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.