August i, 1885.] The Australasian Scientific Magazine. 
19 
TEMPERATURE. 
BY 
HEYKIM NABI COSMOS. 
“Assimilation in plants,” says Pfeffer, “ is only possible between a mean 
specific minimum and maximum of temperature ; between these two 
extremes lies an optimum for the development of the species.” This 
demonstrates that similar laws alike exist for plants as for animals, and 
that hybridisation is the general, not the exceptional, law for variation of 
species in plants. 
The publication of Humboldt’s “ Essay on the Geography of Plants ” 
(1805) first formally drew the attention of botanists to the connection 
between the distribution of plants and the distribution of heat on the 
surface of the earth. As an advance is made from the Equator towards the 
pole in either hemisphere, the mean annual temperature declines, and in 
succession a series of vegetable zones is encountered, merging gradually 
into each other, though each, where best marked, is perfectly distinguished 
from its successor. In the tropics there are the palms, which give so 
striking a characteristic to the landscape, the broad-leaved bananas, and 
great climbing plants throwing themselves from stem to stem, like the 
rigging of a ship. Next follows a zone of evergreen woods, in which the 
orange and citron come to perfection. Beyond this, another of deciduous 
trees, the oak, the chestnut, and the fruit-trees, which we have acclimatised 
in our orchards. Here the great climbers of the tropics are replaced by 
the hop and the ivy. Still farther is a belt of conifers — firs, larches, 
pines, and other needle-leaved trees, and these lead up to a range of 
birches, becoming more and more stunted, merging into a region of 
mosses and saxifrages, but which at length has neither tree nor shrub ; 
and, finally, as the perpetual polar ices are reached, the red snow algae 
show the last trace of vegetable organisation. 
A similar sequence of facts had long previously been observed by Tourne- 
fort in an ascent of Mount Ararat. The distribution of vegetation from the 
base to the top of the mountain bears a general resemblance to the distri- 
bution along the base towards the polar regions. These facts were generalised 
by subsequent observers. It was established that there exists an analogy 
between horizontal distribution on the surface of the globe, and vertical 
distribution at different altitudes above the level of the sea* Even in the 
tropics, if a mountain be sufficiently high a short ascent suffices to carry us 
from the characteristic endogenous growths at its foot through a zone of 
evergreens into one of deciduous trees, and thence again into one of 
conifers, the vegetation declining through mosses and lichens, till we reach 
the region of perpetual snow. 
In these cases horizontal and vertical distribution present a striking 
botanical resemblance. There is likewise so clear a meteorological 
analogy that it is impossible to avoid the conclusion that the 
