1880. | Zoology. 449 
remaining year by year at the present average figure, would be 
increased over two-thirds each year. ny one familiar with 
geometrical ratios will understand the inevitable result. In the 
second year we should find these pests nearly three times as 
numerous as now, and with that astounding acceleration of 
increase characteristic of geometrical progression, they would 
multiply until in about twelve years we should have the entire 
State carpeted with insects, one to the square inch over our whole 
territory. I have so arranged this computation as to exclude the 
insoluble question of the relative value of birds and predaceous 
or parasitic insects, unless we suppose that birds eat an undue 
proportion of beneficial species. 
“Take another view of this matter. According to the compu- 
tation of Mr. Walsh, the average damage done by insects in 
Illinois amounts to twenty millions dollars a year, Large figures 
certainly ; but when we find that this means only about fifty-six 
cents an acre, we begin to see their probability. Few intelligent 
farmers or gardeners would refuse an offer to insure complete 
protection, year after year, against insects of all sorts, for twenty- 
five cents an acre per annum, and we will, therefore, place the 
damage at one-half the above amount—ten million dollars per 
annum. 
“Suppose that, as a consequence of this investigation, we are 
able to take measures which shall result in the increase, by so 
much as one per cent. of the efficacy of birds as an insect-police, 
the effect would be a diminution of the above injury to the 
amount of sixty-six thousand dollars per annum, equivalent to 
the addition of over one and a-half million dollars to the perma- 
“Compared with these numbers, my 7500 insects a year seem 
certainly many times too few, and similar criticisms might very 
Probably be made on other items of the estimate. I prefer, how- 
ever, to put these matters with a moderation which will command 
young mocking-bird (Mimus polyglottus), raised from the nest by my nephew, 
TA 
Robert Forbes, ate about 240 red-legzed grasshoppers daily, equivalent to at least 
480 average insects, , 
