234 ANIMALS OF THE PAST 
the young. The loss of two or three broods 
of sparrows or two or three litters of rabbits 
makes comparatively little difference, as the 
loss is soon supplied, but the death of the 
young of the larger and higher mammals is a 
more serious matter. A factor that has prob- 
ably played an important réle in the extinction 
of animals is the relation that exists between 
various animals, and the relations that also 
exist between animals and plants, so that the 
existence of one is dependent on that of an- 
other. ‘Thus no group of living beings, plants 
or animals, can: be affected without in some 
way affecting others, so that the injury or 
destruction of some plant may result in seri- 
ous harm to some animal. Nearly everyone is 
familiar with the classic example given by Dar- 
win of the effect of cats on the growth of red 
clover. This plant is fertilized by bumble bees 
only, and if the field mice, which destroy the 
nests of the bees, were not kept in check by 
cats, or other small carnivores, their increase 
would lessen the numbers of the bees and this 
in turn would cause a dearth of clover. 
The yuccas present a still more wonderful 
4 
