34 THE STORY-BOOK OF SCIENCE 



are the devourers that will see the end of such a 

 family?" 



"Nine million eggs!" exclaimed Emile. "Is that 

 a great many?" 



' ' Just to count them, one by one, would take nearly 

 a year of ten working hours each day." 



"Whoever counted them had lots of patience," 

 was Emile 's comment. 



' ' They are not counted, ' ' replied Uncle Paul ; ' ' they 

 are weighed, which is quickly done; and from the 

 weight the number is deduced. 



"Like the cod in the sea, the plant-lice are exposed 

 on their rosebushes and alders to numerous chances 

 of destruction. I have told you that they are the 

 daily bread of a multitude of eaters. So, to increase 

 their legions, they have rapid means that are not 

 found in other insects. Instead of laying eggs, very 

 slow in developing, they bring forth living plant-lice, 

 which all, absolutely all, in two weeks have obtained 

 their growth and begin to produce another genera- 

 tion. This is repeated all through the season, that 

 is to say at least half the year, so that the number 

 of generations succeeding one another during this 

 period cannot be less than a dozen. Let us say that 

 one plant-louse produces ten, which is certainly be- 

 low the actual number. Each of these ten plant-lice 

 borne by the first one bears ten more, making one 

 hundred in all; each of these hundred bears ten, 

 in all one thousand ; each of the thousand bears ten, 

 in all ten thousand ; and so on, multiplying always by 

 ten, eleven times. Here is the same calculation as 

 the dervish's grain of wheat, which grew with such 



