Gillichthys mirabilis— BAMLOW 
be supported by more than one line of evidence. 
The gradient of meristic characters in mira- 
bilis follows the temperature gradient of the 
late summer period. The number of rays in the 
dorsal and anal fins increases from north to 
south in the Gulf of California, whereas over 
the same latitude on the Pacific Coast the oppo- 
site relation holds, and the counts tend to de- 
crease from north to south. The same reversal 
of trends between the Gulf and outer coast oc- 
curs in the pectoral fin. 
The fin indices decrease from north to south 
on the Pacific Coast, but continue to decrease 
in the Gulf from south to north. Thus a con- 
tinuous dine in the meristic characters exists 
from. San Francisco south, on the Pacific Coast, 
and then north up into the Gulf of California. 
It might be argued that the late summer tem- 
perature gradient in the Gulf is opposite to that 
of the outer coast, and that this environmenal 
difference directly accounts for the reversal of 
the fin-index gradient. This cannot be the case, 
because the counts are determined while the 
eggs and larvae develop during late winter 
through early summer; then the temperature 
gradients of the Gulf and Pacific Coast are 
parallel, warmer to the south. If any correlation 
exists between environmental temperatures and 
number of countable elements, it is with the 
gradient of extremely warm temperatures found 
late in the summer, not those occurring during 
early development. 
This dine, however, might reflect nothing 
more than the progressive physical separation 
between populations. 
The fin index for seta in the upper Gulf of 
California continues the trend noted in mira- 
bilis , that is, for lower indices toward the north 
in the Gulf. The average value for seta there 
is about 82 as compared to about 95 for adja- 
cent populations of mirabilis. The temperature 
situation that prevails in the habitat of seta 
could be regarded as an extension of the gra- 
dient seen in the habitats of mirabilis from 
south to north in the Gulf. The habitat of seta 
is located on the coast in the high intertidal 
zone among black lava flows, boulders, and 
streams of seepage water (Barlow, 196L*). Cli- 
matically this environment is even more rigor- 
ous, and surely experiences higher summer 
67 
temperatures than do the nearby sloughs where 
mirabilis occurs. 
Returning to the fin indices of mirabilis, 
specimens from the Salton Sea, compared to 
their parental stock, have much higher counts 
in their dorsal and anal fins, and in their pec- 
toral fins as well The differences probably can 
be attributed to retardation of developmental 
rate. If the interaction between these fins be- 
haved as expected from observations on the 
geographical dine, the pectoral fin counts should 
have decreased while the median fin counts in- 
creased. As shown by the fin index, the rela- 
tionship between median and paired fins, re- 
markably, is unchanged in these fish. 
Fluctuations in the mean number of seg- 
mented rays from year to year in a given popu- 
lation are in the same direction for the median 
fins and the pectoral fins. In other words, the 
changes in the number of elements in paired 
and median fins are positively correlated in their 
response to environmental changes. Colder years 
induce the formation of more rays in all of 
these fins. 
In each fish, however, the number of pectoral 
fin rays appears to be inherited independently 
from the number of median fin rays. In a given 
year class, the fish with more numerous rays in 
the median fins have, on the average, the same 
number of pectoral fin rays as do fish with less 
median fin rays. Thus the median fin and the 
pectoral fin elements respond to the environ- 
ment in the same way, but independently of 
each other. 
From the foregoing evidence it seems reason- 
able to conclude that the change in relationship 
between the number of elements in the median 
fins and in the pectoral fin, the fin index, over 
the geographic range of mirabilis indicates ge- 
netic divergence. 
Analogous differences between inter- and 
i ntra- population al variation have been observed 
in other gobies (Johnsen, 1936: 8-9) . In dif- 
ferent populations of Gobius flavescens and G. 
minutus , the vertebral counts (median fin counts 
follow vertebral counts) are negatively correlated 
with pectoral fin counts. In contrast, these meris- 
tic characters are positively correlated within a 
given population; larger fish have more verte- 
brae and average more pectoral fin rays as well. 
