236
‘
THE BIOLOGY OF FISHES
_
!
It would appear, therefore, that the physical changes
f '
during the Chalk period had a profound inuence on the
!
mode of life and characters of shes.
They led to the
appearance
of new forms of both a sedentary and pelagic
_
type, coastal and deep-sea forms, which did not exist
previously. And this led, again, to the appearance
of new
'
freshwater forms.
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2. MODERN PERIODS
What happened to the Earth during the Chalk and
Tertiary periods we can but faintly imagine. The land—
'
‘
masses appear to have been in a more or less uid condition,
rolling together in places and rising subsequently there into
huge mountains, thus causing many parts to be submerged
under the sea for long periods. The changes cannot be
"
referred to ice—sheets, like those of thelater Glacial epochs,
but seem rather to have come from deep breaks in the lower_
lying magma. Whatever may have been the causes, we
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_
have to picture the continents slowly becoming separated
from one another, so that Africa is no longer connected with
,
“
South America and the North Atlantic basin is opento
:
the south.
The northern part, on the other hand, is closed
-_Ï î —
»_
Eastern States until late in the Ceozoic period, and Scandi—
navia, Greenland, and -Canada_were probably .united until
'
'
!
the rst Glacial epoch.
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,
_
‘
:
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Î
”: ' With
the breakdown of the-bûdgebetWeen Africa and
'
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South;Arxïericagit might .be supposed that 'a =considerable
:“
into the“: North Atlantic took place from the , .
so‘uth,
the
This was probably
—
theïcaSe, but -it_çhäS ;to be=îremembered
waters of
the
Were already well populated with
,
the 'pelagic
had been an
*
'
eastern..- entrance: over '—'Syria,gand
before. the
'
'
!
=? *
,
Atlantic: openédl »
is_'É0t very
therefore,
-
'
betWeen
.For;example, the
'Èrotosp‘hyrænidæ which-seem:1
have-given; rise .to :..qùit—e
…
;: