RELATIVE FREQUENCY OF EARTHWORMS IN DIFFERENT REGIONS OF THE WORLD
It will be of use for various purposes to be considered later to
arrive at a comprehensive view of the relative numbers of species
and genera of earthworms in the four quarters of the globe. And in
making this general census we shall not take into consideration the
purely aquatic forms, but shall limit ourselves to the earthworms,
_sensu stricto_, or Megadrili, of which, however, it is true that
some members are actually lake and river dwellers. This latter fact
will not, however, interfere with the usefulness of the comparative
survey.
Two preliminary remarks are necessary. The opinions of naturalists
vary as to the limits of genera; and a species may be a species to
one and a mere variety to another. Thus it will be impossible to give
a summary of the facts to be enumerated presently, which will be
either absolutely accurate or which will satisfy everyone in every
detail. But it is asserted that the following survey is substantially
correct.
In the second place it is often possible to eliminate from the
fauna of a given region those species and even genera which have
been accidentally imported, a matter which will receive careful
consideration on a later page. Such forms are therefore, in those
cases at any rate where the evidence seems to be overwhelming,
withdrawn from the list. In other cases, particularly in the Eastern
region of the world, it has been found less easy to rectify the
catalogues by removing what Dr Michaelsen has termed 'peregrine'
forms.
We shall commence with a census of South America; the entire
Continent will be divided for the present purpose into three
divisions, viz. South America, Central and North America, and in the
third place the West Indian Islands.
In South America we find that the bulk of the indigenous earthworms
belong to the family Geoscolecidae and to a definite sub-family,
viz. Geoscolecinae. These genera are _Onychochaeta_ with one
species, _Hesperoscolex_ of which one species is known from the
area, _Periscolex_ with one species, _Anteoides_ with two species,
_Pontoscolex_ one species. _Opisthodrilus_ with two species,
_Rhinodrilus_ (including either as synonyms or as sub-genera,
_Thamnodrilus_, _Anteus_, _Tykonus_, _Urobenus_ and _Aptodrilus_)
with no less than 49 species: _Andiodrilus_ with five species,
_Holoscolex_ with one species, _Glossoscolex_ ten species,
_Fimoscolex_, _Andiorrhinus_ and _Enantiodrilus_ with one species
apiece.
Thus of this sub-family of Geoscolecidae we have in South America
a large number of genera and a much larger number of species. Of
a second sub-family of Geoscolecidae there are three species of
_Criodrilus_ found in the South American Continent.
We now turn to the Megascolecidae of which a large number of species
occur within the area now under consideration. The bulk of these
belong to the sub-family Acanthodrilinae and they are as follows:
Of the genus _Notiodrilus_ there are ten species, of _Microscolex_
two species, of _Chilota_ 19, of _Yagansia_ 13.
A second sub-family Trigastrinae also occurs in this Continent and
the following genera are permanent inhabitants, viz.:--
_Dichogaster_ (= _Benhamia_) with only three species, of which
two at least are found elsewhere, and of which therefore the
autochthonism is doubtful.
Finally, there is the sub-family Ocnerodrilinae comprising the
following genera: _Kerria_ with ten species, _Ocnerodrilus_ (with
sub-genera _Liodrilus_, _Ilyogenia_ and _Haplodrilus_) four species,
which again are rather doubtful indigenes of the South American
Continent.
Leaving aside certain species (of the genera _Lumbricus_,
_Pheretima_, etc.) which are clearly not indigenous, the South
American Continent harbours 150 kinds of earthworms which are
distributed in some 19 genera. But of these a few species (_e.g._
_Onychochaeta windlei_, _Kerria macdonaldi_) stray into neighbouring
regions, _i.e._ the West Indies and California. It is doubtful
therefore whether they are to be referred to as limited to one of
these regions and accidentally imported into the others, or whether
they are genuine inhabitants of both.
The South American Continent shares with the West Indies the
following genera, but the species (except in the case of
_Onychochaeta windlei_, _Glossoscolex peregrinus_) are distinct;
these genera are _Hesperoscolex_, _Pontoscolex_, _Dichogaster_, and
_Ocnerodrilus_. _Diachaeta_ is limited to the West Indies.
The following South American genera are also found in Central and
warmer North America (Mexico, California), viz. _Hesperoscolex_,
_Periscolex_, _Rhinodrilus_, _Pontoscolex_, _Andiodrilus_,
_Glossoscolex_, _Notiodrilus_, _Microscolex_, _Dichogaster_, _Kerria_
and _Ocnerodrilus_. But with the exception of _Hesperoscolex_, which
seems to belong rather to Central America and the West Indies,
_Microscolex_ and _Pontoscolex_ which are world-wide and whose
original home is therefore difficult to fix, and _Dichogaster_
and _Ocnerodrilus_ which would seem rather to be rare immigrants
(perhaps not truly indigenous) into South America, these genera are
practically distinctively South American.
Thus we may fairly say that the genera _Anteoides_, _Opisthodrilus_,
_Andiodrilus_, _Holoscolex_, _Glossoscolex_, _Rhinodrilus_,
_Andiorrhinus_, _Fimoscolex_, _Enantiodrilus_, _Notiodrilus_,
_Chilota_, _Yagansia_ and _Kerria_ are at least very distinctive of
S. America and that they are represented by the large majority of
species found in that continent, the total being 120 or slightly more.
Leaving the American Continent and adjacent islands and archipelagos,
the next great land-mass to receive attention will be the Continent
of Africa. In giving a census of the species and genera of earthworms
which inhabit this quarter of the globe it must be premised that
the facts relate only to Africa south of the Sahara. But little is
known of the genera which occur in Algeria and Morocco, but from what
little is known it is clear that they should come into consideration
in connection with the fauna of Europe and not with that of tropical
and south temperate Africa.
We have in the first place to consider an entirely peculiar family,
of fair extent in genera and species, which is limited to this region
of the world; that is to say with one apparent exception which is
clearly only apparent. The genus _Eudrilus_ is one of the few kinds
of worms that turns up in collections from every tropical part of
the world and even at times from more temperate countries. It is one
of those 'peregrine' forms, as Dr Michaelsen has termed them, which
possess unusual facilities for extending their range. Presumably its
real home is Africa. This family is known as the Eudrilidae though
by some it is only regarded as a sub-family of the Megascolecidae.
In this family we have the following genera: _Eudriloides_
with 11 species, _Platydrilus_ with 11 species, _Megachaetina_
with two species, _Reithrodrilus_ with one, _Stuhlmannia_ with
five species, _Notykus_ and _Bogertia_ with one species each,
_Metadrilus_ also with but one species, _Pareudrilus_ with perhaps
five, _Libyodrilus_ with one and _Nemertodrilus_ with two species,
_Metschaina_ with two species, _Eudrilus_ with two (or possibly
more?) species, _Parascolex_ with four species, _Preussiella_ with
two, _Buttneriodrilus_ with two, _Eminoscolex_ with 16 species,
_Hyperiodrilus_, _Heliodrilus_, _Alvania_, _Rosadrilus_, _Kaffania_,
_Euscolex_, _Metascolex_, _Beddardiella_, _Gardullaria_ with only one
species to each genus, _Bettonia_ with three species, _Teleudrilus_
with 15 species, _Polytoreutus_ with 22 species, _Iridodrilus_
with two, _Malodrilus_ also with two, _Neumanniella_ with eight,
_Eupolytoreutus_ with two and _Teleutoreutus_ with only one species.
This completes the list of the Eudrilidae. We will take the huge
family Megascolecidae next; and we find in tropical Africa that the
genus _Dichogaster_ alone contains at least 93 species confined to
Africa, as well as a few which it shares in common with America,
and the common and widely spread _D. bolavi_ which has even made
its way to Europe. Of the sub-family Ocnerodrilinae we have the
genus _Gordiodrilus_ with seven species, a genus which also occurs
in Madagascar and the West Indies: _Nannodrilus_ with three
species, _Diaphorodrilus_ with one species and a few examples of
_Ocnerodrilus_ and its sub-genera, some of which are also forms that
occur elsewhere in the world, for example _Nematogenia panamensis_.
Of _Pygmæodrilus_ there are eight or nine species.
Of the sub-family Acanthodrilinae the Cape region of South Africa
harbours some seven species of the genus _Notiodrilus_, of which one
however is a West African form. The allied _Chilota_ has 13 species,
and there is a peculiar genus _Holoscolex_ near to _Yagansia_ with
one species. We next have to deal with the Geoscolecidae, of which
a sub-family, Microchaetinae, is mainly found in Africa, the rest
being found in the neighbouring Madagascar and some few in the East.
_Microchaetus_ contains about 14 species, _Tritogenia_ perhaps
three, _Callidrilus_ two, and the genus _Glyphidrilus_, mainly found
in Asia, has one species in the region now under consideration. In
addition to these Geoscolecids there is the peculiar and largely
aquatic _Alma_ with six or seven species in East, West, and Central,
Africa, and in the Nile region.
