Earthworms and Their Allies

by Frank E. Beddard

It is not the place here to give a general sketch of the division of Biology termed Zoogeography; but a few general conclusions must be laid before the reader in order to render what follows intelligible. It is universally agreed that the range in space (and in time also) of a given species of animal (or plant) is as much a part of its scientific definition as are its anatomical characters. A description for instance of Acanthodrilus ungulatus is incomplete without a reference to the fact that it occurs in, and is confined to, the island of New Caledonia.

Each continent or island or part of a continent and part of an island has its own peculiar inhabitants as well as some others which range beyond its confines. Thus as we have seen the genus Hyperiodrilus is confined to the tropical West of Africa while the genus Dichogaster also found in that region is also met with in other parts of Africa as well as in certain parts of America and of the East. In this way the entire globe may be mapped out into regions characterised by their inhabitants and these regions may also be further subdivided. The commonly accepted regions were originally devised by Mr Sclater and are known as the Palaearctic, Nearctic, Neotropic, Ethiopian, Oriental (Mr Sclater's name was 'Indian'), and Australian. These regions were originally formed to convey the facts relative to the distribution of Passerine birds only; but it is generally held that they apply also to the distribution of vertebrates generally. The science of zoogeography does not however end with the display of maps conveying graphically the mere facts of distribution of this group and that. Its business is also to enquire into the causes of the affinities between the faunas of different regions or the varying degree of remoteness which those faunas may show. On the one hand the varying powers of dispersal and the means of extending their range possessed by different animals have to be considered, and on the other hand geological changes in the relative position of land masses have to be taken into account.

The specific identity between the earthworms of Great Britain and the adjacent part of the continent of Europe would be very difficult to understand were we only acquainted with the fact that salt water is fatal to these animals. But we also know from geology that it was only at a very recent date that England was cut off from union with the continent. Thus an identity of fauna was to be expected. On the other hand we are confronted with a very great difference between the earthworms of eastern tropical Africa and of the adjacent island of Madagascar. In the latter we have as a prevalent form the genus Kynotus; in the former continent many Geoscolecidae but no Kynotus. It is believed that the separation of Madagascar from the mainland was at an earlier date than that of Great Britain from Europe. We must however be cautious before slipping into what might seem a case of arguing in a circle. It will however probably not be disputed that Madagascar was severed earlier than England.

We will now attempt to map out the world into a series of regions characterised by their earthworm inhabitants and see how far these regions agree with those rendered necessary by the distribution of some other animals.

We can to begin with accept the Palaearctic region. The region however will be a little different from that usually accepted. For we must probably exclude Japan, whose earthworm fauna contains the characteristically Eastern genus Pheretima. Otherwise we have a region characterised by the family Lumbricidae, which is really limited to it, and by just a few traces of other genera such as Hormogaster among the Geoscolecidae and Sparganophilus which however is possibly an accidental immigrant. This region is certainly quite clear. Now according to some persons such as Prof. Heilprin the northern part of America should be joined with Europe and Asia to form an Holarctic region; while by most authors, the separate name of Nearctic is given to the north of the New World. With regard to the terrestrial Oligochaeta it appears to me that this part of the world is possibly to be excluded altogether as possessing no indigenous worms.

In considering the distribution of the Mammalia Sir Ray Lankester excluded New Zealand from his view as never having possessed any indigenous mammalian fauna, and termed this part of the world Atheriogaea. In the same way it is possible that the northern part of the United States and Canada, whose earthworm fauna consists of species of Lumbricidae identical with those of Europe, may possibly be also a region to be excluded in the present survey and spoken of as 'Ascolecogaea.' In the southern part of the United States we shall find genera which will be considered presently. On the other hand it is equally conceivable that this part of the world lost its earthworm fauna through excessive glaciation in the ice age, the forms having been driven south and are now only gradually making their way northwards again. In this case the modern earthworm population which appears to be absent from large tracts of Canada will be simply due to involuntary migration. These two views must be left for further development.

In any case the southern parts of the United States seem to be separable as a distinct region from South America and to be characterised by the sub-family Diplocardiinae, the genus Diplocardia extending as far northwards as the state of Illinois. The distinctness of such a region however from Central America and the West Indies is marred by the abundance of Ocnerodrilus of which Dr Eisen has described so many forms. On the other hand the West Indies are closely allied in their earthworm fauna to tropical South America, sharing with that region several forms of Geoscolecids belonging in both cases invariably to the sub-family Geoscolecinae. The bulk of the latter are undoubtedly tropical South American in range and there is no doubt whatever about the distinctness of this part of the world as a separate region. There is moreover a further puzzle which confronts us who are trying to delimit an American region or regions. In North America are species of the genus Argilophilus which is referred by Michaelsen to the genus Plutellus which comes from the East and at least one species of Megascolides, also an Eastern genus.

