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From this brief statement of facts some inferences of interest can be drawn. It is in the first place plain that every part of the world except the extreme north and south has a considerable fauna of earthworms. The one exception would appear to be the northern part of the North American continent. Here we meet with members of the family Lumbricidae which are however species that are met with in the Euro-Asiatic province and are thus to be regarded as possibly later immigrants introduced probably by man. Thus temperature short of a constantly frozen condition of the ground is not a bar to the existence of earthworms. Even a freezing of the ground for lengthy periods is not a complete obstacle to the existence of those Annelids; for I have myself received examples of Lumbricidae from the arctic island of Kolguev. Moreover the temperate regions would seem to be as fully populated in the way of individuals, and even of species, as are the tropical regions. Indeed as to individuals it seems that the temperate regions are more fully supplied than much of the tropics. This however is not quite the object of the present section to discuss. We are here concerned with the relative frequency of genera and species. There are according to a recent estimate of the Rev. H. Friend some forty species recognisable in Great Britain. And as already has been stated the earthworms of Europe amount to perhaps 130, – at any rate well over one hundred. In tropical America there are hardly more. But in the latter case the number of genera is very greatly in excess of that of Europe. We cannot however say that an abundance of generic types is quite characteristic of the tropics. For the Eastern Archipelago, though rich in species, is but poor in genera, not possessing more than half a dozen or so. And on the other hand the temperate climate of New Zealand has produced a very considerable series of genera, much more than those of the islands of the East and nearly as many as those of, for instance, Central America and the West Indies.

This conclusion is in its turn contradicted by the conditions observable in Chili and the temperate regions of South America, where the number of species is large but the number of genera small. In short no general laws, in the present state of our knowledge, can be laid down as to the connection between species and genera on the one hand and climatic conditions on the other. In this department of our subject we cannot do more than has already been done, i. e. to state the actual facts. One is tempted in comparing the rich fauna of tropical Africa with the very limited fauna of Madagascar to associate a richness of types with extent of land surface. In the two cases cited this conclusion is obvious. It may also be extended – if we confine ourselves to species and not to genera. For the two great islands of New Zealand have not between them more than fifty species of earthworms, while Australia has four or five times that number. It will be noticed however that we cannot associate poverty of generic differentiation with limited land masses; for New Zealand has a large number of generic types, very many more than the huge Euro-Asiatic tract of continent.

The Range of Genera

We have seen, and shall again refer to the fact, that individual species of earthworms have not as a rule a range over a great extent of country, save only in those cases such as Pheretima heterochaeta which belong to that physiological section of these worms called 'peregrine' forms; these appear to possess some means of extending their range by the assistance of man which is denied to other forms. Apart from these instances, which do not come under the present category, it is only Lumbricus and its immediate allies, Helodrilus, etc., of which certain species are found to exist over wide tracts of land. There are however many genera which have a wide range and which may be contrasted with others in which the range is very limited. The two extremes are moreover connected by forms with an intermediate range. There is no doubt whatever that the genus with the widest range is Notiodrilus of which species are found throughout the antarctic region, viz., in Patagonia, the islands of the Antarctic Ocean, the Cape of Good Hope, New Zealand, and also further to the north, sometimes even to and beyond the tropics in America, Australia, and Africa. There is no other genus of which the genuine extension (i. e. not in any way due to man) is so great as this genus Notiodrilus. And this fact gains much significance from the now generally accepted view that in its anatomical structure Notiodrilus comes near to the original type of earthworm.

Perhaps the next most widely distributed genus is Helodrilus of the family Lumbricidae which occupies Europe and Asia to the extreme east, and is thought also to be indigenous to certain parts of North America. But this range, though equally wide perhaps in mileage, is less impressive than that of Notiodrilus, since the land areas inhabited by the genus are continuous – almost so if we accept North America as its real habitat. Here we have a case precisely the opposite of that of Notiodrilus; for while there are reasons for regarding Notiodrilus as an ancient form of Lumbricid, there are equally good reasons for regarding the Lumbricidae as the most modern family of earthworms.

To find other instances of widely spread genera we must recur to the great family Megascolecidae. There are practically no Geoscolecidae which have a really extensive range. The only instances are Criodrilus and its ally Sparganophilus which occur in America, whether North or South, and in Europe; but as these forms are at least largely aquatic the facts are not quite comparable with those now under consideration.

