ON THE ORIGIN OF GENERA. 119 



Isthmian Oscines, seventeen only hold the first rank, by virtue of 

 their additional, the tenth primary quill, while this feature marks 

 one hundred and twenty-eight species of Palestine. As we rapidly 

 follow the line to the point where its extreme is manifested, in the 

 family of the Thrushes or Turdidse, Panama is left but two soli- 

 tary pioneers of these songsters of the North, while seventy-five 

 species represent the family in Palestine. 



The comparison between different faunse exhibits an apparent 

 gradation in some other groups equally curious. Thus, the true 

 Cyi^rinidffi in the Palaearctic region reach a great development, 

 and produce the highest number of teeth on their pharyngeal jaws 

 known, as well as attains the greatest bulk and importance. The 

 number of these teeth is usually seven to five in the inner row ; 

 only two or three genera exhibit only four on both. In the Ne- 

 arctic region the number of teeth is almost always 4 — 4, more 

 rarely 4 — 5, and very seldom as high as 5 — 5. The species of the 

 family are excessively numerous, but are, with scarcely any excep- 

 tion, of small size and weak organism. These statements apply to 

 those of the eastern district of the region between the Eocky 

 Mountains and the Atlantic. Similar tyi^es occur in the northern 

 region of the Neotropical — Mexico, but in no great numbers. 

 Farther south the family disappears, its place being supplied by 

 the generalized family of Characinidae. 



I have already alluded to the great variety of the highest or 

 pentamerous carnivorous beetles in the Palaearctic region. They 

 are extremely abundant in the Nearctic, while the intermediate 

 territory, the Sonoran and Mexican sub-districts, are the head- 

 quarters of the next lower form, the Tenebrionidae, which have 

 the tarsal Joints 4 — 5. These give place in the Neotropical to the 

 multitudes of the still lower series — those with the joints 4 — 4 and 

 3 — 3 — Tetramera and Trimera. 



The preceding comparisons indicate that an inherent difference 

 between the types of a continent exists at the present time, though 

 the difference is subordinated to a universal distribution of the 

 higher groups throughout the earth. Has this state of things ex- 

 isted for any long period, or is it a result of different progress in 

 the same group since the human period ? This brings us neces- 

 sarily to a consideration of the truths of paleontology, especially 

 of the last periods, which have been already urged by Darwin. 

 Thus the present fauna of Australia was j^receded in the Post-Pli- 

 ocene and Pliocene by forms possessing similar peculiarities, and 



