SELF-DIVISION AND REGENERATION. BUDDING AND REGENERATION. AUTOTOMY.
THEORY OF AUTOTOMY
Self-division, as a means of propagation, is of widespread occurrence in
the animal kingdom. In some cases the animal simply breaks into pieces
and subsequently regeneration takes place in the same way as when the
animal is cut into pieces by artificial means. In other cases the parts
are gradually separated, and during this time new parts are formed by a
process resembling that of regeneration after separation. A few
zoologists have tried to show how the process of regeneration before
separation has been derived from regeneration following self-division.
It is our purpose to examine here the evidence in favor of this
hypothesis.
A study of the forms that propagate by means of self-division shows that
the process is present in many groups of the animal kingdom. In the
unicellular forms this method is universally present; and in the
multicellular forms the division of the individual cells is looked upon
as a process similar to the method of propagation in the protozoa. The
sponges do not multiply by self-division. In the cœlenterates, on the
other hand, we find this mode of propagation present in most forms.
Hydra appears rarely, if at all, to divide by a cross-division, and,
although one or two cases of longitudinal division have been described,
it is not improbable that they have been started by the accidental
splitting of the oral end. The hydromedusæ, _Stomobrachium mirabile_,
_Phialidium variabile_, _Gastroblasta Raffælei_, are known to increase
by division.[66] Several actinians and many corals divide
longitudinally, while the scyphistoma of the scyphomedusæ produce
free-swimming ephyras by cross-divisions of the fixed strobila stage.
The ctenophors do not divide.
It is known that several fresh-water planarians propagate by division,
the tail-end breaking off in the region behind the old pharynx. In one
form,[67] and possibly in others, regeneration may begin before the
separation takes place. Many of the rhabdocœlous planarians increase by
cross-division--the separation taking place more nearly in the middle of
the body. In these forms the parts develop new organs more or less
completely before they separate. In the trematodes self-division does
not take place. The division of the body of the tapeworm into
proglottids may represent a process of self-division, but the
proglottids do not regenerate after separation.
The nemertians break up readily into pieces, if roughly treated or if
the conditions of life are unfavorable, but this can scarcely be spoken
of as a process of voluntary self-division. Regeneration takes place in
some species, but imperfectly or not at all in others.
In the group of annelids we find many cases of self-division, especially
in marine polychætes and in fresh-water oligochætes. One of the most
interesting forms, belonging to the first group, is the palolo worm in
which the swimming headless form, that is set free by division, serves
to distribute the sexual products. Subsequently it appears that the
piece dies without regenerating a new head. If we examine more in detail
some of the cases of self-division in annelids, we find the following
interesting facts. In nereis the posterior region of the body undergoes
great changes of structure, the new worm being known under a different
name, viz. heteronereis. In this part of the worm, eggs (or sperm) are
produced, but it does not separate from the anterior end as a distinct
individual. In the family of scyllids the changes that take place in the
posterior or sexual end of the body are often accompanied by non-sexual
modes of fission. In some species the changes that take place are like
those in nereis, and no separation occurs; in other species the sexual
region becomes separated from the anterior or non-sexual regions. In
scyllis a new head develops, _after separation_, on the sexual or
posterior piece. A new tail is also regenerated by the non-sexual or
anterior piece, and as many new segments are formed as are lost. The new
posterior region may again produce sexual cells, and again separate. In
autolytus a new head develops on the posterior piece _before it
separates_. A region of proliferation is also found at the posterior end
of the anterior part. In some species new individuals develop in this
zone of proliferation, and a chain of as many as sixteen worms may be
present before the one first formed drops off. A still more complicated
process is found in myriana. The region just in front of the anus
elongates, and gives rise to a large number of segments. These form a
new individual with the head at the anterior end. Then another series of
segments is proliferated at the posterior end of the old, or anterior
worm, and just in front of the first-formed individual. This region also
makes a new individual. The process continuing, a chain of individuals
is produced, with the oldest individual at the posterior end and the
youngest at the anterior end of the series. Each individual grows
larger, and produces more segments at its posterior end. Reproductive
organs appear in each individual, and when the germ-cells are mature the
chain breaks up.
None of the earthworms propagate by self-division, although
occasionally, under unfavorable conditions, pieces may pinch off at the
posterior end.[68] Lumbriculus, on the other hand, propagates by
self-division, although it has been disputed whether the division takes
place without the intervention of an external injury or disturbance of
some sort, or whether the division may take place entirely from internal
causes, that is, spontaneously. Von Wagner has shown that at certain
seasons lumbriculus breaks up much more readily than at other times,
which may only mean that it is more sensitive to stimuli at one time
than at another.