The genus Dichogaster (which includes as synonyms Benhamia, Millsonia, Microdrilus) is unquestionably indigenous to tropical Africa and certain parts of America including the West Indies. It has been also met with in the East; but as the species there occurring, such as for example the species originally described by myself as Microdrilus saliens, are of small size, an accidental introduction is quite possible, and it is by no means certain that it has not occurred. In any case the genus is known to possess species which are undoubtedly to be reckoned among peregrine forms – such as D. bolavi, which has turned up in Europe. Gordiodrilus and also Ocnerodrilus with its sub-genera have very much the same range as has Dichogaster. It is to be noted however that these forms are circumtropical, and that their distribution is thus less continuous than that of Notiodrilus; they do not however show the markedly discontinuous range of certain other genera of Megascolecidae. For instance Octochaetus is well known from New Zealand, and, not occurring in the intermediate tracts, is again met with in India. Hoplochaetella is believed by Michaelsen to present us with another precisely similar instance. Then also the genera Woodwardia and Notoscolex are to be found in Australia and again (absent from the immense tract lying in between) in Ceylon. Megascolex has much the same range, showing also this marked and remarkable discontinuity. Stranger still, perhaps, is the range of Plutellus and Megascolides, of which the former, chiefly found in Australia and Tasmania, not only extends its habitat to Ceylon but also to North America; it is there represented by Eisen's species Argilophilus marmoratus, referred by him, and not unnaturally, to a distinct genus, but placed by Michaelsen in Plutellus. Megascolides is Australian and from the North Island of New Zealand, where its species were regarded by Benham as of a distinct genus, Tokea. There is also one form, Megascolides americanus, in the western region of North America.

The two genera Yagansia and Chilota, closely related to Notiodrilus, have a range which is short of that of Notiodrilus, and we shall see later that there are reasons for regarding these genera as derived from Notiodrilus. They are met with only in the south of South America, and in the Cape of Good Hope region.

The range of Microscolex seems to be much the same as that of Notiodrilus; but it is a little uncertain how far the genus is really autochthonous in the countries where it occurs; and in any case it differs from Notiodrilus in occurring in Europe, where the species has been named for a long time M. phosphoreus. We do not positively know whether this is 'peregrine' in Europe or not.

The range of the antarctic Acanthodrilinae is in a sense continuous; for they argue the former northward extension of the antarctic continent and in any case they occupy neighbouring land masses. In Octochaetus and Plutellus the case is different and one of real discontinuity. There are however cases of wide range which is also actually continuous and such is afforded by the genus Pheretima. This genus appears to be possibly indigenous to Australia; in any case it reaches from the Solomon Islands on the east to India towards the west, being found in all intermediate continents, while it reaches Japan on the north side of this large area.

There are other genera which extend their range over a considerable area, but which are not so widely distributed as these which we have just been considering. Thus Diporochaeta is chiefly Australian but also reaches even the South Island of New Zealand and the southward lying antarctic islands. Desmogaster and Eupolygaster among the Moniligastridae range from Burmah in the east to Sumatra and Borneo further east, though they are not recorded from intermediate islands. Perionyx is found in Burmah, India, Zanzibar, Sumatra, and Java. There are other examples of genera which have much the same range as those enumerated. Finally there are those which are confined to one land mass and very often to a restricted region of that. Thus Kynotus is confined to Madagascar, all the genera of Eudrilidae to tropical Africa, some of them, e. g. Beddardiella and Euscolex, to very limited tracts, others to wider or less wide areas in that continent. Maoridrilus is only found in New Zealand, to the South Island of which also is confined the genus Neodrilus. To the Cape region of Africa is limited Microchaetus; and to a belt running across the northern part of the tropical region and extending down the Nile, the remarkable, partly aquatic, Alma.

As a kind of appendix to these facts and conclusions we shall next deal with certain widely spread forms that have been already referred to, with the range of different genera over great land masses of the world, and with the earthworms of oceanic islands.

CHAPTER VI
PEREGRINE FORMS

Dr Michaelsen has used this term to describe those species which possess some powers of migration over the sea, denied to the majority of worms, and probably due to the direct interference of man. Thus we find in collections of earthworms from various parts of the world not only examples of forms which do not come from other parts of the world, but also a few which occur in many or even most of such collections. It is for example to be actually expected that a collection of earthworms made in South America, the Philippine Islands, or Australia will contain examples of the apparently ubiquitous Pontoscolex corethrurus. This is what has actually happened in cases of which I have personal knowledge, as well as in many others recorded in the literature of the subject. I have myself received this worm from the three parts of the world mentioned, and also from Hawaii. Others have increased its known range to other parts of the South American continent, to Central America, the West Indies, the islands of Sumatra, Java, Borneo, Celebes, Mauritius, and Madagascar, etc. It is in fact found everywhere in the tropics. With this range may be contrasted that of another genus of the same family (Geoscolecidae), viz. Kynotus, which, though consisting of many species, is not found outside of the Madagascar district. It should be added that Pontoscolex does not appear to contain more than two species, the one not mentioned in the above survey of its distribution being P. insignis of Kinberg, which is apparently the same as P. liljeborgi of Eisen, and is limited to certain parts of America.

