IX 
WARNING COLORATION AND MIMICRY 
253 
fact, that insect-eating birds only learn by experience to 
distinguish the edible from the inedible butterflies, and in 
doing so necessarily sacrifice a certain number of the latter. 
The quantity of insectivorous birds in tropical America is 
enormous; and the number of young birds which every year 
have to learn wisdom by experience, as regards the species of 
butterflies to be caught or to be avoided, is so great that the 
sacrifice of life of the inedible species must be considerable, 
and, to a comparatively weak or scarce species, of vital im¬ 
portance. The number thus sacrificed will be fixed by the 
quantity of young birds, and by the number of experiences 
requisite to cause them to avoid the inedible species for the 
future, and not at all by the numbers of individuals of which 
each species consists. Hence, if two species are so much 
alike as to be mistaken for one another, the fixed number 
annually sacrificed by inexperienced birds will be divided be¬ 
tween them, and both will benefit. But if the two species are 
very unequal in numbers, the benefit Anil be comparatively 
slight for the more abundant species, but very great for the rare 
one. To the latter it may make all the difference betAveen 
safety and destruction. 
To give a rough numerical example. Let us suppose that 
in a given limited district there are two species of Heliconidae, 
one consisting of only 1000, the other of 100,000 individuals, 
and that the quota required annually in the same district for 
the instruction of young insectivorous birds is 500. By the 
larger species this loss Avill be hardly felt; to the smaller it 
Avill mean the most dreadful persecution resulting in a 
loss of half the total population. But, let the tAvo species 
become superficially alike, so that the birds see no difference 
between them. The quota of 500 will iioav be taken from a 
combined population of 101,000 butterflies, and if propor¬ 
tionate numbers of each suffer, then the Aveak species AA'ill 
only lose five individuals instead of 500 as it did before. 
Noav Ave knoAV that the different species of Heliconidie are 
not equally abundant, some being quite rare ; so that the 
benefit to be derived in these latter cases would be very im¬ 
portant. A slight inferiority in rapidity of flight or in powers 
of eluding attack might also be a cause of danger to an in¬ 
edible species of scanty numbers, and in this case too the being 
