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.