REGENERATION AND LIABILITY TO INJURY
There is a widespread belief amongst zoologists that a definite relation
exists between the liability of an animal to injury and its power of
regeneration. It is also supposed that those individual parts of an
animal that are more exposed to accidental injury, or to the attacks of
enemies, are the parts in which regeneration is best developed, and
conversely, that those parts of the body that are rarely or never
injured do not possess the power of regeneration.
Not only do we find this belief implied in many ways, but we find this
point of view definitely taken by several eminent writers, and in some
cases carried so far that the process of regeneration itself is supposed
to be accounted for by the liability of the parts to injury. In order
that it may not appear that I have exaggerated the widespread occurrence
of this belief, a few examples may be cited.
Réaumur in 1742 pointed out that regeneration is especially
characteristic of those animals whose body is liable to be broken, or,
as in the earthworm, subject to the attacks of enemies. Bonnet (1745)
thought that such a connection exists as has just been stated, and that
the animals that possess the power of regeneration have been endowed
with germs set aside for this very purpose. He further believed that
there would be in each animal that regenerates as many of these germs as
the number of times that it is liable to be injured during its natural
life. Darwin in his book on _Animals and Plants under Domestication_
says: “In the case of those animals that may be bisected, or chopped
into pieces, and of which every fragment will reproduce the whole, the
power of regrowth must be diffused throughout the whole body.
Nevertheless, there seems to be much truth in the view maintained by
Professor Lessona[36] that this capacity is generally a localized and
special one serving to replace parts which are eminently liable to be
lost in each particular animal. The most striking case in favor of this
view is that the terrestrial salamander, according to Lessona, cannot
reproduce lost parts, whilst another species of the same genus, the
aquatic salamander, has extraordinary powers of regrowth, as we have
just seen; and this animal is eminently liable to have its limbs, tail,
eyes, and jaws bitten off by other tritons.”
Lang, referring to the brittleness of the tails of lizards, points out
that this is a very useful character, since the bird of prey that has
struck at a lizard gets hold of only the last part of the animal to
disappear under cover; the lizard escapes by breaking off its tail. The
brittleness of the tail is, therefore, an adaptive character that has
become fixed by long inheritance.
To this example may be added that of certain land snails in the
Philippine Islands. The individuals of the genus helicarion live on
trees in damp forests, often in great droves. They are very active, and
creep with unusual swiftness over the stems and leaves of the trees.
Semper has recorded that all the species observed by him have the
remarkable power of breaking off the tail (foot) close behind the shell,
if the tail is roughly grasped. A convulsive movement is made until the
tail comes off, and the snail drops to the ground, where it is concealed
by the leaves. Semper adds that in this way the snails often escaped
from him and from his collectors, leaving nothing behind but their
tails. The tail is said to be the most obvious part of the animal, and
it is assumed that this is, therefore, the part that a reptile or bird
would first attack.[37] Lang states that in this case external
influences have produced an extraordinarily well-developed sensitiveness
in the animal, so that it reacts to the external stimulus by voluntarily
throwing off the tail. It would be, of course, of small advantage to be
able to throw off the tail unless the power of regenerating the lost
organ existed, or was acquired at the same time as the extreme
sensitiveness that brings about the reaction. Lang does not state,
however, explicitly that he believes the regenerative power to have
arisen through the exposure of the tail of the lizard and the tail of
the snail to injury, although he thinks that the mechanism by means of
which these parts are thrown off has been acquired in this way. Several
other writers have, however, used these same cases to illustrate the
supposed principle of liability to injury and power of regeneration.
Weismann in his book on _The Germ Plasm_ has adopted the principle of a
connection between regeneration and liability to injury and has carried
it much farther than other writers. We can, therefore, most profitably
make a careful examination of Weismann’s position. His general idea may
be gathered from the following quotation:[38] “The dissimilarity,
moreover, as regards the power of regeneration _in various members of
the same species_, also indicates that adaptation is an important factor
in the process. In proteus, which in other respects possesses so slight
a capacity for regeneration, the gills grow again rapidly when they have
been cut off. In lizards again this power is confined to the tail, and
the limbs cannot become restored. In these animals, however, the tail is
obviously far more likely to become mutilated than are the limbs, which,
as a matter of fact, are seldom lost, although individuals with stumps
of legs are occasionally met with. The physiological importance of the
tail of a lizard consists in the fact that it preserves the animal from
total destruction, for pursuers will generally aim at the long trailing
tail,[39] and thus the animal often escapes, as the tail breaks off when
it is firmly seized. It is, in fact, as Leydig was the first to point
out, specially adapted for breaking off, the bodies of the caudal
vertebræ from the seventh onward being provided with a special plane of
fracture so that they easily break into two transversely. Now if this
capability of fracture is provided for by a special arrangement and
modification of the parts of the tail, we shall not be making too daring
an inference if we regard the regenerative power of the tail as _a
special adaptation, produced by selection, of this particular part of
the body, the frequent loss of which is in a certain measure provided
for_, and not as the outcome of an unknown ‘regenerative power’
possessed by the entire animal. This arrangement would not have been
provided if the part had been of no, or of only slight, physiological
importance, as is the case in snakes and chelonians, although these
animals are as highly organized as lizards. The reason that the limbs of
lizards are not replaced is, I believe, due to the fact that these
animals are seldom seized by the leg, owing to their extremely rapid
movements.” Overlooking the numerous cases of the regeneration of
internal organs that have been known for several years, and basing his
conclusion on a small, unconvincing experiment of his own on the lungs
of a few salamanders, Weismann concludes: “Hence there is no such thing
as a general power of regeneration; in each kind of animal this power is
graduated according to the need of regeneration in the part under
consideration; that is to say, the degree in which it is present is
mainly in proportion to the liability of the part to injury.”
