104 



GENERAL ZOOLOGY 



but also success in leaving progeny." The struggle results 

 from the tendency of living things to increase more rapidly 

 than the means of subsistence. Professor 

 Jordan, of Leland Stanford Junior Uni- 

 versity, says, " If the eggs of a common 

 house-fly should develop, and each of its 

 progeny should find the food and tempera- 

 ture it needed, with no loss and no des- 

 truction, 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 illustration: "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 



FIG. 55. Caddice-Flies. Enlarged 