The pieces into which lumbriculus breaks up regenerate after separation.
In another form, _Ctenodrilus monostylos_, division takes place first in
the middle of the body behind a cross-septum. Each half may again divide
in the same way, and the same process may be repeated again and again
until some of the pieces are reduced to a single segment. A new anterior
and posterior end may then develop on each piece. In _Ctenodrilus
pardalis_ each segment of the middle region of the body constricts from
the one in front and from the one behind, and each produces a new head
at its anterior end and an anal opening at its posterior end. The worm
then breaks up into a number of separate worms. In this series,
self-division of the individual is not associated with the development
of sexual forms, but seems to be a purely non-sexual method of
reproduction. In the leeches self-division does not occur, and no cases
are known in the mollusks.
In the echinoderms several forms reproduce by voluntary self-division.
In the brittle-stars some forms divide by the disk separating into two
parts, one having two and the other three of the old arms. Each piece of
the disk then regenerates the missing part of the disk as well as the
additional arms. In the starfishes the arms may be thrown off if
injured, and, while in certain forms the lost arm does not regenerate a
new disk, yet, according to several writers, it may in other species
regenerate a new animal. Dalyell observed a process of self-division in
a holothurian, each part producing a new individual, and more recent
observers have confirmed this discovery.
No cases of self-division are known in the groups of myriapods, insects,
crustaceans, spiders, polyzoans, brachiopods, enteropneusta, or
vertebrates.
Before discussing the general problems connected with the preceding
cases, I should like to point out that it is certainly a striking fact
that in all, or nearly all, of these cases of self-division, the
separation takes place in the shortest axis, without regard to the
structure of the animal. A law similar to that enunciated in connection
with the division of the cell seems to hold for the organism as a whole:
namely, division takes place, as a rule, in the shortest diameter of the
form. The protozoa are, in a sense, excluded, since being unicellular
forms they come under the rule for the division of the cell. In the
cœlenterates we find the actinians and corals, that have short,
cylindrical bodies, dividing from the oral to the aboral end, while the
longer scyphistoma divides transversely. The flat, bell-shaped medusa,
gastroblasta, divides in an oral-aboral plane. The flat-worms and
annelids divide transversely, and, therefore, in the plane of least
resistance. The most important illustrations of this principle are
furnished by the echinoderms. Those brittle-stars that divide through
the disk do so in the shortest direction, that is, from the oral to the
aboral side, whilst the holothurians that are long, cylindrical forms
divide across the body and, therefore, in a structural plane at right
angles to that of the brittle-stars. It may be claimed that in all these
cases the plane of division is that in which the animal is most likely
to be broken in two by external agents, but this is, I think, only a
coincidence, and the result is really due to internal conditions. The
division is brought about in most cases, and perhaps in all, by the
contraction of the muscles; and the arrangement of the muscles in
connection with the form of the body is the real cause of the
phenomenon.
Returning to the general question of the occurrence of the process of
division in the different groups, we find that in nearly all of them in
which self-division occurs it is found in a number of different forms in
the same group. The process seems to be characteristic of whole groups
rather than of species, and so far as evidence of this sort has any
value it points to the conclusion that the process is not necessarily a
special case of adaptation to the surroundings, because the species that
divide may live under very diverse conditions.
A further examination of the facts throws a certain amount of light on
the relation between the processes of self-division and of regeneration.
The following questions may serve to guide us in our examination:--
(i) Is regeneration found only in those groups in which self-division
takes place as a means of propagation; or, conversely, does
self-division only occur in those groups that have the power of
regeneration?
(ii) Is regeneration confined, in the groups that make use of
self-division as a means of propagation, to those regions of the body
where the self-division takes place?
(iii) Is regeneration as extensive in the groups that do not propagate
by self-division as in those that do?
(iv) Can we account, in any way, for the presence of self-division in
certain groups, and for its absence in others?
(v) What relation exists between the forms that prepare for subsequent
self-division and those that do not?
The first question is easily answered. Regeneration is also found in
nearly all the other groups that do not propagate by self-division,--as,
for instance, the mollusks, vertebrates, etc. The second half of the
question may also be answered. All the groups that propagate by
self-division have also the power of regeneration.[69]
In answer to the second question there is ample evidence showing that
regeneration is by no means confined to those regions of the body in
which the self-division occurs.
