PRESIDENT'S ADDRESS. 19 



Unionidae having existed as early as the Cretaceous and even the 

 Jurassic period. 



II. THE EURYG^AN OR EURASIATIC REALM. 



The European or Palaearctic Realm is the largest of all, and 

 embraces the entire northern portion of the Old World. Its south 

 ern limits nearly coincide with the tropic of Cancer in the lowlands, 

 and its isotherm projected therefrom in the more rugged countries. 

 In Africa it extends into the Desert of Sahara, and in Asia it is 

 limited by the Himalaya Mountains and their spurs. It possesses 

 members of 31 families of terrestrial mammals, 55 of birds (accord 

 ing to Wallace), 25 of reptiles, 9 of amphibians (according to Giin- 

 ther), and 16 of fresh -water fishes. None of these families, how 

 ever, are continuous over the entire area and at the same time pe 

 culiar to it. It is true that several families are restricted within 

 its limits, such as the Trogonophidae and Ophiomoridae among 

 reptiles, and the Comephoridae among fishes, but these a're very 

 limited in their distribution; the family Comephoridae, e.g., is 

 represented by a single species, confined to a single lake (Baikal) 

 of Siberia. The realm is characterized, therefore, rather by what 

 it has not than by what it has among families, and is based mainly 

 on the structural modifications of minor value (generic or specific) 

 of its constituents. 



III. THE INDOG^EAN REALM. 



The Indian or Oriental Realm is of less extent than either of 

 the two preceding ones, but is nevertheless richer than either in the 

 number of species. It extends from the Himalayan range on the 

 north to the Indian Ocean on the south, and toward the S. E. is 

 limited by the narrow but deep strait which intervenes between 

 Celebes Island and its dependencies on the one hand, and Borneo 

 on the other, and also between the island of Lombok on the 

 cne hand and Bali on the other; it thus includes the penin 

 sulas of Hither and Farther India, and the Indo-Malayan Archi- 