Summing up the genera which are found in tropical and South Africa we
find that there are 44 which are abundant in, or entirely confined
to, the region. In addition to these four or so occur in Africa but
are either more abundant elsewhere or (as in the case of _Chilota_
and _Notiodrilus_) are equally distinctive of other parts of the
world. The number of species may be estimated at 270, possibly rather
more. Clearly therefore this part of the world is much richer than
South America, both in numbers of genera actually found, and peculiar
to the country, and numbers of species.
Passing from Africa the next definite quarter of the globe which
will detain us here is the island of Madagascar, so remarkable for
the Mammalian fauna which characterises it, for its lemurs, peculiar
Insectivora and Carnivora, and above all by reason of the absence of
the prevalent African types such as antelopes, zebras, rhinoceros
etc. It is probable that a good deal still remains for discovery
among the earthworms of this island; but a considerable number are
already known and they are as follows:
The Eudrilidae are completely absent, a state of affairs which is
paralleled by the absence of antelopes among mammals.
The sub-family Acanthodrilinae of the family Megascolecidae are
represented by four species of the genus _Notiodrilus_, with which
perhaps _Maheina braueri_ from the Seychelles should be included as
it presents small differences from _Notiodrilus_.
Among the Megascolecinae a good many species of _Pheretima_ have been
collected both on Madagascar and on certain adjacent islands; but
these, with one possible exception, are forms which occur elsewhere
and are often indeed very widely distributed 'peregrines,' so that
it is hardly permissible to place them among the real inhabitants of
Madagascar. The same arguments hold in the case of _Lampito mauritii_
and the ubiquitous _Eudrilus_ and _Pontoscolex_. But there is the
peculiar _Howascolex_.
Among the Ocnerodrilinae a distinct species of _Gordiodrilus_ occurs,
and an obviously introduced _Ocnerodrilus_.
It is among the Geoscolecidae that the most characteristic forms are
met with. These belong exclusively to a genus of the Microchaetinae,
_Kynotus_, which is found nowhere else but in Madagascar, where it
is represented by at least twelve different species.
We have therefore in Madagascar and the surrounding islands only two
peculiar genera, only four genera which can be regarded as endemic,
and only about seventeen peculiar species.
Passing eastward and crossing the Indian Ocean we come to the
Continent of Asia, and we shall first direct attention to the
peninsula of India and adjacent parts of Burmah and the island of
Ceylon, of which there has been accumulated a great deal of knowledge
concerning the Oligochaetous fauna.
This quarter of the globe differs quite as much from any that have
been hitherto considered as they do among themselves. We have left
behind us the Geoscolecidae with the exception of the ubiquitous and
clearly peregrine _Pontoscolex_, and the genus _Glyphidrilus_, which,
being aquatic at times, is perhaps hardly to be considered in the
present survey.
We have also in this Indian region the equally ubiquitous _Eudrilus
eugeniae_ which need not of course detain us further. The Lumbricidae
are for the most part of European forms with the exception of
_Helodrilus indicus_ conceivably an actual inhabitant of India.
It is among the Megascolecidae that the vast majority of the
forms endemic in India are to be found. We shall take this family
according to its sub-families. In the first place we note that the
sub-family Acanthodrilinae is totally unrepresented. The large
sub-family Megascolecinae has very numerous representatives. Of
the genus _Megascolex_ itself there are some 30 species, of which,
however, two or three are looked upon as varieties. Of the allied
genus _Notoscolex_ there are ten species and of _Perionyx_ about
13. The genera _Megascolides_, _Diporochaeta_, _Spenceriella_ and
_Woodwardia_ have only six species between them of which three belong
to _Megascolides_. _Plutellus_ has five species in this region.
_Lampito_, which is a widely spread form with but one species _L.
mauritii_, may or may not find here its original home. It also occurs
in Africa and Madagascar. _Pheretima_ is represented by no less
than 12 species, of which at any rate the very great majority are
peregrine forms, and not to be safely regarded as forming an integral
part of the fauna of the Indian peninsula and adjoining countries.
