8o VIGNETTES FROM NATURE. 



North America in the west have no very 

 large birds, no very large terrestrial reptiles, 

 no very large amphibians ; and their lower 

 mammals are all small and skulking species 

 mere rats, hedgehogs, shrews, and moles, 

 instead of big sloths, kangaroos, or me- 

 gatheria. In the oceans, where mammals 

 are at some disadvantage, and where they 

 have not yet firmly established themselves, 

 big fish still abound. Nevertheless, even 

 here, the whales, porpoises, walruses, seals, 

 manatees, and other aquatic mammals are 

 pushing them hard ; and we know that the 

 sharks and rays, the monsters of their kind, 

 as well as many other big tribes of fish, are 

 now in their decadence. Thus it would 

 seem that everywhere some animals of the 

 dominant types are the largest, and that 

 mammals, therefore, now tend all over the 

 world to replace the large members of inferior 

 groups. 



