THE
EVOLUTION THEORY
VOLUME II
THE
EVOLUTION THEORY
BY
DR. AUGUST WEISMANN
PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU
TRANSLATED WITH THE AUTHOR'S CO-OPERATION
BY
J. ARTHUR THOMSON
REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN
AND
MARGARET R. THOMSON
ILLUSTRATED
IN TWO VOLUMES
VOL. II
LONDON
EDWARD ARNOLD
41 & 43 MADDOX STREET, BOND STREET, W.
1904
_All rights reserved_
CONTENTS
LECTURE PAGE
XX. REGENERATION 1
XXI. REGENERATION (_continued_) 23
XXII. SHARE OF THE PARENTS IN THE BUILDING UP OF THE
OFFSPRING 37
XXIII. EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY
OF FUNCTIONAL MODIFICATIONS 62
XXIV. OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS
ARE NOT TRANSMITTED 80
XXV. GERMINAL SELECTION 113
XXVI. GERMINAL SELECTION (_continued_) 136
XXVII. THE BIOGENETIC LAW 159
XXVIII. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 192
XXIX. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (_continued_) 210
XXX. IN-BREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION,
AND THEIR CONSEQUENCES 238
XXXI. THE INFLUENCES OF ENVIRONMENT 265
XXXII. INFLUENCE OF ISOLATION ON THE FORMATION OF
SPECIES 280
XXXIII. ORIGIN OF THE SPECIFIC TYPE 299
XXXIV. ORIGIN OF THE SPECIFIC TYPE (_continued_) 330
XXXV. THE ORIGIN AND THE EXTINCTION OF SPECIES 346
XXXVI. SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION 364
INDEX 397
LIST OF ILLUSTRATIONS
FIGURE PAGE
35 _B_ (repeated). _Hydra viridis_, the Green Freshwater Polyp. 4
96. A Planarian cut transversely into nine pieces 6
97. A Planarian which has been divided into two by a
longitudinal cut 14
98. The leg of a Crab, adapted for self-mutilation or
autotomy 17
99. Regeneration of the lens in a Newt's eye 21
100. Regeneration of Planarians 25
101. A Starfish arm 27
76 (repeated). Diagram of the maturation divisions of the ovum 39
82 (repeated). Fertilization in the Lily 59
91 (repeated). Hind-leg of a Grasshopper 83
102. Brush and comb on the leg of a Bee 84
103. Claw on the leg of a 'Beach-fly' 85
104. Digging leg of the Mole-cricket 86
105. Ovary of a fertile Queen-Ant and ovaries of a Worker 91
106. Three Workers of the same species of Indian Ant 97
107 _A_, _B_. Larva of a Caddis-fly 105
107 _C_. Leptocephalus stage of an American Eel 133
108. Nauplius larva of one of the lower Crustaceans 161
109 _A_, _B_. Metamorphosis of one of the higher Crustacea,
a Shrimp 162
109 _C_. Second Zoæa stage 163
109 _D_, _E_. Mysis-stage and fully-formed Shrimp 164
70 (repeated). Daphnella 166
110. The largest of the Daphnids (_Leptodora hyalina_), with
summer ova beneath the shell 166
111. Nauplius larva from the winter egg of _Leptodora hyalina_ 167
112. Development of the parasitic Crustacean _Sacculina
carcini_ 168, 242
113. The two sexes of the parasitic Crustacean _Chondracanthus
gibbosus_ 170
114. Zoæa-larva of a Crab 171
115. Caterpillar of the Humming-bird Hawk-moth _Macroglossa
stellatarum_ 178
3 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 178
4 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 179
8 (repeated). Caterpillars of the Buckthorn Hawk-moth 179
116. Development of the eye-spots in the caterpillar of the
Elephant Hawk-moth _Chærocampa elpenor_ 180
117. Caterpillar of the Bed-straw Hawk-moth _Deilephila galii_ 181
118. Two stages in the life-history of the Spurge Hawk-moth
_Deilephila euphorbiæ_ 182
119. Caterpillar of the Poplar Hawk-moth _Smerinthus populi_ 184
120. _A_, Symmetrical, and _B_, asymmetrical curve of frequency 207
121. Life-cycle of _Coccidium lithobii_ 214
122. Conjugation of a Coccidium (_Adelea ovata_) 216
123. Conjugation of _Coccidium proprium_ 218
79 (repeated). The two maturation divisions of the 'drone eggs' 236
124. Alternation of generations in a Gall-wasp 245
125. The two kinds of galls formed by the species 246
126. Ovipositor and ovum of the two generations of the same
species of Gall-wasp 247
127. Life-cycle of the Vine-pest (_Phylloxera vastatrix_) 249
128. Heterostylism 254
38 (repeated). A fragment of a Lichen 261
129. Aberration of _Arctia caja_, produced by low temperature 276
130. Skeleton of a Greenland Whale, with the contour of the body 313
131. Peridineæ: species of _Ceratium_ 325
LECTURE XX
REGENERATION
Budding and division--Every theory of regeneration in
the meantime only provisional, a mere 'portmanteau
theory'--Regeneration not a primary character--Volvox--Hydra--Vital
affinities--Planarians--Heteromorphoses--Enemies of
Hydroid-colonies--Regeneration in Plants--In Amphibians--In
Earthworms--Different degrees of regenerative capacity according
to the liability of the part to injury--Different results of
longitudinal halving in Earthworms and in Planarians--Regeneration
in Birds--The disappearance of the power of regeneration is very
slow--Morgan's experiments on Hermit-crabs--Autotomy in Crustaceans
and Insects--Regeneration of the lens in Triton.
