126 DARWINISM AND THE PROBLEMS OF LIFE 



how could the sharks develop to such a size, as they 

 only appear at a time when the reptiles had long 

 dominated the sea, and had been a danger to the 

 ancestors — probably much smaller — of the sharks ? 

 We can hardly admit that the sharks would be 

 developed so much more rapidly than the great marine 

 lizards. In fact, we saw in the first chapter, in the 

 instance of the fox and the hare, that two species that 

 live in a bioccenosis cannot extirpate each other, as 

 the strengthening of one species involves at the same 

 time more protection for the other. 



An effort has been made to explain by means of 

 an example how a species that has lived with another 

 for thousands of years may at last bring about its 

 destruction. The process is supposed to have taken 

 place between the machserodus and the glyptodon, 

 mammals of the American Tertiary period. 



The glyptodons were animals about three yards 

 long, something like the modern armadilloes, which 

 developed a powerful armour as they increased in 

 size, and this protected the bearer, like the shell of 

 a turtle, and was very thick. It afforded excellent 

 protection against most enemies, but not against the 

 machairodus, a tiger that also grew bigger and bigger, 

 and stuck its enormously long canine-teeth, with edges 

 like razors, into the glyptodont, in order to suck its 

 blood, as its enormous teeth did not allow it to rip 

 it up. We can easily understand how armour and 

 teeth were improved by natural selection ; in other 

 words, how those glyptodons lived longest and re- 

 produced most whose armour was impenetrable to 



