ean or native vessel, there is ample time
in two or three hundred years for the very different conditions to have
established a marked diversity in the characters of the species. This is
the more probable because there is also a true rat of the Old World genus
Mus, which is said to differ slightly from any known species; and as this
genus is not a native of the American continents we are sure that it must
have been recently introduced into the Galapagos. There can be little doubt
therefore that the islands are completely destitute of truly indigenous
mammalia; and frogs and toads, the only tropical representatives of the
Amphibia, are equally unknown.

_Reptiles._--Reptiles, however, which at first sight appear as unsuited as
mammals to pass over a wide expanse of ocean, abound in the Galapagos,
though the species are not very numerous. They consist of land-tortoises,
lizards and snakes. The tortoises consist of two peculiar species, _Testudo
microphyes_, found in most of the islands, and _T. {279} abingdonii_
recently discovered on Abingdon Island, as well as one extinct species, _T.
ephippium_, found on Indefatigable Island. These are all of very large
size, like the gigantic tortoises of the Mascarene Islands, from which,
however, they differ in structural characters; and Dr. Guenther believes
that they have been originally derived from the American continent.[60]
Considering the well known tenacity of life of these animals, and the large
number of allied forms which have aquatic or sub-aquatic habits, it is not
a very extravagant supposition that some ancestral form, carried out to sea
by a flood, was once or twice safely drifted as far as the Galapagos, and
thus originated the races which now inhabit them.

The lizards are five in number; a peculiar species of gecko,
_Phyllodactylus galapagensis_, and four species of the American family
Iguanidae. Two of these are distinct species of the genus Tropidurus, the
other two being large, and so very distinct as to be classed in peculiar
genera. One of these is aquatic and found in all the islands, swimming in
the sea at some distance from the shore and feeding on seaweed; the other
is terrestrial, and is confined to the four central islands. These last
were originally described as _Amblyrhynchus cristatus_ by Mr. Bell, and _A.
subcristatus_ by Gray; they were afterwards placed in two other genera
Trachycephalus and Oreocephalus (_see_ Brit. Mus. Catalogue of Lizards),
wh