Page:Handbook for Boys.djvu/139

118 different habits and characteristics inhabiting the elevated regions of this same zone. It must be remembered that even under the tropics some of the highest mountains carry a perpetual snow-cap. There is therefore all possible gradations of climate from sea-level to the top of such mountains, even at the equator, and plant life is as a result as varied as is climate. Each zone, whether determined by latitude or by altitude, possesses a distinctive flora.

But altitude and latitude are not the only factors which have been instrumental in determining the plants found in any particular locality. This old earth of ours has not always been as we see her to-day. The nature we know and observe is quite different from that which existed in earlier ages of the earth's history. The plants, the trees, and the flowers that existed upon the earth during the age when our coal was being deposited were very different from those we now have. There has been a change, but, strange as it may seem, there are in some places upon the earth to-day some of the same species of plants which were abundant during the coal-forming periods. These are among the oldest representatives of the plant world now extant. Then we are told that there was a period when the north temperate zone was covered with a great ice field which crowded down as fax as southern Pannsylvania and central Ohio. This naturally brought about a profound change in the location and character of the plants of this region. There are in the Black Hills of Dakota species of plants which have no relatives anywhere in the prairie region, and no means is known by which these representatives of a Rocky Mountain family could find their way into the Black Hills, save that, previous to the ice age, this species was generally scattered over the territory, and that, during the ice age, the species was perpetuated in the hills but was killed out between there and the Rocky Mountains where it is found in abundance. These are some of the natural reasons for the existence of varied plants in different localities. They are sufficient to explain the reason for the existence of local floras.

But nature has provided untold ways for the perpetuation as well as the dispersal of plants for the purpose of, so far as possible, enabling the plants of the world to take possession of all parts of the earth's surface. If this adjustment were complete, the plants would be practically alike all over the surface of the earth, but we have already explained why this cannot be and why we have a different flora in each zone, whether it be marked by lines of latitude or height of the