We have endeavoured to explain the handing on of the complement
of heritable qualities from one generation to another as due to a
continuity of the germ-plasm, and we assumed that the germ-cells never
arise except from cells in the 'germ-track'; that is, from cells which
are equipped, from the fertilized egg-cell onwards, with a complete
sample of slumbering germ-plasm, and are thereby enabled to become
germ-cells, and, subsequently, new individuals, in which the aggregate
of inherited primary constituents implied in the germ-plasm can again
attain to development.
We have now to consider other cases of inheritance in relation to the
same problem--the origin of their hereditary equipment.
We know, of course, that new individuals may arise apart from
germ-cells, that, in many of the lower animals and in plants, they may
arise by budding and fission.
For both these cases the germ-plasm theory will suffice, with a
somewhat modified form of the same assumption which we made in regard
to the formation of germ-cells. The origin of a new individual by
budding seems often, indeed, to proceed from any set of somatic cells
in the mother animal; but somatic cells, if they contain solely the
determinants controlling themselves, cannot possibly give rise to a
complete new individual, since this presupposes the presence of _all_
the determinants of the species. But as these determinants cannot be
formed _de novo_, the budding cells must contain in addition to the
usual controlling somatic determinants, idioplasm in a latent, inactive
state, which only becomes active under certain internal or external
influences, and then gives rise to the formation of a bud. The source
of this accessory idioplasm must, however, be looked for only in the
egg-cell.
In plants this bud-idioplasm must be complete germ-plasm, because
the budding starts only from one kind of cell, the cambium-cells;
but in animals in which--as it seems--it always proceeds from at
least two different kinds of cells--those of the ectoderm and those
of the endoderm--the matter is more complex. In this case these two
kinds of cells will contain as bud-idioplasm two different groups of
determinants, which mutually complete each other and form perfect
germ-plasm, and only the co-operation of these two sets will give rise
to the formation of a bud. I will not, however, go further into detail
in regard to these relations, for the theory can do nothing more here
than formulate what has been observed; it is hardly in a position to
help us to a better understanding of the facts.
The case is not much clearer in regard to the processes which lead to
the replacing of lost parts. The manifold phenomena of regeneration
can also be brought into harmony with the theory, if we attribute to
those cells from which the replacing or entire reconstruction of the
lost part arises an 'accessory-idioplasm,' which, at least, contains
the determinants indispensable to the building up of the part. It is
possible that the assumed accessory idioplasm frequently contains
a much larger complex of determinants, and that it depends on the
liberating stimuli which, and how many of these, will become active.
If we take a survey of regenerative phenomena in the animal kingdom,
it strikes us at once that the capacity is very different in different
species, extraordinarily great in some and very slight in others.
In general it is greater in lower animals than in higher, but,
nevertheless, the degree of differentiation cannot be the only factor
that determines the capacity for regeneration. That unicellular
organisms can completely replace lost parts, that even a piece of an
infusorian can reconstruct the whole animal if only the piece contain
a part of the nucleus, we have already seen when discussing the
significance of the nuclear substance. In this case the nucleus must
contain the complete germ-plasm, that is, the collective determinants
of the species, and these induce the reconstruction of the lost part,
though they do so in a way that is still entirely obscure to us. In the
meantime, our interpretation will not carry us further, either here or
in regard to any other order of vital phenomena. To go further would be
little short of propounding a causal theory of life itself; it would
mean having a complete and real 'explanation' of what 'life' is. As yet
no one has been able to claim this position. We can see the different
stages through which every organism passes, and that they arise one
out of the other; we can even penetrate down to the succession of those
delicate and marvellously complex processes which effect nuclear and
cell-division; but we are still far from being able to deduce, except
quite empirically, from the present state of a cell what the succeeding
one will be, that is, from being able to understand the succession of
events as a necessary nexus which could be predicted. How a biophor
comes to develop from itself the phenomena of life is quite unknown to
us; we know neither the interaction of the ultimate material particles
nor the forces which bring it about; we cannot tell what moves the
hordes of different kinds of biophors to range themselves together in
a particular order, what molecular displacements and variations arise
from this, or what influence the external world has, and so forth.
We see only the visible outcome of an endless number of invisible
movements--growth, division, multiplication, reconstruction, and
differentiation.
