226
,
THE BIOLOGY OF FISHES
'
the great zone of coal beds was laid down, thus where
:
land was of a marshy, freshwater nature, we have to
the salt and fresh waters of the northern hemispheré as
mixed up together in close proximity to one another.
chief centres where disturbances have been most
have evidently Iain along this carboniferous zone and its
Ï'f
immediate neighbourhoôd to the north and south,
the Rocky Mountains and Great Lakes of America
Britain and Europe to the Caspian, and further across Persia ;:ÿ'E
«
and
to the indian Archipelago.
:
_It is not necessary to suppose that any great fall
:
rise of the land occurred.
The deposits were
laid down locally, as if extensive oodings and
*
taken
place within the land—locked regions, f'_ =
.
perhaps— 'dùe'to volcanic agencies or the invasions of ice- [ 'ÎfË
sheets}!
With the presence of different kinds of water
clOsdby, these oodings would inevitably lead to a heavy
Ï
,
mortality among the more or less sedentary shes, whilst
_
‘”î
:
the active and pelagic shes would more likely escape. We
may thus believe that the extinction of the armoured
_
-
Ganoids '- and Dipnoi was due to natural causes; other
_
_
fâ1‘masWere£also blotted out, in later periods in a similar
'
Way;
Speaking metaph0rically,we may say that a struggle
Was’going on between the sedentary and the=pelagic forms, ;, ; f:
"
,
and that when the physical
forms surviVed. The extinction
—
though it was probably spread
of
:
Of all the numerousçGanoids
that lived in thè‘Î
_.
T he few
_]
thé—Ï
they have been
to a
PeÏ10d _f:_
7
_'
,
,
Î Ï
Wenœdin0twonderthat lâche 1 sedèntary shes wele
ïÏ_'fjt1meaftert1mecaughtmiheir retreats and Wiped Out,
:sôïheof the pelagic fér'm’s— were
iie îiPalæoniscidæ' d‘esée
_
ÊThePalæomscrdæ
their _