THE DIFFERENTIATION OF SPECIES. 69 



hurried from it; but I ought, perhaps, to be thankful that I 

 obtained sufficient materials to establish this most remarkable 

 fact in the distribution of organic beings. . . . My attention 

 was first thoroughly aroused by comparing together the numer- 

 ous specimens, shot by myself and several other parties on 

 board, of the mocking thrushes, when to my astonishment, I 

 discovered that all those from Charles Island belonged to one 

 species (Mimus trifasciatus) ; all from Albemarle Island to 

 M. parvulus ; and all from James and Chatham islands (be- 

 tween which two other islands are situated as connecting links) 

 belonged to M. melanotis.''^ 



In 1 890 I exammed the specimens of Tropidurus collected by 

 the United States Fish Commission steamer "Albatross," in 

 1888, on eight islands. The material consisted of 128 speci- 

 mens. I was not a little astonished to find that nearly every 

 island contained a peculiar race or species of this lizard, and 

 that not a single island contained more than one race or 

 species.^ Meanwhile Ridgway ^ had studied the birds collected 

 by the "Albatross." He found that Nesomimus had also 

 peculiar species on Hood and Abingdon. 



It was this peculiar distribution of the species on the differ- 

 ent islands, which convmced me that the Galapagos Islands 

 could not be of volcanic origin, lifted out of the ocean ; but 

 that they must have originated through subsidence. Only by 

 such an assumption the harmonic distribution of the fauna 

 could be understood. To secure a better basis for the opinion 

 it was necessary to make very extensive collections on each of 

 the islands and to find out all the details of distribution. Dur- 

 ing my stay all the islands, with the exception of Narborough, 

 Wenman, and Culpepper, were visited. The result was exactly 

 as had been anticipated. 



^ Mr. Ridgway has shown now that also the specimens from James and 

 Chatham are different. 



^ Baur, G.: Das Variiren der Eidechsen-Gattung Tropidurus auf den Galapagos 

 Inseln. Biol. Centralbl., vol. X, 1890, pp. 475-483. 



3 Ridgway, R.: Birds collected on the Galapagos Islands in 18S8. Proc. Un. 

 St. Nat. Mies., vol. XII, 1889, pp. 101-108. — Description of twenty-two new 

 species of birds from the Galapagos Islands. Proc. Uti. St. Nat. AIus., vol. XVII, 

 1894, pp. 357-370. 



