‘ANIMALS DEPENDENT inci inane ee 
known. The agen of some Bt iods au instance, 
eat the leaves of one plant only; the Peacock (Vanessa 
io) and the Small Tortoiseshell (V. urtice) are cases 
in point. The caterpillars of both these species feed 
exclusively on the Common Nettle (Urtica dioica). 
Should the efforts of farmers and gardeners succeed 
in exterminating this unwelcome plant, these two 
butterflies would disappear from the Earth. Some- 
times absolute mutual dependence is found on both 
the animal and vegetable sides. The American Yucca 
filamentosa, often grown in our gardens, depends 
solely on the little moth Pronuba yuccasella for its 
pollination, just as the insect is absolutely dependent 
on the plant (see p. 80), and other species of Yucca 
have each its particular dependent moth, which feeds 
on no other plant, and whose flowers are pollinated 
by no other. 
Apart from such special cases, the general depend- 
ence of animals upon plants is obvious, and is by no 
means confined to food-supply. Animals of all grades, 
from human beings to Caddis Worms, construct 
houses of vegetable materials; trees are the chosen 
home of large sections of our fauna, and the herbs of 
the field are the world for millions of tiny beings. 
There’s never a leaf or a blade too mean 
To be some happy creature's palace. 
Turning to the other side of the picture, no such 
general dependence of the plant world upon the 
animal world is found, but the inter-relations of the 
two are many and varied, and in the absence of animals 
of one kind or. another whole groups of plants would 
become extinct. The cases where plants derive their 
