PROOFS FROM EMBRYOLOGY, OR COMPARISON IN THE ONTOGENIC SERIES.
It is a curious and most significant fact that the successive stages of
the development of the _individual_ in the higher forms of any group
(ontogenic series) resemble the stages of increasing complexity of
differentiated structure in ascending the animal scale in that group
(taxonomic series), and especially the forms and structure of animals
of that group in successive geological epochs (phylogenic series). In
other words, the individual higher animal in embryonic development
passes through temporary stages, which are similar in many respects to
permanent or mature conditions in some of the lower forms in the same
group. To give one example for the sake of clearness: The frog, in its
early stages of embryonic development, is essentially a fish, and if
it stopped at this stage would be so called and classed. But it does
not stop; for this is a temporary stage, not a permanent condition.
It passes through the fish stage and through several other temporary
stages, which we shall explain hereafter, and onward to the highest
condition attained by amphibians. Now, if we could trace perfectly
the successive forms of amphibians, back through the geological epochs
to their origin in the Carboniferous, the resemblance of this series
to the stages of the development of a frog would doubtless be still
closer. Surely this fact, if it be a fact, is wholly inexplicable
except by the theory of derivation or evolution. The embryo of a
higher animal of any group passes _now_ through stages represented by
lower forms, because in its evolution (phylogeny) its ancestors _did
actually have these forms_. From this point of view the ontogenic
series (individual history) is a brief recapitulation, as it were, from
memory, of the main points of the philogenic series, or family history.
We say brief recapitulation of the _main_ points, because many minor
points are dropped out. Even some main points of the earliest stages of
the family history may be dropped out of this sort of inherited memory.
This resemblance between the three series must not, however, be
exaggerated. Not only are many steps of phylogeny, especially in
its early stages, dropped out in the ontogeny, but, of course, many
adaptive modifications for the peculiar conditions of embryonic life
are added. But it is remarkable how even these--for example the
umbilical cord and placenta of the mammalian embryo--are often only
modifications of egg-organs of lower animals, and not wholly new
additions. It is the similarity in spite of adaptive modifications that
shows the family history.
We will now illustrate by a few striking examples.
We can not do better than to take, again, as our first example, the
development of tailless amphibians, and dwell a little more upon it:
=1. Ontogeny of Tailless Amphibians.=--It is well known that the
embryo or larva of a frog or toad, when first hatched, is a legless,
tail-swimming, water-breathing, gill-breathing animal. It is
essentially a fish, and would be so classed if it remained in this
condition. The fish retains permanently this form, but the frog passes
on. Next, it forms first one pair and then another pair of legs;
and meanwhile it begins to breathe also by lungs. At this stage it
breathes equally by lungs and by gills; i. e., both air and water.
Now, the lower forms of amphibians, such as siredon, menobranchus,
siren, etc., retain permanently this form, and are therefore called
_perennibranchs_, but the frog still passes on. Then the gills
gradually dry up as the lungs develop, and they now breathe wholly by
lungs, but still retain the tail. Now this is the permanent, mature
condition of many amphibians, such as the triton, the salamander, etc.,
which are therefore called _caducibranchs_, but the frog still passes
on. Finally, it loses the tail, or rather its tail is absorbed and its
material used in further development, and it becomes a perfect frog,
the highest order (_anoura_) of this class.
Thus, then, in ontogeny the fish goes no further than the fish stages.
The perennibranch passes through the fish stage to the perennibranch
amphibian. The caducibranch takes first the fish-form, then the
perennibranch-form, and finally the caducibranch-form, but goes no
further. Last, the anoura takes first the fish-form, then that of the
perennibranch, then that of the caducibranch, and finally becomes
anoura. This is shown in the diagram, which must be read upward, line
by line.
[Illustration:
FISH. PERENNIBRANCH. CADUCIBRANCH. ANOURA.
^
FISH. PERENNIBRANCH. CADUCIBRANCH.--------^
^
FISH. PERENNIBRANCH-----------^
^
FISH.------------^
Diagram showing the stages of development of amphibians. (To be read
upward.)]
Now, this is undoubtedly the order of succession of forms in geological
times--i. e., in the phylogenic series. This series is indicated by
the arrows in the diagram. Fishes first appeared in the Devonian
and Upper Silurian in very reptilian or rather amphibian forms.
