These several zones become sixteen in number when considered with reference to both the northern and southern hemispheres. And a like division of isothermals is made in the case of all our mountain systems, extending in both directions from the equator. In ascending our equatorial, tropical, and sub-tropical mountains, we find, of course, at their several bases, the temperature of the zones in which they respectively lie; from two thousand to three thousand feet, we reach the next higher zone, and so on, at about the same ratio of altitude, until we ascend to the polar zone or the line of perpetual ice and snow. The peak of Teneriffe, for instance, lies in the sub-tropical zone, but, at the elevation named, we meet with the vegetation which characterizes the warm temperate zone. And this holds true of all our mountain systems, in all latitudes, and at all altitudes, in all parts of the globe. They all present the same or strikingly similar characteristics in plant life, with such variations and modifications only as might be accounted for, were all the influencing conditions and surrounding circumstances, modifying geographical distribution, known to us. From the lowest to the highest regions in which vegetation flourishes, this rule, with slight exceptions only, will be found to obtain, and it is in this direction that the observations of the scientific, as well as practical botanist, should hereafter be extended. Humboldt noticed this characteristic feature of the earth's vegetation quite early in his explorations, and accordingly divided the tropical mountains, as the earth's surface was then divided, into three separate zones, the tropical, the temperate, and the frigid. But a closer classification now distinguishes them into the same number of zones as are marked, in approximate isotherms, on the earth's surface. Mr. Renfrey gives us further statistics of great value respecting these several plant zones of the globe, all of which fit so admirably into our theory of plant-distribution, that we can hardly see how the most prejudiced mind can resist the force of its application. Among the most important of these statistical facts are tables giving the comparative rain-falls in the different plant zones of the old and new worlds, and the classes of vegetation peculiar to each of them. The Equatorial zone, for instance, is characterized by extreme luxuriance in growth, owing no doubt to the great heat and abundant moisture therein, and exhibits a vegetation which is peculiar to itself, and which could only thrive under the hygrometric, thermometric, telluric, and other conditions of that extensive zone.