Of the sub-family Octochaetinae the type genus _Octochaetus_ has
ten species in this region and _Eutyphoeus_, which is restricted to
the region, has 15 species or perhaps rather more. _Hoplochaetella_
has but one. Of the Trigastrinae there are four or five examples
of the genus _Dichogaster_ which occur within the region now
under consideration; but it is doubtful whether they are truly
indigenous. On the other hand _Eudichogaster_, closely allied both to
_Dichogaster_ and _Trigaster_, seems to be confined to this part of
the world where it is represented by five species. Finally we come to
the last sub-family--that of the Ocnerodrilinae, which is represented
by a species apiece of the genera _Ocnerodrilus_, _Nematogenia_ and
_Gordiodrilus_. The latter species _G. travancorensis_ is alone
to be regarded as endemic and it is very near to the African _G.
zanzibaricus_, described some years since by the present author.
The remaining family of terricolous Oligochaeta found in India is
the family Moniligastridae which is practically limited to this part
of the world and consists of at least twenty species distributed
among the genera _Moniligaster_, _Eupolygaster_, _Desmogaster_ and
_Drawida_, the majority belonging to the last-named genus.
This quarter of the globe is therefore inhabited by 18 genera which
are certainly truly endemic, and which comprise between them about
120 species. But only four or five genera are peculiar.
The remainder of the Asiatic Continent is not very well explored
with regard to its earthworm inhabitants. It seems clear however
that the southern and coastal region of China and Japan with the
Malay peninsula are really continuous with the mass of islands which
lie between India and Australia and form together a tract of land
which is characterised by an Oligochaetous fauna differing from both
that of India on the one hand and Australia on the other. We shall
therefore consider this huge portion of the globe as one region
comparable to the other divisions that have been hitherto considered.
With reference to the Lumbricidae and Geoscolecidae the same remarks
may be made as in the case of India. The indigenous forms of the
latter family are to be looked upon as outside of the present survey
since they are largely or entirely aquatic forms. _Pontoscolex
corethrurus_, and _Eudrilus eugeniae_, need not detain us for reasons
already amply stated. We now come to the great family Megascolecidae.
Of this family the genus _Pheretima_ stands at the head; and of the
200 or so species that have been or can be assigned to this genus
all, with merely two or three exceptions, are natives of the Eastern
Archipelago and adjoining mainlands of Asia. Of other Megascolecinae
the region has yielded the following genera. In Java one species
of _Woodwardia_ (_W. javanica_) has lately been described. It is
regarded by Michaelsen however as doubtfully indigenous. And the same
remark may be made of _Perionyx_. _Plionogaster_, however, with four
or five species, is limited, as far as our present knowledge goes, to
the Philippines and to neighbouring islands. The Acanthodrilinae and
Octochaetinae are totally absent from this part of the world, there
being no record even of peregrine species of these sub-families.
The sub-family Trigastrinae is not however unrepresented; for of
_Dichogaster_ several species occur, such as _D. malayana_ and _D.
saliens_. But these are by no means certainly to be looked upon as
real natives of the situations within this area where they occur.
This completes the scanty list of genera found in the region under
consideration; for one Moniligastrid (_M. barwelli_) is hardly to
be looked upon as indigenous. We have therefore to record here but
five genera, of which only two are certainly indigenous and probably
also confined to the region; they contain between them certainly two
hundred species.
We next come to the Continent of Australia. The earthworm fauna is
again quite without Geoscolecidae and of course Lumbricidae. As to
the former there are not even doubtful cases like _Glyphidrilus_ of
the East; for we find only recorded _Pontoscolex corethrurus_, a
species concerning whose extraordinary powers of migration there is
no possible doubt. The same may be said of _Eudrilus_ also recorded
from Australia. Here the Megascolecidae are as conspicuous as in the
old world generally. We find, however, a great many members of the
sub-family Acanthodrilinae. Of the antarctic genus _Notiodrilus_
there are some six species. _Microscolex_ occurs; but the real
habitat of this genus is very doubtful. _Diplotrema_, with one
species, _D. fragilis_, is not only indigenous to, but confined to,
Australia.
The most prevalent sub-family is that of the Megascolecinae. Of
the genus _Pheretima_ there are two species which may or may not be
truly indigenous. Of _Plutellus_ there are forty species; there are
over thirty of _Diporochaeta_, while _Notoscolex_ (with which Dr
Michaelsen associates _Digaster_, _Didymogaster_, _Perissogaster_)
includes more than forty species, _Fletcherodrilus_ has but one
species, _Megascolex_ has seventy species, _Woodwardia_ sixteen,
_Spenceriella_ five, and _Megascolides_ seventeen. There are four
species of _Perionyx_ which were at one time regarded by Michaelsen
as necessitating a new but allied genus _Perionychella_; the two are
now merged. Thus there are not far short of 150 species and eleven
genera represented, of which only two are limited to Australia.
