60 EVOLUTION AND ANIMAL LIFE 



with no loss and no destruction, the people of the city in whie^ 

 it happened would suffocate under the plague of flies. When- 

 ever any species of insect develops a large percentage of the egg€ 

 laid, it becomes at once a plague. Thus originate plagues of 

 locusts, grasshoppers, and caterpillars. But the crowd of life 

 renders these plagues rare. Scavenger-beetles and bacteria 

 destroy the decaying flesh where the fly would lay its eggs. 

 IMinute creatures, bacteria, protozoa, other insects, are parasitic 

 within the larva itself. ]\lillions of flies starve to death. Mil- 

 lions more are eaten by birds and predaceous insects. The 

 final result is that from year to year the number of flies does 

 not increase. Linnaeus once said that " three flies will devour 

 a dead horse as quickly as a lion.^' Quite as soon would three 

 bacteria with their descendants reach the same result. "Even 

 slow-breeding man," saj^s Darwin, "has doubled in twenty-five 

 years. At this rate in less than a thousand vears there literally 

 would not be standing room for his progeny. The elephant is 

 reckoned the slowest breeder of all animals. It begins breeding 

 when thirty years old and goes on breeding until ninet}^ years 

 old, bringing forth six young in the interval and surviving to be 

 a hundred years old. If this be so, after about 800 years there 

 should be 19,000,000 elephants alive descended from the first 

 pair." A few years of still further multiplication without check, 

 and every foot of the earth would be covered by elephants. 



Similar calculations may be made in regard to any species of 

 animal or plant whatsoever. Each one increases at a rate which 

 without checks would make it soon cover the earth. Yet the 

 number of individuals in a state of nature in any species re- 

 mains about stationary. With the interference of man, in 

 many species the numbers slowly diminish; very few increase. 

 There are about as many squirrels in the forest one year as 

 another, as many butterflies in the field, as many frogs in the 

 pond. Wolves, bears, deer, ducks, singing birds, fishes, all suf- 

 fer from man's attacks or man's neglect and grow fewer year 

 by year. It is manifest that the tendency to reproduce by 

 geometric ratio meets everywhere with a corresponding check. 

 This check is known as the Struggle for Existence. 



The struggle for existence is threefold: (a) Among indiAdduals 

 of one species, as wolf against wolf or sparrow against sparrow, 

 (b) between individuals of different species, as rabbit with wolf 

 or blue-bird with sparrow ; (c) with the conditions in Uf e — as 



