308 PRINCIPLES OF ANIMAL BIOLOGY 



all of which was described in the chapter on taxonomy. The whole 

 evolution process is a change of species, carrying with it necessarily the 

 changes of genera and higher categories. 



An important feature of this process, as it relates to geographic dis- 

 tribution, is that new species have been arising all the time since living 

 things first existed. New species are originating at this very moment, 

 and may be expected to continue to come into existence in the indefinite 

 future. Also important is the fact that new species have taken their 

 origin everywhere, in all parts of the earth which support life. Time 

 and place thus enter in an important way into all questions of present- 

 day distribution. The range occupied by each species becomes a center 

 of dispersal from which its descendants tend to spread, and these centers 

 have existed all over the earth and through long periods of time. 



Starting much earlier than the evolution of life was the development 

 of the earth. With the early stages of this process we are not concerned; 

 but those parts of the earth's evolution which were contemporaneous 

 with the evolution of living things are very important in geographic dis- 

 tributiom The changes which affected distribution have been largely 

 the rise and fall or other shifts of position of the land, and changes of 

 climate. The question of permanence of continents is an important one. 

 Most zoogeographers have held that the continents have always been, in 

 general, about where they now are; but there is another theory, that of 

 continental drift, according to which continents, floating on the plastic 

 interior of the earth, have moved horizontally. A common example of 

 this drift is the alleged separation of Africa and South America. Many 

 European and some American geologists have supported the drift theory, 

 but distribution of animals has seemed to most biologists to call for more 

 nearly permanent continents. 



Regardless of the general position of continents, their shapes have 

 changed. Many areas of dry land teem with fossils belonging to classes 

 which are strictly marine. Such areas must once have been under the 

 sea. Michigan, for example, contains many extinct corals, though it is 

 now hundreds of miles from any salt water. Even high mountains have 

 arisen out of the ocean. Land has also sunk, and areas which were the 

 shores of an ocean have become its bottom. Broken shore lines are a 

 common result of the sinking of hilly or eroded land. 



Changes of climate have also been frequent. Michigan and most 

 neighboring areas have been under glaciers more than once. At the 

 other extreme, more northern regions have been tropical, as indicated 

 by luxuriant plant growth preserved as fossils. Humid areas have 

 become dry, swamps have become dry plains, forests have been con- 

 verted into grasslands. These changes must have affected the distri- 

 bution of animals profoundly. 



