Tropic South-American Elements. 305 
than it is to that of the West Indies, and includes types of the former, which 
are not known to reach the West Indies. 
The facts here considered relative to the composition and the distribution 
of the flora of Central America and Mexico demonstrate that, apart from the 
peculiar Mexican element and the southward extensions of northern types, there 
are two other distinct elements, 'namely, the tropic and the Andine, 
The relationship existing between the past and present distribution of species 
in Mexico, Central America and the West Indies, as controlled by the geologic 
and physiologic changes accompanying elevation and subsidence, and the change 
of climate, as well, as the relative distribution of the land and water areas is 
here briefly set forth. It should be stated that a separate description of the 
various West Indian elements has been purposely omitted from the foregoing 
discussion because of ignorance, but will now be referred to in the following 
description of the factors underlying and determining the distribution of plants 
in Mexico, Central America and the islands forming the Greater Antilles. 
According to the evidence that we have, the evolution of new forms was 
most rapid in Mexico, for from the facts we must believe, that the cacti, agaves 
and yuccas originated there. The period at which this development took place 
must have been subsequent to the upper Cretaceous period for the greater 
part of Mexico except a narrow strip along the west coast was under the 
ocean during the lower Cretaceous. During the upper Cretaceous period, Mexico 
was elevated above the sea and during the lower Tertiary was connected with 
the West Indian islands. During the upper Tertiary, the West Indian bridge 
was severed and the North American continent reached its present shape. 
It was during the Tertiary, that the physiographic and geologic changes were 
taking place in Mexico when the valleys were filled to their brim by erosion 
and volcanic discharges, recorded in an earlier part of this work. We have, 
therefore, no reason to doubt that during these physiographic upheavals, that 
Mutation was active in the development of new species and that many of the 
forms of vegetal life peculiar to the Mexican plateau then had their rise as 
new forms. Let us briefly trace the history of the vegetation of North America 
as illuminated by the discussion of the facts presented above. 
If ORTMANN’s ') views as to the distribution of the land and water decapods 
are correct, many tropic forms came into Mexico during the upper Cretaceous 
when Mexico was connected with ‚South America and the West Indies by a 
land bridge which was severed in early Tertiary times. But the writer believes, 
that the tropic element of the continental flora was of later derivation than this, 
although many forms no doubt migrated northward during the upper Cretaceous 
Period, 
Origin of the Antillean Flora. The West Indies and Central America 
constitute an evolutionary region of their own distinct from the North American 
nn ABBREREN 
1) ORTMAsN, Dr. A. E.: The geographical Distribution of freshwater Decapods bearing upon 
Ancient Geography. Proceedings American Philosophical Society XLI: 267. April—December 1902. 
Harshbe rger, Survey N.-America. a 
