Gillkhthys mirabilis-— Barlow 
69 
reflect the degree of physical separation between 
populations. The less the exchange of genetic 
material, the greater the opportunity for dis- 
similarities to arise and persist. 
If we are to understand why the variation 
is in a particular direction and not random, 
then we must first seek correlations between 
environmental factors and the observed geo- 
graphic cline. Temperature and illumination are 
the only obvious parameters suitable to such an 
inquiry. The only evidence available relates to 
temperature differences, so illumination will 
not be treated. This raises the problem, inherent 
in such an approach, of formulating a self- 
realizing assumption. 
I have already pointed out the crude correla- 
tion between the dines of meristic characters 
reported on and the gradient of temperatures 
found late in the summer. In another article 
(Barlow, 1961b) , I have argued that relatively 
stable differences in the means of counts be- 
tween populations probably reflect physiological 
differences, most likely temperature responses, 
that are genetically determined. Changes in 
counts between populations, as suggested by 
Hubbs (1928), might constitute examples of 
the well-known Baldwin effect (Baldwin, 
1896): phenotypic modification of the counts 
(resulting from physiological adjustment) be- 
comes genetically augmented and characterizes 
the population. A similar argument might hold 
for the differences in variances that apparently 
are associated with different photoperiods. 
Peculiarities of G. mirabilis from the Salton Sea 
The Salton Sea is a saline lake situated in the 
Salton Sink, a northward extension of the Gulf 
of California basin. According to Carpelan 
(1958), it has a surface area of about 340 
square miles, but a maximum and highly vari- 
able depth of only 12 m. The salinity of the 
water is about 33 %©, although the Salton Sea 
is not of marine origin; it has been formed by 
the inflow and evaporation of Colorado River 
water. Relative to ocean water, sulfate (2.7 
times more concentrated) and calcium (1.9) 
ions are especially abundant, whereas potassium 
(0.58) and magnesium (0.75) are appreciably 
less concentrated. The annual maximum and 
minimum surface water temperatures are usu- 
ally about 36C and 10C, respectively. Great 
daily fluctuation in temperature occurs, espe- 
cially in shallow water (Barlow, 1958). 
The specimens of mirabilis from the Salton 
Sea differ from those on the outer coast in just 
the ways one would predict for fish whose 
growth had been drastically retarded. All fin ray 
counts and the vertebral counts are high (but 
the relationship between the fin counts is un- 
changed ) , supernumerary spiny rays are formed, 
scalation is incomplete, the head and its related 
parts are small, the fins are low, and the com- 
pletion of the interorbital canal is delayed. 
Anomalies, such as a continuous slit for the 
anterior and posterior nates, an abbreviated 
postorbital canal, and flaps on the jaws, are 
holdovers of conditions usually seen only in 
postlarvae. 
The unusual combination of salts in the water 
of the Salton Sea probably interferes with the 
development of the fish. The water temperatures 
are not to be considered responsible, for they are 
moderate during that period of early develop- 
ment when the characters are determined. In- 
deed, the temperatures are comparable to those 
of habitats along the sea coasts. Other water 
conditions (oxygen tension, pH, and illumina- 
tion) also are similar to those encountered by 
naturally occurring populations. The Salton Sea 
fish at times are heavily infested with a mono- 
genetic trematode, but so are coastal populations. 
Besides, many of the characters are determined 
before hatching and so could not be modified 
by debilitation caused by parasites (Hubbs, 
1927). 
The possibility exists, of course, that the 
mirabilis in the Salton Sea have diverged ge- 
netically, and that the structural differences are 
the result of such genetic change. The original 
stock consisted of merely 500 fish, of which 
only a fraction could have reproduced success- 
fully. By chance alone, certain alleles must have 
been lost, others fixed, even if all 500 fish are 
assumed to have reproduced (Wright, 1951). 
The population expanded rapidly to a large 
number of fish, as did later introductions of 
other species. In a large population, genetic 
drift would no longer be operative (Wright, 
1951). During the 25 years after introduction, 
selection has been effected, for the most part, 