As long as we are so far from an understanding of life no theory of
regeneration can be anything more than a 'portmanteau theory,' as
Delage once expressed himself in relation to the whole theory of
inheritance, a theory which is like a portmanteau in that one can
only take out of it what has previously been put in. If we wish to
explain the renewal of the aboral band of cilia in a Stentor, we
first pack our trunk, in this case the nucleus of the Infusorian,
with the determinants of the ciliated region, and then think of these
as being liberated by the stimulus of wounding, and being brought to
and arranged in the proper place by unknown forces to reconstruct the
ciliary region in some unknown way. No one could be more clearly aware
than I am that this is not an exhaustive causal explanation of the
process itself. Nevertheless, it is not quite without value, inasmuch
as it allows us at least to bring the facts together in rational
order--in this case the dependence of the faculty of regeneration on
the presence of nuclear substance--under a formula which we can use
provisionally, that is, with which we can raise new questions. As soon
as we ascend higher in the series of organisms the theory gains a
greater value, for, while we leave altogether out of account any answer
to the ultimate question, and thus renounce for the present the attempt
to find out how the determinants set to work to call to life the parts
which they control, we are brought face to face with other, in a sense,
preliminary questions which we _can_ solve, and the solution of which
seems to me at least not entirely without value.
The first of these questions runs thus: Is the power of regeneration a
fundamental, primary character of every living being in the sense that
it is present everywhere in equal strength, independently of external
conditions, and thus is an inevitable outcome of the primary characters
of the living substance? Or is it, though primaeval in its beginnings,
a phenomenon of adaptation, which depends on a special mechanism, and
does not occur everywhere in equal extent and potency?
We have already become acquainted with some facts which must incline us
to the latter view. The globular Alga-colonies of _Volvox_ (Fig. 63)
consist of two kinds of cells, of which only one kind, the reproductive
cells, possess the power of reproducing the whole, the others, the
flagellate, or, as we called them, somatic cells, being only able to
produce their like, but never the whole.
New investigations which have been carried out by Dr. Otto Hübner in my
Institute have placed these facts beyond doubt. We may conclude that,
in this case, a disintegration of the germ-plasm has taken place during
ontogeny, by means of differential cell-division, so that only the
reproductive cells receive the complete germ-plasm, while the somatic
cells receive only the determinants necessary to their own specific
differentiation, the somatic determinants.
In this case regeneration and reproduction coincide; there is no
regeneration except the origin of a new individual from a reproductive
cell.
[Illustration: FIG. 35 _B_ (repeated). _Hydra viridis_, the Green
Freshwater Polyp. Section through the body-wall, somewhere in the
direction of _ov_ in Fig. 35 _A_. _Eiz_, the ovum lying in the ectoderm
(_ect_), and including zoochlorellæ (_schl_) which have immigrated
from the endoderm (_ent_) through the supporting lamella (_st_). After
Hamann.]
Let us now ascend to the lowest of the Metazoa, for instance, the
freshwater polyp, Hydra (Fig. 35 _A_), and we find a high degree of
regenerative capacity in the restricted sense, for, in addition to
the power of producing germ-cells, that is, cells which, when two
combine in amphimixis, give rise again to a new animal, almost any
part of the polyp can regrow a whole animal. Not only has Hydra been
cut in from two to twenty different pieces, but it has even been
chopped up into innumerable fragments, and yet each of these, under
favourable circumstances, was able to grow again into a complete
animal. Nevertheless, we are not justified in concluding that every
cell possesses the power of reproducing the whole. If, with the help
of a bristle, we turn one of these polyps outside in like the finger
of a glove, and then prevent it turning right again by sticking the
bristle transversely through it, it does not live, but soon dies,
obviously because the cells of the two layers of the body, ectoderm
and endoderm, cannot mutually replace each other, and cannot mutually
produce each other. The inner layer, now turned outwards, cannot resist
the influence of the water, and the outer layer, now turned inwards,
cannot effect digestion; in short, one cannot be transformed into the
other, and we must therefore conclude that both are specialized, that
they no longer contain the complete germ-plasm, but only the specific
determinants of ectoderm and endoderm respectively.
The animal's high regenerative capacity must therefore depend on the
fact that certain cells of the ectoderm are equipped with the complete
determinant-complex of the ectoderm, in the form of an inactive
accessory idioplasm, which is excited to regenerative activity by
the stimulus of wounding, and that, in the same way, the cells of
the endoderm are equipped with the whole determinant-complex of the
endoderm. It need not be decided whether all or only many of the
cells, perhaps the younger ones, are thus adapted for regeneration;
in any case a great many of them must be distributed throughout the
whole body, with perhaps the exception of the tentacles, which are by
themselves unable to reproduce the whole animal. When the animal is
mutilated, the cells of both layers, equipped with their respective
determinant-aggregates, co-operate in reproducing the whole from a part.
It is true that even with these assumptions we only reach the threshold
of a real explanation. For, given that all the determinants of the
species must be present in a fragment, we are not in a position to show
how these set about reconstructing the animal in its integrity, and the
most that we can say is, that it must depend on the specific kind of
stimulus to which each of the cells is exposed through its direct and
more remote environment, which determinants are to be first liberated,
and therefore which parts are to be reconstructed.
