GENERAL ZOOLOGICAL FACTS AND THEORIES 279 



two widely separated regions, it is safe to conclude that the dis- 

 tribution must once have been continuous. Examples of dis- 

 continuously distributed animals are rare. Tapirs inhabit tropi- 

 cal America and the Malay Archipelago; the reed bunting 

 in England reappears in Japan; the white mountain butterfly 

 inhabits the Rocky Mountains of British Columbia and the 

 region of Hudson's Bay, but is absent between these localities. 



GENERAL CONCLUSIONS. " The laws governing the distribution 

 of animals are reducible to three very simple propositions. Every 

 species of animal is found in every part of the earth having con- 

 ditions suitable for its maintenance, unless: 



" (a) Its individuals have been unable to reach this region, 

 through barriers of some sort; or 



" (b) Having reached it, the species is unable to maintain itself, 

 through lack of capacity for adaptation, through severity of 

 competition with other forms, or. through destructive conditions 

 of environment; or 



" (c) Having entered and maintained itself, it has become so 

 altered in the process of adaptation [or as a result of other pro- 

 cesses] as to become a species distinct from the original type " 

 (249, p. 314). 



(2) The Distribution of Animals in Time 



The fossil remains of animals that lived millions of years ago 

 give us authentic records of the fauna present upon the earth's 

 surface at that time. These records, unfortunately, are frag- 

 mentary, since only the hard parts of the animals were preserved, 

 and these, when discovered, are almost always broken and in- 

 complete, making the reconstruction of many parts necessary. 

 The number of species of fossil forms known at the present 

 time is given in parentheses after the descriptions of the phyla 

 of the animal kingdom in Chapter I (p. 5). From the evidence 

 obtained from fossils, paleozoologists have constructed a table 

 (Table XV) showing the geological periods, arranged in the 



