Ixxx REPORT—1858. 
equal climate, augments their productiveness, and enlarges the limits of their 
capacity to support different climatal conditions. Agriculture also effects 
local modifications of climate. The clearance of forests, by diminishing the 
cooling influence of evaporation from leaves, increases the temperature. 
When, by the spread of thorough drainage over Britain, the surface-water is 
at once carried off, instead of remaining on the surface until slowly dispersed 
by evaporation and atmospheric currents, such prompt removal of the raw 
material of mist and cloud may be reasonably expected to be attended with 
a greater average annual amount of solar light and heat. 
Certain species of plants require more special physical conditions for 
health ; others more general conditions; and their extent of diffusion varies 
accordingly. Thus the plants of temperate climates are more widely diffused 
over the surface of the globe, because they are suited to elevated tracts in 
tropical latitudes. 
There is, however, another law which relates to the original appearance, or 
creation, of plants, and which has produced different species flourishing under 
similar physical conditions, in different regions of the globe. ‘Thus the plants 
of the mountains of South America are of distinct species, and for the most 
part of distinct genera, from those of Asia. The plants of the temperate 
latitudes of North America are of distinct species, and some of distinct 
genera, from those of Europe. The Cactee of the hot regions of Mexico are 
represented by the Huphorbiacee in parts of Africa having a similar climate. 
The modes of generalizing the observations on the geographical distribution 
of indigenous plants are various. 
One is by dividing the horizontal range of vegetation into zones, bounded 
by annual isothermal lines, as, 1, the equatorial ; 2, tropical; 3, subtropical ; 
4, warmer temperate; 5, cooler temperate; 6, subarctic; 7, arctie; 
8, polar zones: with temperatures progressively falling from an annual 
isotherm of 79°3 Fahr. to one of 365 for the month of July. 
Another mode is the classification of plants according to the regions of 
altitude; as into those of,—1, Palms; 2, Tree-ferns; 3, Myrtles; 4, Ever- 
greens ; 5, Deciduous trees; 6, Conifers; 7, Alpine shrubs; 8, Alpine herbs. 
But the corresponding altitudes in different countries have frequently dif- 
_ferent, though analogous or representative, species. The presence or other- 
wise of snow on the mountain-tops also influences the character of the plants 
at corresponding altitudes. Thus, forests of tall Conifers flourish in the 
Himalayas at regions of altitude where only stunted specimens of tropical 
plants are found in the mountains of Sumatra. 
A third, and perhaps more truly natural mode of expressing the geogra- 
phical distribution of plants, is by regions defined by the proportion of plant- 
species peculiar to them. When one half, at least, of the known species are 
peculiar to a certain space, it constitutes a ‘ phytogeographie’ region, accord- 
ing to Schouw. In it, also, a fourth part of the genera must be either pecu- 
liar, or so predominating as to be comparatively rare in other regions; and 
