REPTILES AND AMPHIBIANS 1 29 



SO much greater that new variations may again arise 

 and preserve some animals from it. But if the 

 danger arises suddenly, or is suddenly increased, the 

 whole species affected by it must die out, as no 

 single variation is considerable enough to evade it. 



Thus the disappearance of the mice in our illustra- 

 tion is a sudden phenomenon that at once causes the 

 hares to face an excessive number of enemies, which 

 formerly only took part of their food from the hares. 

 There was not time for the hares to change sufficiently 

 to meet the numbers of foxes ; their fertility could 

 not at one stroke be increased enough to cover the 

 enormous disappearance. There was no time for the 

 selection of the fittest. If we assume that something 

 of this kind took place in the case of the ichthyosauri, 

 we can understand their extinction. The most probable 

 contingency is that the sharks came in great numbers 

 from another regioii into the sea where the reptiles 

 were, probably driven out by geological changes. 

 They increased at the expense of the ichthyosauri 

 until the latter were completely extinguished. But 

 the sharks could not suddenly reduce their fertility 

 and size, and therefore had to go themselves on 

 account of the lack of food. 



We see, then, that a species can be extirpated by 

 some event that occurs suddenly, in this case by the 

 immigration of sharks. Physical changes have a 

 similar effect. If vast steppes took the place of the 

 rich vegetation of the Jurassic and Cretaceous periods, 

 the herbivorous dinosauri would be without food, and 

 would be destroyed. We do not know anything about 



