§ 106. As implied at the outset, the character of a region, when unfavourable to any species, sufficiently accounts for the absence of this species; and thus its absence is not inconsistent with the hypothesis that each species was originally placed in the regions most favourable to it. But the absence of a species from regions that _are_ favourable to it cannot be thus accounted for. Were plants and animals localized wholly with reference to the fitness of their constitutions to surrounding conditions, we might expect Floras to be similar, and Faunas to be similar, where the conditions are similar; and we might expect dissimilarities among Floras and among Faunas, proportionate to the dissimilarities of their conditions. But we do not find such anticipations verified. Mr. Darwin says that "in the Southern hemisphere, 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." Still more striking are the contrasts which Mr. Darwin points out between adjacent areas that are totally cut off from each other. "No two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, isthmus of Panama." On opposite sides of high mountain-chains, also, there are marked differences in the organic forms--differences not so marked as where the barriers are absolutely impassable, but much more marked than are necessitated by unlikenesses of physical conditions.