154 BOARD OF AGRICULTURE. [Pub. Doc. 



of the woolly aphis of the apple, each female of a given 

 species should give rise to one hundred young, and if each 

 of these should reach maturity without mishap and also 

 give rise to one hundred young each, and so on for ten 

 generations, the number in the last generation, determined 

 by the rule of geometrical progression, would amount to 

 one quintillion. Most people have very little conception 

 of high numbers, and to gain a clearer idea of these figures 

 let us bear in mind that one of these plant lice is about one 

 tenth of an inch in length, and that ten of them, placed in 

 a line touching each other, would reach an inch, one hun- 

 dred and twenty so placed would reach a foot, and 633,600 

 would reach a mile. If we should divide one quintillion, 

 the number in the last generation, by 633,600, the number 

 required to reach a mile, it would give the number of miles 

 that whole generation would reach if all those aphids were 

 placed in a line and just touching each other, and this dis- 

 tance is 157,828,282,282 miles. This number again is 

 beyond our comprehension. The earth is about 25,000 

 miles in circumference, and this string of plant lice would 

 reach around the world more than 6,000,000 times, or it 

 would reach from the earth to the moon more than 655,000 

 times, and it would reach from the earth to the sun 1,715 

 times. 



I have called your attention in this way, that you may 

 appreciate how very prolific plant lice are ; and there must 

 be a mortality among them every year that is perfectly 

 appalling, and concerning which we know as yet almost 

 nothing. Occasionally there is a season a little more favor- 

 able to the lice, as the past summer has been, when they 

 multiply and cover every green thing ; but even then the 

 mortality among the lice must be beyond all comprehension. 

 Wliat all the conditions are that cause the destruction of so 

 many plant lice it is impossible to say ; but we know that 

 the young of certain beetles feed voraciously upon them and 

 assist greatly in reducing their numbers, but I can scarcely 

 believe that there are not other forces at work that aid in 

 holding them in check. 



Six years ago I performed an experiment on plant lice on 

 roses. Twelve potted rose bushes of ditt'erent varieties were 



