REGULATION OF FORM AND STRUCTURES 119
rolling about in the water and the skin is thrown out into
spinous processes which give it a curious appearance (Pl. Xl),
z, 3). The tail consequently develops but little or not-at all.
But the little sh makes use of the dorsal and anal ns,
which are not abnormal to begin with, to such an extent
that they become the principal organs of propulsion, and-the
_
muscles operating them develop right back to the vertebral '
column.
.
.
,
,
These examples indicate that the regulation of the
structures in shes has been, is still, and always will be,
imperfect. The little sh, if it is to swim freely, has to
organise and co-ordinate many
parts and
processes, which pe”r'sist‘ in nearly the same condition as
when they rst originaœd. Any change, any improvement
or differentiation, can only come from less or greater move—
ment, and then the environment plays its part. And -_at
.
V1‘ious,periods the organism has given Up the attempt=to
remain a free-swimming animal and has regulated its
structures in relation to
on the bottom or onland,,
.
_