torn to
pieces. Their toughness must therefore be referred to selection, which
would favour the trees with slightly thicker leaves, though we cannot
calculate with any exactness how great the first stages of increase in
thickness must have been. Our hypothesis receives further support from
the fact that, in many such trees, the leaves are drawn out into a
beak-like prolongation (Stahl and Haberlandt) which facilitates the
rapid falling off of the rain water, and also from the fact that the
leaves, while they are still young, hang limply down in bunches which
offer the least possible resistance to the rain. Thus there are here
three adaptations which can only be interpreted as due to selection.
The initial stages of these adaptations must undoubtedly have had
selection-value.

But even in regard to this case we are reasoning in a circle, not giving
"proofs," and no one who does not wish to believe in the selection-value
of the initial stages can be forced to do so. Among the many pieces of
presumptive evidence a particularly weighty one seems to me to be THE
SMALLNESS OF THE STEPS OF PROGRESS which we can observe in certain
cases, as for instance in leaf-imitation among butterflies, and
in mimicry generally. The resemblance to a leaf, for instance of a
particular Kallima, seems to us so close as to be deceptive, and yet we
find in another individual, or it may be in many others, a spot added
which increases the resemblance, and which could not have become fixed
unless the increased deceptiveness so produced had frequently led to
the overlooking of its much persecuted possessor. But if we take the
selection-value of the initial stages for granted, we are confronted
with the further question which I myself formulated many years ago: How
does it happen THAT THE NECESSARY BEGINNINGS OF A USEFUL VARIATION ARE
ALWAYS PRESENT? How could insects which live upon or among green leaves
become all green, while those that live on bark become brown? How have
the desert animals become yellow and the Arctic animals white? Why were
the necessary variations always present? How could the green locust lay
brown eggs, or the privet caterpillar develop white and lilac-coloured
lines on its green skin?

It is of no use answering to this that the question is wrongly
formulated (Plate, "Selektionsprinzip u. Probleme der Artbildung" (3rd
edition), Leipzig, 1908.) and that it is the converse that is true; that
the process of selection takes plac