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 rea] 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 
