184 ANIMALS OP THE PAST 



with ashes, killing off the vegetation, may have brought 

 about or hastened the extermination of some of our 

 western mammals during Tertiary time. Nearly every- 

 one is familiar with the classic example given by Darwin 

 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 example of 

 the dependence of plants on animals, for their existence 

 hangs on that of a small moth whose peculiar structure 

 and habits bring about the fertilization of the flower. 

 The two probably developed side by side until their 

 present state of inter-dependence was reached, when 

 the extinction of the one would probably bring about 

 that of the other. 



It is this inter-dependence of living things that makes 

 the outcome of any direct interference with the natural 

 order of things more or less problematical, and some- 

 times brings about results quite different from what 

 were expected or intended. 



The gamekeepers on the grouse moors of Scotland 

 systematically killed off all birds of prey because they 

 caught some of the grouse, but this is believed to have 

 caused far more harm than good through permitting 

 weak and sickly birds, that would otherwise have fallen 

 a prey to hawks, to live and disseminate the grouse 

 distemper. 



The destruction of sheep by coyotes led the State of 

 California to place a bounty on the heads of these 

 animals, with the result that in eighteen months the 

 State was called upon to pay out $187,485. As a result 

 of the war on coyotes the animals on which they fed, 



