10 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. XI. 
of Inagua entered by way of the Greater Antilles and not through the northern Bahamas (pl. E). 
The following diagram expresses these conclusions graphically: 
sitet CARIBAEAE. _-7 BAHAMANAE. 
ieee ee inugta ae 
Costaricenses ? ?------- LS VIPAR At: o teredees 
, 
ooo 2% lS aes OR ere 
Even between islands rising from a common bank covered by water of considerable depth, 
the local forms of Agave have undergone differentiation for the most part; but the small extent 
of differentiation between these forms, or even between the groups of species characteristic of 
the larger island groups, points to rather slow plasticity in the genus during the time which 
must have elapsed since their isolation (pls. D and E)—probably at an early period in modern 
geological time. The scant occurrence of Agave in South America may also indicate that this 
period antedates its passage of the Isthmus and supports the conclusion that the Antillanae 
rather than the Viviparae stand nearest to the parent stock. 
This analysis does not require a discussion of the union and disruption of the continent 
before Tertiary time, or perhaps even during any part of that period. It is significant that 
Agave is so slightly represented in South America when compared with such Tertiary genera as 
Fagus. Geologists and physiographers, though they differ in willingness to admit elevations and 
depressions corresponding to the deepest present channels, find reason for believing in unions and 
disruptions of some sort; and a Pacific faunal element, found in the Gulf and Caribbean deposits, 
shows that the waters from the west must have entered when these were laid down. The West 
Indian agaves do not controvert Spencer’s conclusion! that the deep channels separating 
these islands are to be interpreted as erosion canyons formed above the level of the sea in a land 
elevated after the Matanzas limestones and the Lafayette loams had been silted into still earlier 
canyon deeps at the end of the Tertiary period. 
The mental image that may be formed of the Caribbean region in early Quaternary time 
is that of a rather low tableland stretching from what is now Yucatan and Central America to 
South America; bordered by a volcanic range the peaks of which now project some thousands 
of feet above the water, though the present difference of level between their summits and the 
intervening deeps may perhaps be due to crumpling as well as general elevation and attendant 
erosion; and including extensive lagoons corresponding to the present Caribbean deeps, with 
the Gulf of Mexico to the north constituting a great dead sea. Its climate, as Spencer 
surmises, may have been comparable with that of the Mexican plateau to-day, with torrential 
rainfall on its limiting mountains. No sounding data, as they are now obtainable, may be 
depended on to outline this connection of the continents, because elevations and subsidences 
can hardly be assumed to have been uniform throughout, but a restoration of the contours 
now marking 1,200 or 1,500 fathoms of depth gives an approximate suggestion of the extent 
of the land at this time (pl. D). 
Onto this Pleistocene plateau, already sinking and soon cut off from Yucatan by the 
opening of a connection between the Gulf and the Caribbean Sea (pl. E), may be pictured the 
dry-shod passage of the primal form from which are descended the dominant and characteristic 
fleshy-leaved agaves of the West Indies, if not also of another form from which the now unique 
and localized xerophytic Inaguan group has sprung. These forms, making their way to the 
east and south around the widening seas and heading the fiords into which the settling 
canyons were passing, were undifferentiated, apart from local variation, until the filling by the 
sea of what is now the Anegada channel divided them into stocks from which have been derived 
the Antillanae and subsequently the Antillares to the north and the Caribaeae and probably 
1 Bull. Geol. Sac. America, vol. 6, 1895, p. 128, etc. 

