estigations
have shown that these, too, have an adaptive significance (Haecker).
The same thing has been shown by Schuett 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 re