Before attempting to grapple with the remarkable facts implied by the distribution of this genus, it will be well to survey the whole group of Oligochaeta and to reduce to as short a space as possible the total series of facts which are of the same nature.

A case, even more striking than that of Pontoscolex, is afforded by the Eudrilid genus Eudrilus. As with Pontoscolex there are two species of this genus, one, E. pallidus, being confined to West Africa, the remaining one, E. eugeniae, being world-wide in range. This latter species has received the following names, viz. E. decipiens, E. lacazii, E. peregrinus, E. sylvicola, E. boyeri, E. jullieni, E. erudiens, and E. roseus; they appear to be all synonyms of the name originally given by Kinberg who however did not recognise the distinctness of the form as a genus. It is now known as Eudrilus eugeniae. The variety of names given to supposed different forms (for two of which I am myself responsible) is due to the fact that in earlier days when nothing was known about the geographical distribution of this group of animals it was thought by no means unreasonable that a given genus represented by several species should range over the globe. This fact coupled with imperfect description of structural details led to the multiplication of supposed species, a position which is no longer tenable. This worm is quite as abundant in gatherings from all parts of the world as is Pontoscolex corethrurus; and in addition to the countries inhabited by the latter, Eudrilus eugeniae has been met with in New Caledonia: tropical Africa is probably its original home.

The two families that have been hitherto considered offer no further instances, among their many species, of worms with so wide a range as those just dealt with. There are indeed one or two forms, e. g. Criodrilus and Glyphidrilus, which have a considerable range though not nearly equalling that of Eudrilus and Pontoscolex. These are, however, aquatic forms and the range of aquatic forms is determined as far as we can see by a different series of causes to that of terrestrial forms, which are referred to later.

Among the Moniligastridae we have apparently an instance of a peregrine form. The genus itself has its headquarters in Ceylon and extends a little way in other eastern regions; there is, however, one species, Moniligaster bahamenis, described some years since from the Bahamas which must surely be an example of a peregrine form, particularly since it is probably identical with M. japonicus whose name is indicative of its habitat.

Among the huge family of the Megascolecidae there are a considerable number of species which apparently possess the same facilities for making their way in different directions and across seas from the locality that is thought to be their real home.

Of the very many genera, however, of which this family is composed, a comparatively small number are thus peregrine in habit at times. All the species known which are distributed broadcast, more or less, over the tropics belong to the genera Pheretima, Microscolex, Dichogaster, Megascolex, Perionyx, Ocnerodrilus, Kerria. These several genera are placed in order of frequency of exotic occurrence. Indeed of the two latter genera their frequent life in fresh water may really remove them from the present category altogether. In addition to these are some perhaps more questionable instances, such as the genus Gordiodrilus which, prevalently West African, has also been found in the West Indies, in East Africa, and in India and Madagascar. These instances I propose to leave out of consideration in the present sketch. The most obviously peregrine genus of all those enumerated is Pheretima, which according to my experience turns up in almost all gatherings of earthworms from any part of the tropical or even sometimes temperate regions of the world. It seems to be fairly well settled that this extensive genus has its real home in the islands of the Eastern Archipelago, perhaps extending a little in various directions from that centre. But examples of the genus have been found in almost all other regions. And what is especially to the point in considering the facts, as will be pointed out with more emphasis later, the assumedly peregrine species do not differ from those found in the real district in which the genus is indigenous.

Dr Cognetti de Martiis enumerates in the Neotropical region, that is in South and Central America and the West Indies, the following species: Pheretima biserialis, P. californica, P. capensis, P. elongata, P. hawayana, P. hesperidum, P. heterochaeta, P. houlleti, P. posthuma, P. rodericensis, P. schmardae and P. violacea. Of these twelve species it is quite certain that the last six occur in the East, where they are doubtless indigenous. So too do the species P. biserialis, P. capensis, and P. hawayana. The synonymy of the different species of this large genus is not yet in a completely settled condition. But in the meantime it is in my opinion quite possible that both P. hesperidum and P. californica are identical with species also occurring in the East. There remains the somewhat doubtful P. elongata from Peru which has not been very fully described. There is thus no convincing evidence of species really indigenous to and confined to any part of America. Some of these species also occur in many other parts of the world. For instance, P. heterochaeta is very widely spread indeed, occurring as it does in Australia, New Caledonia, Madagascar, and even England (in hothouses). This species indeed is the most prevalent of all Pheretimas and seems to be abundant in gatherings of earthworms from various localities as are Eudrilus and Pontoscolex.