That there are at work regulative forces, such as we were already
compelled to assume in regard to the division and regeneration of
unicellular organisms, as to the nature of which we cannot yet make
any definite statement, but which we may call 'polarities,' or, as I
prefer to say, 'affinities,' is shown by countless experiments which
have been made, particularly with the freshwater polyp. Thus Rand cut
off the anterior end of the polyp with its circle of tentacles, and the
excised disk of living substance lengthened in a transverse direction,
so that half the tentacles came to lie to the right, the other half to
the left, while the body developed between these two groups, so that
they became further and further separated from each other, till finally
the original transverse axis of the animal became the longitudinal
axis. One group of tentacles survived and surrounded the new mouth,
while the other at the opposite aboral pole, the new foot, died off.
This total change of structure in the polyp, as to the arrangement of
its main parts, points to unknown forces, which cannot depend on the
determinants as such, but on the vital characters of the living parts,
and on the interactions of these with one another.
[Illustration: FIG. 96. A Planarian cut transversely into nine pieces.
The regeneration of seven of these into entire animals is shown. After
Morgan.]
The same holds true of all the lower Metazoa that have highly developed
regenerative capacity, not only of polyps, but of worms such as the
Planarians. Through the experiments of Loeb, Morgan, Voigt, Bickford,
and others, we know that these animals respond to almost every
mutilation by complete reconstruction, that they may, for instance, as
is indicated in Fig. 96, be cut transversely into nine or ten pieces
with the result that each of these pieces grows again to a whole
animal, unless external influences are unfavourable and prevent it.
Something similar happens if the head be cut off a Tubularia-polyp, it
forms a new head with proboscis and tentacles. It does so, at least,
if the stalk of the polyp be left in the normal position; but if it
be stuck into the sand in the reverse position a head arises at the
end which is uppermost, where the roots arose previously, and the
previous head-end now sends out roots. By suspending a beheaded stalk
horizontally in the water a head can be caused to develop at each end
of the stalk, so that we must assume that every part of the polyp is,
under some circumstances, capable of developing a head, and that it
must be 'circumstances'--in this case gravity, contact with earth or
with water, and the mutual influence of the parts of the animal upon
each other--which decide what is to be produced. Loeb, who was the
first to observe this form of regeneration, called it heteromorphosis,
to express the fact that particular parts of the animal might be
produced at quite different places from those originally intended for
them.
It would certainly be erroneous to range these cases of heteromorphosis
against the determinant theory, but they certainly do not afford any
special evidence of its validity as an interpretation, for all that we
can say here again is that all, or at least many, cells of the animal
must contain the full determinant-complex of the ectoderm, and others
those of the endoderm, and that particular groups of determinants
become active when they are affected by certain external or internal
liberating stimuli. In regard to such animals the theory is hardly more
convincing than the rival theory, that the faculty of regeneration is a
general property of living substance, which does not attain to equally
full expression everywhere, because it is met by ever-increasing
difficulties involved in the increasing complexity of structure. The
validity of the theory only begins to be seen when we deal with cases
where it is demonstrable that every part cannot bring forth every
other, where the power of regeneration is limited, and occurs only in
definite parts in a definite degree, and can only start from particular
parts. Here the assumption of a general and primary regenerative
capacity fails. Any one who insists, as O. Hertwig does, that the
idioplasm in all cells of the body is the same, can always plead that,
in the cases in which regeneration does not occur, the fault lies,
not in the regenerative capacity, but in the absence of the adequate
liberating stimuli, and at first sight it does seem as if this position
were unassailable. We shall find, however, that there are facts which
make Hertwig's interpretation quite untenable.
My own view is that the regenerative capacity is not something
primary, but rather an adaptation to the organism's susceptibility to
injury, that is, a power which occurs in organisms in varying degrees,
proportionate to the degree and frequency of their liability to injury.
Regeneration prevents the injured animal from perishing, or from
living on in a mutilated state, and in this lies an advantage for the
maintenance of the species, which is the greater the more frequently
injuries occur in the species, and the more they menace its life
directly or indirectly. A certain degree of regenerative capacity is
thus indispensable to all multicellular animals, even to the highest
among them. We ourselves, for instance, could not escape the numerous
dangers of infection by bacilli and other micro-organisms if our
protective outer skin did not possess the faculty of regeneration,
at least so far that it can close up a wound and fill up with
cicatrice-tissue a place where a piece of skin has been excised.
Obviously, then, the mechanism which evokes regeneration must have
been preserved in some degree and in some parts at every stage of the
phyletic development, and must have been strengthened or weakened
according to the needs of the relevant organism, being concentrated
in certain parts which were much exposed to injury and withdrawn from
other rarely threatened parts. Thus the great diversity which we can
now observe in the strength and localization of the regenerative
capacity has been brought about. But all this can only be regarded as
adaptation.
