But even in organisms of more than microscopic size there must frequently
be minute, even microscopic differences which set going the process of
selection, and regulate its progress to the highest possible perfection.

Many tropical trees possess thick, leathery leaves, as a protection against
the force of the tropical rain drops. The DIRECT influence of the rain
cannot be the cause of this power of resistance, for the leaves, while they
were still thin, would simply have been 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