ught 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 importance. 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 between them, and both will benefit.
But if the two species are very unequal in numbers, the benefit will be
comparatively slight for the more abundant species, but very great for
the rare one. To the latter it may make all the difference between
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 will be hardly felt; to
the smaller it will mean the most dreadful persecution resulting in a
loss of half the total population. But, let the two species become
superficially alike, so that the birds see no difference between them.
The quota of 500 will now be taken from a combined population of 101,000
butterflies, and if proportionate numbers of each suffer, then the weak
species will only lose five individuals instead of 500 as it did before.
Now we know that the different species of Heliconidae are not equally
abundant, some being quite rare; so that the benefit to be derived in
these latter cases would be very important. A slight inferiority in
rapidity of flight or in powers of eluding attack might also be a cause
of danger to an inedible species of scanty numbers, and in this case too
the being merged in another much more abundant species, by similarity
of external appearance, would be an advantage.

The question of fact remains. Do young birds pursue and capture these
distasteful butterflies till they have learned by bitter experience what
species to avoid? On this point Dr. Mueller has fortunately been able to
obtain some direct evidence, by capturing several Acraeas and
Heliconidae which had evidently been seized by birds but had afterwards
escaped