216
THE iBI‘OLOGY OF FISHES
b‘lotted out in the Pàlæozoic and Mesozoic periods. But
,
the modern Dipnoi, which have retained several important
_
and primitive features of the Arthrodirans and CtenodontS
'
(Phaneropleuron), are represented as having maintained
_
£
some connection with the earliest forms.
'
'
'
The principal connections between ancient and modern
,
days have, however, been maintained by the free-swimming,
Î"
pelagic forms of the Teleosts and Elasmobranchs ; in both
,
cases the lines are unbroken and unmistakable if we accept
'
«
'
the seemingly inevitable conclusion, that the osseous f0rms
'
preceded the cartilaginous.
'
1
The geological periods to the right aïe marked off as if
of nearly equal duration, but they are only intended to give
:
some idea of the times when the various groups arose and
'
_
"
—ourished.
'
F0r the duration of the periods reference should
'
'
be made..to p. 202.
The Palæozoic period lasted about
‘
ten timeS:as longas the—Mesozoic, the latter at least twice
as long as the Cenozoic and recent together. The oldest *_
,
of the living forms (Clupeids, Rays, and Sharks) go back
,
about 20 million years, which may
be taken as the longest
time for the formation of the species of the “present day.
_.
But with Such a long period “at their disposal and the
numerous changes in the physical cOnditions
'
during this
_
-
period“, we can readily— understand how the shes have
'
become so varied and dierent from one another...
'
1__
The principal groups “mentioned "—- are
'
but a few words
d1v1s1ons gwen
Ï
"”Of’ the Teleostei;
"
All
classrcatronrequ1re a
;
:
-
'_g'eneral'
“which do
n0tt in
;(1i'9'04) had several ? ;
of
etc., and this
'
'
:_
system had the
advantage of revealing“the
_
'
,
difc'ulti‘ësf 1ntheway0f Ïa':.natural system. Tate Regan_
_
_
;(19é7'ïahd:t19ï3)
the dumping grôund,’
_
Whil’St
Goodrich (I_90ô)’; cut the GrOrdian kn0ts by arranging _'
‘_
; theshesÎaçcôiçdirig‘toï3types. There. is no doubt‘that the
'
'
'
and m0st praictical'so long as we
,‘deeply‘fint‘o problems of‘phy10gehy; ,
'
THE iBI‘OLOGY OF FISHES
b‘lotted out in the Pàlæozoic and Mesozoic periods. But
,
the modern Dipnoi, which have retained several important
_
and primitive features of the Arthrodirans and CtenodontS
'
(Phaneropleuron), are represented as having maintained
_
£
some connection with the earliest forms.
'
'
'
The principal connections between ancient and modern
,
days have, however, been maintained by the free-swimming,
Î"
pelagic forms of the Teleosts and Elasmobranchs ; in both
,
cases the lines are unbroken and unmistakable if we accept
'
«
'
the seemingly inevitable conclusion, that the osseous f0rms
'
preceded the cartilaginous.
'
1
The geological periods to the right aïe marked off as if
of nearly equal duration, but they are only intended to give
:
some idea of the times when the various groups arose and
'
_
"
—ourished.
'
F0r the duration of the periods reference should
'
'
be made..to p. 202.
The Palæozoic period lasted about
‘
ten timeS:as longas the—Mesozoic, the latter at least twice
as long as the Cenozoic and recent together. The oldest *_
,
of the living forms (Clupeids, Rays, and Sharks) go back
,
about 20 million years, which may
be taken as the longest
time for the formation of the species of the “present day.
_.
But with Such a long period “at their disposal and the
numerous changes in the physical cOnditions
'
during this
_
-
period“, we can readily— understand how the shes have
'
become so varied and dierent from one another...
'
1__
The principal groups “mentioned "—- are
'
but a few words
d1v1s1ons gwen
Ï
"”Of’ the Teleostei;
"
All
classrcatronrequ1re a
;
:
-
'_g'eneral'
“which do
n0tt in
;(1i'9'04) had several ? ;
of
etc., and this
'
'
:_
system had the
advantage of revealing“the
_
'
,
difc'ulti‘ësf 1ntheway0f Ïa':.natural system. Tate Regan_
_
_
;(19é7'ïahd:t19ï3)
the dumping grôund,’
_
Whil’St
Goodrich (I_90ô)’; cut the GrOrdian kn0ts by arranging _'
‘_
; theshesÎaçcôiçdirig‘toï3types. There. is no doubt‘that the
'
'
'
and m0st praictical'so long as we
,‘deeply‘fint‘o problems of‘phy10gehy; ,
'