After arriving at this conclusion the following admission is a decided
anticlimax: “The question, however, arises as to whether the capacity of
each part for regeneration results from special process of adaptation,
or whether regeneration occurs as the mere outcomewhich is to some
extent unforeseen--of the physical nature of an animal. Some statements
which have been made on this subject seem hardly to admit of any but the
latter explanation.” After showing that some newts confined in aquaria
attacked each other, “and several times one of them seized another by
the lower jaw, and tugged and bit at it so violently _that it would have
been torn off had I not separated the animals_,”[40] and after referring
to the regeneration of the stork’s beak, Weismann concludes: “Such
cases, the accuracy of which can scarcely be doubted, indicate that the
capacity for regeneration does not depend only on the special adaptation
of a particular organ, but that a general power also exists which
belongs to the whole organism, and to a certain extent affects many and
perhaps even all parts. By virtue of this power, moreover, simple organs
can be replaced when they are not specially adapted for regeneration.”
The perplexity of the reader, as a result of this temporary vacillation
on Weismann’s part, is hardly set straight by the general conclusion
that follows on the same page: “We are, therefore, led to infer that the
general capacity of all parts for regeneration may have been acquired by
selection in the lower and simpler forms, and that it gradually
decreased in the course of phylogeny in correspondence with the increase
in complexity of organization; but that it may, on the other hand, be
increased by special selective processes in each stage of its
degeneration, in the case of certain parts which are physiologically
important and are at the same time frequently exposed to loss.”
There are certain statements of facts in the same chapter that are
incorrect, and the argument is so loose and vague that it is difficult
to tell just what is really meant. As a misstatement of fact I may
select the following case: It is stated that lumbriculus does not have
the power of regenerating laterally if cut in two, and it is argued that
a small animal of this form could rarely be injured at the side without
cutting the animal completely in two. As a matter of fact, lumbriculus
can regenerate laterally, and very perfectly, as any one can verify if
he takes the trouble to perform the experiment; but, of course, if the
whole animal is split in two lengthwise the pieces die, or if a very
long piece is split from one side the remaining piece usually
disintegrates. If, however, the anterior end is split in two for a short
distance, or if a piece is partially split in two, the half remaining in
contact with the rest of the piece completes itself laterally. The same
result follows also in the earthworm.
As an example of looseness of expression I may quote the following from
Weismann: “A useless or almost useless rudimentary part may often be
injured or torn off _without causing processes of selection to occur
which would produce in it a capacity for regeneration_. The tail of a
lizard again, which is very liable to injury, becomes regenerated
because, as we have seen, it is of great importance to the individual
and if lost its owner is placed at a disadvantage.” And as an example of
vagueness, the following statement commends itself: “Finally the
complexity of the individual parts constitutes the third factor which is
concerned in regulating the regenerative power of the part in question;
for the more complex the structure is, the longer and the more
energetically the process of selection must act in order to provide the
mechanism of regeneration, which consists in the equipment of a large
number of different kinds of cells with the supplementary determinants
which are accurately graduated and regulated as regards their power of
multiplication.”
Without attempting to disentangle the ideas that are involved in these
sentences, let us rather attempt to get a general conception of
Weismann’s views. In a later paper (1900), in reply to certain
criticisms, he has stated his position somewhat more lucidly. In the
following statement I have tried to give the essential part of his
hypotheses: Weismann believes the process of regeneration to be
regulated by “natural selection”; in fact, he states that it has arisen
through such a process in the lower animals--since they are more subject
to injury--and that it has been lost in the higher forms except where,
on account of injury, it has been retained in certain parts. Thus when
Weismann speaks of regeneration as being an adaptation of the organism
to its environment, we must understand him to mean that this adaptation
is the result of the action of natural selection. We should be on our
guard not to be misled by the statement that because regeneration is
useful to the animal, it has been acquired by natural selection, since
it is possible that regeneration might be more or less useful without in
any way involving the idea that natural selection is the originator of
this or of any other adaptation. It will be seen, therefore, that in
order to meet Weismann on his own ground it will be necessary to have a
clear understanding in regard to the relation of regeneration to
Darwin’s principle of natural selection. With Weismann’s special
hypothesis of the “mechanism,” so-called, by which regeneration is made
possible we have here nothing to do, but may consider it on its own
merits in another chapter.