Then in the Carboniferous, fishes still continuing, there appeared
the lowest--i. e., most fish-like--forms of amphibians. _These were
undoubtedly perennibranchs._ In the Permian and Triassic higher forms
appeared, which were certainly caducibranch. Finally, only in the
Tertiary, so far as we yet know, do the highest form (anoura) appear.
The general similarity of the three series is complete. If we read the
diagram horizontally, we have the ontogenic series; if diagonally with
the arrows, we have both the taxonomic and the phylogenic series.
=2. Aortic Arches.=--But some will, perhaps, say that these stages
in the ontogeny are only examples of adaptive modifications--like
modifications for like conditions of life--and had better be accounted
for in this way, without reference to family history. We will,
therefore, take another example, which can not be thus accounted
for--an example in which there is no possible use _now_ for the
peculiar form or structure which we find. For this purpose we take the
case of the _course of circulation in vertebrates_.
[Illustration: FIG. 36.--Showing heart and outgoing blood-vessels of a
lizard (after Owen). The arrows show the course of the blood.]
[Illustration: FIG. 37.--A, heart and gill-arches of a fish; B, one
arch, with fringe (after Owen); H, the heart.]
If one examines the large vessels _going out_ from the heart of a
lizard, he will find _six aortic arches_--i. e., three on each side.
These all unite below to form the one descending abdominal aorta. This
is shown in the accompanying figure (Fig. 36), in which _a a′ a″_ and
_b b′ b″_ are the six arches. Now, there is no conceivable use in
having so many aortic arches. We know this, because there is but one in
birds and mammals, and the circulation is as effective, nay, much more
effective in these than in reptiles. The explanation of this anomaly is
revealed at once as soon as we examine the circulation of a _fish_,
which is shown in the accompanying figure (Fig. 37). The multiplication
of the aortic arches is here, of course, necessary, for they are the
_gill-arches_. The whole of the blood passes through these arches, to
be aërated in the gill-fringes. The use of this peculiar structure is
here obvious enough. If a lizard were ever a fish, and afterward turned
into a lizard, changing its gill-respiration for lung-respiration,
then, of course, the useless gill-arches would remain to tell the
story. Now, although a lizard never was a fish, in its _individual_
history or ontogeny, it was a fish in its family history or phylogeny,
and therefore it yet retains, by heredity, this curious and _useless_
structure as evidence of its ancestry.
[Illustration: FIG. 38.]
[Illustration: FIG. 39.
FIGS. 38, 39.--Diagrams showing the change of the course of blood
in the development of a frog. 38. The tadpole stage. 39. The
mature condition. H, heart; G G′ G″, external gills; _g g′ g″_,
internal gills; _c c_, connecting branches in the tadpole; _p p_,
pulmonary branches.
]
That this is the true explanation is demonstrated by the fact that in
amphibians this very change actually takes place before our eyes in
the _individual history_. We have already seen that the individual
frog, in its tadpole state, is a gill-breather. It has therefore its
gill-arches (Fig. 38), three on each side, like a fish, and for the
same reason, viz., the aëration of the blood. But when its gills dry
up and lung-respiration is established, its now useless gill-arches
still remain as aortic arches, to attest their previous condition (Fig.
39). Now, the lizard undoubtedly came from an air-breathing, tailed
amphibian, and therefore inherited this form of arterial distribution.
In both lizard and amphibian the ultimate cause is an origin from
fishes, in which such arches are obviously necessary. The diagrams,
Figs. 38 and 39, are illustrations somewhat idealized, showing the
manner in which the change actually takes place in air-breathing
amphibians. Fig. 38 represents the tadpole stage, and Fig. 39 the
mature condition. In the former the gills are mostly external, G G′,
etc., but also internal, _g g′_, as in the fish. Observe in this
condition the small connecting vessels, _c c′_. When the external gills
dry up, these are enlarged, and the whole of the blood passes through
them, as shown in Fig. 39. It is seen, also, in Fig. 38, that a small
branch, _p_, goes from the lower gill-arches to the yet rudimentary
lung, _l_. When the gill-fringes have disappeared, the whole of the
blood of the lower arch goes through the now enlarged pulmonary branch
to the lungs, L, now in full activity, and the remainder of this arch
disappears, as shown by the dotted lines in Fig. 39.
The change which actually took place in the family history of the
lizard probably differed from the above only in being more simple, the
gills being only internal like the fish. The external gills complicate
the process a little in the case of the frog, but the principle is
precisely the same.
