32 NATURAL HISTORY 



Peninsula with its adjacent islands. In early geological times mem- 

 bers of this genus were widespread and abundant, whereas now, due 

 to the disappearance of former land connections, there are but two 

 widely isolated species in existence. 



The distribution of animals is bound up in their food supply. 

 Hence carnivorous animals are restricted to areas wiiere the animals 

 on which they prey live. Often a biological barrier is created by the 

 presence of animals which are parasitic on a given form. The tsetse 

 fly, Glossina, which frequents the river bottoms and shores of lakes 

 in certain parts of Africa, prevents the ranging of other than native 

 cattle in these areas because of the fact that they transmit a blood 

 parasite fatal to such animals. Man himself is most active in both 

 creating and breaking down barriers. He introduces new animals 

 and plants either purposely or by chance into areas where they thrive 

 and replace other species, or by building dams, irrigating, deforesta- 

 tion, or accidentally burning over areas, he destroys one kind of life 

 perhaps never to replace it with another. 



Successions and Their Causes 



Succession means that in a given area organisms succeed one an- 

 other because of changes in the environment, migration taking place 

 so that they may reach conditions favorable to their development. 

 An example of plant succession may be seen in almost any pond that 

 is gradually drying up. In deep water there are a few submerged 

 aquatic plants ; in water from 6 to 8 feet deep floating plants such as 

 pond lilies are found ; in shallow water from 1 to 4 feet deep, cat- 

 tails and reeds are abundant ; while at the edge we find a meadow of 

 sedges and some bushy plants. As the pond becomes drier, these 

 plants slowly push outward until eventually it may be completely 

 filled with plants which build up soil, making first a swamp and 

 eventually a meadow, while around the edge of the former pond will 

 now be a forest of trees and bushes. In the tropical oceans different 

 corals succeed each other, growing on the skeletons of other species, 

 thus building their way into shallow and warmer water, or along the 

 ocean shore colonial diatoms may occur, to be followed by hydroids and 

 seaweeds, the latter becoming a dominant climax formation, a group of 

 species that are better fitted to survive in that habitat than any others. 



Erosion, which carries away the original inhabitants, or a deposit 

 of new soil by running water, wind, or other agencies, gives oppor- 

 tunity for the establishment of new life in a region thus devastated. 



