mplex 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 (Hacker). The
same thing has been shown by Schutt 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 (Chun, "Reise der Valdivia",
Leipzig, 1904.) 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 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