INTRODUCTORY. 74 3 
fly, concerning whose injurious propensities there can 
be no manner of doubt. We save our cabbages 
for the time, perhaps, but we sweep away the 
appointed food of many other creatures, which 
Nature had appointed to restrain the increase 
of the butterfly within due limits. Now, these 
creatures must either find for themselves a different 
food, or die. In all probability they choose 
the former alternative, and attach themselves as 
parasites to some other being, very likely of a bene- 
ficial character, whose labours for good they now 
restrict just as they formerly restricted those of its 
predecessor for evil. And so we should lose in one 
way what we had gained in another, while the 
secondary and indirect consequences of our act might 
very possibly prove of a still more harmful and 
disastrous nature. 
SomE birds, of course, are far more useful than 
others, and with these I shall deal in the following 
pages, setting forth and laying stress upon their services, 
and at the same time showing as fairly and impartially 
as possible the mischief by which those services are 
sometimes qualified. Before entering upon this task, 
however, I must once more remind the reader that an 
insect killed and a grain of corn destroyed by the 
same bird do not counterbalance one another, but 
that, speaking with moderation, the insect killed is 
equivalent to forty or fifty grains of corn saved ; 
with this difference, that the loss of the latter is 
immediately felt, while the gain caused by the 
