ECOLOGICAL 205 



insects, such as bees, butterflies, and two-winged flies, visit flowers 

 and unconsciously secure cross-pollination. Without pollination the 

 possible seeds or ovules cannot in ordinary cases become real 

 seeds that will germinate; and cross-pollination tends to 

 secure, not only more seeds, but a better quality. Thus one of 

 the most important instances of the Balance of Nature is that 

 between flowering plants and their insect visitors. This is not 

 affected by the fact that some of the plants that are most valu- 

 able to man, such as cereals, are pollinated by the wind-borne 

 pollen-grains. 



We have said enough to illustrate the biological idea of the 

 Balance of Nature, necessarily referred to in other connections 

 in the book. Different kinds of living creatures have evolved together 

 and become mutually dependent, so that increase or decrease on 

 one side of the correlation inevitably affects the other. This is of 

 great practical importance, warning man against upsetting what 

 has been long established and automatically adjusted in Nature. 

 The destruction of insectivorous birds means multiplication of 

 injurious insects; the introduction of rabbits into a new country 

 where their natural enemies are not represented leads to agricultural 

 disaster; the careless introduction of weeds into new surroundings 

 where they are not kept down has often been calamitous; even the 

 apparently irreproachable destruction of poisonous snakes may be 

 soon followed by a plague of small rodents which they helped to 

 keep within bounds. Ignorance is usually very costly, and not least 

 when it disturbs the Balance of Nature. 



DETAILED ILLUSTRATION: BIRDS AS POLLINATORS 



We have dwelt on the familiar linkage between flowers and their 

 appropriate insect visitors, such as bees and butterflies. On ends of 

 their own, the search for nectar in particular, the insects fly from 

 blossom to blossom and, as Aristotle noticed, they often keep for a 

 time to the same kind of flower. Bees, as Darwin said, are good 

 botanists, and if they visit in succession various members of the 

 same species, they effect cross-fertilisation. 



In default of insect visitors there is the possibility of fertilisation 

 by wind-borne pollen, as in the case of pine-trees and many grasses. 

 Or the plant may be able to effect self-fertilisation, as in the case 

 of the common edible pea. There is also the remarkable possi- 

 bility, illustrated by the dandelion and a few others, that the 

 flower may become parthenogenetic, the egg-cells developing 

 without being fertilised at all. But the likelihood is that if the 

 appropriate insects ceased their visits, many of our familiar wild 

 flowers would disappear from the face of the earth, through 



