rudimentary structures, but what may be more descriptively
designated--in accordance with the theory of descent--obsolescent or
vestigial structures. It is, however, of great importance to add that
these structures are of such general occurrence throughout both the
vegetable and animal kingdoms, that, as Darwin has observed, it is
almost impossible to point to a single species which does not present
one or more of them. In other words, it is almost impossible to find a
single species which does not in this way bear some record of its own
descent from other species; and the more closely the structure of any
species is examined anatomically, the more numerous are such records
found to be. Thus, for example, of all organisms that of man has been
most minutely investigated by anatomists; and therefore I think it will
be instructive to conclude this chapter by giving a list of the more
noteworthy vestigial structures which are known to occur in the human
body. I will take only those which are found in adult man, reserving for
the next chapter those which occur in a transitory manner during earlier
periods of his life. But, even as thus restricted, the number of
obsolescent structures which we all present in our own persons is so
remarkable, that their combined testimony to our descent from a
quadrumanous ancestry appears to me in itself conclusive. I mean, that
even if these structures stood alone, or apart from any more general
evidences of our family relationships, they would be sufficient to prove
our parentage. Nevertheless, it is desirable to remark that of course
these special evidences which I am about to detail do not stand alone.
Not only is there the general analogy furnished by the general proof of
evolution elsewhere, but there is likewise the more special
correspondence between the whole of our anatomy and that of our nearest
zoological allies. Now the force of this latter consideration is so
enormous, that no one who has not studied human anatomy can be in a
position to appreciate it. For without special study it is impossible to
form any adequate idea of the intricacy of structure which is presented
by the human form. Yet it is found that this enormously intricate
organization is repeated in all its details in the bodies of the higher
apes. There is no bone, muscle, nerve, or vessel of any importance in
the one which is not answered to by the other. Hence there are hundreds
of thousands of instances of the most detailed correspondence, without
there being any instances to the contrary, if we pay due regard to
vestigial characters. The entire corporeal structure of man is an exact
anatomical copy of that which we find in the ape.
My object, then, here is to limit attention to those features of our
corporeal structure which, having become useless on account of our
change in attitude and habits, are in process of becoming obsolete, and
therefore occur as mere vestigial records of a former state of things.
For example, throughout the vertebrated series, from fish to mammals,
there occurs in the inner corner of the eye a semi-transparent eye-lid,
which is called the nictitating membrane. The object of this structure
is to sweep rapidly, every now and then, over the external surface of
the eye, apparently in order to keep the surface clean. But although the
membrane occurs in all classes of the sub-kingdom, it is more prevalent
in some than in others--e.g. in birds than in mammals. Even, however,
where it does not occur of a size and mobility to be of any use, it is
usually represented, in animals above fishes, by a functionless
rudiment, as here depicted in the case of man. (Fig. 10.)
[Illustration: FIG. 10.--Illustrations of the nictitating membrane
in the various animals named drawn from nature. The letter N
indicates the membrane in each case. In man it is called the _plica
semilunaris_, and is represented in the two lower drawings under
this name. In the case of the shark (_Galeus_) the muscular
mechanism is shown as dissected.]
Now the organization of man presents so many vestigial structures thus
referring to various stages of his long ancestral history, that it would
be tedious so much as to enumerate them. Therefore I will yet further
limit the list of vestigial structures to be given as examples, by not
only restricting these to cases which occur in our own organization; but
of them I shall mention only such as refer us to the very last stage of
our ancestral history--viz. structures which have become obsolescent
since the time when our distinctively human branch of the family tree
diverged from that of our immediate forefathers, the Quadrumana.
(1) _Muscles of the external ear._--These, which are of large size and
functional use in quadrupeds, we retain in a dwindled and useless
condition (Fig. 11). This is likewise the case in anthropoid apes; but
in not a few other Quadrumana (e.g. baboons, macacus, magots, &c.)
degeneration has not proceeded so far, and the ears are voluntarily
moveable.
[Illustration: FIG. 11.--Rudimentary, or vestigial and useless,
muscles of the human ear. (From _Gray's Anatomy_.)]
(2) _Panniculus carnosis._--A large number of the mammalia are able to
move their skin by means of sub-cutaneous muscle--as we see, for
instance, in a horse, when thus protecting himself against the sucking
of flies. We, in common with the Quadrumana, possess an active remnant
of such a muscle in the skin of the forehead, whereby we draw up the
eyebrows; but we are no longer able to use other considerable remnants
of it, in the scalp and elsewhere,--or, more correctly, it is rarely
that we meet with persons who can. But most of the Quadrumana (including
the anthropoids) are still able to do so. There are also many other
vestigial muscles, which occur only in a small percentage of human
beings, but which, when they do occur, present unmistakeable homologies
with normal muscles in some of the Quadrumana and still lower
animals[5].