In answer to the third question, it may be stated that although, in the
groups that propagate by self-division, regeneration may be present in
nearly all parts of the body, the same phenomenon occurs in other groups
that do not propagate by division.
The fourth question offers many difficulties, and our answer will depend
largely upon what we mean by “_accounting for_” the process in certain
groups. If the question is interpreted to ask, Why does an animal
divide? no answer can be given. If it is meant to ask, Can we see how
the process would be difficult, or even impossible, in certain groups
and not in others? then an approximate answer may be given, or at least
an hypothesis formed. In the first place, the power of regeneration must
be present in the region at which the self-division takes place in order
that the result may lead to the formation of new individuals, or else be
acquired in that region along with the acquirement of the means for
division. A leech is not much more complicated than a marine annelid,
yet it has little or no power of regeneration; hence, perhaps,
propagation by division could not be acquired by the leeches until they
had first acquired the power to regenerate. In the second place, in
certain forms a separation of the body into two parts would lead to the
death of one or of both parts, owing to the dependence of the different
regions upon each other. In forms like the vertebrates, insects,
crustacea, etc., we can readily see why this would be the case. Hence
propagation by means of self-division could not be acquired, since the
division itself would lead to the destruction of the organism. In the
third place, the structure of the body may be such that the process of
self-division would be mechanically impossible. A hard outer coat, like
that of the sea-urchin, combined with a weak development of the
musculature of the body, would effectively prevent the self-division of
the animal.
The fifth question has many sides. It involves us on the one hand in a
historical question of the origin of self-division, and on the other
hand in a discussion of the stimulus that brings about, not only the
division, but the changes that precede the division in those cases in
which the new part develops before division takes place.
Several zoologists have held that the process of self-division followed
by regeneration has been the starting-point for the process of
propagation preceded by regeneration. Von Kennel, for instance,
maintains that self-division in some of the annelids has arisen in this
way. He says: “We recognize everywhere in the animal kingdom the power
of organisms to replace lost parts, and we call this regeneration. It
may be developed in very different degrees in animals, and, as a rule,
only those parts of the body have the power of regeneration that still
possess the organs that are essential for independent existence. The
higher the organization of the animal, so much the less is its power of
regeneration, perhaps, because the division of labor of the different
organs has gone so far that extensive injuries cannot be repaired....
There is no doubt that this power is adaptive, in a high degree, to
preserve the species under unfavorable conditions, so that they are much
better off in the battle for existence than are the animals that live
under the same conditions but have not the power of regeneration.... The
power of regeneration that gives the animal a better chance in the
battle for existence and, therefore, makes more certain the continuance
and the distribution of the species will be, as is well known from
numerous observations, in a high degree inherited, indeed even increased
so that its descendants will possess that power in a higher degree than
their forefathers; and, in consequence, a much smaller stimulus (motive)
suffices, than at first, to bring about the division of the parts.”
After showing, according to the usual formula, that the process of
regeneration is useful, and, _therefore_, would come under the guidance
of natural selection, von Kennel proceeds to show how the result is
connected with an external stimulus! He asks: “Can accidental injuries
account for the result (viz. for the division in lumbriculus,
planarians, and starfish), since how few starfish are there with
regenerating arms in comparison with the enormous number of uninjured
individuals? Should we not rather look for the external stimuli that
have initiated the process of self-division?” “Animals that have
developed the power of regeneration by a long process of inheritance
will have acquired along with this the property of easier reaction to
all external adverse conditions. In a sense the sensitiveness for such
stimuli is sharpened, and the animal responds at once by breaking up. In
the same way the ear of a good musician becomes more sensitive through
practice. If we think of the same stimulus as regularly recurring, and
as always answered in the same way, then we may look upon it as a normal
condition of the life of the animal and its response as also a normal
process in the animal. If, for instance, the breaking into pieces of
lumbriculus is a consequence of the approach of cold weather or of other
external conditions, then the organization of this animal must react by
breaking up in consequence of its adaptation to the conditions acquired
through heredity. The self-division becomes a normal process under
normally recurring conditions. If the organism has been accustomed to
respond through numerous generations, and, therefore, its sensitiveness
has become highly developed, it will be seen that it may be influenced
by the slightest change in the unfavorable conditions, and although, at
first, the change may not be sufficiently strong to cause the animal to
divide, yet the introductory changes leading to the division may be
started, which will in turn make the division, when it occurs, easier
and the animal that possesses this responsiveness more likely to
survive. This would be the case if a slow process of constriction took
place, so that, at the time of separation, no wounds of any size are
formed.” “By a further transfer of the phenomenon, a partial, or even a
complete, regeneration may set in before division takes place.” “We find
changes like this in the series of forms, _Lumbriculus_, _Ctenodrilus
monostylos_, _Ctenodrilus pardalis_, _Nais_, _Chætogaster_. It appears
in a high degree probable that the series has originated in the way
described. Perhaps zoologists will find after some thousands of years
that lumbriculus propagates as does nais at present.” In this way von
Kennel tries to show how the process of regeneration, that takes place
before division, has been evolved from a simple process of breaking up
in response to unfavorable conditions. The imaginary process touches on
debatable ground, to say the least, at every turn, and until some of the
principles involved have been put on a safer basis, it would be
unprofitable to discuss the argument at any length.
