ORGANIC AGENCIES. 123 



the great cats, lions, tigers, jaguars ; among birds, tou- 

 cans, parrots, trogons, ostriches ; among reptiles, croco- 

 dilians and pythons ; and among corals, the reef-builders, 

 characterizing the tropics ; while the musk-ox, white 

 bear, seals, walrus, ducks and geese, characterize the 

 polar regions yet we can not make a zonal arrangement 

 of families as easily as we can with plants. But, confin- 

 ing our attention to species or even genera, animal forms 

 are subject to the same laws as those of plants : 1. All 

 animal species are limited in range ; 2. The range of 

 species is more limited than that of genera, and of genera 

 than that of families, etc.; 3. Contiguous ranges grad- 

 uate into each other by overlapping, the species inter- 

 mingling and coexisting 011 the margins ; 4. Each species 

 reaches a maximum of number and vigor in middle regions 

 and dies out on the borders ; 5. But in specific character 

 they seem to remain substantially the same throughout 

 their range, and do not change or transmute into other 

 species on the borders ; 6. Physical conditions may limit 

 their range, but do not seem to change them into other 

 species, though varieties may be formed in this way ; 

 7. Here, again, it is as if species originated, no matter 

 how, in the places where we find them, and have spread 

 in all directions as far as physical conditions and struggle 

 with other species would allow. All that we shall say 

 hereafter will apply equally to animals and plants. 



Continental Faunas and Floras. If there were no 

 barriers to the spread of species around the earth on the 

 same zone, there can be no doubt that they would thus 

 spread, and faunas and floras would be arranged in a 

 series of temperature zones from the equator to the poles, 

 containing the same species all around. But the oceans 

 are impassable barriers between the continents, and there- 

 fore the faunas and floras of different continents are sub- 

 stantially different. It is, again, as if they originated on 

 the continents where we find them, and have been pre- 