I should like to submit a few examples to show that the regenerative
capacity is by no means uniformly distributed, and that, as far as we
can see, it is greater or less in correspondence with the needs of the
animal, both in regard to the whole and to particular parts.
It must first be pointed out that those lower Metazoa, like the
Hydroid polyps in particular, which are endowed with such a high and
general power of regeneration, do actually require this for their
safety; they are not only soft, easily injured and torn, but they
are most severely decimated by many enemies. In the beginning of May
I found on the walls of the harbour at Marseilles whole forests of
polyp-stocks of the genera _Campanularia_, _Gonothyræa_, and _Obelia_,
all large and splendidly developed, with thousands of individual polyps
and medusoids, but in a very short time the great majority of the
polyps were eaten up by little spectre-shrimps (Caprellids) and other
crustaceans, worms, and numerous other enemies, and towards the end of
May it was no longer possible to find a fine well-grown colony. It must
therefore be of decisive importance for these species if the stems and
branches, which are spared because protected by horny tubes, possess
the faculty of transforming their simple soft parts into polyp-heads,
or of giving off buds which become polyps, or even of growing a new
stock from the twigs which have been half-eaten and bitten loose from
the stock and have fallen to the ground. If, finally, a torn-off
polyp-stalk (of _Tubularia_) falls to the ground with the wrong side
up, the end which is now the lower will send out roots, and the end
now uppermost will give off a new head. This also appears to us
adaptive, and does not surprise us, since we have been long accustomed
to recognize that what is adapted to an end will realize this if it be
possible at all. Think again of the innumerable adaptations in colour
and form which we discussed in the earlier lectures. I hope later to
be able to show in more detail how it comes to pass that necessity
gives rise to adaptation. In regard to the case of the polyps, we can
understand that, as far as a high degree of regeneration and budding
was possible in these animals at all, it could not but be developed.
Regeneration and budding complete each other in this case, for the
former brings about in the individual 'person' what the latter does
in the colony, namely, a _Restitutio in integrum_. It is readily
intelligible that the former was not difficult to establish where the
latter--the capacity of budding--was already in existence.
It seems at first sight very striking that the higher plants, which all
depend upon budding, and which form plant-colonies (corms) in the same
sense as the polyps form animal-colonies, only possess the faculty of
true regeneration in a very low degree, although they are extremely
liable to injury.
We see from this that the two capacities are not co-extensive, that
germ-plasm may be contained in numerous cells of the body in a latent
state, and yet that regeneration of each and every detailed defect
may not be possible. This is the case in the higher plants in regard
to most of their parts. A leaf in which a hole has been cut does not
close the hole with new cell-material; a fern frond from which some
of the pinnules have been cut off does not grow new ones, but remains
mutilated. Even leaves which, if laid on damp earth, readily give off
buds which grow to new plants, as the Begonias do, do not replace a
piece cut out of the leaf; they are not at all adapted to regeneration.
From the standpoint of utility this is readily intelligible. It was,
so to speak, not worth Nature's while to make such adaptations in the
case of leaves or blossoms, partly because these are very transient
structures, and partly because they are rapidly and easily replaceable
by the development of others of the same kind. Moreover, the leaf in
which we have cut a hole continues to function, but the polyp whose
mouth and tentacles we have cut off could no longer take nourishment
unless it were adapted for regeneration. But that this adaptation
_could_ have been made in the case of plants is proved by the
root-tips which are formed anew when they are injured, and the closing
of wounds on the stem by a 'callus.'
I shall return to plants when we are dealing with the mechanism
of regeneration, but I must now direct more attention to animals,
inquiring further into the question as to whether the faculty of
regeneration is correlated with the degree of liability to injury to
which the animal is exposed, and with the biological importance of
the injured part, for this must be the case if regeneration be really
regulated by adaptation.
Hardly any other vertebrate has attained such celebrity on account of
its high regenerative capacity as the water-newt, species of the genus
_Triton_. It can regrow not only its tail, but the legs and their parts
if they are cut off. Spallanzani saw the legs grow six times, after
he had cut them off six times. In the blind newt (_Proteus_) of the
Krainer caves, a near relative of the common newt, the leg regenerated
only after a year and a half, although the animal stands on a lower
stage of organization than the newt, and thus should rather replace
lost parts more easily. But Proteus lives sheltered from danger in
dark, still caves, while Triton is exposed to numerous enemies which
bite off pieces from its tail or legs; and the legs are its chief means
of locomotion, without which it would have difficulty in procuring
food. It is different with the elongated eel-like newt of the marshes
of South Carolina, _Siren lacertina_. This animal moves by wriggling
its very muscular trunk, after the manner of an eel, and in consequence
of the disuse of its hind legs it has almost completely lost them. Even
the fore-legs have become small and weak, and possess only two toes,
and these do not regrow if they are bitten off, or only do so very
slowly.
