238 ANIMALS OF THE PAST 



source of food and would be brought into a 

 silent struggle with forms already in posses- 

 sion, while the destruction of one set of plants 

 would be to the advantage of those with which 

 they came into competition and to the disad- 

 vantage of vegetation that was protected by 

 the shade. Finally, these changed conditions 

 would react in various ways on the smaller 

 birds and mammals, the general effect being, 

 to use a well-worn simile, like that of casting 

 a stone into a quiet pool and setting in motion 

 ripples that sooner or later reach to every part 

 of the margin. 



It is scarcely necessary to warn the reader 

 that for the most part this is purely conjectu- 

 ral, for from the nature of the case it is bound 

 to be so. But it is one of the characteristics 

 of educated man that he wishes to know the 

 why and wherefore of everything, and is in a 

 condition of mental unhappiness until he has 

 at least formulated some theory which seems 

 to harmonize with the visible facts. And 

 from the few glimpses we get of the extinction 

 of animals from natural causes we must formu- 

 late a theory to fit the continued extermina- 



