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- 
