VII 



Relations Betiveen Plants and Animals 141 



young horses and cattle — then the latter would become 

 feral, and this would certainly greatly alter the vegetation ; 

 this again would largely affect the insects ; and this the 

 insectivorous birds, and so onwards in ever-increasing circles 

 of complexity.'' So, too, with the terrible tsetse fly, which, 

 as Livingstone pointed out, renders pasturage and animal 

 transport impossible within its region, so condemning civili- 

 sation to a lower type. 



But perhaps his best illustration is the most familiar 

 one, which should never become trite to us. " Plants and 

 animals, remote in the scale of nature, are bound together 

 by a web of complex relations. ... I have found from 

 experiments that humble-bees are almost indispensable to 

 the fertilisation of the heartsease ( Viola tricolor)^ for other 

 bees do not visit this flower. I have also found that the 

 visits of bees are necessary for the fertilisation of some 

 kinds of clover — thus, 100 heads of red clover {Trifoliiiin 

 pratetise) produced 27,000 seeds, but the same number of 

 protected heads produced not a single seed. Humble-bees 

 alone visit red clover, as other bees cannot reach the nectar. 

 . . . Hence we may infer as highly probable that, if the 

 whole genus of humble-bees became extinct or very rare in 

 England, the heartsease and red clover would become very 

 rare, or wholly disappear. The number of humble-bees in 

 any district depends in a great measure on the number of 

 tield-mice, which destroy their combs and nests ; and 

 Colonel Newman, who has long attended to the habits of 

 humble-bees, believes that more than two -thirds of them 

 are thus destroyed all over England." 1 Now the number 

 of mice is largely dependent, as every one knows, on the 

 number of cats ; and Colonel Newman says, " Near 

 villages and small towns I have found the nests of humble- 

 bees more numerous than elsewhere, which I attribute to 

 ^ Origin of Species, chap. iii. Lond. 1859. 