As already explained (pages 82-85), the large gap between fishes and
reptiles, as regards mode of respiration, is completely filled both
in the taxonomic series--i. e., in ganoids, dipnoi, and the mature
condition of the different orders of amphibians--and in the ontogeny
of the higher amphibians. Now, we add that the same is true of the
arterial distribution. We have just traced the change in the ontogeny
of the frog, but the steps of the same change are traceable in passing
from the typical fish (teleosts), through dipnoi and amphibians to
reptiles. Thus, again, the phylogeny, the taxonomy, and the ontogeny,
are in complete accord.
But the argument for evolution does not stop here. If birds and
mammals have come from reptiles, and therefore from fishes, we may
expect to find some evidences of the same kind still lingering in
the great arteries. And such we do find. It is a most curious and
significant fact that, in the early embryonic condition of birds and
mammals, including man himself, we find on each side of the neck
several gill-slits, each with its gill-arch, and therefore _several
aortic arches on each side_, precisely similar to what we have already
described. These arches are subsequently, some of them, obliterated;
some modified to form the one aortic arch, and some of them still more
modified to form the other great arteries coming from the heart to
supply the head and forelimbs.
[Illustration:
FIG. 40.--Diagram of mammalian heart. _a_, aorta; _p_, pulmonary
artery; _scs′c′_, subclavium on each side; _cc′_, carotids on
each side.
]
[Illustration: FIG. 41.--Ideal diagram representing the primitive
aortic arches (after Rathke).]
This is so beautiful and convincing an example, and one so generally
unfamiliar, to even intelligent persons, not especially acquainted with
biology, that it is best to explain it more fully. In Fig. 40 we give
a mammalian heart and outgoing vessels, very slightly modified, so as
to suggest the process of change. In Fig. 41 we give an ideal diagram
representing the primitive aortic arches as they exist in the embryo of
mammals, birds, and reptiles. It represents, also, substantially, the
arches as they exist in the _mature_ condition in the most reptilian
fishes (dipnoi) and in some sharks, except that in these the arches
are of course furnished with gill-fringes. We will use this figure,
therefore, to represent both the embryonic condition of air-breathing
vertebrates and the mature condition of some fishes. The place of
the heart is indicated by the dotted circle. Fig. 36, on page 134,
shows what these arches become in reptiles (lizard). It is seen that
the two upper arches on each side are obliterated, as indeed they
already are in some teleost fishes. Fig. 42 shows what they become in
birds. The two upper arches are, of course, obliterated. The others
are all modified, each in a manner which may be readily understood
by comparison with Fig. 41. Finally, Fig. 43 shows what they become
in mammals and in man. In the bird (Fig. 42) the first pair of arches
become the two pulmonary arteries as they do also in the lizard. The
second pair become on the right side (left of the diagram) the aortic
arch, on the left side (right of the diagram) the left subclavian,
_s′c′_ (the right subclavian, _sc_, is a branch of the aortic arch).
The third pair become carotids, _cc_, while the fourth and fifth, as
already said, are aborted. In the mammal (Fig. 43), on the left side
(right of the diagram) the first arch becomes the pulmonary artery,
_p_. In the fœtus the continuation of this arch forms the ductus
arteriosus, which is afterward obliterated, as shown in the dotted
line. The second arch becomes the aortic arch, the third the left
exterior carotid. On the right side (left of the diagram) the first
arch becomes aborted; the second, the right subclavian, _sc_ (the
_left_ subclavian, _s′c′_, is a branch of the aortic arch); and the
third, the right carotid. Nos. 4 and 5, on both sides, as usual, are
aborted.
[Illustration: FIG. 42.--Modified for bird.]
[Illustration: FIG. 43.--Modified for mammal.]
[Illustration: FIG. 44.--Lancelet (Amphioxus lanceolatus). Magnified
two and one-half times.]
See, then, the gradual process of change through the whole vertebrate
department. In the lowest of all vertebrates, if vertebrate it may
be called (for what corresponds to its backbone is an unjointed,
fibrous cord), the amphioxus or lancelet (Fig. 44), there are about
forty gill-arches on each side. As we rise in the scale of fishes
these are reduced in number. In the lamprey, there are seven; in the
sharks, usually five; in ordinary fishes (teleosts), there are four or
sometimes only three on each side, the others being aborted. Thus far
the change is only by diminution of number in accordance with a law
universal in biology, that decrease in the number of identical organs
is evidence of advance in the grade of organization, provided that it
be associated with more perfect structure of the organ. The further
change is one of adaptive modification. In some reptiles (lizard) the
three gill-arches on each side all retain the form of aortic arches;
in some reptiles only two retain this form. In birds and mammals only
one arch is retained, in the form of aortic arch, the others being
modified to form the great outgoing vessels of the heart, or else
aborted. It may be well to observe that in birds the one aortic arch
turns to the right, while in mammals it turns to the left. This is
positive evidence that mammals could not have come from birds, nor
_vice versa_. They both came from reptiles, and, of the many reptilian
arches, a right one was retained by the bird branch, and a left one by
the mammalian.
