wi] THE FERTILISATION OF FLOWERS. 57 
the plant to be open during the day-time. For the 
same reason plants which depend upon the bees and 
butterflies close their petals as soon as the sun sinks 
in the west. Then, too, observe the manner in which 
various flowers are hung on the stalk in order to 
protect their honey from deterioration by the access 
of rain; or those flowers in which the honey is not 
thus protected—such as the Daisy and Pimpernel— 
how they close their petals at the approach of rain. 
Such researches have proved an intimate connection 
between animals and plants which previously seemed 
to lack the most shadowy relation one to the other. 
What, for instance, could seem more ridiculous than 
the assertion that the clover-crop in any district 
depends upon the number of old maids there? Yet 
such is really the case, for the clover is dependent 
chiefly upon the humble-bees for fertilisation—in fact 
one species, the red clover, is fertilised by humble- 
bees alone, Now it is well known that the nests and 
combs of humble-bees are pillaged and destroyed 
by field-mice, which in turn are destroyed by cats, 
The number of cats in a district is largely determined 
by the number of old maids there resident, so that 
if in a given district there are a large number of old 
maids, there will also be a good supply of cats. If 
a good supply of cats the field-mice will be held in 
check and their numbers reduced. If field-mice are 
not abundant the humble-bees will thrive, and ferti- 
lise more clover. There seems at first sight some- 
thing humorous about this connection, yet there are 
many similar instances which Mr. Darwin has brought 
to light. 
