1925] 
St over: A Synopsis of the Amphibia of California 
261 
During the brief period in which the stream runs clear the water 
is often warmed by the sun to a considerable degree where the stream 
is shallow r and lacks a close border of trees. Such warming of course 
also raises the temperature and increases the rate of development in 
egg masses or larvae resting on the stream bottom. 
The drying up of the creek in summer constitutes the greatest 
menace to an amphibian such as hoylu. As the stream becomes slowly 
restricted, there is a gradual concentration of the frogs in the pools. 
When these become shallow a large number of the individuals fall 
easy prey to raccoons and herons and other species of animals which 
feed upon frogs. The few frogs which escape probably do so by 
taking refuge in the deeper permanent pools, or perhaps in crevices 
in the bank ; and the small size of the adults may be a response to the 
need of being able to take shelter in small holes available in such 
places. But observation is needed on this latter point. 
It is also probable that, as a creek dries up progressively from 
the lower reaches toward the headv/aters, the frogs tend to move up 
the stream course as is indicated in the case of Hyla arenicolor. 
The presence of Tooylii in the streams of the Transition Zone of the 
northern Coast Ranges and its apparent absence in the streams in this 
zone in the Sierra Nevada are probably due to the seasonal differences 
in behavior of the streams in the two regions. In the humid portions 
of the Coast Ranges the stream grades are, in general, low, and the 
slopes of the drainage basins are covered with large amounts of humus 
and vegetational grow T th. When rain falls much of it is absorbed at 
once, without heavy run-off, and this water is later given up grad- 
ually to the creeks. In consequence of this, the streams are largely 
perennial and hence suited to the continuance of a stream-inhabiting 
amphibian such as the present species. Boylii , where it lives in the 
humid coast Transition Zone, is assured of sufficient moisture and, at 
the same time, of a stream current not dangerous to its eggs or larvae. 
The streams in the Transition Zone of the Sierra Nevada are different 
in character. Their courses are boulder strewn and precipitous, but 
these features in themselves are not necessarily of a character to deter 
a stream-inhabiting frog. The Sierran streams become swollen in 
early summer with the melting of the snows at the higher levels, and 
the rate of flow of the water in that season is so swift that amphibian 
larvae without special adaptive structures for holding their position 
in the stream would quickly be swept downstream and killed. It is 
this feature rather than the low temperature of the water which is the 
