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 Mr. 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 Newman, 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 would 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 following account of the struggle between trees in the 
forests of Denmark, from the researches of M. Hansten- 