We should never lose sight of the fact that the arranging of a series
like that beginning with lumbriculus and ending with chætogaster is a
purely arbitrary process and does not rest on any historical knowledge
of how the different methods originated or how they stand related, and
no one really supposes, of course, that these forms have descended from
each other but at most that the more complicated processes may have been
at first like those shown in other forms. Even this involves assumptions
that are far from being established, and it seems folly to pile up
assumption on top of assumption in order to build what is little more
than a castle in the air.
_REGENERATION AND BUDDING_
In several groups of animals a process of budding takes place that
presents certain features not unlike those of self-division. It is
difficult, in fact, to draw a sharp line between budding and
self-division, and although several writers have attempted to make a
distinction between the two processes, it cannot be said that their
definitions have been entirely successful. It is possible to make a
distinction in certain cases that may be adopted as typical, but the
same differences may not suffice in other cases. For instance, the
development of a new individual at the side of the body of hydra is a
typical example of budding, while the breaking up of lumbriculus or of a
planarian into pieces that form new individuals is a typical example of
division. In a general way the difference in the two processes involves
the idea that a bud begins as a small part of the parent animal, and
increases in size until it attains a typical form. It may remain
permanently connected with the parent, or be separated off. By division
we mean the breaking up of an organism into two or more pieces that
become new individuals, the sum-total of the products of the division
representing the original organism. Von Kennel first sharply formulated
this distinction, and it has been also supported by von Wagner, who has
attempted to make the distinction a hard and fast one;[70] but as von
Bock has pointed out, there are forms like pyrosoma and salpa in which
the non-sexual method of propagation partakes of both peculiarities, and
in _Syllis ramosa_ the individuals appear to bud from the sides, while
in other annelids a process of division takes place. Von Bock assumes,
therefore, as more probable, that budding and self-division are only
different phenomena of the same fundamental process. It might be better,
I think, to go even further in order to clear this statement from a
possible historical implication, and state only that the two processes
involve some of the same factors.
Budding occurs in several groups of the animal kingdom. There are
numerous cases in the protozoa, such, for instance, as that in
noctiluca. In the sponges buds are formed that go to build up a colony
in most instances. In the cœlenterates cases of lateral budding are
found in nearly all the main groups, and in one and the same individual,
as in the scyphistoma of aurelia, in fact both budding and division
occur. In the polyzoa, in the ascidians, and in cephalodiscus lateral
budding takes place. In the rhabdocoel turbellarians, and in some of the
annelids, we find chains of new individuals produced by a process that
is often spoken of as budding. It is convenient, however, to distinguish
these cases of axial budding from those of lateral budding; for, while
they both involve an increase in the products over that of the original
animal, the axial relations in lateral buds are established in a new
plane, while in axial budding the main axis of the new animal is a part
of that of the old, and this difference may involve different factors.
The process of budding does not occur in the insects, spiders,
crustaceans, mollusks, ctenophores, brachiopods, nematodes, vertebrates,
or in several other smaller groups.
This examination shows that there are groups in which both processes
take place, viz. cœlenterates, planarians, annelids; and others in which
budding alone takes place, viz. ascidians, polyzoa, cephalodiscus; and
one group at least in which division, but not budding, takes place, the
echinoderms. It is obvious that from the occurrence of the process of
budding in the animal kingdom we cannot infer anything as to its
relation to division or to regeneration.
