210 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



continuance of its kind. The crowning instances are to be found in in- 

 terrelations between plants and animals which secure cross-fertilisation 

 and the distribution of seeds. To both of these Darwin devoted much 

 attention, and they were always favourite subjects with him. 



Everyone knows that flowering plants and flower-visiting insects 

 have grown up throughout long ages together, in alternate influence 

 and mutual perfecting. They are now fitted to one another as hand 

 to glove. The insects visit the flowers for food; in so doing they carry 

 the fertilising golden dust from blossom to blossom, so that the 

 possible seeds become real seeds. 



In 1793 a Berlin naturalist, Christian Konrad Sprengel, like 

 Darwin in his perception of the web of life, published a pioneer book 

 entitled The Secret of Nature Discovered in the Structure and Fertili- 

 zation of Flowers, in which he showed that most flowers have 

 nectar which insects enjoy; that by the insects' visits pollination is 

 secured; that there is no detail of the flower without its meaning 

 the colour is a flag to attract the insect's eye, conspicuous spots are 

 honey-guides to the explorers, there are arrangements for keeping the 

 pollen dry and for dusting it on the insects, and so on. If Sprengel 

 had only discovered the utility of the cross-fertilisation, which Darwin 

 proved experimentally, his work could hardly have been overlooked 

 for nearly seventy years. In 1841 it came into Darwin's hands, and 

 impressed him as being "full of truth," although "with some little 

 nonsense." In Darwin's work Sprengel had his long-delayed reward. 



Darwin's instance of the connection between cats and clover. 

 One of Darwin's instances of the web of life given in connection with 

 the pollination of flowers has become familiar all over the world. 

 It should never become trite to us and it should never be regarded as 

 more than a particularly clear illustration of a general fact. " 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 heart's- 

 ease (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 pro- 

 duced 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 heart's-ease and red clover would become 



