ZOOGEOGRAPHY 307 



Distribution as a condition is thus affected by extinction ; distribution as a 

 process is not so affected. 



That whole races of animals may be exterminated is very evident 

 in the effect which man has had upon the fauna. In North America, 

 the passenger pigeon, the Carolina parrakeet and other species have 

 Jaeen exterminated, other forms are on the verge of .extinction, and 

 numerous species have been greatly reduced in numbers. It is equally 

 evident that extinction occurs in nature. The rocks and soil contain 

 fossil remains of thousands of forms which are no longer to be found alive. 

 There is no reason to believe that the causes of extinction in the past 

 were in the main different from those that prevail at present. It has 

 been noted that diseases, droughts, exceptionally cold weather, multipli- 

 cation of enemies, changes in the substances held in the water in the 

 case of aquatic forms, increased moisture, and other agencies destroy 

 wild animals now. Specifically it may be pointed out that during ex- 

 ceptionally severe winters the quail in southern Michigan become greatly 

 depleted in numbers; that the disease rinderpest which swept across 

 Africa a number of years ago is said to have destroyed 65 per cent, 

 of the hoofed animals of that continent; that during winters characterized 

 b}' excessive sno'w^all the deer in northern Michigan perish in considerable 

 numbers because they are unable to move about freely and thus are 

 more Hable to attack by wolves; and that during exceptional droughts on 

 the pampas of South Ainerica and Africa large numbers of wild animals 

 die of thirst, or because, being compelled to get water at a few places, they 

 are more readily attacked by carnivorous species. The causes of extinc- 

 tion are very often obscure either because they work imperceptibly or 

 because they cannot be inferred from the remains of the animals. How- 

 ever, from the fact that animals are morphologically and physiologically 

 dependent upon their environment, it may be safely concluded that just 

 as the range is limited by environmental conditions, so any change in 

 these conditions in the inhabited area will if great enough lead to extinc- 

 tion; and if the change is coextensive with the range total extermination 

 of the race will result. 



Evolution. — Many of the facts of distribution are capable of inter- 

 pretation by the assumption that evolution has operated with the other 

 factors. If each kind of animal has arisen from a preexisting kind, 

 then each group of related animals must have had an ancestral form, and 

 if the component parts of the groups are wide-spread the range of the 

 ancestral form may be considered to be the center of dispersal of the 

 group. The facts of distribution can apparently be interpreted only 

 on this basis. 



Accepting evolution, along with the other factors which can be 

 recognized, the method of distribution is generally conceived to be as 

 follows. The ancestral form tends to spread in all directions. In some 



