288
THE BIOLOGY OF F_ISHES
_
1eferstothenatural‘fäôñdii'ans in M€X1CO, the temperature '
exper1mentsof
mind.
Behind
*
1
the var1ab1htyli‘e
and the differences
'
has: to be remembered that these
families came Originally from warmer ‘ 5_
eveh though some like the Salmonoids have adapted
-
__
to more northerly“ climates.
tendency of
ÏÇ
ŸlËthe males to mature oney‘ear earlier—,which is a
well—
established fact
species, both freshwater and
:
marine, "may have come from this chang'e-ofcliîrrâte or be
.
g__
s1mply an attribute of
“
maleness,” accordingto'Ïithe theory
of Geddes and Th0mson. Amongthe sh represented on
,
_' :Ï
the
therefore, the males have an extra
‘äyéar*s grouP, whilst the
females,Ÿ
;=Ï=t
Part ot‘lîe ,dispf0portion. in the
numberêÏ‘Èof-the
for-iii this way_ and__
the;dispfôportior‘iäWill go On:i_ncreasing, as in, theSvmss
lakes,: unless some- means. .
securmgpl‘ûpêÏ
'The-experimeñts of—Thumm are of specralinterest
1; '
,
.
…
—
_
-_
Ë:«_ä
:
With regard to the marine;
_for'—a disproportion füîider"naturalconditions :
is
though much
writtengnthe
subject When—we take— thenaturallawmtocons1deraüon ,“ _;
males mature! ear—lier, 1tseemsthatontheWh@1€
_
”"};
"
the numb€sareapprommatelyequalbutthemalescheo
m@rerapldlywhstthereseemsnonaturhmütotheage
:'
andgrowth0fthefemle
ReturmngonœmOf6totheñœrestmgCyprinodonts
seven
"
â”—from a smgle parrmgW1ththemale
_
It ha3f;been found—that
;
Ï
,
Îl€
later
Which-is in
'
agreementW1th
Thomson—’S-theory‘
"
'
variability
' ïÂ_Ï
99i‘ons; Lofthe
—.
:;Ànot-her
'
ÿ
….
—,_
curmus
the
.