It is only possible to deal with the forms that occur within the
borders of the United States, as even individual stations of
common Mexican forms are little if at all known. These United
States forms represent a northern extension of an abundant
Mexican display. The group EUMAMILLARIA, containing twelve of
the thirty-one forms defined as occurring north of the Rio
Grande, makes the feeblest extension northward, at no place being
found far from the boundary, and all the twelve are Mexican forms
which extend but slightly into the United States. Only five of
the forms are found east of the Pecos: heyderi, the most widely
distributed EUMAMILLARIA, extending from the southeastern border
of Texas westward along the whole Mexican boundary except in
California; hemisphaericus, extending through southern Texas and
southern New Mexico; meiacanthus, also along the Mexican border
of Texas and New Mexico; texanus, a low ground form of the Rio
Grande Valley, extending from the mouth of the river to El Paso,
and suggesting a connection with the West Indian stellatus; and
sphaericus, another low ground valley form of similar range, but
apparently only extending up the Rio Grande to the region of
Eagle Pass.
The Pecos forms the eastern boundary of five other EUMAMILLARIA
forms: micromeris, extending northward from Coahuila and
Chihuahua, apparently only in the mountains between the Pecos and
El Paso; wrightii, of similar narrow northward extension, but
ranging further northward on the high plains of the Upper Pecos
in New Mexico; denudatus, also with a narrow northward extension
west of the Pecos; lasiacanthus, extending from Chihuahua with a
northern limit between the Pecos and Arizona; and grahami, a
Sonoran type which has spread between the Pecos and southeastern
California.
The ten preceding forms have evidently entered our borders from
the highlands of Sonora and Chihuahua, with the exception of the
Rio Grande Valley forms, texanus and sphaericus. Another
species, tetrancistrus, is also a Sonoran type which has reached
the eastern slopes of the mountains of southeastern California,
and extended through western Arizona to southern Nevada and
southern Utah, the most extended northern range of any
EUMAMILLARIA. The twelfth form, goodrichii, is Lower
Californian, and extends into California only in San Diego
County. A summarized statement of the distribution of our twelve
EUMAMILLARIA would be that two of them have extended from the low
grounds of Coahuila and Chihuahua and spread along the valley of
the Rio Grande; nine have come from the high grounds of Chihuahua
and Sonora, four of which have extended eastward to the low
levels of southeastern Texas; four have kept to the highlands
west of the Pecos, and one has kept to the Colorado Valley and
its tributaries, while one has a short northern extension from
Lower California.
The nineteen forms of CORYPHANTHA are decidedly more northern in
their distribution, and are our characteristic representatives of
the genus Cactus. Ten of these, however, are but northern
extensions of Mexican forms, and six of the ten have simply that
tongue-like northern extension in the mountains between the Pecos
and the Upper Rio Grande (above. El Paso), viz.: dasyacanthus,
tuberculosus, scheerii (which has also spread somewhat east of
the Pecos), and the three pectinate and closely related forms
radians, echinus, and scolymoides. Of the four remaining Mexican
forms, macromeris is a low ground Rio Grande Valley form,
extending from above El Paso well towards the Lower Rio Grande;
potsii just crosses the border in the neighborhood of Laredo; and
radiosus and neo-mexicanus have by far the greatest northern
extension, stretching from Sonora and Chihuahua to southern Utah
and central Colorado, and eastward to the Guadalupe River of
Texas.
The nine remaining coryphanths are distinctly forms of the United
States, occupying two well-marked regions, viz.: the northern
plains, and the desert region of western Arizona and adjacent
California, Nevada, and Utah. In the former region is found the
widespread viviparus, which extends from the southern borders of
British America to the plains of eastern Colorado and western
Kansas, and even crosses the Rocky Mountain divide into northern
Idaho and northeastern Washington; and missouriensis, which also
ranges from the high prairies of the Upper Missouri to the same
southern limit, and is continued southward into Texas in its
varieties similis and robustior.
In the Arizona desert region, four distinct but closely allied
forms have become differentiated from the strong radiosus stock,
viz.: arizonicus, deserti, alversoni, and chloranthus, all of
which might be regarded as distinct species. In southeastern
Texas is found an isolated form, sulcatus, occurring between the
Brazos and Nueces rivers. That viviparus must be regarded as a
strong northern extension of the radiosus stock can not be
doubted, as the low depressed cespitose northern form seems to
merge southward so gradually into the simple more robust ovate to
cylindrical forms of radiosus as to suggest the propriety of
regarding them all as specifically identical.
The result of a closer inspection of the distribution of these
nearly related forms is worthy of note. C. viviparus extends
from British America and the Upper Missouri to eastern Colorado
and western Kansas; neo-mexicanus (the form most nearly related
to viviparus) extends from central Colorado and southern Utah
into Mexico; at the southeastern edge of this range begins
radiosus and extends eastward through southern Texas; from the
western edge of neo-mexicanus the form arizonicus extends
westward into southern California, touching chloranthus at its
Utah limit, and at its California extension reaching alversoni
and deserti, the latter of which extends northward into the
desert region of southeastern California and adjacent Nevada.
Taking this type as of Mexican origin, it seems to have entered
the United States from Sonora and Chihuahua, and to have spread
in three directions, viz.: eastward through southern Texas;
westward and northwestward into southern California and southern,
Utah; and northward to the head waters of the Missouri and
British America, though we would limit the northern extension of
the present specific type to central Colorado, and would regard
the still more northern forms as of the same origin but entitled
to specific rank.