It has been pointed out that in the flowering plants, in which the
growth takes place by means of buds, the power of terminal regeneration
is very slightly developed, and its absence is sometimes accounted for
on the ground that the new growth takes place by means of the
development of lateral buds. If by this statement it is meant that buds
being present the suppression of regeneration in other regions may
occur, then there may be a certain amount of truth in the statement. If,
however, it is intended to mean that _because_ a plant has acquired the
power of reproducing new parts by means of buds it has, therefore, lost
the power to regenerate in other ways (or has never developed the power
to regenerate), then the argument is, I think, fallacious; for we find
even in flowering plants that the new buds sometimes arise from the new
part, or callus, that forms over the cut-end, and this process resembles
a real regenerative process. We also find that hydroids that produce
lateral buds also regenerate freely from an exposed end. But we are at
present so much in the dark in regard to the causes that bring about
budding in organisms that a discussion of the possibilities would hardly
be profitable.
_AUTOTOMY_
The process of autotomy differs only in degree from the process of
self-division. In autotomy the part thrown off does not produce a new
animal. The breaking off of the tail of the lizard at the base, if the
outer part is injured, is an example of a typical process of autotomy.
The throwing off of the crab’s leg, if the leg is injured, is also
another typical case of autotomy. There is a definite breaking-joint at
the base of the crab’s leg at which the separation always takes place
(Fig. 45, A 1-1). The breaking-joint is in the middle of the second
segment from the base of the leg, where there is found, on the outside
of the leg, a ring-like groove that marks the place of rupture. A
comparison of the legs of the crab with the walking legs of the
crayfish, or of the lobster, shows that the groove in the crab’s legs
corresponds to a joint in the legs of the two other forms. In the
crayfish and lobster the walking legs generally break off at this same
level, although by no means as easily or with as much certainty as in
the crab. The first pair of legs of the crayfish and lobster, carrying
the large claws, have also a breaking-joint at the base of the leg
similar to that in the crab’s leg, and these legs break off in the
living animal always at the breaking-joint.
[Illustration: FIG. 45.--_A._ After Andrews. Base of leg of crab to show
breaking-joint, 1-1. _B._ After Fredericq. Diagram of leg of crab to
show how autotomy takes place. _C._ After Andrews. Longitudinal section
of base of leg to show in-turned chitinous plate at breaking-joint.]
Réaumur first recorded that if the leg of a crayfish or of a crab is cut
off outside of the breaking-joint it is always thrown off later at the
breaking-joint. Fredericq has made a careful examination of the way in
which the leg is thrown off in the crab, _Carcinus mænas_. He found that
the breaking does not take place at the weakest part of the leg; for
the leg of a dead crab will support a weight of 3½ to 5 kilograms,
which represents about one hundred times the weight of the crab’s body.
When the weight is increased to a point at which the leg breaks, it does
so between the body and the first segment or between the first and
second segments. When it breaks off in this way, the edges are ragged
and are left in a lacerated condition; but when the leg is thrown off by
the animal at the breaking-joint, there is left a smooth surface covered
over, except in the centre, by a thin cuticle. Through the opening in
the centre of this cuticle a nerve and a blood vessel pass to the distal
part of the leg. Very little bleeding takes place when the leg is thrown
off, but if the leg is cut off or broken off at any other level the
bleeding is much greater. Fredericq studied the physiological side of
the process and found that it is the result of a reflex nervous act. He
found that if the brain of the animal is destroyed the leg may still be
thrown off, but if the ventral cord is destroyed the reflex action does
not take place. The reflex is brought about ordinarily by an injury to
the leg that starts a nerve impulse to the ventral nerve-cord, and from
this a returning impulse is sent to the muscles of the same leg, causing
the muscles in the region of the breaking-joint to contract violently,
and the result of their contraction is to break off the leg. If the
muscles are first injured, the leg cannot be thrown off. Andrews, who
has studied the structure of the breaking-joint in the spider-crab; has
found that there is a plane of separation extending inwards from the
groove on the surface. This plane is made by a narrow space between two
chitinous membranes that are continuous at their outer ends with the
general chitinous covering of the leg (Fig. 45, _C_). When the leg
breaks off, one-half of the double membrane is left attached to the base
of the leg and the other to the part that is lost. This in-turned
membrane seems to correspond to the in-turning of the surface cuticle in
the region of the joints. The arrangement of the muscles at the
breaking-joint is shown in Fig. 45, _B_. The upper muscle is the
extensor muscle of the leg, and through its contraction the breaking off
takes place. When it contracts the leg is brought against the side of
the body, which is supposed to offer the resistance necessary to break
off the leg. If the leg is held by an enemy, this may offer sufficient
resistance for the muscle to bring about the breaking.
