- Alternate Glacial periods in the North and South. IN considering 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 wholly accounted for by climatal and other physical conditions. Of late, almost every author who has studied the subject has come to this conclu- sion. The case of America alone would almost suffice to prove its truth; for if we exclude the arctic and northern temperate parts, all authors agree that one of the most fun- damental divisions in geographical distribution is that be- tween the New and Old Worlds; yet if we travel over the vast American continent, from the central parts of the United States to its extreme southern point, we meet with the most diversified conditions ; humid districts, arid deserts, lofty mountains, grassy plains, forests, marshes, lakes, and great rivers, under almost every temperature. There is hardly a climate or condition in the Old World which can- not be paralleled in the New—at least as closely as the same species generally require. No doubt small areas can be pointed out in the Old World hotter than any in the New World; but these are not inhabited by a fauna different from that of the surrounding districts; for it is rare to find a group of organisms confined to a small area, of which the con- ditions are peculiar in only a slight degree. Notwithstand- ing this general parallelism in the conditions of the Old and New Worlds, how widely different are their living pro- ductions ! 395