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University of California Publications in Zoology [Vol. 27 
embryos collected on February 15 hatched on February 28), so that 
hatching probably occurs by early February in years of normal rain- 
fall. Meanwhile aquatic vegetation and insects are developing so that 
by the time the larvae are ready to feed a bounteous supply is avail- 
able to them. Growth is therefore rapid. Continuance in the larval 
state, such as occurs with the “axolotls” in the lakes of Colorado and 
of the Valley of Mexico, would be impossible for the larvae of A. 
calif orniense in the average pond which dries up by May or June. 
Metamorphosis therefore takes place in mid-May or early June, and 
the young salamanders must at once seek shelter in mammal burrows 
or in such other underground retreats as may offer them protection 
against desiccation. Subsequent foraging is probably done at night 
when the relative humidity, particularly in the stratum of air in 
contact with the ground, is high enough to permit a moist-skinned 
salamander to venture abroad without danger of desiccation. 
The population of Ambystoma calif orniense in California is 
isolated from the parent stock, “tigrinum,” by an area several hun- 
dred miles in extent, including the Sierra Nevada and Great Basin. 
The query arises then as to the probable path by w T hich the calif or- 
niense stock reached California. Two routes seem possible: (1) a 
southern route, by way of the Mohave and Colorado' deserts, at a time 
when those areas were less arid than they are at present (see chapter 
on Hyla arenicolor, p. 213), and (2) a northeasterly route by way 
of the northern Great Basin. The southern route may be objected to 
on the ground that species which reached California by that path, 
such as Hyla arenicolor and Bufo cognatus ssp., are to be found at the 
present time in the region south of Tehachapi. As stated in the para- 
graph on “Range,” Ambystoma calif orniense has never been found 
south of the southern San Joaquin Valley. Had it arrived through 
Arizona and southeastern California w r e w r ould expect to find it repre- 
sented on the coastal plain of southern California. The northern 
route, therefore, seems the more probable one. At the present time 
tigrinum is to be found in Colorado', Montana, and Utah. It occurs 
commonly in some of the high lakes in the Rocky Mountains, as at 
Lake Solitude, altitude 9,000 feet, Wasatch County, Utah (Van Den- 
burgh, 1915, p. 101), and the lakes on Grand Mesa, altitude 11,000 
feet, Mesa County, Colorado (J. D. McDonald, MS). From these 
localities the species could easily have spread, around the western 
margin of the Great Basin to the northeasterly part of California. 
Tigrinum also occurs locally in northern Arizona, as on San Francisco 
