great outlay to keep the stream open. But a natural remedy has now been
found in planting willows on the banks. The roots of these trees
penetrate the bed of the stream in every direction, and the water-cress,
unable to obtain the requisite amount of nourishment, gradually
disappears.

_Increase of Organisms in a Geometrical Ratio_.

The facts which have now been adduced, sufficiently prove that there is
a continual competition, and struggle, and war going on in nature, and
that each species of animal and plant affects many others in complex and
often unexpected ways. We will now proceed to show the fundamental cause
of this struggle, and to prove that it is ever acting over the whole
field of nature, and that no single species of animal or plant can
possibly escape from it. This results from the fact of the rapid
increase, in a geometrical ratio, of all the species of animals and
plants. In the lower orders this increase is especially rapid, a single
flesh-fly (Musca carnaria) producing 20,000 larvae, and these growing so
quickly that they reach their full size in five days; hence the great
Swedish naturalist, Linnaeus, asserted that a dead horse would be
devoured by three of these flies as quickly as by a lion. Each of these
larvae remains in the pupa state about five or six days, so that each
parent fly may be increased ten thousand-fold in a fortnight. Supposing
they went on increasing at this rate during only three months of summer,
there would result one hundred millions of millions of millions for each
fly at the commencement of summer,--a number greater probably than
exists at any one time in the whole world. And this is only one species,
while there are thousands of other species increasing also at an
enormous rate; so that, if they were unchecked, the whole atmosphere
would be dense with flies, and all animal food and much of animal life