THE 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 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 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 



