But this idea of plants coming primarily from a single pair of progenitors, and each primordial pair branching off into diversified offspring, as in the case of the cabbage, assumed to be the original ancestor of all the turnips and ruta-bagas, may be an article of botanical faith, but never of experimental proof. "_Entia non sunt multiplicanda præter necessitatem_" is an old and well-approved maxim, applicable alike to the countless myriads of living organisms, as to the innumerable crystalline forms to be found everywhere in nature. Nothing is produced without the necessary conditions on which its production depends. "Necessity," in its primitive signification, is a term of the very widest meaning, and most universal application. It applies as well to the course of nature as to the course of human events--to the laws of vegetable and animal growth as to the inevitable march and order of celestial movements. As applied to any form of life-manifestation it implies a law of development and growth, as well as the physiological conditions without which vital manifestations are impossible. For law, in a physiological sense, is that mode of vital action by which effects are invariably and inevitably produced.[15] And this law is just as dependent on necessary vital conditions as vital manifestations are dependent on a physiological law. There must always be this reciprocal dependence and relationship between conditioning causes and effects. Whenever and wherever the necessary vital conditions exist, the physiological law takes effect, and the requisite vital manifestation is witnessed. And this is no doubt as true of animal as of vegetable life.
The earth's surface has been divided into eight separate zones, each of which is distinguished by its peculiar or characteristic fauna and flora. Their order, measured from the geographical equator, is as follows;
1. The Equatorial Zone, extending from 0° to 15°. 2. " Tropical " " " 15° " 23°. 3. " Sub-tropical " " " 23° " 34°. 4. " Warm Temperate " " " 34° " 45°. 5. " Cold " " " 45° " 58°. 6. " Sub-arctic " " " 58° " 66°. 7. " Arctic " " " 66° " 72°. 8. " Polar " " " 72° " 82°.
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.