In order to have before us the material for a discussion of the possible
influence of natural selection on regeneration, let us first examine the
facts that bear on the question of the liability of the parts to injury
and their power to regenerate, and in this connection the questions
concerning the renewal of parts that are thrown off by the animals
themselves in response to an external stimulus are worthy of careful
consideration. A comparison between the regeneration of these parts with
that of other parts of the same animal gives also important data.
Furthermore, a comparison may be made between different parts of the
same animal, or between the same parts of different animals living under
similar or dissimilar conditions.
There are only a few cases known in which a systematic examination has
been carried out of the power of regeneration of the different parts of
the body of the same animal. Spallanzani’s results show that those
salamanders that can regenerate their fore legs can regenerate their
hind legs also. Towle, who has examined in my laboratory the
regeneration of a number of American newts and salamanders, finds also
that both the fore and hind legs regenerate in the same forms. The tail
and the external gills, in those newts with gills, also regenerate. It
has also been shown in triton that the eye regenerates if a portion of
the bulb is left. Broussonet first showed (1786) that the fins of fish
have the power to regenerate, although, strangely enough, Fraisse and
Weismann state that very little power of regeneration is present in the
fins of fish. I have found that the fins of several kinds of fish
regenerate, belonging to widely different families.[41] In _Fundulus
heteroclitus_ I have found that the pectoral, pelvic, caudal, anal, and
dorsal fins have the power of regeneration. In reptiles the feet do not
regenerate,--at least no cases are known,--but the tail of lizards has
this power well developed. In birds neither the wings nor the feet
regenerate, but Fraisse has described the case of a stork in which, the
lower jaw being broken off, and the upper being cut off at the same
level, both regenerated. Bordage has recorded the regeneration of the
beak of the domesticated fighting cocks (of the Malay breed) of
Mauritius. In the mammals neither the legs, nor the tail, nor the jaws
regenerate, although several of the internal organs, as described in the
next chapter, have extensive powers of regeneration.
The best opportunity to examine the regenerative power in similar organs
of the same animal is found in forms like the crustacea, myriapods, and
insects, in which external appendages are repeated in each or many
segments of the body. In decapod crustacea, including shrimps, lobsters,
crayfish, crabs, hermit-crabs, etc., regeneration takes place in the
walking legs of all the forms that have been examined, and this includes
members of many genera and families. I have made an examination of the
regeneration of the appendages (Fig. 37) of the hermit-crab. In this
animal, which lives in an appropriated snail’s shell, only the anterior
part of the body projects from the shell. The part that protrudes is
covered by a hard cuticle, while the part of the body covered by the
shell is quite soft. Three pairs of legs are protruded from the shell.
The first pair with large claws
[Illustration: FIG. 37.--Appendages of Hermit-crab (_Eupagurus
longicarpus_). _A._ Third walking leg. _B._ Next to last thoracic leg.
_B¹._ Last thoracic leg. _C_, _C¹_, _C²_. Three abdominal appendages
of male. _D._ Telson and sixth segment with last pair of abdominal
appendages. _E._ Regeneration of new leg from cut-end outside of
“breaking-joint.” _F._ Leg regenerating from cut made inside of
“breaking-joint.” _G._ Leg regenerating from cut made very near the
body.]
are used for procuring food, and as organs of offence and defence; the
second and third pairs are used for walking. The following two pairs,
that correspond to the last two pairs of walking legs of crabs and
crayfishes, are small, and are used by the animal in bracing itself
against the shell. The first three pairs of legs have an arrangement at
the base, the “breaking-joint,” by means of which the leg is thrown off,
if injured. The last two pairs of thoracic legs cannot be thrown off.
The first three pairs of legs are often lost under natural conditions.
In an examination of 188 individuals I found that 21 (or 11 per cent)
had lost one or more legs. If one of the first three legs is injured,
except in the outer segment, it is thrown off at the breaking-joint,
and a new leg regenerates from the broken-off end of the stump that is
left. The new leg does not become full size, and is of little use until
the crab has moulted at least once. The leg breaks off so close to the
body, and the part inside of the breaking-joint is so well protected by
the bases of the other legs, that it is scarcely possible that the leg
could be torn off inside of the breaking-joint, and, as a matter of
observation, all crabs that are found regenerating their legs under
natural conditions do so from the breaking-joint. If, however, by means
of small scissors, the leg is cut off quite near the body, a new leg
regenerates from the cut-end, even when the leg is cut off at its very
base. The breaking-joint would thoroughly protect from injury the part
of the leg that lies nearer to the body, and yet from this inner part a
new leg is regenerated. Moreover, the new leg is perfect in every
respect, even to the formation of a new breaking-joint. In this case we
have a demonstration that there need be no connection between the
liability of a part to injury and its power of regeneration.
