184 GENERAL ZOOLOGY 



This term is used by Darwin in "a large and metaphorical 

 sense, including dependence of one being on another, and 

 including (which is more important) not only the life of the 

 individual but also success in leaving progeny." The strug- 

 gle results from the tendency of living things to increase 

 more rapidly than the means of subsistence. Professor 

 Jordan of Stanford University says : 



If the eggs of a common house fly should develop, and each of 

 its progeny should find the food and temperature it needed, with 

 no loss and no destruction, the people of a city in which this might 

 happen could not get away soon enough to escape suffocation 

 from a plague of flies. 



Professor Thomson of Edinburgh, Scotland, gives this illus- 

 tration : 



A female aphis, often producing one offspring per hour for days 

 together, might in a season be the ancestor of a progeny of atomies 

 which would weigh down five hundred millions of stout men. 



A rapid increase would be noted on the part of any animal, 

 were the various checks to its multiplication removed. 

 The struggle for life goes on between different individuals 

 of the same species, as in a swarm of grasshoppers; be- 

 tween individuals of different species, as grasshoppers and 

 insect-eating birds ; and between living organisms and the 

 conditions of existence, such as temperature, winds, moist- 

 ure, and food supply. 



The caddis flies are, in their immature stage, aquatic lar- 

 vae which build protective cases composed of grains of sand, 

 or bits of straw, or leaves (Fig. 99). These cases afford con- 

 cealment and protection to the young. Applying the prin- 

 ciple of natural selection here, we may say that those 

 caddis flies which varied in the direction of protective cases 

 have survived, and those which did not have been devoured 

 or otherwise destroyed ; hence a race of case-building cad- 