In all the figures illustrating this subject, we have left out the
great _incoming_ vessels or veins, because we are not here concerned
with them, they not being transformed gill-arches.
Last of all, it may be well to stop a moment to show the cogency of
this evidence. If it were a question of the origin of some structure
not only useful (for all structures selected by Nature must be useful)
but the _best imaginable_, like the eye or the ear, for example; then,
if _we examined only the highest form_ or _the finished article_, there
are two ways in which it is possible to explain the adaptive structure.
We may either suppose that it was made at once out of hand, by some
intelligent contriver; or else that it was slowly made by a process
of evolution, becoming more and more perfect by a selection of only
the most perfect from generation to generation. But in the case of the
six aortic arches of the lizard, we are shut up to the one explanation
only, viz., by slow process of evolution. One arch is all that is
necessary, as is plainly shown by the use of only one in the more
perfect circulation of birds and mammals. If the thing were done out
of hand, unconditioned by the previous structure in fishes, to have
made six was surely but a bungling piece of work.
[Illustration: FIG. 45.--Fish-brain. A, side view; B, top view.]
[Illustration: FIG. 46.--Reptile-brain. A, side view; B, top view.]
[Illustration: FIG. 47.--Bird-brain. A, side view; B, top view.]
[Illustration: FIG. 48.--Mammal-brain. A, top view; B, side view.]
[Illustration: FIG. 49.--Man’s brain. A, side view; B, top view.]
=3. Vertebrate Brain.=--Another excellent example is the structure of
the vertebrate brain. The brain of an average fish is represented in
Fig. 45. It consists of four or five swellings, or ganglia, strung
along, one beyond another. Commencing behind, these are, first, the
medulla, _m_; then the cerebellum, _cb_; then the optic lobes, _ol_;
then the cerebrum and thalamus combined, _cr_; and last, the olfactive
lobes, _of_. Of these, it will be observed, the optic lobe is the
largest in the brain of the fish (Fig. 45). In the brain of the reptile
(Fig. 46) we have the same serial arrangement, of the same parts,
only that the cerebrum has now become the dominant part instead of
the optic lobes. In the average bird (Fig. 47) the cerebrum has grown
so large that it extends backward, and partly covers the optic lobes.
In the lower mammals (marsupials), the brain is much the same in this
respect, as in birds--i. e., the cerebrum only partly covers the optic
lobes, so that, looked at from above, the whole series of ganglia are
still visible. But in the average mammal (Fig. 48) the cerebrum is so
enlarged that it covers entirely the optic lobes and encroaches on the
cerebellum behind and the olfactive lobes in front. In some monkeys,
indeed, the cerebellum is nearly or even quite covered. Finally, in
man (Fig. 49), the cerebrum has grown so enormously that it covers
every other part and completely conceals them from view when the brain
is looked at from above. In front it not only covers but has grown far
beyond the olfactive lobes; behind it extends beyond and overhangs the
cerebellum; on the sides it overhangs and covers all. Looked at from
above, nothing is seen but this great ganglion. The ideal section (Fig.
50) represents all these stages diagrammatically in one figure. After
what has been said, the figure will be readily understood.
[Illustration: FIG. 50.--Ideal section showing all the above stages.]
[Illustration: FIG. 51.--Sub-fish Stage. _th_, thalamus; _ol_, optic
lobe; _m_, medulla.]
[Illustration:
FIG. 52.--Fish-stage. _of_, olfactive lobe; _cr_, cerebrum; _th_,
thalamus; _ol_, optic lobe; _cb_, cerebellum; _m_, medulla.
]
[Illustration: FIG. 53.--Reptile-stage.]
