Earthworms and Their Allies

On the other hand the genus Dichogaster offers quite different facts, which are in contradiction to those just enumerated. This genus as already said is very characteristic of tropical Africa, and a large preponderance of the known species are confined to that continent. Although there is some variation in structural characters among the many species which compose this genus, there is but little doubt that they are all rightly referred to one genus with perhaps some doubtful, though not very striking, exceptions. In any case the utmost divergence of structure between worms usually placed together in this genus is nowhere near to that which separates the genera of Eudrilidae from each other. Of the African members of the genus the species are pretty evenly divided between the eastern and western halves of the continent; they are, like the Eudrilidae, tropical in range, not occurring to the southward, where their place is taken by the Acanthodrilinae and Geoscolecidae. There are it is true a few species, such as D. gracilis and D. bolavi, which are common to the two sides of Africa; but in these cases we clearly have to do with those rather mysterious species which can apparently unduly extend their range and which are known as peregrine forms; for they also occur in other parts of the world besides Africa. We have therefore in Dichogaster the case of a genus which ranges all over the tropical parts of Africa, but whose species are not common to the Atlantic and Indian shores of that continent.

We will now contrast these conditions, which exemplify certain facts shown by the characteristic Oligochaeta of tropical Africa, with those which obtain in Europe. In this region of the world the prevalent and practically the only genera which need be taken into consideration in surveying the Oligochaetous fauna from the present point of view, are Lumbricus and the genus Allolobophora of Eisen which has been variously rearranged into genera and sub-genera known by the names of Helodrilus, Bimastos, Octolasium, etc. The structural differences which divide these genera and sub-genera are not great; in any case they do not exhibit such a wide range of variation from each other as do two such Eudrilid genera as Stuhlmannia and Hyperiodrilus. We find the genera mentioned not only in Europe but extending themselves over more or less of Asia, even occurring in Japan; while the North American continent contains also representatives of the same. Not only do we find this community of genera over vast extents of country greater in diameter than the African continent, but there are also many species which range as widely or nearly as widely as the case may be as the genus to which they belong. Thus the species of Allolobophora (we do not trouble about the newer sub-divisions as they hardly affect the facts to be emphasised), A. caliginosa, A. longa, A. rubida, A. chlorotica, A. octaedra, A. constricta, A. beddardi, Lumbricus terrestris, L. castaneus, have an enormously wide range over what is generally termed the Palaearctic region, extending also in some cases into the Nearctic. It is true no doubt that the majority, indeed perhaps all, of these are, like certain species of Dichogaster mentioned above, among those forms termed peregrine which have the capability of living in every quarter of the globe to which they have apparently been conveyed by man. But there remain many species which have a very extended habitat in the northern hemisphere, and in any case the genera and the species are there truly indigenous and widely spread.

It would thus appear that the capability for independent migration varies greatly among earthworms. Of the types selected for consideration the Eudrilidae are the slowest movers; the genus Dichogaster comes next, while the power of migration possessed by the genera Allolobophora and Lumbricus is very much greater. Assuming for the moment the correctness of this inference it is clear that it will influence many other propositions connected with the relative age of the families of these worms and with many problems of geographical distribution. It appears to us that this simple explanation is the correct one. But to show this it will be necessary to eliminate other possible explanations. It might be urged that the wider range of the genus Dichogaster and the still wider range of the genus Allolobophora (shown by community of species in widely distant localities) was evidence merely of relative age, that the older groups have had more time to travel and that the newer groups have not had so long a time to spread themselves over their habitat. On this hypothesis the genera of Eudrilidae would be geologically much newer than the genus Dichogaster and similar statements might be made for the other forms here under consideration. As already explained we cannot attempt to answer this question in the only way in which it can be really satisfactorily answered, by a reference to fossil forms; for there are no fossils to refer to. So far as comparative anatomy enables us to arrive towards a solution of the question, it would appear that the genus Dichogaster belongs to a more ancient race than either of the other two groups considered, and that of these latter the Lumbricidae are the most modern. Moreover we associate not only a wide, but also a discontinuous, distribution with an archaic race; and for this reason also we should place the genus Dichogaster in the position of being the most ancient of these Oligochaeta. For the genus occurs in Central America and in certain parts of the East as well as in Africa. So that we can fairly dismiss the view that the Lumbricids by virtue of their greater range over a given area are the most ancient type and that their range is associated merely with their antiquity. Nor does it appear that geographical or meteorological consideration can have had effect in the present instances. For conditions favourable to earthworms prevail in tropical Africa, as in Europe and much of North Asia.

