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 role 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 



