Where moisture is lacking， no plants can grow. Where there is but little moisture， only those plants can grow which are able to make the best possible use of the available moisture. In dry regions the plants are few and so constructed that little moisture can be evaporated from them. In dry desert regions， except where water finds its way to the surface in considerable quantity， forming oases， there can be but little forage for animals， and what there is， is scattered. The desert animal must therefore be a wanderer， able to subsist upon meager fare.
This is true of the human inhabitants of the desert as well. They must rove about in small bands living in tents， picking up a precarious living for themselves and their animals， and they must be hardyA WATER HOLE IN THE DESERT.
and capable of withstanding privation. They must move rapidly and carefully over the long distances separating the patches where food can be found. They must therefore be fine horsemen， like the Arabs of Arabia and the Sahara， or strong and swift runners， like the Indians of the southwestern United States.
The life of man on the oases， although much less miserable than that on the desert， is subject to great disadvantages. These spots are of limited extent and frequently of limited moisture. They are often separated by almost untraversable areas from the other inhabitants of the world. There can be but little commerce or intercourse with the rest ofmankind. Although the oasis may appeal with a poetic charm to theA DESERT AND OASIS.
Notice the oasis at the foot of the gully where a spring comes to the surface.
dweller in the desert， yet to the inhabitant of a fertile country it is but a sorry place.
In regions where there is plenty of moisture， sunlight and heat， the growth of plants and animals is abundant and is only limited by space and food. Here life is at its best.
119.Life on Islands. -Islands which rise from the continentalshelves were probably at one time connected with the continents， but have since been separated by the submergence of the intervening low- land. The animals and plants of such islands are similar to those of the adjacent large land masses. But oceanic islands possess only those types of plants and animals which originally were able to float or fly to them over the surrounding water expanse. Indigenous mammals， ex- cept certain species of bat， are wanting. Birds are abundant.
On the tropical islands the cocoanut palm is the main supply of vegetable food， clothing and building material. Many of the species of both plants and animals are different from those of the nearest continent and even of the adjacent islands. So complete has been theisolation of the life of these islands for
THE DODO.
Although the dodo is extinct， sufficient remains have been found to enable scientists to tell how it looked.
so long a time that it has been possible for great differences in species to develop. Large unwieldy birds unableto fly or run rapidly have been found on some oceanic islands， the dodoof Mauritius， now extinct， being one of the most notable.
The absence of predatory animals has probably made the development of such forms possible. The great species of tortoise from the Galapagos Islands perhaps owes its development to the same cause. Nowhere else have such huge tortoises been found. The remarkable fauna and flora found on oceanic islands may be regarded as due to their geographical isolation.
120.Life as Affected by Man. -Wherever man has establishedhimself， he has become a dominant factor in the distribution and ex-istence of plants and animals. Forests are cut down， swamps a r e d r a i n e d a n d s t r e a m s dammed. Shade-loving plants suddenly find themselves ex- posed to the full glare of the sun， plants which need much water find themselves in a dryORIGINAL FLORA SUPPLANTED BY NEW PLANTS.
soil， and other plants which need a dry soil are flooded by the impeded streams. They cannot stand these sudden changes of environment and die out. The plow overturns the sod and the fields are sown with seeds the natural home of which may have been thousands of miles away across the sea.
In the course of years the original flora of the region is represented by only a few species inhabiting places man has not deemed it worth his while to cultivate. New plants suited to man"s wants have taken the place of those which through thousands of years of struggle have shown themselves the best adapted to the geographical conditions in the region.
The animals share the same fate as the plants. Domestic animals replace the wild denizens of the country. Only in inaccessible and waste places are a few lone remnants of the native fauna left. If the area which man enters is limited and bounded by impassable barriers， as in the case of islands， certain animals may be entirely exterminated， as wolves and bears have been in England. If the animal is a unique species， its extermination from its native island may mean its total destruction， as in the case of the dodo of Mauritius.
121.Forestry. -One of the notable ways in which modern man hasaffected the life and industries of the earth is in his treatment of the forests. In North America， before the coming of the white man， there were probably extensive areas where the growth of forests had been checked by fires set by the Indians. The prairie regions were probably much enlarged by the annual grass fires. Tree-covered areas， too， wereBAD FORESTRY.
The débris was left to feed the forest fires， and all the standing timber was ruined.
often burned over， and the growth of the trees checked， in order to make hunting less difficult. The greater part of the country was， how- ever， covered with thrifty forests.