and conclude that there is no such thing as a 'biogenetic law' or
recapitulation of the phylogeny in the ontogeny. Not only is there
such a recapitulation, but--as F. Müller and Haeckel have already
said--ontogeny is nothing but a recapitulation of the phylogeny,
only with innumerable subtractions and interpolations, additions
and displacements of the organ-stages both in time and place. It
would be a great mistake to conclude from the fact of these manifold
alterations that the whole proposition of the recapitulation of the
phylogeny in the ontogeny is erroneous, or at least valueless. If its
only use were to enable us to read the racial history of a species out
of its germinal history, it is intelligible enough that we might be
led to give it up in despair, but I think that the main thing is to
get some insight into the history of the ontogeny, and there can be
no doubt that this can have been built up on no other foundation than
upon the racial history. What is new could only have arisen from what
was already in existence, and everything in ontogeny, not only the
palingenetic stages which still represent in some measure the facies of
fully-formed ancestral stages, but also the cœnogenetic stages, like
the pupa-stage we have already discussed, have arisen historically,
nothing _de novo_, but all in connexion with what was already present.
But what was first present was in all cases the stages of the ancestral
forms.
It is undoubtedly of the greatest value to be able to penetrate more
and more deeply into embryonic development, and to discover more
precisely the changes that have taken place throughout its course in
the originally existing material of ancestral forms. But it must not
be forgotten that, all transformations notwithstanding, so much of
the racial history is still very plainly indicated in the germinal
history, that this must always remain for us a most important source
from which to draw conclusions in regard to the phyletic development of
any animal group. I admit that these conclusions have sometimes been
drawn with too great confidence, but even if we cannot regard as well
founded Haeckel's view that in the ontogeny of Man there are fourteen
different ancestral stages recognizable, a protist stage, a gastræa
stage, a prochordate, an acranial, a cyclostome, a fish-stage, and so
on, we must recognize that the unicellular stage of ontogeny, with
which even now the development of every human being begins, undoubtedly
repeats the facies of an ancestor, although greatly altered; for we
must be descended from unicellular organisms. The essential part of
this ancestral stage is thus preserved in the ontogeny, and only what
is special and in some measure due to chance, that is, to adaptation to
special conditions of existence, has been modified.
It has been supposed that the proposition that phylogeny is
recapitulated in the ontogeny is disproved, because the ontogenetic
stage must always contain within it the primordia of the later stages
which have been added since the corresponding phylogenetic stage. It
is certain that the egg-cell or the sperm-cell of Man contains, though
in a form not recognizable by us, all the determinants of the perfect
human body, but this neither affects its nature as a cell nor its
particular form as ovum or spermatozoon. It is essentials that are
important in this comparison, not accessories. Neither can I agree
with Hensen's argument when he says that the 'recapitulation-idea' is
erroneous, because the actual course of ontogeny is the 'best and only
possible one,' which, apart from previous history altogether, must of
necessity be followed. Certainly the actual course is the best, and
under the given circumstances the only possible one, but that does not
exclude recapitulation, on the contrary it implies it, for ontogeny
could at no time have arisen from a _tabula rasa_, but only from what
was historically existent.
I do not propose to examine each of Haeckel's ancestral stages in
Man's pedigree, or to estimate the degree of probability with which
they may be deduced from the ontogeny; but that Man's ancestry does,
in a general way, include such a series of phyletic stages may be
admitted, even if we grant that many of these stages are now no longer
represented in the ontogeny as stages of the developing organism as
a whole, but only by stages of individual organs or group of organs.
Thus it may be disputed whether there is still a fish-stage in
human development, but it cannot be disputed that the rudiments of
'gill-arches' and 'gill-clefts,' which are peculiar to one stage of
human ontogeny, give us every ground for concluding that we possessed
fish-like ancestors.
As we now know that the history of a given mode of embryogenesis
has involved numerous time-displacements of the organ-rudiments, we
must attach all the more weight to the developmental history of the
individual parts and characters, in which the phylogeny can often be
read more clearly than in the stages of the organism as a whole, and we
can probably find out important laws in this way.
