10 BOTANICAL GAZETTE [JANUARY 
In the Tertiary period an ocean rolled its waves over these 
plains that are now covered by the savannas of Llanos and the 
forests of the Orinoco and the Amazon. When at last the bot- 
tom of the ocean got above the sea level, plants from the sur- 
rounding highland immigrated, and the resulting vegetation got 
its stamp from the physical conditions. Where the ground all 
the year round was moist and the rainfall abundant arose the 
dense forests and marshy land along the Orinoco and other riv- 
ers; and where the ground was dry, at all events periodically, 
the savannas resulted. In the forests and on the marshland the 
number of species certainly is very large on account of the 
extraordinarily favorable physical conditions of the place, while 
in the savannas, on account of the dryness, the development of 
species has been much slower. If then we compare the savan- 
nas with the campos of Brazil, the main cause of the difference 
in richness, the existence of which I must suppose to be real, 
seems to spring only from the difference pointed out in the age 
of the two vegetations. On the highland of Brazil, vegetation 
surely has existed as early as any plants existed on earth, but 
on the savannas of Venezuela they first arrived after the Ter- 
tiary period. Here in the north of Europe it is the glacial 
period, in Venezuela it is the late uplift of the earth’s crust that 
is the cause of the comparative poverty of species. - 
Thus we see how both physical and historical factors may 
act in many ways upon the flora of different parts of the world 
and imprint upon them a different character in respect to the 
richness of species. It is not at all easy to determine the 
respective influence of the two groups of factors ; only we may 
suppose that as to the richness of species the historical ones are 
the most important. Now I should like, as far as in this short 
hous it is possible, to instance how physical facts act upon the 
tropical vegetation, and give it another look than that of our 
northices one. For that purpose I shall speak briefly of the three 
vegetation formations already mentioned, and most general of 
all: the forest, the scrub, and the savanna; but before I do so, I 
shall remind you of the physical factors, light and heat, both of 
