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." 1 



In 1890 I examined 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. 2 Meanwhile Ridgway 3 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 convinced 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. 



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

 Chatham are different. 



2 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 1888. Proc. Un. 

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

 species of birds from the Galapagos Islands. Proc. Un. St. Nat. Mus., vol. XVII, 

 1894, pp. 357-370- 