Earthworms are exposed to much persecution; not only birds, such as
blackbirds and some woodpeckers, but, above all, the moles prey upon
them, and Dahl has shown that moles often lay up stores of worms in
winter which they have half crippled by a bite, while even Réaumur
knew that moles frequently only half devoured earthworms. It was
thus an obvious advantage to earthworms that a part of the animal
should be able to regrow a whole, and accordingly we find a fairly
well-developed regenerative capacity among them. But it varies greatly
in the different species, and it would be interesting if we knew the
conditions of life well enough to be able to decide whether the faculty
of regeneration rises and falls in proportion to the dangers to which
the species is exposed. Unfortunately we are far from this as yet; we
only know that, in the common earthworms of the genera _Lumbricus_ and
_Allolobophora_, the faculty of regeneration is still very limited,
for at most two worms, and sometimes only one, can develop from an
animal cut into two pieces. Cutting into a greater number of pieces
does not yield a larger number of worms, but usually only one, and
often none at all.
This corresponds to the behaviour of their enemies, which may often
bite off a piece or tear it away when the worm attempts to escape, but
never cut it up into pieces. The regenerative capacity is more highly
developed in the genus _Allurus_, more highly still in the worms of the
genus _Criodrilus_ which lives in the mud at the bottom of lakes, and
most highly of all in the genus _Lumbriculus_ which lives at the bottom
of small ponds. Long ago Bonnet cut up a specimen of _Lumbriculus_ into
twenty-six pieces, of about two millimetres in length, and he observed
most of these grow to complete worms again. His experiments have often
been repeated in recent times, and have been extended and made more
precise in many ways. Von Bülow was able to get whole animals from
pieces consisting of from four to five somatic segments, and with eight
or nine segments he almost invariably succeeded. A _Lumbriculus_ which
he had cut into fourteen pieces, one of which only measured 3.5 mm. in
length, gave rise to thirteen complete worms with head and tail; only
one piece perished.
These worms have little enemies with sharp jaws which may gnaw at them
behind or before but cannot swallow them whole. Lyonet, famous for his
analytic dissection of the wood-caterpillar (_Cossus ligniperda_),
observed when he was feeding the larvæ of dragon-flies with these
Lumbriculid worms that 'the anterior end of some whose posterior end
had been gnawed away by the larvæ continued to live on the ground.' We
can thus understand why a high power of regeneration is of use to these
worms, and at the same time why it is advantageous to them to contract
so that they break in pieces on very slight irritation, but to this we
shall refer again.
The very diverse potency of the faculty of regeneration in animals
belonging to the same small group, and nearly, if not quite, at the
same level of organization, seems to show clearly that we have here to
do with adaptation to different conditions of life, although we cannot
demonstrate this in detail. It would certainly be erroneous to regard
the conditions of life as uniform, since the worms in question not
only live in different places--in the earth, in mud, or in water--and
are thus exposed to different enemies, and since they may also be
quite different in regard to size and speed, in means of defence, and
possibly also of defiance, as is indeed in some measure demonstrable.
We meet with the same thing in a group of still smaller worms, Rösel's
'water-snakelets,' species of the genus _Nais_. These, too, behave in a
variety of ways in the matter of regeneration, for while many species,
such as _Nais proboscidea_ and _Nais serpentina_ will, if cut into
two or three pieces, become two or three worms respectively, Bonnet
expressly mentions an unnamed species of _Nais_ which does not bear
cutting up at all, and even dies if its head be cut off.
Thus neither the degree of organization nor the relationship alone
determines the strength of the regenerative capacity. And as nearly
related species may behave quite differently in this respect, so also
do the different parts of one and the same animal; and here, too, the
strength of the capacity seems to depend on the more frequent or rarer
injury of the relevant part and on its importance in the maintenance of
life. Let us take a few examples.
Parts which, in the natural life of the animal, are never injured, show
in many cases no power of regeneration. This is so in regard to the
internal parts of the newt, whose regenerative capacity is otherwise
so high. I cut half or nearly the whole of a lung away from newts
anæsthetized with ether; the wound closed, _but no renewal of the organ
took place_. The same thing happened when a piece of the spermatic duct
or of the oviduct was cut away. It is true that the kidney enlarges in
higher animals when a piece has been cut out, by the proliferation of
the remaining tissues, but that is a mere physiological substitution,
evoked by the increased functional stimulus, due to the accumulation in
the blood of the constituents of the urine. Such substitution depends
on the growth of parts already existing, and it occurs in man when one
kidney is removed, for the other, as is well known, may then grow to
double its normal size. This is mere hypertrophy of the part that is
left, it is not regeneration in the morphological sense, and it is not
comparable to the re-formation of a cut-off leg in the salamander, or
of a head in the worm, where the growth is not a mere increase of the
remaining stump, but a new formation. It would be regeneration if a
new kidney developed from the remnants of the kidney-tissue, or, in
the liver, if new lobes grew in place of those which were cut off. But
neither of these things happens, and, as far as I am aware, nothing of
the kind has ever been observed, nothing more than new formation of
liver-cells through increase of existing ones; that, however, is not
regeneration in the morphological sense.
