82 EVOLUTION OF LIVING ORGANISMS. 



opposite coasts of the isthmus of Panama differ con- 

 siderably, but differ far less than does the fauna of the 

 Mediterranean from that of the Red Sea, these seas 

 having been separated for a much longer time. In the 

 Sandwich Islands there are some 300 species of the genus 

 Achatinella, almost every valley having its own peculiar 

 form of this mollusc. A similar variety of structure has 

 been described by Sarasin among the land molluscs of 

 the island of Celebes, where species become subdivided 

 into an astonishing number of local races still united by 

 transitional forms. Islands afford excellent illustrations 

 of divergence through isolation. While their fauna and 

 flora bear a general resemblance to those of the nearest 

 mainland, each island or archipelago usually acquires a 

 remarkable number of peculiar forms. For instance, 

 almost everyone of the West Indian islands has its repre 

 sentative species of the golden oriole. The Galapagos 

 archipelago has its own reptiles, insects, and land mol- 

 luscs ; out of some thirty species of land birds about 

 twenty-eight are peculiar : while each one of these 

 islands has developed its own race of the gigantic land 

 tortoise. 



Another kind of isolation is that brought about by 

 parasitism. Since every parasite tends to restrict itself 

 to one particular kind of host, being often actually trans- 

 mitted from one individual to another, a parasitic species 

 becomes split np into as many divergent races as there 

 are varieties of host, until finally each host may acquire 

 its own peculiar species of parasite. A somewhat similar 

 subdivision and specialisation takes place in plants fer- 

 tilised by insects. The flowers tend to become special- 

 ised in structure and colour to attract particular kinds 

 of insects, while the insects undergo a corresponding 

 specialisation in order to derive nourishment from the 

 flowers. 



Physiological isolation may result from incompati- 

 bility of habits or temperament, or from sterility. 