Now, it is a most remarkable fact that substantially these same
stages, which are permanent conditions in the taxonomic series, are
passed through as transient stages in the embryonic development of the
human brain, and in the order given above. The very early condition
of the human brain is represented in Fig. 51. It is evidently nothing
more than the intercranial continuation of the spinal cord, enlarged
a little into three swellings or ganglia. These are the early
representatives of the medulla, the optic lobes, and the thalamus;
which last may be regarded as the basal and most fundamental part of
the cerebrum. This stage may be regarded as lower than that of the
ordinary fish. I have called it, therefore, the _sub-fish stage_.
The cerebellum is a subsequent outgrowth from the medulla, as is
the cerebrum and olfactive lobes from the thalamus. Fig. 52 may be
said, therefore, to represent fairly the fish-stage. Henceforward the
principal growth is in the cerebrum and cerebellum, both of which are
subsequent outgrowths of the original simple ganglia, the medulla, and
the thalamus. The cerebrum especially increases steadily in relative
size, first becoming larger than but not covering the optic lobes (Fig.
53). This represents the reptilian stage. Next, by further growth,
it covers partly the optic lobes (Fig. 54). This may be called the
bird-stage. Then it covers wholly the optic lobes, and encroaches on
the cerebellum behind and olfactive lobes in front (Fig. 55). This is
the mammalian stage. Finally, it covers and overhangs all, and thus
assumes the human stage (Fig. 56).
[Illustration:
FIG. 54.--Bird-stage. _of_, olfactive lobe; _cr_, cerebrum; _th_,
thalamus; _ol_, optic lobe; _cb_, cerebellum; _m_, medulla.
]
[Illustration: FIG. 55.--Mammalian stage.]
[Illustration: FIG. 56.--Human stage.]
We have spoken thus far only of relative _size_; but progressive
changes take place also in complexity of structure--i. e., in the
depth and number of convolutions of the cerebrum and cerebellum. The
cerebrums of fish, of reptile, bird, and lower mammals are smooth.
About the middle of the mammalian series it begins to be convoluted.
These convolutions become deeper and more numerous as we go upward in
the scale, until they reach the highest degree in the human brain.
The object of these inequalities is to increase the surface of gray
matter--i. e., the extent of the force-generating as compared with
the force-transmitting part of the brain, or battery as compared with
conducting-wire. Now, in embryonic development the human brain passes
also through these stages of increasing complexity of organization.
Here also the ontogenic is similar to the taxonomic series.
Now, why should this peculiar order be observed in the building of the
individual brain? We find the answer, the only conceivable scientific
answer to this question, in the fact that _this is the order of the
building of the vertebrate brain by evolution_ throughout geological
history. We have already seen that fishes were the only vertebrates
living in the Devonian times. The first form of brain, therefore, was
that characteristic of that class. Then reptiles were introduced;
then birds and marsupials; then true mammals; and, lastly, man. The
different styles of brains characteristic of these classes were,
therefore, successively made by evolution from earlier and simpler
forms. In phylogeny this order was observed because these successive
forms were necessary for perfect adaptation to the environment at each
step. In taxonomy we find the same order, because, as already explained
(page 11), every stage of advance in phylogeny is still represented
in existing forms. In ontogeny we have still the same order,
because ancestral characteristics are inherited, and family history
recapitulated in the individual history.
[Illustration: FIG. 57.--A, brain of extinct Ichthyornis; B, modern
tern.]
[Illustration: FIG. 58.--A, brain of Eocene dinoceras; B, Miocene
brontothere; C, modern horse.]
But not only is this order found in the evolution of the whole
vertebrate department, but something of the same kind is found also
in the evolution of _each class_. The earliest reptiles, the earliest
birds, and the earliest mammals had smaller and less perfectly
organized brains than their nearest congeners of the present day.
This is shown in the accompanying figures (Figs. 57 and 58). To
carry out one example more perfectly: In the history of the horse
family, in connection with the changes of skeletal structure already
described (page 108), we have also corresponding changes in the
size and structure of the brain; _pari passu_ with the improvement
of the mechanism we have also increased engine-power and increased
muscular energy and therefore increased activity and grace. The brain
of a modern horse, though not very large, is remarkable for the
complexity of its convolutions. The great energy, activity, and nervous
excitability of the horse are the result of this structure.
=Cephalization.=--Thus, in going up the phylogenic, the taxonomic,
or the ontogenic series, we find a gradual process of development
headward, brainward, cerebrumward; or, more generally, we might say
that in all organic evolution we find an increasing dominance of the
higher over the lower, and of the highest over all. For example, in
the lowest plane of either series we find first the different systems
imperfectly or not at all differentiated. Then, as differentiation
of these progress, we find an increased dominance of the highest
system--the _nervous system_; then in the nervous system, the
increasing dominance of its highest part--the _brain_; then in the
brain the increasing dominance of its highest ganglion--the _cerebrum_;
and, lastly, in the cerebrum the increasing dominance of its highest
substance--the exterior gray matter--as shown by the increasing number
and depth of the convolutions. This whole process may be called
_cephalization_.