[5] See especially Mr. John Wood's papers, _Proc. R. S._, xiii to
xvi, and xviii; also _Journ. Anat._, i and iii. In this connexion
Darwin refers to M. Richard, _Annls. d. Sc. Nat. Zoolg._, tom.
xviii, p. 13, 1852.
(3) _Feet._--It is observable that in the infant the feet have a strong
deflection inwards, so that the soles in considerable measure face one
another. This peculiarity, which is even more marked in the embryo than
in the infant (see p. 153), and which becomes gradually less and less
conspicuous even before the child begins to walk, appears to me a highly
suggestive peculiarity. For it plainly refers to the condition of
things in the Quadrumana, seeing that in all these animals the feet are
similarly curved inwards, to facilitate the grasping of branches. And
even when walking on the ground apes and monkeys employ to a great
extent the outside edges of their feet, as does also a child when
learning to walk. The feet of a young child are also extraordinarily
mobile in all directions, as are those of apes. In order to show these
points, I here introduce comparative drawings of a young ape and the
portrait of a young male child. These drawings, moreover, serve at the
same time to illustrate two other vestigial characters, which have
often been previously noticed with regard to the infant's foot. I allude
to the incurved form of the legs, and the lateral extension of the great
toe, whereby it approaches the thumb-like character of this organ in the
Quadrumana. As in the case of the incurved position of the legs and
feet, so in this case of the lateral extensibility of the great toe, the
peculiarity is even more marked in embryonic than in infant life. For,
as Prof. Wyman has remarked with regard to the foetus when about an inch
in length, "The great toe is shorter than the others; and, instead of
being parallel to them, is projected at an angle from the side of the
foot, thus corresponding with the permanent condition of this part in
the Quadrumana[6]." So that this organ, which, according to Owen, "is
perhaps the most characteristic peculiarity in the human structure,"
when traced back to the early stages of its development, is found to
present a notably less degree of peculiarity.
[6] _Proc. Nat. Hist. Soc._, Boston, 1863.
[Illustration: FIG. 12.--Portrait of a young male gorilla (after
Hartmann).]
[Illustration: FIG. 13.--Portrait of a young male child.
Photographed from life, when the mobile feet were for a short time
at rest in a position of extreme inflection.]
(4) _Hands._--Dr. Louis Robinson has recently observed that the grasping
power of the whole human hand is so surprisingly great at birth, and
during the first few weeks of infancy, as to be far in excess of present
requirements on the part of a young child. Hence he concludes that it
refers us to our quadrumanous ancestry--the young of anthropoid apes
being endowed with similar powers of grasping, in order to hold on to
the hair of the mother when she is using her arms for the purposes of
locomotion. This inference appears to me justifiable, inasmuch as no
other explanation can be given of the comparatively inordinate muscular
force of an infant's grip. For experiments showed that very young babies
are able to support their own weight, by holding on to a horizontal bar,
for a period varying from one half to more than two minutes[7]. With his
kind permission I here reproduce one of Dr. Robinson's instantaneous,
and hitherto unpublished, photographs of a very young infant. This
photograph was taken after the above paragraph (3) was written, and I
introduce it here because it serves to show incidentally--and perhaps
even better than the preceding figure--the points there mentioned with
regard to the feet and great toes. Again, as Dr. Robinson observes, the
attitude, and the disproportionately large development of the arms as
compared with the legs, give all the photographs a striking resemblance
to a picture of the chimpanzee "Sally" at the Zoological Gardens. For
"invariably the thighs are bent nearly at right angles to the body, and
in no case did the lower limbs hang down and take the attitude of the
erect position." He adds, "In many cases no sign of distress is evinced,
and no cry uttered, until the grasp begins to give way."
[7] _Nineteenth Century_, November, 1891.
[Illustration: FIG. 14.--An infant, three weeks old, supporting its
own weight for over two minutes. The attitude of the lower limbs,
feet, and toes, is strikingly simian. Reproduced from an
instantaneous photograph, kindly given for the purpose by Dr. L.
Robinson.]
(5) _Tail._--The absence of a tail in man is popularly supposed to
constitute a difficulty against the doctrine of his quadrumanous
descent. As a matter of fact, however, the absence of an external tail
in man is precisely what this doctrine would expect, seeing that the
nearest allies of man in the quadrumanous series are likewise destitute
of an external tail. Far, then, from this deficiency in man constituting
any difficulty to be accounted for, if the case were not so--i. e. if
man _did_ possess an external tail,--the difficulty would be to
understand how he had managed to retain an organ which had been
renounced by his most recent ancestors. Nevertheless, as the anthropoid
apes continue to present the rudimentary vestiges of a tail in a few
caudal vertebrae below the integuments, we might well expect to find a
similar state of matters in the case of man. And this is just what we do
find, as a glance at these two comparative illustrations will show.
