WHY DO ANIMALS BECOME EXTINCT? 221 
back to the little Eocene Hyracothere. So in 
a general way it may be said that much of 
what at the first glance we might term extinc- 
tion is really the replacement of one set of 
animals by another better adapted to surround- 
ing conditions. 
Again, there are many cases of animals, and 
particularly of large animals, so peculiar in 
their make up, so very obviously adapted to 
their own special surroundings that it requires 
little imagination to see that it would have 
_ been a difficult matter for them to have re- 
_ sponded to even a slight change in the world | 
about them. Such great and necessarily slug- 
_ gish brutes as Brontosaurus and Diplodocus, 
_ with their tefis of flesh, small heads, and feeble 
Se 
teeth;-Were’ obviously reared in easy circum- 
stances, and unfitted to succeed in any strenu- 
ous struggle for existence. Stegosaurus, with 
his bizarre array of plates and spines, and huge- 
headed Triceratops, had evidently carried spe- 
cialization to an extreme, while in turn the 
carnivorous forms must have required an abun- 
dant supply of slow and easily captured prey. 
Coming down to a more recent epoch, when 