In still another way the same thing may be shown. If the crab is
anæsthetized, and a leg cut off outside of the breaking-joint, it is
not, at the time, thrown off--the nervous system, through whose action
the breaking off takes place, being temporarily thrown out of order.
After recovery, although the leg is thrown off in a large number of
cases, it is sometimes retained. In such cases it is found that from the
cut-end the missing part is regenerated. In this case also we find that
regeneration takes place from a part of the leg that can never
regenerate under natural circumstances.
The third and fourth legs of the hermit-crab cannot be thrown off, but
they have the power of regeneration at any level at which they may be
cut off. They are in a position where they can seldom be injured, and I
have never found them absent or injured in crabs caught in their natural
environment. The soft abdomen is protected by the snail’s shell. At the
end of the abdomen the last pair of abdominal appendages serve as
anchors to hold the crab in the shell. These appendages are large and
very hard, and can seldom be injured unless the abdomen itself is
broken, and under these circumstances the crab dies. Yet if these
appendages are cut off they regenerate perfectly, and after a single
moult cannot be distinguished from normal ones.
The more anterior abdominal appendages are present only on one side of
the adult, although they are present on both sides of the larva, and, to
judge from a comparison with other crustacea, these appendages have
degenerated completely on one side, and have become rudimentary in the
male, even on the side on which they are present. They too will
regenerate if they are cut off. In the female these appendages are used
to carry the eggs, and are, therefore, of use. They also have a similar
power of regeneration. The maxillæ and maxillipeds of the hermit-crab
have likewise the power of regeneration, as have also the two pairs of
antennæ and the eyes.
In other decapod crustacea also it has been shown that the power of
regeneration of the appendages is well developed. It has been long known
that the crayfish and the lobster can regenerate lost parts. The first
pair of legs, or chelæ, in these forms has a breaking-joint, at which
the leg can be thrown off, yet in the crayfish I have seen that if the
leg is cut off inside of the breaking-joint it will regenerate. The four
pairs of walking legs do not possess a breaking-joint, but may be thrown
off in some cases at a corresponding level. They regenerate from this
level, as well as nearer the body and farther beyond this region.
Przibram has recently shown that, in a number of crustacea, regeneration
of the appendages takes place, even when the entire leg is extirpated as
completely as possible.
Newport has shown that the myriapods can regenerate their legs, and it
is known that several forms have the power of breaking off their legs in
a definite region at the base if the legs are injured, and I have
observed in _Cermatia forceps_ that this takes place even when the
animal is thrown into a killing fluid. Newport (’44) has also shown that
when the legs of a caterpillar are cut off new ones regenerate during
the pupa stage. It has been long known[42] that the legs of mantis can
regenerate, and Bordage, who has recently examined the question more
fully, has shown that a breaking-joint is present at the base of the
leg. The tarsus of the cockroach also regenerates, producing only four,
instead of the five, characteristic segments.[43]
A number of writers have recorded the regeneration of the legs of
spiders.[44] Schultz, who has recently examined more thoroughly the
regeneration of the legs in some spiders, finds that the leg is renewed
if cut off at any level. He removed the leg most often at the
metatarsus, but also at the tibia, and generally between two joints. In
some cases the leg was cut off at the coxa, at which level it is
generally found to be lost under natural conditions. Wagner observed in
tarantula that when the leg is removed at any other place than at the
coxa, the animal brings the wounded leg to its jaws, and bites it off
down to the coxa. In the _Epeiridæ_, that Schultz chiefly made use of,
this never happened. He observed, however, even in these forms, that
when the leg is cut off at the coxa it regenerates better than
[Illustration: FIG. 38.--_A-F._ After King. _A._ Starfish with four arms
regenerating at different levels. _B._ Three arms regenerating from
disk. _C._ Arm split in two producing two arms. _D._ Arm cut off
obliquely, regenerating at right angles to cut-surface. _E._ Starfish
split between two arms, producing two new arms from split. _F._ An arm,
with a small piece of disk attached, regenerating three new arms. _G._
After P. and F. Sarasin, Starfish (_Linckia multiformis_) with four new
arms springing from end of one arm. Interpreted as a new starfish, but
probably only multiple arms (see _C_, above).]
when cut off at any other level. Schultz states that we see here an
excellent example of how regeneration is influenced by natural
selection, since regeneration takes place best where the leg is most
often broken off. On the other hand, the author hastens to add that
since regeneration also takes place when the leg is cut off at any other
level, this shows that the power to regenerate is characteristic of all
parts of the organism, and is not _merely_ a phenomenon of adaptation,
as Weismann believes. It seems highly improbable that a spider could
ever lose a leg in the middle of a segment, _i.e._ between two joints,
since the segments are hard and strong and the joints much weaker; but
nevertheless the leg has the power to regenerate also from the middle of
the segment, if cut off in this region.