Shall the process stop here? When evolution is transferred from the
animal to the human plane, from the physiological to the psychical,
from the involuntary and necessary to the voluntary and free, shall not
the same law hold good? Yes! all social evolution, all culture, all
education, whether of the race or the individual, must follow the same
law. All _psychical advance is a cephalization_--i. e., an increasing
dominance of the higher over the lower and of the highest over all; of
the mind over the body, and in the mind of the higher faculties over
the lower; and, finally, the subordination of the whole to the highest
moral purpose.
[Illustration: FIG. 59.--Homocercal tail-fin. A, form; B, structure.]
[Illustration: FIG. 60.--Heterocercal or vertebrated tail-fin. A, form;
B, structure.]
=4. Fish-Tails.=--Still another and last example: It has long been
noticed that there are among fishes two styles of tail-fins. These
are the even-lobed, or homocercal (Fig. 59), and the uneven-lobed,
or heterocercal (Fig. 60). The one is characteristic of ordinary
fishes (teleosts), the other of sharks and some other orders. In
_structure_ the difference is even more fundamental than in _form_.
In the former style the backbone stops abruptly in a series of short,
enlarged joints, and thence sends off rays to form the tail-fin
(Fig. 59, B); in the latter the backbone runs through the fin to its
very point, growing slenderer by degrees, and giving off rays above
and below from each joint, but the rays on the lower side are much
longer (Fig. 60, B). This style of fin is, therefore, _vertebrated_,
the other _non-vertebrated_. Figs. 59 and 60 show these two styles in
form and structure. But there is still another style found only in the
lowest and most generalized forms of fishes. In these the tail-fin is
vertebrated and yet symmetrical. This style is shown in Fig. 61, A and
B.
[Illustration: FIG. 61.--Vertebrated but symmetrical fin. A, form; B,
structure.]
Now, in the development of a teleost fish (Fig. 58), as has been shown
by Alexander Agassiz,[24] the tail-fin is first like Fig. 61; then
becomes heterocercal, like Fig. 60; and, finally, becomes homocercal
like Fig. 59. Why so? Not because there is any special advantage in
this succession of forms; for the changes take place either in the egg
or else in very early embryonic states. The answer is found in the
fact that _this is the order of change in the phylogenic series_. The
earliest fish-tails were either like Fig. 61 or Fig. 60; never like
Fig. 59. The earliest of all were almost certainly like Fig. 61; then
they became like Fig. 60; and, finally, only much later in geological
history (Jurassic or Cretaceous), they became like Fig. 59. This order
of change is still retained in the embryonic development of the last
introduced and most specialized order of existing fishes. The family
history is repeated in the individual history.
[Illustration: FIG. 62.--Tail of the Archæopteryx.]
[Illustration: FIG. 63.--Tail of a modern bird.]
Similar changes have taken place in the form and structure of birds’
tails. The earliest bird known--the Jurassic Archæopteryx--had a long
reptilian tail of twenty-one joints, each joint bearing a feather on
each side, right and left (Fig. 62). In the typical modern bird, on the
contrary, the tail-joints are diminished in number, shortened up, and
enlarged, and give out long feathers, fan-like, to form the so-called
tail (Fig. 63). The Archæopteryx’ tail is _vertebrated_, the typical
bird’s _non-vertebrated_. This shortening up of the tail did not take
place at once, but gradually. The Cretaceous birds, intermediate in
time, had tails intermediate in structure. The Hesperornis of Marsh
had twelve joints. At first--in Jurassic--the tail is fully a half
of the whole vertebral column. It then gradually shortens up until it
becomes the aborted organ of typical modern birds. Now, in embryonic
development, the tail of the modern typical bird _passes through all
these stages_. At first the tail is nearly one half the whole vertebral
column; then, as development goes on, while the rest of the body grows,
the growth of the tail stops, and thus finally becomes the aborted
organ we now find. The ontogeny still passes through the stages of the
phylogeny. The same is true of all tailless animals. The frog is tailed
in the larval condition, because its ancestors were tailed amphibians.
