32
THE BIOLOGY OF FlSHES
from east to west round the world—«in all possible seas
and lakes.
‘The signicance of regarding the haunts of shes in this
'
broad perspective lies in the conclusion it indicates. The :.
difculties encountered by the shes in becoming adapted
to their sùrroundings have arisen, in the rst place, from the
physical conditions, temperature, light, pressure, and the
’
‘
more
subtle chemical conditions, salinity and gaseous
'
contents of the water.
The habits of rising to the surface
to obtain more‘ oxygen, of swimming fast or sloW or 'resting
on the bottom, of shunning the light—few shes like to
bask‘ in the sun like lizards——changingfrom fresh to salt
water and so on, are elementary adaptations of even deeper
’
.
importance than the biological conditions, =—'that is,. the
,
relations to
-
.
,
,
,
zi“
DEFISHPS
'
.
_
habits
… anf
thb1gy Wmu…1ud th
_
The population "ef::the“sèà is
mom”
nd no parallel among higher
an1mals,not even amongthose that live in the sea. For this
Î_Ï
is not easy to interpret the meaning of
: the$e _
through :the intensive investigations of recentyearg
‘
15%—
Byway .9fgint_r_oduCtiona 1tmaybe f saldthattheeggsare
_
,
àtÏ ähÔfher of several
"
other '=paçesgandthe5äçtgäitsgewandé‘fgr…n—a f…. Some of
movement1s pass1ve,some
!
and delibe'rate,
;
…
', _and some
CompulsorY, as if the
__animal Wer—e in no _Waÿ;a=frée
Diiculties arise when
-
to . determine “the causesj beh—ind‘these
Ï’
To._simpli£y matters,
migfàti0ns of .the
,
,
:
-.
be kept apartand
_
_,
…
treated SeparateIY. -;During_ .theSe the _Shes are not; quite ,
.and_thezcauses‘ operating are Of 5 adierent-f nature
‘
_
ï
ordiary movements?
…
r
…