Climate as Affecting Migration

That excessively rigorous climatic conditions affect the range of earthworms as well as fresh-water forms is quite clear from the conditions which obtain in the most northern climes. At any rate in those regions where physical conditions render it impossible for these Annelids to have their being. A perpetual mantle of snow and a temperature far below freezing point are absolute barriers to the extension of range. And yet there are some few Oligochaeta which do not in the least suffer from a somewhat milder taste of such conditions. Thus species of Enchytraeidae have been met with on glaciers and even found in frozen water, while a few earthworms have been brought from the island of Kolguev. These however are quite exceptions to the general sterility as regards earthworms of the excessively cold regions. We have already seen that there are no general facts to be deduced as concerning the relative abundance of terrestrial worms in the tropics and in more temperate climes. Tropical Africa is, it is true, rich in genera and species; but on the other hand tropical East Indies have but few genera inhabiting their numerous islands. Temperate England has very few genera and not a large number of species; temperate New Zealand has a considerable number of different indigenous genera. When however we leave this general aspect of the question and consider separate families and genera, there seems to be some little relation between climate and distribution and thus some effect of climate in acting as a barrier to migration. For example, though continuity of land surface permits of the tropical African Eudrilidae ranging southwards as far as the Cape they are not met with so far as we know in the most southern parts of Africa; nor are the South American Geoscolecidae found in Patagonia or northward beyond Central America. These instances do really look like an influence of climate upon range. On the other hand we must be careful to eliminate the possibility of another explanation and that is the impossibility of successful migration owing to the previous occupation of the ground with abundant other forms. The very same countries would appear to show that this explanation is unnecessary. For the prevalent genus of the southern tracts of South America Notiodrilus extends its way northward as does the same genus from temperate to tropical Africa and Madagascar.

It looks very much, therefore, as if certain Oligochaeta are dependent upon climate for their range, and as if others were at least more independent of climatic conditions. And there are other facts which support this view. The same opinion is supported by the phenomena of involuntary migration, a subject which has been considered also separately under the head of 'Peregrine forms.' The great prevalence of Lumbricidae accidentally imported into many parts of the world shows that temperature is no real bar to their voluntary migration. On the other hand the fact that specimens of the East Indian genus Pheretima though commonly imported accidentally into the warmer regions of the world have not been able to make good a footing in Europe, save in greenhouses, shows that this genus is affected in its range by questions of climate. These facts suggest another inference of great interest which can only be mentioned tentatively, and not put forward as a demonstrated conclusion. Seeing that Lumbricus (sensu lato) can comfortably take up its home in warm extra-European countries, but yet that it has evidently not spread to those countries in the course of nature but by man's interference, it seems possible that time alone has prevented this; and that therefore this family Lumbricidae is one of the most recently evolved families of Oligochaeta. Certain structural features support this way of looking at the matter. The same arguments precisely apply to the genus Pheretima, which is also regarded by most systematists as a recently developed race of earthworms. Anyhow the conclusion which the facts seem to warrant is that the effects of climate in influencing distribution are seen to have an unequal effect upon earthworms, some genera being debarred by climatic conditions while others are indifferent to the same.

Mountain Ranges and the Migrationof Earthworms

In many groups of animals the interposition of a lofty chain of mountains presents an insuperable barrier to migration. The barrier is effective for more than one reason. Lack of vegetation and a differing climate are among the more obvious causes which render Alpine chains important as affecting distribution. There is plenty of evidence in the way of positive fact that mountains are not necessarily barriers to the spread of earthworms. The recent explorations of the Ruwenzori chain of mountains in Africa have resulted in the collection of a considerable number of species, some of which come from great altitudes (e. g. 4000 metres and slightly upwards), and one species, viz. Dichogaster duwonica, which Dr Cognetti de Martiis described from the foot of the glacier Elena. I have in my temporary possession a number of examples of the eastern genus Pheretima, some of which are new species from lofty areas in the Philippine Islands. There are plenty of other examples pointing to a like conclusion. It is noteworthy that these forms which have been met with at lofty heights are not essentially different from the plain living forms. One cannot exactly speak, at any rate in the present state of our knowledge, of anything like an Alpine fauna.

It is in fact clear enough that whatever may prove to be the case with regard to particular species, a mountain range is not necessarily a barrier to the dispersal of generic types.

