646 The Outline of Science 



solvent humus-acids of the soil down to the buried surface. Their 

 castings on the hill-slopes are carried down by wind and rain 

 and go to swell the alluvium of the distant valleys. 



Linkages Securing Survival 



The most important linkage in the world is that between 

 many flowering plants and their insect visitors, as we have seen 

 (see BOTANY). The insects carry the fertilising pollen from one 

 blossom to another and bring about not merely fertilisation but 

 cross-fertilisation, which increases the yield and the quality of 

 the seed. Unless the egg-cell, inside the ovule inside the ovary of 

 the flower, be fertilised by a male nucleus from the pollen-grain, 

 the possible seed will not become a real seed capable of develop- 

 ment or germination. Some flowers, like peas, are self -pollinat- 

 ing; in some other cases, like pine-trees, the pollen is carried by 

 the wind, but most flowers are cross-pollinated by insects, and it 

 has been proved experimentally that cross-pollination is best. 



Cats and Clover 



In illustrating the linkage between flowers and their 

 welcome insect visitors (for there are others that do nothing but 

 harm) , Darwin told the "cats and clover" story, which soon spread 

 round the world. Round a hundred heads of the purple clover 

 he put muslin bags, so that air got in and sunlight got in, but no 

 insects. From these hundred heads he got not a single real seed, 

 while from another hundred heads without muslin bags he 

 obtained 27,000 seeds. These heads had been visited by the 

 humble-bee, which effects cross-fertilisation. So the more humble- 

 bees the better next year's clover crop. 



But the nests of the humble-bees are rifled by the field-mice 

 or voles, which are fond of the delicate white grubs. Therefore, 

 the more field-mice the fewer humble-bees, and the poorer next 

 year's clover crop. 



But in the neighbourhood of villages there are fewer field- 