The formation of the new part takes place somewhat differently,
according to Schultz, when the leg is amputated between two segments
than when cut off at the coxa. In the latter case, there is produced
from the cut-end of the last segment a solid rod which, as it grows
longer, bends on itself several times. Joints appear in the rod,
beginning at the base. The leg is set free at the next moult. If the leg
is cut off nearer the distal end a smaller rod is formed, that extends
straight forward, or may be thrown into a series of folds. It lies,
however, inside of the last segment, since the surface exposed by the
cut is quickly covered over by a chitinous covering. The piece is set
free at the next moult.
Loeb has found that if the body of the pycnogonid, _Phoxichilidium
maxillare_, is cut in two there regenerates from the posterior end of
the anterior half a new body-like outgrowth.
Without attempting to describe the many cases in worms and mollusks in
which there is no obvious connection between the power of the part to
regenerate and its liability to injury, but where it is more difficult
to show that it may not exist, let us pass to an examination of the
regeneration of the starfish. It has been known since the time of
Réaumur that starfish have the power of regenerating new arms if the old
ones are lost. It has been stated that in certain starfishes an arm
itself can produce a new starfish,--Haeckel (’78), P. and R. Sarasin
(’88), von Martens (’84), and Sars (’75),--but this has been denied by
other observers. In several species of starfishes, the separated arm
does not regenerate; but if a portion, even a small piece, of the disk
is left with the arm, a new disk and arms may develop (Fig. 38, _F_).
When the arm of _Asterias vulgaris_ is injured it pinches off in many
cases at its base, and a new arm grows out from the short stump that
remains. When these starfishes regenerate new arms in their natural
environment, the new arms almost always arise from this breaking
region.[45] Thus King found out of 1914 individuals of _Asterias
vulgaris_ collected at random, 206, or 10.7 per cent, had one or more
new arms, and all these except one arose from near the disk. In other
species it appears that the outer portions of the arm may be broken off
without the rest of the arm being thrown off. King has found that in
asterias, regeneration takes place more rapidly from the base than at a
more distal level. It may appear, at first thought, that the more rapid
regeneration of the arm at the place at which it is usually thrown off
may be associated with its more frequent loss at this region--in other
words, that the more rapid regeneration has been acquired by the region
at which the arm is generally broken off. This interpretation is,
however, excluded by the fact that, in general, the nearer to the base
the arm is cut off, so much the more rapid is its regeneration. In other
words, the more rapid regeneration of the arm at the base is only a part
of a general law that holds throughout the arm. If the proposition is
reversed, and it is claimed that the arm has acquired the property of
breaking off at the base, because it regenerates more rapidly at that
level, the following fact recorded by King is of importance, viz. that,
although the arm regenerates faster at the base, yet a new arm is not
any sooner produced in this way, since there is more to be produced and
the new arm from the base may never catch up to one growing less rapidly
from a more distal cut-surface, but having a nearer goal to reach.
The results of our examination show that those forms that are liable to
have certain parts of their bodies injured are able to regenerate not
only these parts, but at the same time other parts of the body that are
not subject to injury. The most remarkable instance of this sort is
found in those animals having breaking-joints. In these forms, we find
that regeneration takes place both proximal and distal to this region.
If the power of regeneration is connected with the liability of a part
to injury, this fact is inexplicable.
Turning now to the question as to whether regeneration takes place in
those species that are subject to injury more frequently or better than
in other species, we find that the data are not very complete or
satisfactory for such an examination. It is not easy to ascertain to
what extent different animals are exposed to injury. If we pass in
review the main groups of the animal kingdom, we can at least glean some
interesting facts in this connection.
In the protozoa nucleated pieces have been found to regenerate in all
forms that have been examined, including amœba, difflugia,
thalassicolla, paramœcium, stentor, and a number of other ciliate
infusoria.
In the sponges it has been found by Oscar Schmidt that pieces may
produce new individuals, but how widely this occurs in the group is not
known. In the cœlenterates many forms are known to regenerate, and it is
not improbable that in one way or another the process occurs throughout
the group. The hydroid forms, hydra, tubularia, parypha, eudendrium,
antennularia, hydractinia, podocoryne, etc., the jelly-fish,
gonionemus, and certain members of the family _Thaumantidæ_, have been
found to regenerate. Amongst the _Scyphozoa_, metridium, cerianthus, and
the scyphistoma of aurelia regenerate, and the jelly-fishes belonging to
this group have a limited amount of regenerative power.
