development occur even among the lower Crustaceans, and I was thinking
at the time of the Daphnids, which leave the egg as fully formed little
animals, already equipped with all their segments and limbs. The
nauplius stage is passed through in the egg, and it is an interesting
indication that the ancestors of the modern species were in the way of
moulting, that this embryo nauplius moults within the egg by forming
a fine cuticle which is shed after a time. If it be asked why there
should be direct development in the case of these small and not very
complex water-fleas, while related species, the Branchiopods, which are
much richer in segments and in limbs, should emerge from the egg in
the form of a nauplius, and then pass through a longer larval period,
we may answer that the reason probably lies in the fact that, in the
former case, very few eggs are produced, sometimes only one, often two,
seldom more than a dozen, that these eggs can thus be relatively well
equipped with yolk, and that the formation of the little body which
bears only from seven to nine pairs of limbs can be easily completed
within this egg. Other things being equal, the direct development
would always be an advantage, because reproduction can begin sooner in
the young generation and the number of individuals will thus increase
more rapidly. And this is of particular importance in the case of the
water-fleas.
But if it be asked, further, why so few eggs are produced in this
case, and whether these animals have no enemies, we must answer that,
on the contrary, they are preyed upon and eaten in thousands by
fishes and other freshwater animals, but that the drawback of the
scanty production of eggs is counteracted on the one hand by their
habit of reproducing parthenogenetically for the greater part of the
year, and on the other hand by their habit of concealing the eggs in a
special brood-chamber. This is the case not only in the summer eggs,
to which nourishment is conveyed in the brood-chamber from the blood
of the mother (Fig. 70), but also in the winter or 'lasting' eggs,
which receive within the chamber a protecting covering (the shell or
ephippium).
[Illustration: FIG. 70 (repeated). Daphnella. _A_, summer ovum, with an
oil-globule (_Oe_). _B_, winter ovum.]
In almost all the Daphnids the winter egg develops into a perfect
animal just like that to which the summer egg gives rise, although
it no longer receives any nourishment after it passes into the
brood-chamber. But it receives a larger supply of yolk on this account,
so that the nutritive provision within the egg is sufficient to develop
the perfect animal. There is only one exception to this, and it is
of special theoretical interest, because it shows more plainly than
any other fact that the greater or less degree of condensation in the
ontogeny depends upon the combined effect of the external conditions of
life. The largest of the Daphnidæ, _Leptodora hyalina_, a beautifully
transparent inhabitant of our lakes, which measures about a centimetre
in length (Fig. 110), also emerges from the summer egg as a perfect
animal, but from the winter egg, which floats freely in the water and
has only a small provision of yolk, it emerges as a nauplius, which
then undergoes larval metamorphosis before it becomes a perfect animal
(Fig. 111).
[Illustration: FIG. 110. The largest of the Daphnids (_Leptodora
hyalina_), with summer ova (_Ei_) beneath the shell (_Sch_). _I-IX_,
the appendages. _II_, the oars (second antennæ) which always remain
biramose in Daphnids. _sb_, setæ. _ov_, ovaries. _Schl_, œsophagus.
_Ma_, stomach. _a_, anus. _H_, heart. _Au_, eye. _nG_, natural size.]
Fritz Müller concluded from the repetition of the nauplius form in all
orders of Crustaceans that the primitive form of the Crustacean must
have been a nauplius, and that from it all the modern Crustaceans must
have evolved phyletically by the addition of segments varying in number
and differentiation. Now, however, it is doubted whether there ever
were nauplioid types capable of reproduction. But even if the nauplii
only _represent what have been the larval_ forms from very early times,
they are equally important in illustrating the relations between
ontogeny and phylogeny; they at any rate represent the primitive
pre-cambrian larval form from which all modern Crustaceans are derived.
This is borne out not only by the facts to which we have already
referred, but also by those Crustacean-groups which have diverged far
from the usual Crustacean habit and type.
[Illustration: FIG. 111. Nauplius larva from the winter egg of
_Leptodora hyalina_; after Sars.]
