HISTORICAL ZOOGEOGRAPHY 101 



tionably been derived in the two widely separated regions from the 

 common ancestral form of the intermediate seas. 6 The same may be 

 true of genera. The snail genus, Potamides, of brackish water (India, 

 Africa, California) , is derived from the marine Cerithium. It is much 

 more probable that different Cerithium species have developed into 

 the various Potamides species under the influence of brackish water, 

 in the different parts of the world, than that all the species of Potamides 

 are to be derived from a single ancestral form. It would perhaps be 

 better not to speak of the genus Potamides but to refer to Potamides— 

 forms of the various Cerithium species. Such examples of widely sepa- 

 rated similar forms of common derivation are rare. 



The center of origin is sufficiently obvious in many groups of ani- 

 mals. Thus the numerous species of snails of the family Achatinellidae 

 are confined to the Hawaiian Islands, and neither living nor fossil 

 forms of this group are to be found elsewhere. It must have arisen here 

 from ancestors related to the Polynesian PartulaJ The same general 

 situation holds for the Hawaiian family of birds, the Drepanididae 

 (cf. p. 88 and 520). 



Another example of a natural group with continuous range is af- 

 forded by the penguins. The sea is not a barrier for them, and though 

 their headquarters are on the antarctic and subantarctic shores, a few 

 species range north to the southern tips of the three southern continents 

 and to the South Island of New Zealand. One species has reached the 

 Galapagos Islands, on the equator, but its northward spread has been 

 favored at this point by the cold waters of the Humboldt Current, 

 Their fossil remains are likewise known only from the southern hemi- 

 sphere, from the Eocene in New Zealand and Seymour Island (Ant- 

 arctica) , and the Miocene of Patagonia. 



Such continuity of distribution is by no means invariable. The 

 ranges of families and genera and even of species are often broken by 

 broad areas in which the forms in question are absent. One may even 

 say that discontinuity is the rule for the higher groups, even perhaps 

 for genera, but certainly for families and orders. The more remote the 

 period of common origin of the related forms, the greater will be the 

 probability of discontinuity of range in the modern survivors. There 

 are numerous instances in which a former continuity of range can be 

 demonstrated to explain the present discontinuity. The range of the 

 Pyrenean shrew, Myogale pyrenaica, is widely separated from that of 

 its relatives in the steppes of southern Russia; but fossil remains of 

 this genus are found in the terrestrial deposits of France, Belgium, 

 England, and Germany, where it has presumably become extinct in 



