CHAPTER VII

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.