Even man himself is endowed with a much more considerable tail, viz.,
eight or nine joints, in his early embryonic condition.[25]
We have taken all our examples from vertebrates, but quite as many
and as good examples might be found among articulates. Insects, in
the larval state, are worm-like in form. Hence it is probable that
the earliest progenitors of this class were worm-like. Again, some
insects have aquatic larvæ. The progenitors of these--in fact, of
all insects--were probably aquatic. Crabs, in a larval condition, are
long-tailed, and we know that the long-tailed crustaceans (Macrourans)
preceded the short-tailed (Brachyourans). Water-breathing animals
preceded air-breathers; the same is true in the ontogeny of the frog,
of many insects, and, we might add, even of mammals. For the breathing
of the _fœtus in utero_ is essentially by exposure of fœtal blood
to the oxygenated blood of the mother in a sort of _gill-fringes_
(placental tufts). But why should we multiply examples? The whole of
embryology, in every department, is made up of examples of the same law.
=Illustration of the Differentiation of the Whole Animal
Kingdom.=--Finally, the law of differentiation in the evolution of
the whole animal kingdom may be well illustrated by means of the
different directions taken in the development of the eggs of all the
various kinds of animals. Suppose, then, we have one thousand eggs,
representing all the different departments, classes, orders, families,
etc., of animals. Many of these may doubtless be identified by form
or size, or some other superficial character, as the eggs of this or
that animal, _but structurally they are all alike_. At first, i. e.,
as germ-cells, they all represent the _earliest condition_ of life on
the earth, and the _lowest forms_ of life _now_. If we now watch their
development, we find that some remain in this first condition without
further change. These we set aside. They are _Protozoa_. The remainder
continue to develop, but at first it would be impossible to say to
which of the several departments or primary groups they each belonged.
Then, by cell-multiplication, the original single cell becomes a
cell-aggregate. It may be compared now to a compound protozoan, such
as Foraminifera. The cell-aggregate then differentiates into layers,
and forms, in fact, a two-layered sac called a gastrula. This is the
structure of some of the lowest cœlenterates, such as the hydra. Thus
far all seem to go together. But now, for the first time, the primary
groups are declared. If it be a vertebrate, for example, the most
fundamental characters--the cerebro-spinal axis, the vertebral column,
and the double cavity, neural and visceral, are outlined. Suppose,
now, we set aside all other departments, and fix our attention on the
vertebrates. At first we could not tell which were mammals, birds,
reptiles, or fishes; but after a while the classes are declared. We
now set aside all other classes and watch the mammals. After a while
the order declares itself. We select the ungulates. Then the family is
declared, say the _Equidæ_; then the genus, _Equus_; and, lastly, the
species, _Caballus_.[26]
The same would be true if we followed any other line of development,
whether in vertebrates or in any other department. Observe, then, that,
in following any one line as we have done, there is an increasing
specialization, and, if we followed all the lines, an increasing
differentiation, like the branching and rebranching of a tree. Now,
this is the type and illustration of what took place in the development
of the animal kingdom. We conclude that the animal kingdom appeared
first as Protozoa, then as living cell-aggregates or compound
protozoans, then as gastrula or two-layered sacs with oral opening.
Then the great primary departments, unless we except the vertebrates,
commenced to separate. This took place before the primordial period;
for in the primordial fauna we have all the departments, except
vertebrates, already declared. This completely explains why it is that
we are able to trace homology only within the limits of each primary
group.
But the question has doubtless already occurred to the thoughtful
reader, “Why should the steps of the phylogeny be repeated in the
ontogeny?” The general answer is doubtless to be found in the law of
heredity--that wonderful law, so characteristic of living things. We
have compared it to a brief recapitulation from memory--the minor
points, especially if they be also early, dropping out. But can we
not explain it further? It is probable that we find a more special
explanation in “_the law of acceleration_,” first brought forward by
Prof. Cope. By the law of heredity each generation repeats the form and
structure of the previous, and in the order in which they successively
appeared. But there is a tendency for each successively-appearing
character to appear a little earlier in each successive generation;
and by this means time is left over for the introduction of still
higher _new_ characters. Thus, characters which were once adult are
pushed back to the young, and then still back to the embryo, and thus
place and time are made for each generation to push on still higher.
The law of acceleration is a sort of young-Americanism in the animal
kingdom. If our boys acquire knowledge and character similar to that of
adults of a few generations back, they will have time while still young
and plastic to press forward to still higher planes.