Thus the sessile Cirrhipedes, with their mollusc-like shells, their
soft, unsegmented bodies, degenerate heads, and their twelve vibratile
food-wafting limbs, emerge from the egg as nauplius larvæ. But the
remarkable parasites on the shore-crabs and the hermit-crab deviate
much further from the type of the rest of the Crustaceans, for they
hang like a sac or formless sausage-like soft mass to the abdomen of
their host, growing into it by fine, pale, root-like threads, through
which they suck up the blood of their hosts (Fig. 112, _C. Sacc._).
They possess neither head, nor thorax, nor abdomen, not even an
indication of segmentation, no limbs of any kind, neither antennæ, nor
mouth parts, nor swimming-legs. Nevertheless they are Crustaceans;
indeed, we can say with certainty that they belong to the order of
Cirrhipedes, for they leave the egg in the form of a nauplius larva
(_A_), with 'horns' on their carapace which no other forms except
themselves and the Cirrhipedes possess. That they are of the same
stock as these is also proved by their further development, for the
nauplius grows first, just as in the case of the Cirrhipedes proper,
into a 'Cypris-like larva' (_B_), so called because it bears a certain
resemblance to the Ostracods of the genus Cypris, and only from this
point do their paths of development diverge. The Cypris-like larva of
the true Cirrhipedes settles down somewhere, attached by its antennæ;
it grows, and its body becomes that of the perfect Cirrhipede; but the
Cypris-like larva of the Sacculinæ bores its way into the inside of a
crab or hermit-crab, at the same time losing its limbs, segmentation,
and its chitinous covering; and within the body of its host it is
transformed into the sac-like organism we have already described. After
a time it emerges again on the surface, and remains attached to the
abdomen of its host (Fig. 112, _C. Sacc._), drawing its nourishment
from the blood which it sucks up by means of its numerous delicate
roots (_W_, _W_).
[Illustration: FIG. 112. Development of the parasitic Crustacean
_Sacculina carcini_, after Delage. _A_, Nauplius stage. _Au_, eye.
_I_, _II_, _III_, the three pairs of appendages. _B_, Cypris-stage.
_VI_-_XI_, the swimming appendages. _C_, mature animal (_Sacc_),
attached to its host, the shore-crab (_Carcinus mænus_), with a
feltwork of fine root-processes enveloping the crab's viscera.
_s_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the
brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).]
From all this we may conclude that certain Cirrhipedes in times long
past adopted a parasitic habit in the Cypris-larva stage, and that they
gradually underwent adaptations to this mode of life, and that these
went further and further, until the animal was transformed into the
singular creature which we now see in the sexually mature form.
The same is the case with the numerous fish-parasites of the order
Copepoda. They all leave the egg as nauplius larvæ, however greatly
they may be modified later on by adaptation to a parasitic habit,
and in them we can still observe, in the fully developed animals,
a whole series of grades of transformation. Thus many genera, like
_Ergasilus_, are distinguished from the free-swimming Copepods only
by the modification of their jaws into piercing and sucking organs,
and of a single pair of antennæ into hooks, by means of which they
attach themselves to the fish on which they feed. In other genera the
degeneration and modification go further; the antennæ, the eye, and
the appendages degenerate more or less, and very remarkable attaching
organs are sometimes developed, in the form of hooks or of knobbed
pincers, or of actual suckers. In several types the degeneration and
modification go so far that the segmentation of the body disappears,
and the animal looks more like an intestinal worm than like a
Crustacean (_Lernæocera_ and others). In all these forms adapted to a
parasitic mode of life it is always only the mature animal which has
been transformed in this manner, for previously it has gone through a
series of stages which are quite similar to those of the free-swimming
Copepods, beginning with the nauplius, and ending with the so-called
Cyclops stage, that is, a larval form which possesses antennæ, eyes,
and swimming-legs similar to our freshwater Copepods of the genus
_Cyclops_.
Here again we see in the ontogeny the repetition of a series of
phyletic stages before the mature form is assumed. Why these stages
should have persisted it is easy enough to understand, for how could an
animal which emerged from the egg as a worm-shaped _Lernæocera_ find a
fresh fish which would serve it as host? Yet these parasites could not
possibly go on preying upon the same fish generation after generation.
