I have not as yet noticed by far the most remarkable feature in the
natural history of this archipelago; it is, that the different
islands to a considerable extent are inhabited by a different set
of beings. My attention was first called to this fact by the
Vice-Governor, Mr. Lawson, declaring that the tortoises differed
from the different islands, and that he could with certainty tell
from which island any one was brought. I did not for some time pay
sufficient attention to this statement, and I had already partially
mingled together the collections from two of the islands. I never
dreamed that islands, about 50 or 60 miles apart, and most of them
in sight of each other, formed of precisely the same rocks, placed
under a quite similar climate, rising to a nearly equal height,
would have been differently tenanted; but we shall soon see that
this is the case. It is the fate of most voyagers, no sooner to
discover what is most interesting in any locality, than they are
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.
The inhabitants, as I have said, state that they can distinguish
the tortoises from the different islands; and that they differ not
only in size, but in other characters. Captain Porter has described
those from Charles and from the nearest island to it, namely, Hood
Island, as having their shells in front thick and turned up like a
Spanish saddle, whilst the tortoises from James Island are rounder,
blacker, and have a better taste when cooked. (17/5. "Voyage in the
U.S. ship Essex" volume 1 page 215.) M. Bibron, moreover, informs
me that he has seen what he considers 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 representative species or races of the
Amblyrhynchus, as well as of the tortoise. My attention 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 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 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
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.