20 DARWINISM chap. 



Such cases as the above may perhaps be thought excep- 

 tional, but there is good reason to believe that they are by no 

 means rare, but are illustrations of what is going on in every 

 part of the world, only it is very difficult for us to trace out 

 the complex reactions that are everywhere occurring. The 

 general impression of the ordinary observer seems to be that 

 wild animals and plants live peaceful lives and have 

 few troubles, each being exactly suited to its place and 

 surroundings, and therefore having no difficulty in maintain- 

 ing itself. Before showing that this view is, everywhere 

 and always, demonstrably untrue, we will consider one other 

 case of the complex relations of distinct organisms adduced 

 by Ml". Darwin, and often quoted for its striking and almost 

 eccentric character. It is now well known that many flowers 

 require to be fertilised by insects in order to produce seed, 

 and this fertilisation can, in some cases, only be effected by 

 one particular species of insect to which the flower has become 

 specially adapted. Two of our common plants, the wild heart's- 

 ease (Viola tricolor) and the red clover (Trifolium pratense), are 

 thus fertilised by humble-bees almost exclusively, and if these 

 insects are prevented from visiting the flowers, they produce 

 either no seed at all or exceedingly few. Now it is known that 

 field-mice destroy the combs and nests of humble-bees, and 

 Colonel Ne^vman, who has paid great attention to these insects, 

 believes that more than two-thirds of all the humble-bees' 

 nests in England are thus destroyed. But the number of 

 mice depends a good deal on the number of cats ; and the same 

 observer says that near villages and towns he has found the 

 nests of humble-bees more numerous than elsewhere, which he 

 attributes to the number of cats that destroy the mice. 

 Hence it follows, that the abundance of red clover and wild 

 heart's-ease in a district will depend on a good supply of cats 

 to kill the mice, which Avould otherwise destroy and keep down 

 the humble-bees and prevent them from fertilising the flowers. 

 A chain of connection has thus been found between such 

 totally distinct organisms as flesh-eating mammalia and sweet- 

 smelling flowers, the abundance or scarcity of the one closely 

 corresponding to that of the other ! 



The followins; account of the strugiilc between trees in the 

 forests of Denmark, from the researches of M. Hansten- 