From the island of Madagascar and neighbouring islands the following species of Pheretima have been obtained and identified by Dr Michaelsen: viz. Pheretima pentacystis, P. peregrinus, P. heterochaeta, P. biserialis, P. rodericensis, P. houlleti, P. robusta, P. mauritiana, P. taprobanae, and P. voeltzkovi. It will be noticed that the majority of these are also included in the list from South America, and that many of them are also found in other parts of the world, and nearly all of them in the East. There remain a few which are doubtful. It is quite possible that P. mauritiana is the same as P. hawayana and P. bermudensis, in which case it has a world-wide range. P. taprobanae is well known as a Ceylon species. P. robusta also occurs in the East Indian islands. There remain P. pentacystis, P. peregrinus, and P. voeltzkovi. P. peregrinus is known from Australia and also from Sumatra, so that that species need not concern us in enumerating those which are possibly endemic. In fact it is only P. pentacystis and P. voeltzkovi which may be really Mascarene.

Another peregrine genus belonging to the sub-family Acanthodrilinae is Microscolex. But the limits of this genus may be regarded as at present rather uncertain. And this difficulty somewhat affects the bearing of the facts to be related, though it hardly affects the value of the facts themselves. Dr Michaelsen referred to the genus in his great work seven well-defined species, and four others not so plainly distinct. Of these eleven, two are confined to New Zealand, four to North and Central America, one to Hawaii, one to Madeira, one to Algeria, while the remaining two are found pretty well over the whole surface of the world. More recently the same authority has somewhat extended his view of the generic characters, so as to include a number of forms found in Patagonia, Cape of Good Hope, and the antarctic region generally, while he has lumped together into two species only, viz. M. phosphoreus and M. dubius, the eleven forms just mentioned, which species therefore are absolutely world-wide in range, and thus form an excellent example of a peregrine form. These species moreover differ from Pontoscolex and some others in that they have been able to establish themselves in Europe. Dr Michaelsen also relates that in the cultivated lands of South West Australia, Microscolex dubius and Helodrilus caliginosus are actually the commonest species; and he calculates that they form together quite 90 % of the earthworms gathered in any locality belonging to this region.

Some of the other Megascolecid peregrine forms will be referred to later. There is no doubt that the family Lumbricidae offers by far the greatest number of peregrine forms and that these are most abundant in collections from extra-European countries, where the collector has searched in cultivated lands. There are at least eight or nine species which are common in many parts of the world though their original home is undoubtedly Europe.

This is a brief review of the facts, more detailed in some cases than in others. It remains to review and compare the results arrived at.

The first general statement that may be made is that this faculty of extending their range beyond the limits assigned by nature is not confined to any one family. For all the chief sub-divisions of the terrestrial Oligochaeta seem to possess it, though in unequal degrees. But the inequality may be more apparent than real. For if it be recollected that the species of the family Megascolecidae are very much more numerous than those of the Eudrilidae or even the Geoscolecidae, the fact that there are more peregrine Megascolecidae will lose some of its importance. With the Lumbricidae the case seems to me to be different. Here the preponderance, not only in species (relatively speaking) but in individuals, is much above that of other families. This preponderance I should be disposed to assign to the newness of the family coupled with the vigour seen in new races. That this is a possible explanation is borne out by the fact that the 'Perichaetidae' (i. e. the genus Pheretima) is the most salient race of peregrine Megascolecidae, and it is now generally held that this group is the most modern of that enormous family.

Another general statement may be made with even more confidence, viz. that it appears to be an undoubted fact that some species are more capable of extending themselves than others. Thus Eudrilus eugeniae occurs everywhere on the great land masses of the globe, except in Europe; it is in fact circummundane in the tropical zone, as is also Pontoscolex. Dichogaster bolavi is again a trifle more restricted in its range, having been recorded from tropical Africa, South America, West Indies, Madagascar, and India. Its occurrence near Hamburg in Europe is also to be noted. A little more restricted still is Nematogenia panamaensis whose range is in Central America, tropical West Africa, and Ceylon. Lastly there are cases such as Pheretima taprobanae which, a native of Ceylon, is also found in Madagascar.

