M. Boussingault, "the country is covered
with immense forest and traversed by numerous rivers; it rains there
almost ceaselessly; and the mean temperature of this moist district
scarcely reaches 78.8 deg. F.... At Payta commence the sandy deserts of
Priura and Sechura; to the constant humidity of Choco succeeds almost at
once an extreme of dryness; and the mean temperature of the coast
increases at the same time by 1.8 deg. F."[57] Even in this selected
favourable instance it might be argued that the part performed in the
change by the presence or absence of forest was comparatively small;
there seems to have been, at the same time, an entire change of soil;
and, in our present ignorance, it would be difficult to say by how much
this of itself is able to affect the climate. Moreover, it is possible
that the humidity of the one district is due to other causes besides the
presence of wood, or even that the presence of wood is itself only an
effect of some more general difference or combination of differences. Be
that as it may, however, we have only to look a little longer at the
table before referred to, to see how little weight can be laid on such
special instances. Let us take five stations, all in this very district
of Choco. Hacquita is eight hundred and twenty feet above Novita, and
their mean temperatures are the same. Alto de Mombu, again, is five
hundred feet higher than Hacquita, and the mean temperature has here
fallen nearly two degrees. Go up another five hundred feet to Tambo de
la Orquita, and again we find no fall in the mean temperature. Go up
some five hundred further to Chami, and there is a fall in the mean
temperature of nearly six degrees. Such numbers are evidently quite
untrustworthy; and hence we may judge how much confidence can be placed
in any generalisation from these South American mean temperatures.

The question is probably considered too simply--too much to the neglect
of concurrent influences. Until we know, for example, somewhat more of
the comparative radiant powers of different soils, we cannot expect any
very definite result. A change of temperature would certainly be
effected by the plantation of such a marshy district as the Sologne,
because, if nothing else were done, the roots might pierce the
impenetrable subsoil, allow the surface-water to drain itself off, and
thus dry the country. But might not the change be quite different if the
soil planted were a shifting sand, which, _fix