As far back as 1873 Würtemberger investigated the fossil ammonites with
special reference to this point. He was concerned even more at that
time with finding proofs of the theory of descent in general, and this
was the first case in which any one succeeded in demonstrating phyletic
transformation-series of species, deposited one above the other in a
corresponding series of geological strata, and connected by transition
forms lying between these. In studying this interesting material, of
which many examples were at his disposal, Würtemberger proved that the
variations which had taken place in the spirally coiled shell in the
course of ages appeared first on the last whorl, and then subsequently
extended to the one before this, and thence to the still younger whorls
of the shell. Meanwhile the last whorl not infrequently exhibited
another new character. Thus, for instance, protuberances on the shell
were shifted in the course of the phylogeny from the last convolution
to the second last, and later to the third last, and so on, while at
the same time the last convolution showed the protuberance changed into
spines. In other words, the new phyletic acquirements first appeared in
the mature animal (in the last-formed whorl or chamber of the shell),
but were subsequently shifted back in the ontogeny to younger stages in
proportion as new transformations of the mature animal appeared. Thus
there was, so to speak, a retraction of the phyletic acquisitions of
the mature animal deeper and deeper into the germinal history of the
species.
About the same time--in the seventies--I obtained similar results
from living species when I was attempting to work out the ontogeny
of the markings on the external skin of the caterpillars of certain
butterflies, and I should like to submit a short account of these.
In one of the early lectures we discussed the protective and defensive
colours of caterpillars in general, and those of caterpillars of the
Sphingidæ in particular. I showed that those naked caterpillars which
live on plants among the grass, or on the grass itself, are often
not only green, like fresh grass-stalks, or yellowish-grey, like dry
ones, but all the larger forms also exhibit light, usually white,
longitudinal lines, which, by mimicking the sharp light reflections on
the grass-stems, heighten the protective resemblance.
We also spoke of the light transverse stripes, often marked with pink
or lilac-blue, of many of the large green caterpillars which live
on trees and bushes, and whose likeness to the leaves is heightened
by this imitation of the lateral veining of a leaf; and finally we
mentioned the warning coloration indicative of unpleasant or nauseous
taste, among which must be classed not only vivid contrasts of colour,
but also specially conspicuous elements of colour such as light
ring-spots upon a dark ground. These different colour schemes which
protect the caterpillars from their enemies are usually only to be
found in the adolescent caterpillar, not in the very small one which
has just emerged from the egg, and the development of the markings in
the individual life clearly shows that the phylogeny of the markings is
more or less obviously contained in the ontogeny.
There are three different schemes of marking which occur in the
caterpillars of hawk-moths or Sphingidæ--longitudinal striping,
obliquely transverse striping, and spots. Longitudinal striping pure
and unmixed is now found only in a few species, for instance in the
caterpillar of the _Macroglossa stellatarum_ (Fig. 115), in which a
white longitudinal line, beginning at the tip of the tail, runs up each
side of the body to the head as a 'sub-dorsal stripe' (_sbd_). These,
with other two similar stripes, effectively secure the fairly large
caterpillar from discovery when it is among grass and herbs.
[Illustration: FIG. 115. Caterpillar of the Humming bird Hawk-moth,
_Macroglossa stellatarum_. _sbd_, the sub-dorsal line.]
Transverse striping occurs as the sole mode of marking in species
which live on bushes and trees whose leaves have strong lateral veins,
such as willows, poplars, oaks, privet, syringa, and so on, and these
markings associated with the leaf-green of their colouring protect them
most effectively from discovery.
The third scheme of marking, namely by spots, occurs in various forms
in species of the genera _Deilephila_ and _Chærocampa_, and it varies
in its biological significance; in many species the spots serve as a
warning colour, by making the caterpillar conspicuous and easily seen
from a distance (_Deilephila galii_, Fig. 117); in others they imitate
the eyes of a larger animal, and have a 'terrifying' effect, as we have
already said (Fig. 4); in still other and rarer cases they heighten the
resemblance of the caterpillar to its food-plant by mimicking parts of
it, as, for instance, the red berries of the buckthorn (_Deilephila
hippophaës_, Fig. 8, _r_).
[Illustration: FIG. 3 (repeated). Full-grown caterpillar of the Eyed
Hawk-moth, _Smerinthus ocellatus_. _sb_, the sub-dorsal stripe.]