The Ocean As a Barrier To Migration

It is very possible that further investigations into the Oligochaeta will prove that there are more marine forms than those which are enumerated in another chapter. Particularly is this likely to be the case among the family Tubificidae and Naididae. For up to the present those forms belonging to those families which are known to be positively marine in their habit show no great difference from allies inhabiting fresh water, and are in one case indeed (Paranais) common to fresh brackish and saline waters. As to earthworms, the number is also extremely limited, and Pontodrilus is up to the present the only genus which is known to inhabit a marine situation almost exclusively. It has, moreover, been shown that both earthworms and their cocoons are susceptible to salt water and are killed thereby. Thus the facilities which these animals possess of crossing tracts of ocean are limited by this fact alone, besides other impediments offered by tracts of water as such. We may in fact entirely discount the possibility of earthworms floating across arms of the sea – of any extent at any rate. For they do not swim or float, but sink in water. Possibly when the alimentary tract was entirely empty of earth the worms might float; but it is always full and even if evacuated during their passage to the bottom waters the body thus freed would hardly rise. However the noxious qualities of sea water to earthworms is a sufficient barrier to their traversing even narrow straits. On the other hand it might be suggested that torn up trees especially with the roots and clinging earth still attached might harbour worms and thus transmit them to foreign shores. It has been suggested that in this or in some similar way the species of Notiodrilus have been wafted from shore to shore of those lands which are washed by the Antarctic Ocean. Dr Benham, however, in criticising this, calls attention to the violent gales and disturbances of the ocean surface which are so prevalent in those stormy regions, and doubts much whether these animals could retain a safe hold upon some travelling tree trunk. Moreover it is only in this antarctic region where the earthworm fauna of the various continents and islands are so very similar.

Facilities of Migration

The above brief account of physical features which affect the range in space of the terrestrial Oligochaeta seem to show that the only really important barrier is the ocean; and even a narrow tract of sea water would, as it appears, act fatally in preventing the successful immigration of a race inhabiting one shore to the opposite shore. On the other hand we do undoubtedly find in different countries – even when separated by a large expanse of ocean – closely related forms. The most striking instance of this is that afforded by a consideration of the antarctic species of Notiodrilus and Chilota. Can this interchange of Oligochaetous faunas be explained by any means which earthworms possess of crossing tracts of sea by the aid of living carriers such as birds? It has been definitely shown that these creatures actually do convey such small animals as Mollusca attached to their feet. Is anything of the kind likely in the case of earthworms? In the first place it may be safely asserted that if it be possible it has not been actually proved. This however might be perhaps put down to the lack of sufficient observation of actual birds and the contents of such masses of soil as are found attached to their feet. A consideration of the habits of earthworms seems to imply that such a mode of transference from country to country is unlikely. In the first place we remark that the general behaviour of earthworms renders this unlikely. Even the smaller kinds, whose bulk would allow of their being carried, are too active in their habits to permit of a safe transference. When disturbed they wriggle and progress with activity. It is not conceivable that they would remain quiescent for sufficient time to allow of a long voyage. But while the bodily transference of adult earthworms seems highly improbable it is conceivable at the first view that their cocoons might be so transferred. We require to know rather more about the cocoons of earthworms before we can accept this view as a possibility; as far as our present knowledge goes it is not likely that these animals can be assisted to emigrate in this way.

For the cocoons are rather bulky for this kind of porterage. Moreover they are apt to be deposited rather deep down and among the roots of grasses, and in situations where they are not so likely to become entangled in the feet of drinking birds. Assuming, however, that these difficulties can be got over there remains another difficulty. A single cocoon among the terrestrial Oligochaeta does not contain a large number of embryos, as has been pointed out on a previous page. It is true that Allolobophora foetida has six within one cocoon, but most of our indigenous forms have but from one to three embryos in a single cocoon. Thus, if successfully imported, it is hardly likely that the developed embryos scattered after their emergence would come together for breeding purposes; and in cocoons with but one embryo the accidental importation in this way would have to be very frequent to produce any result.

The case here is exactly the reverse of that afforded by the aquatic families (or many of them). In these Annelids the attachment of the cocoon to water plants, which are liable to be entangled in the feet of shore-frequenting birds, would tend to favour migration. And in addition to this the cocoons are naturally smaller and often contain a considerable number of embryos. We are to note that the aquatic forms are on the whole distinctly wider in their range than are the earthworms.

CHAPTER IX

THE GEOGRAPHICAL DISTRIBUTION OFEARTHWORMS

The facts referred to and considered in the last chapter lead to further observations upon the geographical distribution of this group of animals and suggest problems for solution.

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.