1925 i Storer: A Synopsis of the Amphibia of California 
17 
The principal reaction^ to the dry climate of California are mani- 
fested in restriction of spawning to the rainy period, in abbreviation 
of the time spent in egg and larval stages, in the thickening of the 
skin in the adults, and in the development of digging spades whereby 
the animals are able to burrow into the ground for protection from 
desiccation during the dry period. 
Cope has already remarked that the western species of Rana are 
thicker skinned than the frogs of eastern United States where the 
climate is more humid. It is of interest to note that within the aurora 
group of pond-inhabiting frogs here in the west, the northern form, 
Rana aurora aurora, which inhabits the northwestern humid coast 
district, has a relatively thin and smooth skin, whereas the southern 
and interior subspecies, Rana aurora draytonii, while also an in- 
habitant of ponds but in regions where the summer air is drier, has 
a roughened and thickened skin, the texture approaching that in some 
of the toads. The young of draytonii have thin, soft skins suggesting 
ancestry in a humid environment ; old adults have the dermal covering 
very rough surfaced and thickened. In Rana catesbeiana, native to 
the humid eastern portion of North America, the skin does not become 
greatly thickened or roughened with age. Bufo alvarius, the toad of 
the Arizona-California desert, which lives along river courses and 
around other perennial sources of water, has a smooth skin (evidently 
an indication of aquaticity) which, however, is thickened, presumably 
as a check against too great loss of water. It is a Bufo which under 
desert conditions has taken on, in part, the habits and appearance 
of a Rana. The frog of the California foothill streams, Rana b. boylii, 
develops, even at a very early stage, a roughened and thickened type 
of skin, and the adults are conspicuously pachydermous. The more 
aquatic member of this group, sierrae, and the not distantly related 
pretiosa, which inhabit lakes or perennial streams in high altitudes 
or more northern latitudes, are smoother and thinner skinned than 
either boylii or muscosa of the hot dry foothill regions in central and 
southern California. 
A second response to dry conditions is found in the terrestrial 
spawning habits of the western species of Plethodontidae. Wilder 
and Dunn (1920) have advanced the theory that the Plethodonts were 
originally inhabitants of mountain brooks and that the land-dwelling 
habit is a secondary development. All of the western species belong- 
ing to this group, so far as their habits are known, are strictly terres- 