Thus all three modes of marking possess a biological value, and
protect the soft and easily wounded animal in some way, and, in the
case of at least two of them, it is clear that they must have arisen
at the very end of the caterpillar's development, since they can only
be effective as the animal is approaching full size, and would be
valueless in the very young caterpillar. The transverse striping only
makes the caterpillar like a leaf when the stripes bear about the same
relation to each other as those on the leaf, and eye-spots can only
scare away lizards and birds when they are of a certain size. Only
longitudinal striping is effective as a protection in the case of young
caterpillars, supposing, that is, that they live in or on the grass
(Fig. 116).
[Illustration: FIG. 4 (repeated). Full-grown caterpillar of the
Elephant Hawk-moth, _Chærocampa elpenor_, in its 'terrifying attitude.']
[Illustration: FIG. 8 (repeated). Caterpillars of the Buckthorn
Hawk-moth, _Deilephila hippophaës_. _A_, Stage III. _B_, Stage V. _r_,
annular spots.]
Let us consider the ontogeny of these different forms of markings,
beginning with the eye-spots. It appears that these develop from a
sub-dorsal stripe, which appears in the young caterpillar in the
second stage of its life, and from it, in the course of the further
development, two pairs of large eye-spots are formed. Even in the
young caterpillar, scarcely one centimetre in length (Fig. 116), it
can be observed that the fine, white sub-dorsal line takes a slight
curve upwards on the fourth and fifth segments (_C_), and on the
lower edge of these curves a black line is laid down (_D_). This is
then continued to the upper side (_E_), and encloses the piece of the
sub-dorsal stripe (_F_ and _G_), and thus there arises a white-centred,
black-framed spot which only requires to grow and to differentiate a
blackish shadow-centre, the pupil (_G_), to give the impression of a
large eye. This occurs as the caterpillar goes on growing, and after
the fourth moult or ecdysis the eyes have already some effect, as the
animal is six centimetres in length, but they become even more perfect
in the fifth and last stage. During this development of the eye-spot
the sub-dorsal stripe disappears completely from the greater part of
the caterpillar, persisting only on the first three segments (Fig. 116,
_B-F_).
[Illustration: FIG. 116. Development of the eye-spots in the
caterpillar of the Elephant Hawk-moth (_Chærocampa elpenor_). _A_,
Stage I, still without marking, simply green. _B_, Stage II, with
sub-dorsal stripe (_sbd_). _C_, sub-dorsal line somewhat later, with
the first hint of the eye-spot (_Au_) on segments 4 and 5. _D_,
eye-spots in Stage III of the caterpillar, somewhat further developed
than in _E_, the third stage. _F_, Stage IV. _G_, the anterior eye-spot
at the same stage.]
When we consider that this stripe in the little caterpillar a
centimetre long, which lives on the large leaves of the vine, or on
the obliquely ribbed willow-herb (_Epilobium hirsutum_), is quite
without protective value, its occurrence at that stage can only be
regarded as a phyletic reminiscence due to the fact that the ancestors
of these species of _Chærocampa_ possessed longitudinal stripes in the
adult state, probably because at that time they lived on plants among
the grass, and that, later, when the species changed their habitat to
plants with broad leaves which had arisen in the meantime, eye-spots
were developed in addition to the green or brown protective colouring
which they retained. Thus the modern development of these spots mirrors
their phyletic evolution very faithfully; on the two segments there
were formed, from pieces of the sub-dorsal line, first white spots
ringed round with black, then unmistakeable eyes with pupils (_C_, _D_,
_G_). This transformation can only have begun in the fairly well-grown
caterpillar, because it was only of any use to it; but later on it was
shunted further back in the ontogeny, from the sixth and fifth to the
fourth and third caterpillar stage, not in its complete development,
but in more and more incipient form; and nowadays the first traces of
eyes, as we have already seen, are visible in the course of the second
stage. The marking of the more remote ancestors, the longitudinal
striping, is now lost in proportion as the eye-spots develop, perhaps
because the former would take away from the full effect of the latter.
The longitudinal stripes are still quite plainly visible on the first
three segments, but these segments are drawn in and are scarcely
noticeable when the caterpillar assumes a defiant attitude (Fig. 4).