There is at present no doubt to be thrown upon the indigeneity of Plutellus. The species according to Dr Eisen show every sign of being genuine inhabitants of California and like certain New Zealand species such as the Tokea esculenta of Benham (referred by Michaelsen to the genus Megascolides) were eaten by the natives. If these genera were forms restricted to North America, that is not only with reference to the rest of America but to the world generally, there would be as I think no doubt about the practicability of making a Nearctic region. As it is, it seems to me to suit the facts of distribution better to regard the whole of the land under consideration as forming one great Neogaean region with three sub-regions, the North American, Central American and West Indian, and tropical South American. This region however will not as I take it include the southernmost extremity of South America. Here in Patagonia and in neighbouring islands we have a different earthworm fauna. It is in fact characterised by the sub-family Acanthodrilinae of which it is true some members of the genus Notiodrilus extend further north. I shall however defer this part of the subject until the more easy delimitations of regions are disposed of.

Tropical Africa is evidently to be included in a third region which will be defined by the Eudrilidae, Microchaetinae among the Geoscolecidae, and by the great prevalence of Dichogaster, a genus whose occurrence in other parts of the tropics is perhaps not yet explained satisfactorily. Also we may record as characteristic of this Ethiopian region a few peculiar genera such as Nannodrilus and Gordiodrilus. Alma being a partly aquatic genus is perhaps less distinctive and as a matter of fact it strays into the Palaearctic region, being found in the lower waters of the Nile. It will be observed that with this exception the limits of the Ethiopian region according to earthworms agrees with that delimitation afforded by a consideration of other groups since it stops short at the Sahara, leaving northern Africa to be referred to the Palaearctic region. At the same time we have an analogy with South America as concerns the southern extremity of the African continent; here we meet with Notiodrilus and allied Acanthodrilinae just as in Patagonia and—as also in that quarter of the world—these forms just stray into the Ethiopian region above—specimens of Notiodrilus being met with in Madagascar as well as in tropical Africa. This bit of Africa as it appears to me must also be cut off from the Ethiopian region and included in an Antarctic region. Madagascar offers a further problem. Are we to include this in Ethiopia or speak of a Malagasy region? Apart from a few forms which are at least possibly to be looked upon as accidental immigrants, such as members of the genera Pheretima and Gordiodrilus, the fauna of Madagascar consists mainly of many species of Kynotus. This genus, a member of the sub-family Microchaetinae, of the family Geoscolecidae, affines Madagascar to Ethiopia and leads me to place both in the same region though we may doubtless speak of a Malagasy sub-region.

We have now to consider the eastern region of the world comprising the two regions known generally to zoogeographers as the Oriental and Australian. Taking a large view of the range of sub-families and genera, and endeavouring to make the great regions of the globe more or less equal, it seems difficult to divide further a region which shall include all of this vast territory, and which may therefore be termed Indo-Australian. For we find as characteristic of the entire stretch of country the great majority of the genera of the huge family Megascolecidae. Indeed the largest sub-family of this family, i.e. the Megascolecinae, is, save for the mysterious occurrence of the genera Plutellus and Megascolides in America, absolutely limited to this area. Another sub-family, that of the Octochaetinae, is limited to it. So far as concerns the others of the sub-families of Megascolecidae it is only the Trigastrinae which occur here (the genus Eudichogaster and a few possibly introduced species of Dichogaster) and a scattered species or two of Notiodrilus of the sub-family Acanthodrilinae. Again there are a few and probably introduced species of the sub-family Ocnerodrilinae. More important still this region has confined to itself the family Moniligastridae; for a species described some years ago by myself from the Bahamas is doubtless an introduced form. We have a complete absence of indigenous Lumbricidae and Geoscolecidae excepting the aquatic Glyphidrilus of the sub-family Microchaetinae. It is true that by taking isolated tracts, even large tracts, of this great regional expanse a sub-division into well characterised regions can be apparently recognised. But in taking such a step we shall be confronted with the curious fact that it is rather neighbouring than widely remote sub-divisions which present the greater differences.