418 CHARLES DARWIN 



specific differences. I have remarked that the marine 

 Amblyrhynchus was larger at Albemarle Island than else- 

 where; and M. Bibron informs me that he has seen two dis- 

 tinct aquatic species of this genus; so that the different 

 islands probably have their representative species or races 

 of the Amblyrhynchus, as well as of the tortoise. My atten- 

 tion was first thoroughly aroused, by comparing together 

 the numerous specimens, shot by myself and several other 

 parties on board, of the mocking-thrushes, when, to my aston- 

 ishment, 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 (between which two other islands are sit- 

 uated, as connecting links) belonged to M. melanotis. These 

 two latter species are closely allied, and would by some 

 ornithologists be considered as only well-marked races or 

 varieties; but the Mimus trifasciatus is very distinct. Un- 

 fortunately most of the specimens of the finch tribe were 

 mingled together; but I have strong reasons to suspect that 

 some of the species of the sub-group Geospiza are confined 

 to separate islands. If the different islands have their repre- 

 sentatives of Geospiza, it may help to explain the singularly 

 large number of the species of this sub-group in this one 

 small archipelago, and as a probable consequence of their 

 numbers, the perfectly graduated series in the size of their 

 beaks. Two species of the sub-group Cactornis, and two of 

 the Camarhynchus, were procured in the archipelago; and 

 of the numerous specimens of these two sub-groups shot by 

 four collectors at James Island, all were found to belong to 

 one species of each; whereas the numerous specimens shot 

 either on Chatham or Charles Island (for the two sets were 

 mingled together) all belonged to the two other species: 

 hence we may feel almost sure that these islands possess > 

 their respective species of these two sub-groups. In land- 

 shells this law of distribution does not appear to hold good. 

 In my very small collection of insects, Mr. Waterhouse 

 remarks, that of those which were ticketed with their local- 

 ity, not one was common to any two of the islands. 



If we now turn to the Flora, we shall find the aboriginal 

 plants of the different islands wonderfully different. I give 



