22 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Many of the varieties of the animals referred to in- 

 habit the same territory, although some are restricted 

 to particular regions. Of geographical varieties or 

 races much is known. As a rule, all widely distributed 

 species present them. Examples are the brown bear of 

 the Northern Hemisphere (Ursus arctos}\ the cobra di 

 capello snake of the warmer parts of Asia (Naja tripu- 

 dians}, and that of Africa (Naja haje). In North Amer- 

 ica the king-snake (Ophibolus getuhis} and the milk- 

 snake {Osceola doliatd} are represented by distinct 

 races in different regions. On the other hand, the 

 copperhead {Ancistrodon contortrix} and the Eastern 

 rattlesnake (Crotalus horridus}, which have a wide 

 range, scarcely vary at all. The chub {Hybopsis bigut- 

 tatus} is an example of a fish distributed everywhere 

 east of the Rocky Mountains, which presents scarcely 

 any variation. 



Variations are not promiscuous or multifarious, but 

 are of certain definite kinds or in certain directions. 

 Thus amid all their varieties, dogs never present black 

 cross-bands on the back like those of the dog-opossum 

 (Thylacinus cynocephalus} of Tasmania, nor do they 

 present ocellated spots like those of the leopard, nor 

 longitudinal stripes like those of certain squirrels. The 

 same is true of the many varieties of cattle {Bos tau- 

 rus}, and of numerous other mammalia. Domestic 

 fowls never vary to blue or green, colors which are 

 common to many other birds ; nor are canaries known 

 to produce blue or red natural sports. All variations 

 are in the first place necessarily restricted by the ex- 

 isting characters of the ancestor; but beyond this it is 

 evident that other conditions determine the nature of 

 the variation. It is not supposable, for instance, that 

 the pale tints of animals which live in dry regions 