I have referred to the slight power of regeneration possessed by the
blind Proteus in regard to its legs or tail, and I connected this with
the absence of enemies in its thinly peopled cave-habitat. But the same
animal can regenerate its gills when these are bitten off, and this is
probably associated with the habit that Proteus has, in common with
other newts with external gills, of nibbling at its neighbour's gills.
Thus, the power of regenerating the gills was retained even when the
animals migrated to the quiet caves of Krain, and were thus secured
from the attacks of other enemies.
In lizards, a leg which has been cut off does not grow again, but an
amputated tail does, and this has quite a definite biological reason,
since the active little animal will seldom be caught by the foot by any
pursuer, but may easily be caught by the tail, which is far behind.
Thus the tail is adapted not only for regeneration, but also for
'autotomy' that is, for breaking off easily when it is caught hold of.
We have already seen that some segmented worms have a very high
regenerative capacity; yet every part cannot produce every other, and
while, in _Lumbriculus_, any piece of from five to nine segments is
able to grow a new head or tail, neither ten nor twenty nor all the
segments together, if they are _halved longitudinally_, can reproduce
the other half, and the cause of this inability does not lie in the
fact that the animal is thereby hindered from taking food, for even the
transversely cut pieces do not feed until they have grown a new head
and tail. The reason must lie in the fact that the primary constituents
for this kind of regeneration are wanting, and they are so because a
longitudinal splitting of this cylindrical and relatively thin animal
never occurs under natural conditions, and thus could not be provided
against by Nature[1].
[1] Morgan maintains that this statement is incorrect, and that
_Lumbriculus_ is capable of lateral regeneration. But if we look into
the matter more closely we find that all he says is, that small gaps
made by cutting a piece out of one side are filled up again, while the
cut pieces perish. If the whole animal be halved, according to Morgan,
both halves die, or if a 'very long piece' be cut out of one side, not
only this piece dies, but also 'the remaining piece.' There is thus, as
I have said, an essential difference between the regenerative capacity
of _Lumbriculus_ and that of _Planaria_.
That regeneration of this kind could have been arranged for if it had
been useful we learn from the Planarians among the flat worms, in which
every piece cut out of the body, large or very small, from the middle,
from the left side, or from the right side of the animal, grows into
a complete Planarian. The animal can be halved longitudinally, as
in Fig. 97, and each half will grow to a whole. This again is quite
intelligible from the biological point of view, for these flat, soft,
and easily torn animals are exposed to all sorts of injuries, and are,
in point of fact, frequently mutilated by enemies which are unable to
swallow them whole. Von Graaf not infrequently found examples of marine
Planarians (_Macrostomum_) which lacked 'a part of the posterior end
or the whole tail region as far as the food-canal,' and of species of
_Monotus_ he found 'very often' in May specimens with the posterior
end split or broken off. Probably the persecutors of these flat-worms
are some species of Crustacean, but, at any rate, so much is proved,
that the Planarians have abundant opportunities of making use of their
faculty of regeneration, and that the species gains an advantage from
it in respect to its preservation.
[Illustration: FIG. 97. _A_, a Planarian, which has been divided into
two by a longitudinal cut. Each half can grow into an entire animal.
_B_, the left half at the beginning of the regenerative process. _C_,
the same completed. After Morgan.]
In contrast to this, worms which live within other animals, and
are thus secure from mutilation, such as the familiar round-worms
(_Nematoda_), have no power of regeneration at all, and do not survive
either longitudinal or transverse division.
Until recently birds were regarded as possessing a very low degree of
regenerative capacity, and, as a matter of fact, they cannot replace
a leg or a wing wholly or in part; but, what is otherwise unheard of
among higher vertebrates, they can renew the whole anterior portion of
the skeleton of the face, the bill, and can indeed almost reconstruct
it with new bones and horny parts. Von Kennel communicated a case of
this kind in regard to a stork, and for a long time this remained an
isolated case, but a few years ago Bordage showed that, in the cocks
which are used in the Island of Bourbon for the favourite sport of
cock-fighting, the bill is regularly renewed when it has been broken
off or shattered. Quite recently Barfurth gave an account of a case
of complete renewal of a broken bill in a parrot. Yet it should not
astonish us that the bill in birds has such a high regenerative
power, for of all parts in a bird it is the one that is most readily
injured; with it the bird defends itself against its enemies and its
rivals, masters its prey, and tears it to pieces, pecks holes in trees
(woodpecker), or climbs (parrot), or digs and burrows in the ground,
or builds its nest, and so on. That the faculty of regeneration could
be developed to so high a degree in relation to this particular part
of the body, while the rest of the very important but rarely injured
parts do not possess it at all, again points to the conclusion that the
faculty of regeneration has an adaptive character.