To secure the existence of the species it was therefore indispensable
that the faculty of swimming should be retained at least in the young
stages; in other words, that the free-swimming ancestral stages should
be preserved in the ontogeny. In all these cases it is therefore beyond
doubt that the germinal history recapitulates a series of stages
comparable to those of the racial history, although not quite unchanged
but adapted to the modern conditions of life, for instance in having
shorter antennæ, smaller eyes, and with four instead of the usual five
swimming-legs. The search for a host does not seem to last long, for
fishes are usually found in large numbers together, and thus the young
parasitic Crustacean does not require to make a long journey before it
finds a refuge.
It is noteworthy that the males of parasitic Crustaceans are not
only much smaller than the females (Fig. 113), but that they are
also much less modified, and resemble the ancestral free-swimming
Copepods to a much greater degree. They usually possess small but
well-developed swimming-legs, and by means of these they seek out the
female, dying after fertilization is accomplished. They are thus not
sessile parasites at all, and have therefore to go through the stages
of the free-swimming Copepods much more completely than the females,
whose task is to accumulate within themselves from the blood of the
fish as much material as possible for the forming of the eggs, and to
produce the largest possible number of these. These therefore greatly
surpass the free-swimming Copepods in fertility, as is evidenced by the
enormous egg-sacs they bear at the posterior end of the body (Fig. 113,
_ei_).
Even among the higher Crustaceans, the so-called Malacostraca, the
germinal history not infrequently exhibits more or less of the racial
history in distinct recapitulation.
[Illustration: FIG. 113. The two sexes of the parasitic Crustacean
_Chondracanthus gibbosus_, enlarged about six times; after Claus.
The main figure is that of the female, whose body bears quaint blunt
processes. At its genital aperture (♂) a dwarf male is situated.
_F_ and _F´_, the two pairs of appendages. _ei_, the long egg-sacs,
portions of which have been cut off in the figure.]
It is true however, as we have already shown, that there are only a
few of the higher Crustaceans which emerge from the egg in the form
of a nauplius; in most of them this stage has been shunted backwards
in the ontogeny, and most of the crabs and hermit-crabs leave the egg
in a higher larval form, that of the so-called Zoæa (Fig. 114). This
term is applied to a larva which already exhibits two main divisions
of the body, a head and thorax portion (cephalothorax, _Cph_) and
an abdomen (_abd_). The cephalothorax is frequently equipped with
remarkable long spines (_st_), and it always bears from five to eight
pairs of limbs, anteriorly the antennæ (_I_ and _II_), then the
mandibles (_III_), further back swimming-legs (_IV_, _V_), and behind
these can be recognized the primordia of the other legs (_VI_-_XIII_),
which will grow freely out later on. Large facetted and stalked eyes
(_Au_) are borne on the head. This Zoæa form is not now found as a
mature Crustacean form, so we cannot maintain with any confidence
that it lived as a mature animal at an earlier period of the earth's
history, but a second still more complex larval form of the higher
Crustaceans is preserved for us in a group of marine Crustaceans, the
Schizopods. These are Crustaceans which, though small, approach in
external appearance our freshwater crayfish, only they have, instead
of the ten walking-legs, biramose swimming-legs, by means of which
they move freely in the water. The number of these branched legs is
even greater than ten, there are sixteen of them (Fig. 109 _D_, p.
164, _VI_-_XIII_). In the aquaria of the Zoological Station at Naples
one may often see these dainty little creatures swimming about in
large companies. Here they are of interest to us chiefly because their
structure occurs in the ontogeny of the highest Crustaceans, the
Decapods; that is, the phyletic stage represented by the Schizopods
appears as an ontogenetic stage, just before the final metamorphosis of
the larva to the perfect animal. This is the case in most of the marine
Decapods, in those forms which do not go through the whole course of
their development within the egg, but emerge as Zoæa larvæ, or even,
as in _Peneus potimirim_, as nauplii. In the last-named species (Fig.