In many crabs the leg is not thrown off if simply held, but only after
an injury. Even the most distal segment may be cut off and the leg
remain attached, and sometimes it is not lost after the distal end of
the next to the last segment is cut off. If a crab is tethered by one
leg it will not throw off its leg in order to escape, unless, in the
crab’s excitement, the leg is twisted or broken. How generally this
holds for all crabs cannot be stated. Herrick says: “Unintentional
experiments in autotomy have often been made by tethering a lobster or a
crab by its large claws. The animal, of course, escapes, leaving only
its leg behind. When lobsters are drawn out of the water by the claws,
or when a claw is pinched by another lobster, or while they are handled
in packing, especially for the winter market, they often ‘cast a claw,’
and the transportation of lobsters at this season is said to be attended
by considerable loss in consequence.” The large claws of the lobster are
quite heavy, the base relatively small at the breaking-joint, and it may
be that simply the weight of the claw, when out of the water, may strain
the leg so that it breaks off,--the leg being injured by its own weight.
The lobster seems to lose its claws quite often under natural
conditions. Rathburn[71] states that “out of a hundred specimens
collected for natural history purposes in Narragansett Bay in 1880,
fully 25 per cent had lost a claw each, and a few both claws.”
Herrick[72] records that “in a total of 725 lobsters captured at Woods
Holl in December and January, 1893-1894, fifty-four, or 7 per cent, had
thrown off one or both claws.”
The autotomy of the arms of the starfish has been often observed.[73]
The arms are thrown off very near the base in many forms. If the animal
is simply held by the arm it does not break off, but if injured it
constricts and falls off. The lost arm does not regenerate a new
starfish in most forms, but, as stated on page 102, there are recorded
some cases in which the arm seems to have this power. King has found
that out of a total of 1914 starfish (_Asterias vulgaris_) there were
206, or 10.76 per cent, that had new arms, and all of these, with one
exception, arose from the base of the arm. The growth of the new arm
from the base takes place more rapidly, as shown in Fig. 38, _A_, than
when the arm is regenerated from a more distal level; but in the latter
case the arm, despite its slower growth, may complete itself before
another does that originates at the same time from the base of the old
arm. There is, therefore, in this respect no obvious advantage, so far
as regeneration is concerned, in throwing off the injured arm nearer to
the disk.
In the brittle-stars (ophiurians) the arm breaks off with greater ease
and at any level. If the arm is simply held and squeezed, it will, in
some forms, break off just proximal to where it is held. If the broken
end is again held, another small piece breaks off, and in this way the
arm may be autotomized, piece by piece, to its very base. Regeneration
may take place from any region, but, as yet, no observations have been
made on the relative rate of growth of the new arm at different levels.
One of the most remarkable cases of autotomy is that in holothurians, in
which the Cuvierian organs, and even the entire viscera, may be ejected
when the animal is disturbed. A new digestive tract is regenerated.[74]
It is known that several of the myriapods lose their legs at a definite
region near the base, and that they have the power of throwing off the
leg in this region if it is injured. I have often observed that the legs
of _Scutigera forceps_ are thrown off if they are held or injured, and
even when the animal is thrown into a killing fluid. Amongst the insects
the plasmids or walking-sticks also throw off their legs at a definite
joint, as described by Scudder, and more recently by Bordage, and still
later by Godelmann. New legs are regenerated from the stump of the old
leg, as has long been known.[75] Other insects do not have the power of
throwing off their legs, and we have only a few observations that show
that the legs if lost can be regenerated. It is known in the cockroach
that the tarsus can regenerate if lost or if cut off, and that fewer
segments are regenerated than are present in the normal animal. Newport
found that the true legs of a caterpillar are regenerated during the
pupa stage if they have been previously cut off.
A further example of autotomy is found in the white ants, which break
off their wings at the base after the nuptial flight. There exists a
definite and pre-formed breaking-plane in this region. The true ants
also lose their wings after the nuptial flight, but there does not seem
to be a definite plane of breaking, so that the process can scarcely be
called one of autotomy. These cases also differ from the other cases of
autotomy in that the lost parts are not renewed.