It does not affect matters to discover cases in which we cannot
recognize this relation between the regenerative capacity of a part
and its importance or its liability to injury. Such instances do not
lessen the convincingness of the positive cases, since we do not know
the exact conditions which may lead to the increase of regenerative
capacity in a part, and, above all, since we do not know the rate at
which such an increase may take place. If adaptation in general depends
upon processes of selection, these processes must also be able to give
rise to an increase in the power of regeneration. On the other hand, it
by no means follows that the disappearance of a faculty of regeneration
which was once present in a part, but which has become superfluous
in the course of time, must take place immediately through natural
selection. For it is the very essence of natural selection that it only
furthers what is useful, and only removes what is injurious; over what
is indifferent it has no power at all. Thus it follows that the faculty
of regeneration, when it has once been present in a part, cannot be
set aside by natural selection (personal selection), for it is in no
way injurious to its possessor. If it gradually decreases and becomes
extinct notwithstanding this, when it is of no further use, as seems to
be to some extent the case in regard to the legs and tail of the blind
Proteus, that must depend on other processes, on those which generally
bring about the gradual disappearance of disused parts or capacities.
We shall attempt to probe to the roots of these processes later on;
for the present let it suffice us to know that, according to our
experience, they go on with exceeding slowness, and that it has taken
whole geological periods to eliminate the legs of the snake-ancestors
so completely as has been done from the structure of most of our modern
snakes, while the Proteus which migrated into the caves of Krain as far
back as the Cretaceous period is indeed blind, but still retains its
eyes under the skin, though in a degenerate condition.
Since the degeneration of disused parts and capacities goes on so
slowly it need not surprise us that we meet many parts which still
possess regenerative capacity, although they are protected from
injury. Thus Morgan found that, in the hermit-crab, the limbs which
are protected within the mollusc shell were quite as ready to regrow
as those which are actually used for walking, and thus are exposed to
possibility of attack, but this proves nothing against the conclusion
we drew from the facts cited above, according to which the faculty of
regeneration comes under the law of adaptation. For the disappearance
of this faculty must take place very much more _slowly than its
growth_. For instance, the development of the tail-fin of the whale has
long been an accomplished fact, while the hind-legs of this colossal
mammal, which were rendered useless by the development of the tail-fin,
still lie concealed in a rudimentary state within the muscles of the
trunk. Yet these limbs must have lost their significance for the animal
exactly at the time that the tail-fin became more powerful. Thus the
retrogression must have taken place more slowly than the progressive
transformation.
It is clear, then, that the faculty of regeneration is not a primary
character of living beings occurring uniformly in all species of
equally high organization and in all parts of an animal in the same
degree; it is a power which occurs in animals of equal complexity in as
varying degrees as in their parts, and which is manifestly regulated by
adaptation. Between parts with the faculty of regeneration and parts
without it there must be an essential difference; there must be present
in the former something that is wanting in the latter, and, according
to our theory, this is the equipment with regeneration-determinants,
that is, with the determinants of the parts which are to be
reconstructed.
If this be really so it should be capable of proof, at least in so
far that we should be able to establish that the power of completing
or re-forming a damaged or lost part is a limited one, localized in
certain parts and cell-layers. This can be actually proved, as may
be seen from numerous cases in which the faculty of regeneration is
associated with autotomy, that is, with the power of breaking off or
dropping off a part of the body. Even in worms we find this power,
as we mentioned before in speaking of the high regenerative capacity
of _Lumbriculus_. This worm reproduces in summer by what is called
'schizogony,' that is, by breaking into two, three, or more pieces, and
it does not seem to require a very strong stimulus, such as pressure
of the end of the worm by the jaws of an insect larva, to start this
rupture; it often follows from quite insignificant friction on the
ground. Certainly the power of regeneration is so great in this animal
that it is out of the question to talk of localizing the primary
constituents of regeneration; almost every broken surface is capable of
regeneration.
But this localization is well illustrated in Insects and Crustaceans,
which possess the power of self-amputation in their appendages,
especially in their legs. As far back as 1826 MacCullock observed
this remarkable power in crabs, and described the mechanism on which
it depends. When the leg is irritated, for instance when it is pinched
at the tip and held fast, it breaks off at a particular place. This
line of breakage lies in the middle of the short second joint (Fig.
98, _A_ and _B_, _s_), just between the insertions of the muscles
(_me_, _mf_, _m_) which extend from this line towards the extremity of
the limb and in the opposite direction towards the body-wall. Between
these muscle-attachments the external skeleton is thin and brittle, and
forms a suture, _s_, which breaks through when the animal contracts
the muscles of the leg convulsively, and thus presses the lower
protuberance (_a_) against a projection (_b_) of the first upper joint.
Crabs require to make a very considerable muscular exertion before they
can throw off the limb, and therefore they can only do it when they are
in full vigour.
[Illustration: FIG. 98. The leg of a Crab, adapted for self-mutilation
or autotomy. _A_, the first three joints of the limb, _I_, _II_, _III_.
_s_, the suture, that is, a thin area on the second joint which is
predisposed to breakage. _mf_, flexor muscle, _me_, extensor muscle,
both inserted at the suture. _B_, the entire leg with its six joints
and with the suture (_s_). Slightly enlarged. After MacCullock.]