II 
TilK STRUGGLE FOR EXISTENCE 
25 
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 lie devoured by three 
of these fliesas 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 would be destroyed by them. To prevent this tremendous 
increase there must be incessant war against these insects, by 
insectivorous birds and reptiles as well as by other insects, in 
the larva as well as in the perfect state, by the action of the 
elements in the form of rain, hail, or drought, and by other 
unknown causes; yet we see nothing of this ever-present war, 
though by its means alone, perhaps, we are saved from famine 
and pestilence. 
Let us now consider a less extreme and more familiar 
case. We possess a considerable number of birds which, 
like the redbreast, sparrow, the four common titmice, the 
thrush, and the blackbird, stay with us all the year round. 
These lay on an average six eggs, but, as several of them have 
