THE EARTHWORMS OF OCEANIC ISLANDS
Oceanic islands are islands that have always been islands, a
definition that seems tautological until we compare it with some
other land masses that may be termed 'islands.' Geology teaches us
in fact that from the point of view of their origin islands may be
divided into two quite sharply contrasted classes. There are those
detached land masses usually lying near to or comparatively near to
some continent, which have been in the course of time detached by the
action of the waves from that continent, such as for instance the
British Isles, which undoubtedly represent a portion of the European
continent which was once quite continuous with Europe. On the other
hand we have the Hawaiian archipelago, St Helena, Fernando Noronha,
and other similar islands, which are more remote in their position
from continents and concerning which it seems clear that they have
originated _de novo_ by the action of submarine volcanos or of the
growth of coral, combined with subsidence, following elevation, or
from several of the causes combined. In any case the islands which
are termed oceanic islands have never formed part of a continent.
They are not relics of previously existing continents. It becomes
a matter of great interest to compare the earthworms which are to
be found upon oceanic islands with those which inhabit continental
islands. Fortunately there are a good many facts at our disposal for
this purpose; and we shall compare the earthworms of the Hawaiian
archipelago with those which are found upon certain small islands
lying to the south of New Zealand, viz. Campbell and Auckland islands
and the more southern Macquarie islands.
The earthworms of the Hawaiian archipelago have been studied by a
good many persons, and altogether a number of species have been
described from that group of islands of which the following is a
list: _Pheretima hawayana_, _P. heterochaeta_, _P. peregrina_,
_P. schmardae_, _P. hesperidum_, _P. morrisi_, _P. perkinsi_,
_P. biserialis_ (= _P. elongata_), _Allolobophora putris_ (=
Kinberg's _Hypogaeon havaicum_), _A. foetida_, _A. caliginosa_, _A.
nordenskiöldi_, _A. limicola_, _A. rosea_, and finally the well-known
_Pontoscolex corethrurus_. Of these species there is only one which
is even possibly a form limited to the Sandwich Islands, and that is
_Pheretima perkinsi_, a species which I myself at first described as
a new form, but which was afterwards regarded as identical with _P.
heterochaeta_ by Michaelsen, and later still resuscitated by Ude. All
the others are found in many parts of the world and not only in the
nearest mainland to the archipelago which we are now considering.
I have had already occasion to speak of some of them as peregrine
forms, especially of _Pontoscolex corethrurus_ which occurs all over
the world.
The conditions which have been recently revealed by an exploration
of the antarctic islands mentioned above are totally different. Dr
Benham has enumerated the following species from those islands,
viz. _Notiodrilus haplocystis_, _N. fallax_, _N. aucklandicus_,
_N. campbellianus_, _N. macquariensis_, _Plagiochaeta plunketi_,
_Rhododrilus cockayni_, _Leptodrilus leptomerus_, _L. magneticus_,
_Plutellus aucklandicus_, _Diporochaeta heterochaeta_, _D.
brachysoma_, _D. helophila_, _D. perionychopsis_ among the
Megascolecidae, besides _Phreodrilus campbellianus_, _Pelodrilus
tuberculatus_, _P. aucklandicus_ and the Lumbricid _Helodrilus
constrictus_. There were also four species of purely aquatic
Oligochaeta which we shall leave aside from the present enumeration,
as their range in space is a matter requiring a different explanation
from that of the terrestrial forms. Here we have a series of worms,
all of which, save the widely spread Lumbricid, are apparently
absolutely indigenous to the islands mentioned since they are all
different as _species_ from those found elsewhere. Indeed there is
a whole genus _Leptodrilus_, consisting, it is true, of but two
species, which is a native of the Campbell and Auckland islands and
of those only. The other genera are found in the antarctic region,
while _Pelodrilus_ is still more widely spread.
These facts as will be observed contrast about as strongly as they
can with those supplied by the fauna of Honolulu and its adjacent
islands. Not only are the worms of the antarctic islands different
species from those found elsewhere, but the majority of them do not
consist of widely ranging peregrine forms. It appears therefore most
probable that these islands are not oceanic islands but a portion of
the former existing northern portion of the antarctic continent. Were
the species _identical_ with those of New Zealand this conclusion
would have of course to be reconsidered. The barriers to migration
(see chap. VIII) explain the contrast recorded in the foregoing
pages.