It has been observed[76] that if the leg of tarantula is cut off at any
other place than at the coxa, the animal bites off the wounded leg with
its jaws down to the coxa. In other spiders this does not occur,
although Schultz has observed that when the legs are lost under natural
conditions they are found broken off in most cases at the coxa. Schultz
has also found that the legs regenerate better from this region than
from any other. It would be rash, I think, to conclude without further
evidence that the habit of tarantula to bite off a wounded leg down to
the coxa has been acquired in connection with the better regeneration of
the leg at this place. It is possible that the injury may excite the
animal to bite off the leg as far as possible, which might be to the
coxal joint. It would certainly be very remarkable if this spider had
acquired the habit in connection with the better regeneration of the leg
at the base, since the leg can presumably also regenerate at any level,
as in the epeirids.
In this same connection I may record that in the hermit-crab I have
often observed that when a leg is cut off outside of the breaking-joint,
if the leg is not thrown off at once, the claws of the first legs catch
hold of the stump and, pulling at the leg, offer sufficient resistance
for the leg to break off at the breaking-joint. I cannot believe that
this instinct has anything to do with the better regeneration of the leg
at the coxal joint, however attractive such an hypothesis may appear.
_THEORIES OF AUTOTOMY_
A number of writers have pointed out that under certain conditions it is
an obvious advantage to the animal to be able to throw off a portion of
the body and thereby escape from an enemy. It has been suggested that if
a crab is seized by the leg, the animal may save its life at times at
the expense of its leg; and since the crab has the power of regenerating
a new leg, it is the gainer in the long run by the sacrifice. The
holothurian, that ejects its viscera, has been supposed to offer a
sufficient reward to its hungry enemy, and escapes paying the death
penalty, at the expense of its digestive tract. Thus, having shown that
the process of autotomy is a useful one, it is claimed that it must have
been acquired through a process of natural selection! An equally common
opinion is that autotomy is an adaptation for regeneration, since in
certain cases, as in that of the crab’s leg, better conditions for
subsequent regeneration occur at the breaking-joint than when the
amputation takes place at any other region. Since less bleeding takes
place when the crab’s leg is thrown off at the breaking-joint, and since
the wound closes more quickly when the arm of the starfish is lost at
the base, it is assumed that we have in both cases an adaptation to meet
accidents, and that the adaptation has been acquired by natural
selection.
A consideration of these questions involves us once more in a discussion
of the theory of natural selection. An attempt has been made in another
place (pages 108-110) to show that we are not justified in assuming that
because a process is useful, therefore it must have been acquired by
means of natural selection. Even if it were granted that the theory of
natural selection is correct, it does not follow that all useful
processes have arisen under its guidance. We may, therefore, leave the
general question aside, and inquire whether the process of autotomy
could have arisen through natural selection (admitting that there is
such a process, for the sake of the present argument), or whether
autotomy must be due to something else.
If we assume that the leg of some individual crayfishes or crabs, for
example, broke off, when injured, more easily at one place than at
another, and that regeneration took place as well, or even better, from
this region than from any other, and if we further assume that those
animals in which this happened would have had a better chance of
survival than their fellows, then it might seem to follow that in time
there would be more of this kind of animal that survived. But even these
assumptions are not enough, for we must also assume that this particular
variation was more likely to occur in the descendants of those that had
it best developed, and that amongst those forms that survived, some had
the same mechanism developed in a still higher degree, and, the process
of selection again taking place, a further advance would be made in the
direction of autotomy. This, I think, is a fair, although brief,
statement of the conventional argument as to how the process of natural
selection takes place. But let us look further and see if the results
could be really carried out in the way imagined, shutting our eyes for
the moment to the number of suppositions that it is necessary to make in
order that the change may occur. It will not be difficult, I believe, to
show that even on these assumptions the result could not be reached. In
the first place, the crabs that are not injured in each generation are
left out of account, and amongst these there will be some, it is true,
that have the particular variation as well developed as the best amongst
those that were injured, and others that have the average condition, but
there will be still others that have the possibilities less highly
developed, and the two latter classes will be, on the hypothesis, more
numerous than those in the first class. The uninjured crabs will also
have an advantage, so far as breeding and resisting the attacks of their
enemies are concerned, as compared with those that have been injured,
and in consequence they, rather than the injured ones, will be more
likely to leave descendants. Even if some of those that have been
injured, and have thrown off the leg at the most advantageous place,
should interbreed with the uninjured crabs, still nothing, or very
little, can be gained, because, on Darwinian principles, intercrossing
of this sort will soon bring back the extreme variations to the average.
