p. 20. 
200 
will only dwell upon one point. If Natural Selection acts 
by the survival of a few favourable variants and the des 
truction of the rest, and if the favourable change in each 
generation is slight, what, it might be asked, is to prevent 
a slightly modified favourable variant from being destroyed? 
Mr. Wallace suggests a solution to this problem. 
“Tropical insectivorous birds very frequently sit on dead branches 
of a lofty tree, or on those which overhang forest paths, gazing 
intently around and darting off at intervals to seize an insect at 
a considerable distance, which they generally return to their station 
to devour. ... At long distances those who slightly resembled the 
Heliconidae might be mistaken for one of the uneatable group, and so 
be passed by and gain another day’s life, which might in many cases 
be sufficient for it to lay a quantity of eggs and leave a numerous 
progeny.”—(Wallace. Contributions to Natural Selection, pp. 80-2.) 
But the chance of escape from birds watching from afar 
does not meet the difficulty presented by large flocks of 
different insectivorous birds hunting together. According 
to Mr. Belt,— 
“Trogons, fly-catchers, tanagers, creepers, woodpeckers, &c., hunt 
together, traversing the forest in flocks of hundreds together, belong 
ing to more than a score different species ; so that whilst they are 
passing over, the trees seem alive with them. Mr. Bates has men 
tioned similar gregarious flocks, met with by him in Brazil; and I never 
went anydistance into the woods around Santo Domingo without seeing 
them. The reason of their association together may be partly for 
protection, but the principal reason appears to be that they play into 
each other’s hands in their search for food. The creepers and wood 
peckers and others drive the insects out of their hiding-places, under 
bark, amongst moss, and in withered leaves. The fly-catchers and 
trogons sit on branches and fly after the larger insects, the fly-catchers 
taking them on the wing, the trogons from off the leaves on which 
they have settled.”—(The Naturalist in Nicaragua, pp. 122-3.) 
Mr. Belt also reminds us that the numerous birds which 
accompany the army ants are ever on the outlook for any 
insect that may fly up.* These facts militate against