1835.1 DISTRIBUTION OF THE ORGANIC BEINGS. .>79 



siders two distinct species of tortoise from the Galapagos, but he 

 does not know from which islands. The specimens that I brought 

 from three islands were young ones : and probably owing to this 

 cause, neither Mr. Gray nor myself could find in them any specific 

 differences. I have remarked that the marine Amblyrhynchus 

 was larger at Albemarle Island than elsewhere; and M. Bibron 

 informs me that he has seen two distinct aquatic species of this 

 genus; so that the different islands probably have their repre- 

 sentative species or races of the Amblyrhynchus, as well as of the 

 tortoise. My attention was first thoroughly aroused, by com- 

 paring together the numerous 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 (between which two other islands are situated, 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. Unfortunately 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 

 representatives 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 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 ^representative 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 locality, not one was 

 common to any two of the islands. 



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



