Among the leading propositions laid down by Arthur Renfrey, Esq., F.R.S.
etc., etc., in the able article prepared by him for "The Physical Atlas of
Natural Phenomena," by Alexander Keith Johnston, Edinburg Edition, 1856,
on "The Geographical Distribution of the most Important Plants Yielding
Food," are the following:--

1. "The primary condition of the existence of any species of plant, is its
absolute creation, of which we know nothing.

2. "But we assume each species to have been _created but once in time and
in place_, and that its present diffusion is the result of its own law of
reproduction under the favorable or restrictive influences of laws
external to it.[14]

3. "The most important of external laws are those relating to climate,
since _any species can flourish only within narrower or wider, but always
fixed limits, of temperature, humidity etc_.,

4. "The climate depends primarily on latitude, since this indicates
distance from the source of heat, and the degree of obliquity of the
heating rays."

There are other governing conditions, of course, such as the average
rain-fall, distance from the equator, the elevation above the sea level in
the various mountain systems of vegetation, etc., including the
hygrometric, thermometric, telluric, and other conditions, of the several
localities in which the different species of vegetation make their