THE AGAVES OF LOWER CALIFORNIA. 39 
resented by a series of closely allied succeeding forms in the 
extensive region over 100 miles wide and 700 miles long 
that it inhabits between western Texas in the United States 
and San Luis Potosi in Mexico. It is not surprising, there- 
fore, that in arid Lower California the genus should be well 
represented and that the number of species thus far collected 
should nearly equal the number of localities from which col- 
lections have been made, nor that none should be known 
to occur in more than a rather narrowly limited region. 
There is no more reason for surprise in the fact that all 
which are yet known should be endemic, for the lower third 
of the Gulf, separating the peninsula from continental Mex- 
ico, is over 1,000 fathoms deep, pointing to a very long iso- 
lation, and 100-fathom depths continue through the remain- 
der, almost to its head, so that connection with the main- 
land to the east appears to have been restricted for a very 
long time to at most a small extension of the desert land 
flanking the mouth of the Colorado River. The coastwise 
islands all stand well up on the 100-fathom shelf and some 
of them are parted from the peninsula by very shoal water. 
The indications, therefore, are that the Lower Californian 
agaves have entered from the north and not by crossing the 
Gulf and that those of the adjacent islands have been de- 
rived from the peninsula; and the general effectiveness of 
water barriers in the spread of the genus is such as to make 
it probable that the species found on the islands reached 
them at a period when they were joined to the adjacent 
land. 
All of the known Lower Californian agaves belong to the 
paniculate subgenus Euagave. To the eye they fall into four 
groups: a compact, broad-leaved, large-toothed form, repre- 
sented by A. Shawii, confined to the northern half of the 
peninsula and, with one exception, to its western coast; a 
more open typically narrow-leaved, often fragile-toothed 
form, comparable with A. deserti of the Colorado Desert, of 
wide occurrence and less uniform appearance than the pre- 
ceding; a form with large recurving lanceolate leaves, mod- 
erately armed, occurring below the middle of the peninsula, 
