Increase in Number of Species 



171 



through waters which normally are not occupied by that species, 

 and mammals may similarly wander long distances into new ter- 

 ritory. The present disjunct distribution of certain conger eels prob- 

 ably came about by such wanderings ( Fig. 72 ) . 



Fig. 72. Known distribution of two species of conger eels of the genus 

 Conger. Note the disjunct African popuhitions of both species. (After Kana- 

 zawa. ) 



The number of barriers is extremely difficult to assess. In general 

 every distinctive ecological area of a continent or an ocean is a 

 barrier to those species which cannot exist in it. However, attempts 

 to correlate patterns of species fission and present ranges with 

 existing barriers encounter difficulties imposed by a dynamic past. 

 As Stebbins (1950) pointed out, colonizations which today seem 

 improbable because of the great distances involved may have oc- 

 curred at some time in the past when the barriers between areas 

 were narrower than at present. Also, areas now disconnected may 

 have been connected in the past. On the basis of present geography 

 the Santa Cruz jay Aphelocoma insuhris appears to be a colonist. 

 It is possible, however, that formerly Santa Cruz Island was con- 

 nected to the Californian mainland and that the Santa Cruz jay is 

 the product of a past range disjunction of the mainland species 

 A. calif ornica race ohscura (Pitelka, 1951). 



Instances of geographic disjunction therefore fall into three 

 categories: range disjunction, colonization, and a great number of 

 doubtful cases which might be the product of either process. 