The process of natural selection could at best only bring about the
result provided all crabs in each generation lose one or more of their
legs, and amongst these only the ones survive that break off the leg at
the most advantageous place; but no such wholesale injury takes place,
as direct observation has shown. At any one time only a small
percentage, about ten per cent, have regenerating legs, and as the time
required completely to regenerate a leg, even in the summer, is quite
long, this percentage must give an approximate idea of the extent of
exposure to injury. It is strange that those who assert off-hand that,
because autotomy is a useful process, therefore it must have been
acquired by natural selection, have not taken the pains to work out how
this could have come about. Had they done so, I cannot but believe they
would have seen how great the difficulties are that stand in the way.
A further difficulty is met when we find that each leg of the crab has
the same mechanism. If we reject as preposterous the idea that natural
selection has developed in each leg the same structure, then we must
suppose that a crab varies in the same direction in all its legs at the
same time; and if this is true it is obvious that the principle of
variation must be a far more important factor in the result than the
picking out of the most extreme variations. The same laws that determine
that one individual varies in a useful direction farther than do other
individuals may, after all, account for the entire series of changes. If
it be replied that natural selection does not take into account the
causes of the differences of individual variation, this is to admit that
it avowedly leaves out of account the very principles that may in
themselves, and without the aid of any such supposed process as natural
selection, bring about the result. The Lamarckian principle of use and
disuse does not give an explanation of autotomy, since the region of the
breaking-joint is not the weakest region of the leg, or the place at
which the leg would be most likely to be injured.
We cannot assume autotomy to be a fundamental character of living
things, since it occurs only under special conditions, and in special
regions of the body. While it might be possible to trace the autotomy of
the legs of the crustacea, myriapods and insects, to a common ancestral
form, yet this is extremely improbable, because the process takes place
in only a relatively few forms in each group. The autotomy of the wings
of white ants that takes place along a preëxisting breaking-line must
certainly have been independently acquired in this group. The breaking
off of the end of the foot in the snail helicarion is also a special
acquirement within the group of mollusca.
Bordage has suggested that the development of the breaking-joint at the
base of the leg of phasmids has been acquired in connection with the
process of moulting. He has observed that during this period the leg
cannot, in some cases, be successfully withdrawn through the small basal
region; and hence, if it could not break off, the animal would remain
anchored to the old exoskeleton. It escapes at the expense of losing its
leg. The animal, having acquired the means of breaking off its leg under
these conditions, might also make use of the same mechanism when the leg
is held or injured, and thereby escape its enemy. The fact that the
crayfish has a breaking-joint only for the large first pair of legs
would seem to be in favor of this interpretation, but the crab has the
same mechanism for the slender walking legs, that one would suppose
could be easily withdrawn from the old covering. It should also be
remembered that we do not know whether the breaking-joint at the base of
the leg of the crab and of the crayfish would act at the time when the
leg is being withdrawn from the old exoskeleton, unless the leg were
first injured outside of the joint.
Our analysis leads to the conclusion that we can neither account for the
phenomenon of autotomy as due to internal causes alone in the sense of
its being a general property of protoplasm, nor to an external cause, in
the sense of a reaction to injury or loss from accident. There would
seem then only one possibility left, namely, that it is a result of both
together, or in other words, a process that the animal has acquired in
connection with the conditions under which it lives, or in other words,
an adaptive response of the organism to its conditions of life.
We are not, however, able at present to push these questions farther,
for, however probable it may seem that animals and plants may acquire
characteristics useful to them in their special conditions of life, and
yet not of sufficient importance to be decisive in a life and death
struggle, still we cannot, at present, state how this could have taken
place in the course of evolution. For, however plausible it may appear
that the useful structure has been built up through an interaction
between the organism and its environment, we cannot afford to leave out
of sight another possibility, viz. that the structure or action may have
appeared independently of the environment, but after it appeared the
organism adopted a new environment to which its new characters made it
better suited. If the latter alternative is true, we should look in vain
if we tried to find out how the interaction of the environment brought
about the adaptation. The relation would not be a causal one, in a
physical sense, but the outcome of a different sort of a relation, viz.
the restriction of the organism to the environment in which it can
remain in existence and leave descendants.