S80 Baron Humboldt on the Laws which are observed 
not change, however, in ascending from the plains toward the 
mountains, in the same manner as they change in approaching 
the pole : this is the case with the Monocotyledones, considered 
in a general view, as well as with the Ferns and Compositae. 
{Proleg, p. 51. and 5S. ; Brown on Congo, p. 5.) 
It may further be remarked, that the development of vege- 
tables of different families, and the distribution of forms, depend 
not on isothermal latitudes, nor on geographical latitudes alone ; 
but that the quotients are not always similar on the same iso- 
thermal line of the temperate zone, in the plains of America 
and of the Old Continent. There exists, under the tropics, a 
very remarkable difference between America, India, and the 
west coasts of Africa. The distribution of organic beings on 
the globe, depends not only on very complicated climatic cir- 
cumstances, but also on geological causes, with which we are en- 
tirely unacquainted, because they are connected with the ori- 
ginal state of our planet. The great Pachydermata are want- 
ing at the present day in the New World, although we find them 
still in abundance in analogous climates, in Africa, and in Asia. 
In the equinoctial zone of Africa, the family of palms is far 
from numerous, compared with the great number of species of 
equinoctial America. These differences, far from deterring us 
from the scrutiny of the laws of nature, ought to excite us 
to study these laws in all their complications. The lines of 
equal heat are not parallel to the equator. They have, as I 
have tried to prove elsewhere, convex summits, and concave 
summits, which are distributed with great regularity over the 
globe, and form different systems along the eastern and wes- 
tern coasts of the two worlds, in the centre of continents, and in 
the neighbourhood of the ocean. It is probable, that when 
philosophical botanists have travelled over a larger extent of 
the globe, we shall find, that often the lines of the maxima 
of dgronpment (the lines taken from the points where the 
fractions are reduced to the smallest denominator,) become 
isothermal lines. In dividing the globe by longitudinal bands 
comprehended between two meridians, and in comparing the 
numerical proportions under the same isothermal latitudes, we 
perceive the existence of different systems of agroupment. We 
^an already, with the actual state of our knowledge, distinguish 