109) the ontogeny contains at least three stages which must have
lived, perhaps not as mature forms, but as primitive larval forms, for
unthinkable ages--the stage of the nauplus (Fig. 109 _A_), that of the
Zoæa (Fig. 109 _B_ and _C_), and that of the Schizopod (Fig. 109, _D_);
from this last the fully developed Decapod Crustacean arises (Fig. 109,
_E_).
We are, therefore, justified in saying that here the racial evolution
is recapitulated in the individual development, although condensed
and shortened in proportion as more numerous stages of the phyletic
development are gone through within the egg, for there the different
stages can succeed each other more rapidly and directly than in a
metamorphosis of the free-swimming larvæ, since these must procure
their own material for their further growth and their metamorphosis,
while the yolk of the egg supplies a store of material which is
sufficient for the production of a whole series of successive stages.
[Illustration: FIG. 114. Zoæa-larva of a Crab, after R. Hertwig.
_I_-_V_, the already functional anterior appendages--antennæ,
mandibles, and swimming-legs. _VI_-_XIII_, rudiments of the posterior
appendages of the cephalothorax (_Cph_). _Abd_, the abdomen. _st_,
spine of the carapace. _Au_, eye. _H_, heart.]
For this reason it inevitably resulted that the sharply defined
characters of the phyletic stages were more and more lost as soon as
they were transferred from larval stages to stages in embryogenesis.
For, in the first place, these sharply defined characters, such as
the spines of the Zoæa larva, or the swimming bristles of the 'oars,'
or the shape of thorax or abdomen characteristic of certain species,
are adapted to a free life, and would be valueless in an embryonic
stage; and secondly, in the transference of the free larval stages
to embryonic development the greatest possible condensation and
abbreviation of the stages must have been striven for, which could only
come about by a continual mutual adaptation of the embryonic parts to
one another, involving the suppression of everything superfluous.
Otherwise the transference of the free stages to the embryogenesis
would have brought no advantage, but rather a most prejudicial
protracting of the development.
We must not, therefore, expect to find the stages of the phylogeny
occurring unaltered in every ontogeny in the way we have found the
nauplius, Zoæa, or Mysis stages in the larval development of the
Decapods. I have noticed already that in the water-fleas (Daphnidæ) and
other Crustaceans without metamorphosis the nauplius stage is still
passed through, but within the egg, and as an embryonic stage, and
this is quite true, but nevertheless it would hardly do to liberate a
nauplius like this from its shell and place it in the water, for the
influence of the water upon the delicate embryonic cells of its body
would soon cause it to swell, and would destroy it utterly. And, even
apart from this, it has no hard and resistant chitinous covering, no
fully-developed appendages, but only the stump-like blunt beginnings
of these without swimming-bristles and without muscles capable of
function, so that it could not even move. Nevertheless it is a nauplius
with all its typical distinctive characters, only it is not a perfect
nauplius capable of life, but rather a 'schema' of one, which must be
retained in the embryogenesis that it may give rise to the later stages.
Shall we therefore say that the statement that phylogeny repeats itself
in ontogeny is false, that the nauplius stage within the embryo is
not a true nauplius at all? That would be pushing precision beyond
reasonable limits, and would obscure our insight into the causal
connexion between phylogeny and ontogeny, which, as we have seen,
undoubtedly exists.
A few years after the appearance of Fritz Müller's work _Für
Darwin_, Haeckel elaborated Müller's idea, and applied it in a much
more comprehensive manner. He formulated it under the name of 'the
fundamental biogenetic law,' and then he used this 'law' to deduce
from the ontogeny of animals, and more particularly of Man, the
paths of evolution along which our modern species have passed in the
course of the earth's history. In doing so the greatest caution was
necessary, since ontogeny is not an actual unaltered recapitulation
of the phylogeny, but an 'abridged' and in most cases--in my own
belief, in all cases--_a greatly modified recapitulation_. Therefore
we cannot simply accept each ontogenetic stage as an ancestral stage,
but must take into consideration all the facts supplied to us by other
departments of biological inquiry which afford help in the decision
of such questions, especially those brought to light by comparative
morphology and by the whole range of comparative embryology.
