112.Ancient Life History. -As the rock layers of the earth are ex- plored, fossils of different kinds of plants and animals are discovered. The fossils of the more recent rock layers correspond very closely to the plants and animals that arefound upon the earth to-day, but the older the layers, the less they correspond. There seems to have been a gradual develop- ment in life forms through the past ages, a fragmentary recordof which is engraved upon cer-
tain of the sedimentary rocks.
PETRIFIED TREES.
Found near Holbrook, Arizona.
Rocks which were formed under different conditions contain different species of life forms, showing that throughout all time the geographic condition has had a marked influence upon plants and animals.
The rocks and fossils show that the geographical conditions of certain areas also have varied greatly. Some regions have been below and above the sea several times. Regions now cold have been warm, and those now dry have been wet, and vice versa. Thus the life in certain areas has suffered great changes by the geographical accidents to which the region has been subjected. The petrified forests near Holbrook, Arizona, show some of the most remarkable tree fossils ever found and indicate that the region has been subjected to remarkable geographical changes.
113.Distribution of Life. -Plants and animals are found wher- ever the conditions are suitable for their existence. In ice-covered ar- eas, like the interior of Greenland, and in exceedingly dry regions, likethe Sahara and certain parts of southwestern United States, there is little life of any kind. With a few such exceptions, however, the surface of the earth is a uni- versal battlefield of plants and animals struggling to exist and to increase. They extend themselves wherever attainable space is opened. But barriers may oppose their spread and geographical accidents may drive them fromGILA MONSTERS.
The most poisonous reptiles of the south-western American desert.
areas which they had heretofore held. The retreat of the sea maycause a change in the position of shore life. In the water a land barrier or an expanse of deep water may prevent the spread of shore forms. On the land a mountain uplift, a desert area, or a water barrier may limit the space occupied by animal and vegetable species.
Certain plants and animals are much more widely distributed than others. Plants like the dandelion and thistle, whose seeds are easily blown about by the wind, spread rapidly, while trees like the oak and chestnut spread slowly. As plants have not the power to move about, they cannot distribute themselves as easily as animals. Certain birds which are strong of flight are found widely distributed over regions separated by barriers impassable to other animals.
Some of the present barriers to life distribution have come into existence in comparatively recent geological time. There is good reason to believe that the British Isles and Europe were formerly connected, and that in very ancient times Australia was joined to Asia. It is also believed that for long ages North and South America were separated bya water barrier and that even after they were once connected, the Isthmus of Panama was again submerged.
These are but a few illustrations of the changes in the earth"s surface which have affected the distribution of animals and plants. Climatic changes like that which brought about the great ice advance of the Glacial Period have affected in a marked degree the distribution of life. It is thus found that when a study is made of the present distribution of life, careful attention must be given to the present and past geographical conditions of the region.
CANADA THISTLE.
One of the most widely distributed of plants.
114.Adaptability of Life. -There is hardly a place on the earth"ssurface not adapted to some form of life. Even upon the ice-bound in- terior of Greenland a microscopical plant and a tiny worm have found a home. The dry desert regions have a few plants with small leaves or, like the cactus, with no true leaves.
This prevents the evaporation of the water from their surfaces and so pro- tects them from drought. To protect them from animals, many of these plants are armed with thorns.
A RATTLESNAKE COILED READY TO SPRING.
The color of these reptiles makes them hardly distinguishable from the surrounding desert.
CACTI.
These are adapted to desert life because they have no leaves from which water can evaporate.
A HERD OF REINDEER.
This animal is of invaluable service to man in polar regions.
Another example of adaptability is the fact that the small animals of the desert are generally of a sandy color, which makes them hardly distinguishable from their desert surroundings. The large ones are swift strong runners, like the antelope and ostrich, or, like the camel, are able to travel for long distances without water.
In the colder regions the plants have the power of rapid growth and germination during the shortseason when the snow has melted away. Then, during the long winter, they lie dormant but unharmed under thesnow and ice. The animals are either able, like the reindeer, to live upon the dry mosses, lichen and stunted bushes, or else upon other animals. Their color, like that of the polar bear, often blends with their surroundings.
Some animals have a wide range of adaptability, like the tiger, which is found from the equator to Siberia. But usually the range of an animal species is much more restricted, since it is seldom able to adapt itself to widely differing conditions. The surrounding region, the elevation, the temperature, the amount of moisture, the soil, the kinds of winds and their force, all have a marked effect upon the fauna (animals) and flora (plants) of a country.
The species that thrive in a region must haveadapted themselves to the existing conditions, yet other animals and plants may be as wellTIGER.
One of the most widely distributed of animals.
A CALIFORNIA RABBIT DRIVE.