=Proofs from Rudimentary and Useless Organs.=--These have to a large
extent been anticipated under previous heads. The tails of birds and
the gill-arches of reptiles are rudimentary. The finger-bones of a
whale’s paddle or a turtle’s flipper may be regarded as useless, at
least so far as the exact number of constituent pieces is concerned;
for an extended surface, without visible joints or separate fingers, is
all that is seen, and apparently all that is required. The splint-bones
of a horse’s foot or the dew-claws of a dog’s foot are certainly
useless. We have already, in speaking of modifications of structure and
of embryonic conditions, given many examples of this kind, but it may
be well to add some striking examples with this special point in view.
If different orders of existing mammals were indeed made by gradual
modification of some generalized primal form, then it is evident that
these useless remnants of once useful parts would be most common in
the most highly modified forms. Now, of all mammals, the whales are
perhaps the most modified or changed from the original mammalian
form--so much modified, in fact, that the popular eye scarcely
recognizes them as mammals at all. Here, then, we might expect, and do
indeed find, many examples:
1. The baleen whales have no teeth, and no use for them. They have
instead a wonderful armature of fringed whalebone plates (baleen),
by means of which they gather their food.[27] Yet the embryo of
the whale has a full set of rudimentary teeth deeply buried in the
jawbone, and formed in the usual way characteristic of mammalian
teeth--i. e., by an infolding of the epithelial surface of the
gum--_but the teeth are never cut_; in fact, they reach their highest
development in mid-embryonic life, and are again absorbed. Why, then,
this waste of developmental energy? Why should teeth be formed only
to be reabsorbed without being cut? The only conceivable answer is,
because the ancestors of the whale, before the family of whales was
fairly established, had teeth which were gradually, from generation to
generation, aborted, because no longer used, the baleen plates having
taken their place. If whales were made at once out of hand as we now
see them, is it conceivable that these useless teeth would have been
given them?
2. Again, many whales have rudimentary pelvic bones, but no hind-limbs.
Why should there be pelvic bones, when the sole object of these bones
is to act as a basis for hind-limbs? In some whales, for example the
right whale, there are also rudiments of hind-legs, but these are
buried beneath the skin and flesh, and therefore, of course, wholly
useless. The only explanation of these facts is that the ancestors of
all the whales before they had become whales were quadrupeds, which
afterward took to the water, and little by little the hind-legs, for
want of use, dwindled away to the useless remnants which we now find.
3. Again, whales seem to be hairless, yet rudimentary hairs are
found in the skin. Their organs of smell are rudimentary, but made
on the pattern of those of mammals, not of fishes--i. e., they are
air-smelling, not water-smelling organs. From all these, as well as
many other facts, it is evident that the whales descended in early
Tertiary times from some marsh-loving, powerful-tailed, short-legged,
scant-haired quadruped by modifications gradually induced by increasing
aquatic habits.
Examples of such rudimentary organs might be multiplied without limit.
As might be expected, some are found even in man. Such, for example,
are the muscles for moving the ear, necessary in animals but useless
in man, and therefore rudimentary. Similarly useless in man are the
scalp-muscle, used by animals to erect the crest or bristles on the
head, and the skin-muscle of the neck and chest, used by animals
for shaking the skin of those parts. Most persons have lost the
power of using these. For my part I can use them all--ear-muscles,
scalp-muscle, skin-muscle--but they serve no useful purpose.
Again, and finally, in man and many mammals we find a slender,
worm-like appendage about three inches long, attached to the cæcum of
the large intestine. Anatomists and physiologists, under the influence
of that philosophy which maintains that every part of the fearfully
and wonderfully made human frame was _directly_ contrived to subserve
some useful purpose, have puzzled themselves to find the use of this.
It probably has no use; on the contrary, it is a continual source of
danger. If the human body had been made at once out of hand, it would
not have been there. How came it, then? It is the rudimentary remnant
of an organ--a greatly enlarged cæcum--which has served, and in some
mammals still serves, a useful purpose. All these cases are survivals;
they are organs which, like many customs in society, have outlived
their usefulness, but still continue by heredity.
But why multiply examples? All along the track of evolution organs
become useless by changes in the habits of their possessors. They are
not, however, shed or dropped bodily at once. No; they are _retained
by heredity_, but _dwindle by disuse_, more and more, until they pass
away entirely. But even when they are entirely gone in the adult, they
are often found still lingering in the embryo. They are among the
most obvious and convincing proofs of the origin of organic forms by
derivation.