148 INTRODUCTION 
ought to be found to have made nearly an equal advance, 
and to be pretty equally dispersed over the earth, for we 
know o*f nothing in their organization that should give 
any considerable advantage in this respect to one over 
another. The species of the two hemispheres should 
also be in general the same. If all the species then, 
originated on the eastern continent, how has it happened 
that those that have reached the western continent, have 
in general, left none of their kind behind them ; or that 
peculiar species exist in small islands or groups of islands 
far removed from other land ? If it be said that in the 
long lapse of ages, species once universally diffused have 
become extinct in particular regions, and that the sur- 
vivors are confined to more limited ranges, we ask how- 
it happens that the testaceous remains discovered by 
geological research, differ as much from existing species 
as the recent species of the two continents differ from 
each other. It seems to us that the facts taken for 
granted in these objections are inconsistent with any 
other theory than that of different foci of creation, and 
that this theory is sustained by all that we know of the 
geological revolutions of the earth, and of the condition 
of the species formerly existing upon it. 
Having thus adopted the theory of distinct zoological 
centres, and admitting that as successive portions of the 
earth's surface emerged from the waters, and became 
adapted to sustain the different classes of animals, those 
races which were fitted for the then existing physical 
condition of things, were brought into being by the pro- 