Having completed the survey of the central and southern land masses
of the globe we next direct attention to the northern land masses,
viz. North and Central America on the one hand, and Europe and
northern Asia on the other. With regard to America we find besides
many species of Lumbricidae the genera _Notiodrilus_, _Microscolex_
among the Acanthodrilinae, _Megascolides_ and _Plutellus_ of the
Megascolecinae, no member at all of the Octochaetinae, a considerable
number of species of _Ocnerodrilus_ and sub-genera belonging to the
Ocnerodrilinae, a good many species of _Dichogaster_ and at least
one of _Trigaster_ among the Trigastrinae, while one sub-family,
that of the Diplocardiinae, is only found here and contains two
genera _Diplocardia_ and _Zapotecia_ with quite ten species between
them of which only one belongs to the last-named genus. There are
no Geoscolecids (except the chiefly aquatic _Sparganophilus_); this
family stops short in the West Indies where their presence has been
briefly referred to in considering the worms of South America. The
vast majority of the genera enumerated here are only found in the
warmer parts of the North American continent. We have therefore
in this division of the world some nine genera of which at least
one, viz. _Diplocardia_ (and its close ally, hardly perhaps to be
separated, _Zapotecia_), is confined to it, while _Trigaster_ only
extends as far south as to the West Indies.
Europe and northern Asia, of which Europe alone and certain
limited tracts of Asia are at all known, contain all the genera
of the family Lumbricidae which are (according to Michaelsen)
_Eiseniella_, _Eisenia_, _Helodrilus_, _Octolasium_, _Lumbricus_
with various sub-genera of _Helodrilus_, such as _Dendrobaena_ and
_Allolobophora_. Besides these forms, which amount to at least 130
species, we find the genus _Hormogaster_, with two species, the only
genus of the Geoscolecid sub-family Hormogastrinae; there is also
a species apiece of the genera _Criodrilus_ and _Sparganophilus_
(sub-family Criodrilinae). But as these are at least largely aquatic
they come under another set of rules and are not important in the
present survey of the earthworms of the world. It will be thus seen
that with these two last exceptions the area in question contains but
six genera of which _all_ are peculiar.
We have now completed the survey of the principal land-masses of
the globe. We shall conclude with a reference to one of the largest
islands of the world, viz. New Zealand, with which will be included
a few outlying islands such as the Aucklands, Snares Island, etc.
The reason for not ignoring these islands as we have ignored certain
other islands of large size, such as Japan, will be apparent from
the peculiarity of the earthworm fauna which they show. From New
Zealand the Moniligastridae, Geoscolecidae, Lumbricidae, and, of
course, the Eudrilidae, are absent, save the ubiquitous _Eudrilus_.
The only family of earthworms which is here represented is that of
the Megascolecidae. Of this family the Acanthodrilinae are very
well represented. We have at any rate seven species of the genus
_Notiodrilus_ of which the bulk are from the small adjacent islands
and not from the mainland. The genus _Rhododrilus_ with nine species
is actually limited to the New Zealand group and so is an allied
genus consisting of two species only, viz. _Leptodrilus_. This
latter genus is confined, so far as present information goes,
to the Auckland and Campbell islands. _Dinodriloides_ with two
species is also limited to New Zealand and to the North Island.
_Maoridrilus_ with some ten species is another native and restricted
genus. So too is the allied _Plagiochaeta_ with numerous setae on
each segment but with the alternating and single nephridial pores of
_Maoridrilus_. There are several species of _Plagiochaeta_ of which
one has been lately regarded by Dr Michaelsen as really falling
within the otherwise Indian genus _Hoplochaetella_, while for another
he has formed the genus _Pereiodrilus_. _Neodrilus_ with but one
species _N. monocystis_ is another peculiar New Zealand genus. The
family Octochaetinae contains only four genera, of which one, viz.
_Dinodrilus_ (with four species), is limited to New Zealand, while
_Octochaetus_ has about five representatives. _Hoplochaetella_
(if Dr Michaelsen's surmise referred to above be correct) has
one species in New Zealand. The Megascolecinae are less numerous
than the Acanthodrilinae, but there are ten species of the genus
_Megascolides_ (which includes Benham's genus _Tokea_), perhaps seven
species of _Diporochaeta_, and two other species which Michaelsen has
removed from the genus _Diporochaeta_ and placed in _Spenceriella_.
One _Plutellus_ (which however may have been introduced) completes
the New Zealand Megascolecines. We have therefore in this part of the
world fifteen genera including between them some 58 species; eight
of the genera are peculiar to the islands.
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.