It may be asserted in the third place that there are no peculiarities of structure shared by all of these peregrine forms which might account for their physiological similarity, except indeed the somewhat negative feature which they have in common, that is of being of small or moderate size. Eudrilus and Pontoscolex are not isolated types in their respective families; nor do they seem to approach each other in any respect. Nor can it fairly be said that these peregrine species are marked by any great variability of structure as compared with other forms, which might allow for their suiting themselves to various climates and conditions. It is true that Eudrilus eugeniae has received many names which might at first argue some variability. But these names have been perhaps given by persons rather under the influence of the idea that remote habitat implied specific difference, and who were thus inclined to see minute differences, and who perhaps were furthermore led astray in the matter by imperfectly accurate descriptions on the part of others. Certainly some of the peregrine species of Pheretima are subject to some variation, particularly in the number and arrangement of their genital pupillae. But this feature is by no means confined to those species and cannot be utilised as in any way an adaptation to wide distribution.

But while we can lay down no general explanation of the phenomenon, it is possible to furnish some explanation of particular cases. Thus the genus Microscolex is the only exotic genus which appears to have established itself in Europe, from which country indeed it was early known as an apparently indigenous inhabitant. We must put this and some similar cases down to ability to do without great heat. It is probable in fact that the original home of Microscolex is the antarctic half of the globe; and this of itself would allow of its establishing a new home in the northern hemisphere, did other circumstances allow of it.

It might be urged that this genus has been able to establish itself in Europe because it has in fact had the chance denied to other species. There are a good many, however, which would in that case be in the same category. Some years ago I received from time to time a very large number of earthworms from the Royal Gardens at Kew which had been accidentally imported thither from many quarters of the globe, among which I described some eighteen or twenty new species including, for instance, the African genus Gordiodrilus. There are plenty of facts of a similar nature and Dr Michaelsen has pointed out that botanical gardens act as centres of dispersion for accidentally introduced Oligochaeta. We must therefore come to the conclusion that temperature is at least one of the causes of a difference in the capability of extending their range shown by the Oligochaeta, a cause which doubtless operates as a check upon extension of range in non-peregrine forms also, and prevents for instance the dispersion of the tropical African Eudrilidae into the region of the Cape.

We may, as it appears to me, confidently look upon indifference to varying temperature as a condition of ability to colonise new countries. But it is obvious that this is not of itself a sufficient cause to explain the facts. Otherwise this country and N. Europe would contain many antarctic earthworms; the only one that has been recorded to my knowledge is Microscolex.

Though an inability to endure a temperate climate may have rendered the movements of peregrine species more limited, the same or rather the exactly opposite cause does not seem to have played any important part in this direction. For it is above all the Lumbricidae, normally dwellers in temperate climates, that are so remarkable for their wide range over the world. Nor can it be convincingly asserted that the extra-Palaearctic Lumbricids are real indigenes of those – often tropical – countries. For if so we should expect them to be at least of different species. Lumbricids however from South America, Australia, etc., are specifically identical with European forms.

There is no doubt that wherever land has been at all long under cultivation in any part of the world that land will be found to produce species of the European genera Lumbricus, Helodrilus, Eisenia, etc. More than this the recently imported European forms will be found to have largely or almost entirely driven out the native species, which have retired more into the interior of the country. There is thus here no barrier placed by temperature. It should be remarked, however, that while these earthworms are most abundant in the less tropical regions, they occur in such tropical districts as Peru, though in less striking numbers. Whether those of North America are really indigenes or not remains perhaps a matter for discussion; but it is at least noteworthy that the vast preponderance of species occurring there are also European and even British. In this particular case, which is on the whole the most emphatic of all the cases of peregrine earthworms, some explanations are possible, or at least have been offered. In the first place it would appear that earthworms are more abundant as individuals in northern countries where the soil is rarely dry for prolonged periods. And as has been already pointed out there is a close relation between earthworms and agriculture. Dunghills are fertile gathering grounds for some species, and ploughed fields and gardens always swarm with several species. In the tropics these animals are not so evident; and the strong rays of the sun appear to drive them further underground and into marshes; this obviously lessens the chance of their accidental transference by man. Furthermore Dr Eisen has pointed out that the European species are apt to have clitella and to be fertile all the year round, which is not always the case with other genera. That naturalist has added to this observation the fact that in rich cultivated soils in California it is impossible to find anything but imported European species, since cultivation itself appears actually to drive away the native forms.