Haeckel was quite well aware of this difficulty, and repeatedly
emphasized it by laying stress on the fact that a 'blurring' of the
phyletic stages of development had arisen through the abridgement of
the phylogeny in the ontogeny, and a 'falsification' of it through
the secondary adaptation of individual ontogenetic stages to new
conditions of life. He therefore distinguished between 'Palingenesis,'
that is, simple though abridged repetition of the ancestral history,
and 'Cœnogenesis,' that is, modification of the racial history by
later adaptation of a few or many stages to new conditions of life.
As an example of cœnogenetic modification, I may cite the pupæ of
butterflies. Since these can neither feed nor move from one spot, they
can at no time have been mature forms, and cannot, therefore, represent
independent ancestors of our modern Lepidoptera; they have originated
through the constantly increasing difference between the structure of
the caterpillar and that of the moth or butterfly. Originally, that is,
among the oldest flying insects, the mature animal could be gradually
prepared within the larva as it grew, so that finally nothing was
necessary but a single moult to set free the wings, which had in the
meantime been growing underneath the skin, and to allow the perfect
insect to emerge, complete in all its parts. This is the case even now
with the grasshoppers and crickets. In these forms the larval mode of
life differs very little, if at all, from that of the perfect insect,
and the main difference between the two is the absence of wings in the
larva. But when the perfect insect adapted itself to conditions of life
quite different from the larval conditions, as was the case with the
nectar-sucking bees and butterflies adapted entirely for flight, while
the larvæ were still adapted exclusively to an abundant diet of leaves
and other parts of plants, and to a very inactive life upon plants, the
two stages of development ultimately diverged so widely in structure
that the transition from one to the other could no longer be made at
a single moulting, and a period of rest had to be interpolated, in
order that the transformation of the body could take place. In this
way arose the stage of the resting and fasting pupa, a 'cœnogenetic'
modification of the last larval stage, _not a recapitulation of an
ancestral form_, but a stage which has been interpolated, or better,
has 'interpolated itself' into the ontogeny on account of the widely
different adaptations of the early and the final stages.
This is a perfectly clear idea, and Haeckel's distinction between
palingenesis and cœnogenesis is undoubtedly justified.
But it is quite a different matter to be able to decide whether
a particular stage or organ has arisen palingenetically or
cœnogenetically with the same certainty as in the case of the
insect-pupa, or even with any degree of probability, and we must admit
that in very many cases, perhaps even in most cases, it is impossible.
This is so chiefly because pure palingenesis is hardly likely to occur
now; the ancestral stages were bound to be modified in any case if
they were to be compressed into the ever-shortening ontogeny of later
descendants, and particularly so if they were to be shunted back into
embryogenesis. In the latter case they would not only be materially
shortened, and, as I have already shown, modified by the mutual
adaptations of the different developing parts, but time-displacements
of embryonic parts and organs would be necessary, as has been very
clearly proved by the excellent recent investigations, which we owe
in particular to Oppel, Mehnert, and Keibel. A shunting forward or
backward of the individual organs takes place--conditioned apparently
by the decreasing or increasing importance of the organ in the finished
state; for in the course of the phylogeny everything may vary, and
not only may a new, somewhat modified, and often more complex stage
be added on at the end of the ontogeny, but each one of the preceding
stages may vary independently, whenever this is required by a change
in its relations to the other stages or organs. Adaptation is effected
at every stage and for every part by the process of selection, for all
parts of the same rank are ceaselessly struggling with one another,
from the lowest vital units, the biophors, up to the highest, the
persons. If we reflect that, in the course of the phylogeny of every
series of species, a number of organs always become superfluous and
begin to disappear in consequence, we can understand what great changes
must take place gradually as such a series of phyletic stages is
compressed into the ontogeny, for all organs which are no longer used
are gradually shifted further and further back in the ontogeny till
ultimately they disappear from it altogether. But, while the primary
constituents of these 'vestiges' play their part in ontogeny for a
shorter and shorter time, new acquisitions are being more and more
highly developed, and thus, in the course of the phylogeny, numerous
time-displacements of the parts and organs in ontogeny must result, so
that ultimately it is impossible to compare a particular stage in the
embryogenesis of a species with a particular ancestral form. _Only the
stages of individual organs can be thus compared and parallelized._