In the platodes we find that all the triclads, thus far examined,
including planaria, phagocata, dendrocœlum, and the land triclad,
bipalium, regenerate. It has been shown that the marine triclads also
regenerate, but less rapidly and extensively, while the marine polyclads
have very limited powers of regeneration. The regeneration of the
trematodes and cestodes has not, so far as I know, been studied, neither
have the nematodes been examined from this point of view.
Some of the nemerteans regenerate, others do not seem to have this
power. A small fresh-water form, tetrastemma, that I examined, did not
regenerate, although some of the pieces, that were filled with eggs,
remained alive for several months.
In the annelids we find a great many forms that regenerate--many marine
polychæta have this power; all oligochæta that have been studied
regenerate; both land forms, like lumbricus, allolobophora, etc., and
fresh-water forms, like lumbriculus, nais, tubifex, etc.
In the crustacea the appendages have the power to regenerate in all the
forms that have been examined.
Several kinds of myriapods, as well as a number of spiders, are known to
regenerate their legs. In the insects, however, only a few forms are
known to have this power,--caterpillars, mantis, and the cockroach. The
large majority of insects, in the imago state, do not seem to be able to
regenerate, although in a few cases regeneration has been found to
occur.[46]
In the mollusks, regeneration of the head takes place under certain
conditions. Spallanzani thought that if the entire head is cut off a new
one regenerates. This conclusion was denied by at least eleven of his
contemporaries, and confirmed by about ten others. It was found later
that the result depends in part on the time of year and in part on the
kind of snail. Carrière, who more recently examined the question, found
that even under the most favorable conditions regeneration does not take
place if the circumœsophageal nerve-commissure is completely removed
with the head, but if a part remains, a new head develops. It has been
stated that a new foot regenerates in helicarion, and I have found that
the foot regenerates also in the fresh-water snails, physa, limnæa, and
planorbis. If the margin of the shell of a lamellibranch or of a snail
is broken off, it is renewed by the mantle. The arms of some of the
cephalopods are known to regenerate, particularly the hectocotylized
arm.
In all the main groups of echinoderms, with one possible exception,
regeneration has been found to take place. Probably all starfishes and
brittle-stars regenerate their arms, and even if cut in two or more
pieces, new starfishes develop. The crinoids regenerate lost arms, and
even parts of the disk; also the visceral mass. The holothurians have
very remarkable powers of regeneration. In some forms regeneration takes
place if the animals are cut in two, or even in more than two pieces.
The remarkable phenomenon of evisceration that take place in certain
holothurians, if they are roughly handled, or kept under unfavorable
conditions, are well known and have been described by a number of
writers. It has even been suggested that the holothurian may save itself
by offering up its viscera to its assailant! Unfortunately for this
view, it has been found that the viscera are unpalatable, at least to
sea-anemones and to fishes. Ludwig and Minchin suggest that the throwing
off of the Cuvierian organs, which are attached to the cloaca, is a
defensive act, and if carried too far, according to the latter writer,
the viscera may also be lost. The holothurians have remarkable
recuperative powers and may regenerate new viscera in a very short time.
The sea-urchins form, perhaps, an exception in this group, since there
are no records of their regenerative power, but no doubt this is because
they have not been as fully investigated as have other forms.
[Illustration: FIG. 39.--_A._ _Amphiuma means_ with left fore and hind
leg regenerating. _B._ _Necturus maculatus_ with right fore leg
beginning to regenerate after eight months. _C._ _Plethedon cinereus._
_A, B, C._ Drawn to same scale.]
In the vertebrates the lower forms, amphioxus, petromyzon, and sharks,
have not been studied in regard to their regenerative power. In the
teleostean fishes the fins of a number of forms are known to regenerate.
It is probable that this takes place in most members of the group.
In the amphibia we find a large number of forms that regenerate their
limbs and tail, and other parts of the body, but limitations appear in
certain forms. The rapid regeneration of the legs in the smaller
urodeles has been often described. In larger forms it takes place more
slowly, at least in large forms having large legs. In proteus the
regeneration may extend over a year and a half, and in necturus it takes
more than a year to make a new limb, at least in animals in confinement.
In the large form, amphiuma, that has extremely small legs, regeneration
takes place much more rapidly than in a form like necturus having much
larger legs (Fig. 39).
