vii Relations Between 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 (Trifolium 

 pratense) 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 

 field-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 

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



