MOVEMENT AND MIGRATION AMONG EARTHWORMS
That earthworms can move upon the surface of the ground at a rapid
pace is probably well enough known to everyone, and that they can
also burrow with considerable celerity. Multiplying the inches of
progress in minutes of time by centuries with the resulting miles, it
is quite clear that there is no reason to suppose that an individual
earthworm might not enormously extend its range under favourable
circumstances. Their powers of locomotion are such that they could
in the course of comparatively few centuries people a continent. As
a matter of fact these animals are frequently very widely spread
upon a given land surface; but on the other hand they are sometimes
equally limited. It behoves us therefore to enquire the reasons
for the possibility of extended migration and the causes which
have led to its restriction. We are now, it must be borne in mind,
considering these animals as purely terrestrial animals moving over
the surface of the land by their own unaided efforts. We leave out
of consideration any possible assistance in crossing water, whether
fresh or salt. We have to consider in fact in the present section
the earthworm inhabitants of larger and smaller tracts of continuous
land such as the African continent, which will serve as an excellent
example wherewith to test the facts and inferences.
And as a 'control' we can compare this continent with the very
different continent of Europe.
As an excellent instance, because of the certitude of specific and
in most cases of generic distinctions, we may take the Eudrilidae as
illustrative of the facts that are to be considered in the present
section. That family consists, as will be remembered, of 33 genera
at most, which have the following more exact range on the African
continent. The genus _Eudriloides_ occurs in British and German East
Africa and has been met with as far south as Mosambique and even
Durban, in which latter locality it has been thought that it is
really an accidentally introduced stranger. _Platydrilus_ is limited
to eastern equatorial Africa, thus not having quite the range of
_Eudriloides_.
The small genera (that is small in numbers of species)
_Reithrodrilus_, _Bogertia_, _Megachaetina_, _Metadrilus_, _Notykus_
have the same limitation of range as the last genus. _Metschaina_
has a wider range from tropical North East to lake Tanganyika.
_Stuhlmannia_ has a wider range still being found as it is in the
Tanganyika district, in tropical North East Africa, and in British
and German East Africa near the coast. _Pareudrilus_ reaches still
further north while _Nemertodrilus_ is limited to the Mosambique
region and to the Orange River district further south. The only
remaining genus of this sub-family of the Eudrilidae is _Libyodrilus_
which is purely West African and equatorial.
Of the remaining genera which are usually grouped together
into a second sub-family, five, viz. _Malodrilus_, _Kaffania_,
_Gardullaria_, _Teleudrilus_ and _Teleutoreutus_, are confined to
tropical North East Africa. _Eminoscolex_ occurs in the same district
but also to the south in the great lake region. The most remarkable
fact about this genus is that one species _E. steindachneri_ comes
from the Cameroons, and another _E. congicus_ from the Congo,
and thus the range of the genus is right across the continent.
_Neumanniella_ has much the same range. _Polytoreutus_ is a purely
equatorial East and Central genus, reaching from the coast to the
lakes. _Bettonia_ known by three species is from British East Africa.
The remaining genera, viz. _Hyperiodrilus_, _Heliodrilus_, _Alvania_,
_Iridodrilus_, _Rosadrilus_, _Euscolex_, _Parascolex_, _Preussiella_,
_Buttneriodrilus_, _Beddardiella_, _Metascolex_, are all West African
and the vast majority equatorial. We thus see that with one exception
the genera of East Africa are totally different from those of West
Africa and that the family as a whole is restricted in its range to
a comparatively small part of the vast African continent. It also
obviously follows, and it is advisable to state this fact however
obvious, that no species are common to the two sides of the continent
except indeed the ubiquitous _Eudrilus_, whose range over the world
has been more than once referred to in this book.
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 MIGRATION OF 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.