Ankyroderma, have anchors which
project considerably beyond the skin, and, according to Oestergren,
serve "to catch plant-particles and other substances" and so mask the
animal. Thus we see that in the Synaptidae the thick and irregular
calcareous bodies of the Holothurians have been modified and
transformed in various ways in adaptation to the footlessness of these
animals, and to the peculiar conditions of their life, and we must
conclude that the earlier stages of these changes presented themselves
to the processes of selection in the form of microscopic variations.
For it is as impossible to think of any origin other than through
selection in this case as in the case of the toughness, and the
"drip-tips" of tropical leaves. And as these last could not have been
produced directly by the beating of the heavy raindrops upon them, so
the calcareous anchors of Synapta cannot have been produced directly
by the friction of the sand and mud at the bottom of the sea, and,
since they are parts whose function is _passive_ the Lamarckian factor
of use and disuse does not come into question. The conclusion is
unavoidable, that the microscopically small variations of the
calcareous bodies in the ancestral forms have been intensified and
accumulated in a particular direction, till they have led to the
formation of the anchor. Whether this has taken place by the action of
natural selection alone, or whether the laws of variation and the
intimate processes within the germ-plasm have cooeperated will become
clear in the discussion of germinal selection. This whole process of
adaptation has obviously taken place within the time that has elapsed
since this group of sea-cucumbers lost their tube-feet, those
characteristic organs of locomotion which occur in no group except the
Echinoderms, and yet have totally disappeared in the Synaptidae. And
after all what would animals that live in sand and mud do with
tube-feet?

(_c_) _Coadaptation_

Darwin pointed out that one of the essential differences between
artificial and natural selection lies in the fact that the former can
modify only a few characters, usually only one at a time, while Nature
preserves in the struggle for existence all the variations of a
species, at the same time and in a purely mechanical way, if they
possess selection-value.

Herbert Spencer, though himself an adherent of the theory of selection,
declared in the beginning of the nineties that in his opinion the