of
this kind, for the question is not merely whether finished adaptations
have selection-value, but whether the first beginnings of these, and
whether the small, I might almost say minimal increments, which have
led up from these beginnings to the perfect adaptation, have also had
selection-value. To this question even one who, like myself, has been
for many years a convinced adherent of the theory of selection, can
only reply: _We must assume so, but we cannot prove it in any case_.
It is not upon demonstrative evidence that we rely when we champion
the doctrine of selection as a scientific truth; we base our argument
on quite other grounds. Undoubtedly there are many apparently
insignificant features, which can nevertheless be shown to be
adaptations--for instance, the thickness of the basin-shaped shell of
the limpets that live among the breakers on the shore. There can be no
doubt that the thickness of these shells, combined with their flat
forms, protects the animals from the force of the waves breaking upon
them,--but how have they become so thick? What proportion of thickness
was sufficient to decide that of two variants of a limpet one should
survive, the other be eliminated? We can say nothing more than that we
infer from the present state of the shell, that it must have varied in
regard to differences in shell-thickness, and that these differences
must have had selection-value,--no proof therefore, but an assumption
which we must show to be convincing.

For a long time the marvellously complex _radiate_ and _lattice-work_
skeletons of Radiolarians were regarded as a mere outflow of "Nature's
infinite wealth of form," as an instance of a purely morphological
character with no biological significance. But recent investigations
have shown that these, too, have an adaptive significance (Häcker).
The same thing has been shown by Schütt in regard to the lowly
unicellular plants, the Peridineae, which abound alike on the surface
of the ocean and in its depths. It has been shown that the long
skeletal processes which grow out from these organisms have
significance not merely as a supporting skeleton, but also as an
extension of the superficial area, which increases the contact with
the water-particles, and prevents the floating organisms from sinking.
It has been established that the processes are considerably shorter in
the colder layers of the ocean, and that they may be twelve times as
long[36] in the warmer layers, thus corresponding to the greater or
smaller amount of friction which takes place in the denser and less
dense layers of the water.

The Peridineae of the warmer ocean layers have thus become long-rayed,
those of the colder layers short-rayed, not through the direct effect
of friction on the protoplasm, but through processes of selection,
which favoured the longer rays in warm water, since they kept the
organism afloat, while those with short rays sank and were eliminated.
If we put the question as to selection-value in this case, and ask how
great the variations in the length of processes must be in order to
possess selection-value; what can we answer except that these
variations must have been minimal, and yet sufficient to prevent too
rapid sinking and consequent elimination? Yet this very case would
give the ideal opportunity for a mathematical calculation of the
minimal selection-value, although of course it is not feasible from
lack of data to carry out the actual calculation.

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 raindrops. 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 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[37] and that it is the converse that is true; that the
process of selection takes place in accordance with the variations
that present themselves. This proposition is undeniably true, but so
also is another, which apparently negatives it: the variation required
has in the majority of cases actually presented itself. Selection
cannot solve this contradiction; it does not call forth the useful
variation, but simply works upon it. The ultimate reason why one and
the same insect should occur in green and in brown, as often happens
in caterpillars and locusts, lies in the fact that variations towards
brown presented themselves, and so also did variations towards green:
_the kernel of the riddle lies in the varying_, and for the present we
can only say, that small variations in different directions present
themselves in every species. Otherwise so many different kinds of
variations could not have arisen. I have endeavoured to explain this
remarkable fact by means of the intimate processes that must take
place within the germ-plasm, and I shall return to the problem when
dealing with "germinal selection."

We have, however, to make still greater demands on variation, for it
is not enough that the necessary variation should occur in isolated
individuals, because in that case there would be small prospect of its
being preserved, notwithstanding its utility. Darwin at first
believed, that even single variations might lead to transformation of
the species, but later he became convinced that this was impossible,
at least without the coöperation of other factors, such as isolation
and sexual selection.

In the case of the _green caterpillars with bright longitudinal
stripes_, numerous individuals exhibiting this useful variation must
have been produced to start with. In all higher, that is,
multicellular organisms, the germ-substance is the source of all
transmissible variations, and this germ-plasm is not a simple
substance but is made up of many primary constituents. The question
can therefore be more precisely stated thus: How does it come about
that in so many cases the useful variations present themselves in
numbers just where they are required, the white oblique lines in the
leaf-caterpillar on the under surface of the body, the accompanying
coloured stripes just above them? And, further, how has it come about
that in grass caterpillars, not oblique but longitudinal stripes,
which are more effective for concealment among grass and plants, have
been evolved? And finally, how is it that the same Hawk-moth
caterpillars, which to-day show oblique stripes, possessed
longitudinal stripes in Tertiary times? We can read this fact from the
history of their development, and I have before attempted to show the
biological significance of this change of colour.[38]

For the present I need only draw the conclusion that one and the same
caterpillar may exhibit the initial stages of both, and that it
depends on the manner in which these marking elements are
_intensified_ and _combined_ by natural selection whether whitish
longitudinal or oblique stripes should result. In this case then the
"useful variations" were actually "always there," and we see that in
the same group of Lepidoptera, e.g. species of Sphingidae, evolution
has occurred in both directions according to whether the form lived
among grass or on broad leaves with oblique lateral veins, and we can
observe even now that the species with oblique stripes have
longitudinal stripes when young, that is to say, while the stripes
have no biological significance. The white places in the skin which
gave rise, probably first as small spots, to this protective marking
could be combined in one way or another according to the requirements
of the species. They must therefore either have possessed
selection-value from the first, or, if this was not the case at their
earliest occurrence, there must have been _some other factors_ which
raised them to the point of selection-value. I shall return to this in
discussing germinal selection. But the case may be followed still
farther, and leads us to the same alternative on a still more secure
basis.

Many years ago I observed in caterpillars of _Smerinthus populi_ (the
poplar hawk-moth), which also possess white oblique stripes, that
certain individuals showed _red spots_ above these stripes; these
spots occurred only on certain segments, and never flowed together to
form continuous stripes. In another species (_Smerinthus tiliae_)
similar blood-red spots unite to form a line-like coloured seam in the
last stage of larval life, while in _S. ocellata_ rust-red spots
appear in individual caterpillars, but more rarely than in _S.
populi_, and they show no tendency to flow together.

Thus we have here the origin of a new character, arising from small
beginnings, at least in _S. tiliae_, in which species the coloured
stripes are a normal specific character. In the other species, _S.
populi_ and _S. ocellata_, we find the beginnings of the same
variation, in one more rarely than in the other, and we can imagine
that, in the course of time, in these two species, coloured lines over
the oblique stripes will arise. In any case these spots are the
elements of variation, out of which coloured lines _may_ be evolved,
if they are combined in this direction through the agency of natural
selection. In _S. populi_ the spots are often small, but sometimes it
seems as though several had united to form large spots. Whether a
process of selection in this direction will arise in _S. populi_ and
_S. ocellata_, or whether it is now going on cannot be determined,
since we cannot tell in advance what biological value the marking
might have for these two species. It is conceivable that the spots may
have no selection-value as far as these species are concerned, and may
therefore disappear again in the course of phylogeny, or, on the other
hand, that they may be changed in another direction, for instance
towards imitation of the rust-red fungoid patches on poplar and willow
leaves. In any case we may regard the smallest spots as the initial
stages of variation, the larger as a cumulative summation of these.
Therefore either these initial stages must already possess
selection-value, or, as I said before: _There must be some other
reason for their cumulative summation_. I should like to give one more
example, in which we can infer, though we cannot directly observe, the
initial stages.

All the Holothurians or sea-cucumbers have in the skin calcereous
bodies of different forms,