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.