In amphiuma the feet are not used by the animal as organs of locomotion,
since they are too small and weak to support the heavy body. They can be
moved by the animal in the same way that the feet are moved in other
forms, and yet are useless for progression. It is said by Schreiber that
the regeneration of the legs of _Triton marmoratus_ is relatively very
slight as compared with that of other forms. Fraisse also found in this
form that an amputated leg did not grow again, only a deformed stump
being produced. The tail also is said to regenerate to only a slight
extent, but, so far as I know, there is nothing peculiar in the life of
this form that makes it less liable to injury than other large
urodeles.[47] Weismann cites the case of proteus, which is said also to
regenerate less well than do other forms. It lives in the caves of
Carniola, where there are few other animals that could attack or injure
it, and to this immunity is ascribed its lack of power of regeneration;
yet Goette states that he observed a regenerating leg in this form, but
that the process was not complete after a year and a half. In necturus
also, which is not protected in any way, regeneration is equally slow.
Frogs are unable to regenerate their limbs, although they are sometimes
lost, but the larval tadpole can regenerate at least its hind legs. In
the lizards the tail regenerates, but at present we do not know of any
connection between this condition and the liability of certain forms to
injury. Turtles and snakes do not regenerate their tails. I do not know
of any observations on crocodiles.
In birds, the legs and wings are not supposed to have the power to
regenerate,[48] but in two forms[49] at least the beak has been found
to possess remarkable powers of regeneration. There are a few very
dubious observations in regard to the regeneration in man of superfluous
digits that had been cut off.[50]
These examples might be added to by others in the groups cited, and also
by examples taken from the smaller groups of the animal kingdom, but
those given will suffice, I think, to show that the power to regenerate
is characteristic of entire groups rather than individual species. When
exceptions occur, we do not find them to be forms that are obviously
protected, but the lack of regeneration can rather be accounted for by
some peculiarity in the structure of the animal. If this is borne in
mind, as well as the fact that protected and unprotected parts of the
same animal regenerate equally well, there is established, I think, a
strong case in favor of the view that there is no necessary connection
between regeneration and liability to injury. We may therefore leave
this side of the question and turn our attention to another
consideration.
It will be granted without argument that the power of replacement of
lost parts is of use to the animal that possesses it, especially if the
animal is liable to injury. Cases of usefulness of this sort are
generally spoken of as adaptations. The most remarkable fact in
connection with these adaptive responses is that they take place, in
some cases at least, in parts of the body where they can never, or at
most very rarely, have taken place before, and the regeneration is as
perfect as when parts liable to injury regenerate. Another important
fact is that in some forms the regeneration is so slow that if the
competition amongst the animals was very keen those with missing legs,
or eyes, or tails, would certainly succumb; yet, if protected, they do
not fail to regenerate. If, therefore, the animal can exist through the
long interval that must elapse before the lost part regenerates, we
cannot assume that the presence of the part is of vital importance to
the animal, and hence its power to regenerate could scarcely be
described as the result of a “battle for existence,” and without this
principle “natural selection” is powerless to bring about its supposed
result.
It is extremely important to observe that some cases, at least, of
regeneration are not adaptive. This is shown in the case where a new
head regenerates at the posterior end of the old one in _Planaria
lugubris_, or where a tail develops at the anterior end of a posterior
piece of an earthworm, or when an antenna develops in place of an eye in
several crustacea. If we admit that these results are due to some inner
laws of the organism, and have nothing to do with the relation of the
organism to its surroundings, may we not apply the same principle to
other cases of regeneration in which the result is useful?
So firm a hold has the Darwinian doctrine of utility over the thoughts
of those who have been trained in this school, that whenever it can be
shown that a structure or a function is useful to an animal, it is
without further question set down as the result of the death struggle
for existence. A number of writers, being satisfied that the process of
regeneration is useful to the animal, have forthwith supposed that,
_therefore_, it must have been acquired by natural selection. Weismann
has been cited as an example, but he is by no means alone in maintaining
this attitude. It would be entirely out of place to enter here into a
discussion of the Darwinian theory, but it may be well worth while to
consider it in connection with the problem of regeneration.
We might consider the problem in each species that we find capable of
regenerating; or, if we find this too narrow a field for our
imagination, we might consider the process of regeneration to have been
“acquired by selection in the lower and simpler forms,” and trace its
subsequent progress as it decreased in the course of phylogeny “in
correspondence with the increase in complexity of organization,” or with
the decrease of exposure to injury. At the risk of adopting the narrower
point of view I shall confine the discussion to the possibility of
regeneration being acquired, or even augmented, through a process of
natural selection in any particular species.
The opportunity to regenerate can only occur if a part is removed by
accident or otherwise. On the Darwinian theory we must suppose that of
all the individuals of each generation that are injured, _in exactly the
same part of the body_, only those have survived or have left more
offspring that have regenerated. In order that selection may take place,
it must be supposed that amongst these individuals injured _in exactly
the same region_, regeneration has been better in some forms than in
others, and that this difference is, or may be, decisive in the
competition of the forms with each other. The theory does not inquire
into the origin of this difference between individuals, but rests on the
assumption of individual differences in the power to regenerate, and
assumes that these differences can be heaped up by the survival and
inbreeding of the successful individuals; _i.e._ it is assumed that, by
this picking out or selection through competition in each generation of
the individuals that regenerate best, the process will become more and
more perfectly carried out in the descendants, until at last each part
has _acquired_ the power of complete regeneration.