In the case of marking with ring-spots, which is found especially in
species of the genus _Deilephila_, the ontogeny discloses that it has
developed phyletically from the sub-dorsal stripe; in the young stage
of this caterpillar also, the sole marking is longitudinal striping;
in _Deilephila zygophylli_, from the steppes of Southern Russia,
this persists apparently through all the stages, but in the others
it disappears almost completely in the later stages, but only on the
segments on which the spot-marking has developed from it. This happens
in a manner similar to that in which the eye-spot in _Chærocampa_
arises, a piece of the white sub-dorsal stripe is enclosed above and
below by a semicircle of black, and later these semicircles unite, and
cut off the portion of the sub-dorsal line, and form a black spot with
a light centre within which a red spot frequently appears (Fig. 117,
_A_).
[Illustration: FIG. 117. Caterpillar of the Bed-straw Hawk-moth
(_Deilephila galii_). _A_, Stage IV, sub-dorsal stripe still distinct,
the annular spots are still incompletely enclosed in it. _B_,
fully-formed caterpillar without trace of a sub-dorsal stripe, but with
ten annular spots.]
In most species these ring-spots occur on many segments (10-12) (Fig.
117, _B_), and in cases where they are of importance in making the
caterpillar conspicuous and easily seen they sometimes form a double
row. But we know one species, _Deilephila hippophaës_, in which only
a single ring-spot exists, and it is a large brick-red spot on the
second last segment, mimicking the red berry of the buckthorn (Fig.
8, _A_ and _B_, _r_). But individuals also occur in which there are,
on the five or six segments in front, smaller ring-spots which become
less distinct the further forward they are, and in most caterpillars
it is possible, on careful examination, to recognize little red dots
on the faded sub-dorsal stripes of these segments (Fig. 8, _B_). We
might be disposed to think, on this account, that the ancestors of
_D. hippophaës_ bore rings on all the segments, and that these had
gradually become vestigial on the majority of them, because they had
lost their earlier biological importance, and now, by adaptation to the
buckthorn, could only be of use on the second last. But when we take
the ontogeny also into account we find in the young caterpillar only a
simple sub-dorsal line, upon which, in the third stage, the red spot of
the tail-horn segment appears (Fig. 8, _A_).
No spots ever occur on the other segments at this stage; they only
appear in the last stage, but as they may be entirely wanting, they
must have arisen as the result of internal laws of correlation, that
is, they must be recapitulations of the hindmost spots which arose
in the phylogeny through natural selection. We may conclude this, at
least, if we believe in the truth of the fundamental proposition of the
biogenetic law, and admit that there is in the ontogeny some more or
less distinct recapitulation of the phylogeny.
[Illustration: FIG. 118. Two stages in the life-history of the Spurge
Hawk-moth (_Deilephila euphorbiæ_). _A_, first stage, the caterpillar
dark blackish-green, without marking. _B_, second stage, the row of
spots is distinctly connected by a light streak, the vestige of the
sub-dorsal stripe.]
This proposition may be recognized as true in the case of _Deilephila_
also, if we compare the different species with one another as regards
their ontogeny. We find here too that not only the sub-dorsal, that is,
the phyletically oldest marking of the Sphingid caterpillars, occurs
everywhere in the young stages, but also that it is being shunted back
to younger and younger stages, in proportion to the degree of the
development of the spot-marking reached in the full-grown caterpillar.
Thus, for instance, in the caterpillar of _Deilephila euphorbiæ_ the
highest form of spot-marking is reached, and in this species the
sub-dorsal line is no longer the sole marking element at any stage.
Leaving out of the question the absolutely unmarked little caterpillar
which emerges from the egg (Fig. 118, _A_), there appears at once in
the second stage a series of ring-spots connected by a fine white
sub-dorsal line (Fig. 118, _B_). In the following stage, the third,
this sub-dorsal line disappears without leaving a trace, and there
remains only the spot-marking, which is subsequently duplicated.
Let us compare with this the ontogeny of the bed-straw hawk-moth,
_Deilephila galii_ (Fig. 117). The full-grown caterpillar possesses
only a single row of ring-spots (_B_), and accordingly the young stages
of the caterpillar up to the fourth show a distinct sub-dorsal line
(_A_), although spots are seen upon it. A still earlier phyletic stage
of development is illustrated by _Deilephila livornica_, in which the
ring-spots are all connected by the sub-dorsal line.
