298 Part III. Chapter 3. 
The Central American states of Guatemala and Costa Rica are according 
to our present knowledge very poor in cactuses. This is evidently due to the 
fact that these states were originally covered with a dense forest, but on the 
dry interior plains species of Peireskia abound, and such epiphytic forms of 
the family as Phyllocactus and Rhipsalis occur here in the moist tropic forest. 
The writer has seen at Orizaba abundant proof of this in the presence of Phyllo- 
cactus growing in the forks of the forest trees. On the other hand, the small 
number of species found on the dry plains of the leeward side of the larger 
West Indian islands is enigmatic, unless one considers, that these islands were 
never intimately connected, even when a land bridge existed with the arid 
Mexican table-lan 
The southern peninsula of Florida, which belongs in many respects as shown 
by many other families to the West Indian Region, has only three species of 
cactuses. 
The fact, that we have in South America another cactaceous headquarters, 
does not militate against the Mexican origin of the family for between arid 
Mexico and arid South America there is no place where the family could thrive. 
Once the forms originating in arid Mexico reached arid South America, where 
we are compelled to considet the conditions somewhat different, we would expect 
to find the rapid multiplication of new forms distinct from those in the north 
such as Zfeiffera and Cariata in Brazil. Perhaps in the past, the Isthmian 
region was not as humid as at present and the Cactaceae had a better chance 
to spread southward. Or, that under similar conditions of environment in such 
widely separated localities, the progenitors of the present cactaceous forms 
mutated along similar lines both in arid Mexico and arid South America, SO 
that the present distribution may be explained by the polygenetic origin of 
the cacti and perhaps in some instances by polyphylesis or the approach in 
morphologic structure of two or more different species, at different places at 
the same time, or at different times, so as to form a new species. Since the 
agricultural Indians have occupied Central America (and vast prehistoric mines, 
as at Copan and Palenque indicate a long occupancy) the original forest has 
been destroyed by fire and by other means over extensive areas and desert 
conditions have been established, where such plants, as Peireskia guatemalensis 
(Fig. 10), form a prominent feature of the flora. 
Yucca-Vegetation. The phytogeographic distribution of the species of the 
genus Yucca is of interest in connection with the Mexican origin of the genuS- 
TRELEASE ') believes that the Mexican table-land is the original home of the entire 
group and that it has followed the customary sweep around the Gulf with a 
reflex wave northwest into the Appalachians. Other species have entered the 
ı) In a letter to the writer dated Jan. 20, 1904. The northern and southern limits of the 
genus Yucca are indicated in the colored map. Beginning in Southern California, the northern 
line runs in a sinuous course across the continent to the month of Chesapeake Bay, while the 
southern line similarly extends from the head of the Bay of Lower California across Mexico t0 
the mouth of the Rio Grande, 