There are so many assumptions in this argument, and so many
possibilities that must be realized in order that the result shall
follow, that, even if the assumptions were correct, one might still
remain sceptical in regard to the possibilities ever becoming realized.
If we examine somewhat more in detail the conditions necessary to bring
about this supposed process, we shall find ample grounds for doubt, and
even, I think, for denial that the results could ever have been brought
about in this way.
In the first place, the assumption that the regeneration of an organ can
be accounted for as a result of the selection of those individual
variations that are somewhat more perfect, rests on the ground that such
variations occur, for the injury itself that acts as a stimulus is not
supposed to have any direct influence on the result, _i.e._ for better
or worse. All that natural selection pretends to do is to build up the
complete power of regeneration by selecting the most successful results
in the right direction. In the end this really goes back to the
assumption that the tissue in itself has power to regenerate more
completely in some individuals than in others. It is just this
difference, if it could be shown to exist, that is the scientific
problem. But, even leaving this criticism to one side, since it is very
generally admitted, it will be clear that in many cases most of the less
complete stages of regeneration that are assumed to occur in the
phyletic series could be, in each case, of very little use to the
individual. It is only the completed organ that can be used; hence the
very basis of the argument falls to the ground. The building up of the
complete regeneration by slowly acquired steps, that cannot be decisive
in the battle for existence, is not a process that can be explained by
the theory.
There is another consideration that is equally important. It is assumed
that those individuals that regenerate better than those that do not,
survive, or at least have more descendants; but it should not be
overlooked that the individuals that are not injured (and they will
belong to both of the above classes) are in even a better position than
are those that have been injured and have only incompletely regenerated.
The uninjured forms, even if they did not crowd out the regenerating
ones, which they should do on the hypothesis, would still intercross
with them, and in so doing bring back to the average the ability of the
organism to regenerate. Here we touch upon a fatal objection to the
theory of natural selection that Darwin himself came to recognize in the
later editions of the _Origin of Species_, namely, that unless a
considerable number of individuals in each generation show the same
variation, the result will be lost by the swamping effects of
intercrossing. If this be granted, there is left very little for
selection to do except to weed out a few unsuccessful competitors, and
if the same causes that gave origin to the new variation on a large
scale should continue to act, it will by itself bring about the result,
and it seems hardly necessary to call in another and questionable
hypothesis.
Finally, a further objection may be stated that in itself is fatal to
the theory. We find the process of regeneration taking place not only at
a few vulnerable points, but in a vast number of regions, and in each
case regenerating only the missing part. The leg of a salamander can
regenerate from every level at which it may be cut off. The leg of a
crab also regenerates at a large number of different levels, and
apparently this holds for all the different appendages. If this result
had been acquired through the action of natural selection, what a vast
process of selection must have taken place in each species! Moreover,
since the regeneration may be complete at each level and in each
appendage without regard to whether one region is more liable to injury
than is another, we find in the actual facts themselves nothing to
suggest or support such a point of view.
If, leaving the adult organism, we examine the facts in regard to
regeneration of the embryo, we find again insurmountable objections to
the view that the process of regeneration can have been produced by
natural selection. The development of whole embryos from each of the
first two or first four blastomeres can scarcely be accounted for by a
process of natural selection, and this is particularly evident in those
cases in which the two blastomeres can only be separated by a difficult
operation and by quite artificial means. If a whole embryo can develop
from an isolated blastomere, or from a part of an embryo without the
process having been acquired by natural selection, why apply the latter
interpretation to the completing of the adult organism?
Several writers on the subject of regeneration in connection with the
process of autotomy (or the reflex throwing off of certain parts of the
body) have, it seems to me, needlessly mixed up the question of the
origin of this mechanism with the power of regeneration. If it should
prove true that in most cases the part is thrown off at the region at
which regeneration takes place to best advantage, it does not follow at
all that regeneration takes place here better than elsewhere, because in
this region a process of selection has most often occurred. The
phenomenon of regeneration in the arm of the starfish, that has been
described on a previous page, shows how futile is an argument of this
sort. If, on the other hand, the autotomy is supposed to have been
acquired in that part of the body where regeneration takes place to best
advantage, then our problem is not concerned with the process of
regeneration at all, but with the origin of autotomy. If the attempt is
made to explain this result also as the outcome of the process of
natural selection acting on individual variations, many of the
criticisms advanced in the preceding pages against the supposed action
of this theory in the case of regeneration can also readily be applied
to the case of autotomy. In Chapter VIII, in which the theories of
autotomy are dealt with, this problem will be more fully discussed.