It can thus hardly be doubted that the biogenetic law is guiding us
aright when we conclude from a comparison of the ontogeny of the
different species of _Deilephila_, that the oldest ancestors of the
genus possessed only the longitudinal stripes, and that from these
small pieces were cut off as ring-spots, and that these were gradually
perfected and ultimately duplicated, while at the same time the
original marking, the longitudinal stripe, was shunted back further and
further in the young stages, until it finally disappeared altogether.
Let us now refer for a moment to the third form of marking in the
caterpillars of the Sphingidæ--transverse striping. This has not arisen
out of the sub-dorsal line, but quite independently and at a later
date. This is proved with great certainty by the ontogeny of species
of the genus _Smerinthus_. The full-grown, and usually also the young
caterpillars, of these species have quite regularly the seven broad
oblique stripes which run in the direction of the tail-horn at equal
intervals on the lateral surfaces of the body (Fig. 3). They are
absent only from the three anterior segments, and upon these a part
of the older marking, the sub-dorsal stripe, has persisted. But we
find this fully developed in the youngest stages of other species. In
_Smerinthus populi_, the little caterpillar, which has no markings at
all when it leaves the egg, very soon shows the white sub-dorsal line,
and simultaneously with it the seven transverse stripes, which cut
obliquely through it; in the older caterpillars the sub-dorsal then
disappears (Fig. 119).
When I was investigating these matters at the beginning of the
seventies I did not succeed in procuring eggs of the species of the
genus _Sphinx_, which likewise almost all exhibit the oblique striping
in their full-grown stages. But from what I knew of the ontogeny of
_Smerinthus_ species I was able to predict that, among the young
stages of _Sphinx_, there must be some with sub-dorsal lines. This
was confirmed later, for Poulton found in _Sphinx convolvuli_ that in
the first stage there are no oblique stripes, but only the sub-dorsal
stripe, while in _Sphinx ligustri_ both kinds of marking were present
at the same time.
[Illustration: FIG. 119. Caterpillar of _Smerinthus populi_, the Poplar
Hawk-moth, at the end of the first stage, showing both the complete
sub-dorsal stripe and the oblique stripes.]
From all these facts, which I have summarized as briefly as possible,
we see that the older phyletic characters are gradually crowded by the
newer into ever-younger stages in the ontogeny, until ultimately they
disappear altogether. We have now to ask to what this phenomenon is
due; is it a simple crowding out of the old and less advantageous by
the new and better characters as a result of natural selection, or is
there some other factor at work? It is clear in regard to these forms
of marking that they can have been developed at first only in the
almost full-grown larva by natural selection, because they are of use
only there, and that, at the same time, the old marking must have been
set aside through the influence of the same factor, in as far as it
prejudiced the effect of the new adaptation. This seems to be indicated
by the persistence of the sub-dorsal line on those segments which are
drawn in when _Chærocampa_ assumes a terrifying attitude, or which do
not bear oblique stripes in the leaf-like caterpillars, e.g. the three
anterior segments in the species of _Sphinx_ and _Smerinthus_. When
newly acquired schemes of marking like the eye-spots of _Chærocampa_
are transmitted from the last stage to the stage before, this can be
explained by following the same train of thought, for the caterpillar
is already of sufficient size to be able to inspire terror with its
eyes; but in still younger stages the spots would not be likely to
have that effect, and yet they occur in quite small animals (20 mm.).
More obvious still is the uselessness of the oblique striping in the
young stages of the _Sphinx_ and _Smerinthus_ caterpillars, for in the
earliest stages of life the caterpillars are much too small to look
like a leaf, and the oblique stripes stand much closer together than
the lateral ribs of any leaf. Moreover, the little green caterpillars
require no further protection when they sit on the under side of a
leaf; they might then very easily be mistaken _in toto_ for a leaf-rib.
_Thus it is certainly not natural selection which effects the shunting
back of the new characters._ Nor can this be caused by the fact that
the new character can only be developed gradually and in several
stages, for the oblique striping at any rate arises in the ontogeny all
at once. There must therefore be some mechanical factor in development
to which is due the fact that characters acquired in the later stages
are gradually transferred to the younger stages. But this shifting
backwards can be checked by the agency of natural selection as soon as
it becomes disadvantageous for the stage concerned.