CLASSIFICATION AND DISTRIBUTION 379 



DISTRIBUTION OF ANIMALS IN SPACE AND TIME 



We have already seen that while organisms are always 

 adapted to the locality in which they live, they are frequently 

 even better fitted for other localities, and their presence in 

 any part of the world must be due to other factors besides 

 fitness. The distribution of organisms on the earth's surface 

 is controlled by three fairly well-known laws: 



1 . The members of a species usually occupy a continuous terri- 

 tory. We do not find some members of a species in one locality 

 and others in a distant region, without finding them also in inter- 

 mediate territory. There are some exceptions to this law, but 

 in the vast majority of instances each species occupies a continu- 

 ous territory around a center of origin. The territory occupied 

 will depend upon many factors of climate, for of course the 

 habitat must be properly fitted to furnish the organism with 

 food, water, and a proper temperature. 



2. All animals and plants can multiply with a rapidity suf- 

 ficient to give them, in a comparatively short time, enough off- 

 spring to cover the face of the earth. The rate of multiplication 

 of different organisms varies very greatly. The codfish may pro- 

 duce 8,000,000 eggs per year, while the elephant produces only 

 a single offspring in two years, and usually not so frequently 

 as that. Among the lower animals and plants, the rate of 

 reproduction is sometimes even greater than the higher num- 

 ber given above. But even the slow rate of the elephant is 

 sufficient, if the multiplication were unchecked, to enable the 

 species to fill the world in a few years. The numerous offspring 

 are always endeavoring to find room for themselves, and food 

 to eat. For this purpose they distribute themselves as widely 

 as possible. 



3. All organisms distribute themselves from the centers, where 

 their reproduction is rapid. All organisms, even those that 

 seem stationary, have some method of dispersing themselves 

 over the earth. The means of dispersal are chiefly the follow- 

 ing: 1. By independent migration. This is true of almost all 