(Fig. 15.) Moreover, during embryonic life, both of the anthropoid apes
and of man, the tail much more closely resembles that of the lower kinds
of quadrumanous animals from which these higher representatives of the
group have descended. For at a certain stage of embryonic life the tail,
both of apes and of human beings, is actually longer than the legs (see
Fig. 16). And at this stage of development, also, the tail admits of
being moved by muscles which later on dwindle away. Occasionally,
however, these muscles persist, and are then described by anatomists as
abnormalities. The following illustrations serve to show the muscles in
question, when thus found in adult man.
[Illustration: FIG. 15.--Sacrum of Gorilla compared with that of
Man, showing the rudimentary tail-bones of each. Drawn from nature
(_R. Coll. Surg. Mus._).]
[Illustration: FIG. 16.--Diagrammatic outline of the human embryo
when about seven weeks old, showing the relations of the limbs and
tail to the trunk (after Allen Thomson), _r_, the radial, and _u_,
the ulnar, border of the hand and fore-arm; _t_, the tibial, and
_f_, the fibular, border of the foot and lower leg; _au_, ear; _s_,
spinal cord; _v_, umbilical cord; _b_, branchial gill-slits; _c_,
tail.]
[Illustration: FIG. 17.--Front and back view of adult human sacrum,
showing abnormal persistence of vestigial tail-muscles. (The first
drawing is copied from Prof. Watson's paper in _Journl. Anat. and
Physiol._, vol. 79: the second is compiled from different
specimens.)]
(6) _Vermiform Appendix of the Caecum._--This is of large size and
functional use in the process of digestion among many herbivorous
animals; while in man it is not only too small to serve any such
purpose, but is even a source of danger to life--many persons dying
every year from inflammation set up by the lodgement in this blind tube
of fruit-stones, &c.
In the orang it is longer than in man (Fig. 18), as it is also in the
human foetus proportionally compared with the adult. (Fig. 19.) In
some of the lower herbivorous animals it is longer than the entire body.
Like vestigial structures in general, however, this one is highly
variable. Thus the above cut (Fig. 19) serves to show that it may
sometimes be almost as short in the orang as it normally is in man--both
the human subjects of this illustration having been normal.
[Illustration: FIG. 18.--_Appendix vermiformis_ in Orang and in Man.
Drawn from dried inflated specimens in the Cambridge Museum by Mr.
J. J. Lister. _Il_, ilium; _Co_, colon; _C_, caecum; W, a window cut
in the wall of the caecum; X X X, the appendix.]
[Illustration: FIG. 19.--The same, showing variation in the Orang.
Drawn from a specimen in the Museum of the Royal College of
Surgeons.]
(7) _Ear._--Mr. Darwin writes:--
The celebrated sculptor, Mr. Woolner, informs me of one little
peculiarity in the external ear, which he has often observed both
in men and women.... The peculiarity consists in a little blunt
point, projecting from the inwardly folded margin, or helix. When
present, it is developed at birth, and, according to Prof. Ludwig
Meyer, more frequently in man than in woman. Mr. Woolner made an
exact model of one such case, and sent me the accompanying
drawing.... The helix obviously consists of the extreme margin of
the ear folded inwards; and the folding appears to be in some
manner connected with the whole external ear being permanently
pressed backwards. In many monkeys, which do not stand high in the
order, as baboons and some species of macacus, the upper portion of
the ear is slightly pointed, and the margin is not at all folded
inwards; but if the margin were to be thus folded, a slight point
would necessarily project towards the centre.... The following
wood-cut is an accurate copy of a photograph of the foetus of an
orang (kindly sent me by Dr. Nitsche), in which it may be seen how
different the pointed outline of the ear is at this period from its
adult condition, when it bears a close general resemblance to that
of man [including even the occasional appearance of the projecting
point shown in the preceding woodcut]. It is evident that the
folding over of the tip of such an ear, unless it changed greatly
during its further development, would give rise to a point
projecting inwards[8].
[8] _Descent of Man_, 2nd ed., pp. 15-16.
[Illustration: FIG. 20.--Human ear, modelled and drawn by Mr.
Woolner. _a_, the projecting point.]
[Illustration: FIG. 21.--Foetus of an Orang. Exact copy of a
photograph, showing the form of the ear at this early stage.]
The following woodcut serves still further to show vestigial
resemblances between the human ear and that of apes. The last two
figures illustrate the general resemblance between the normal ear of
foetal man and the ear of an adult orang-outang. The other two figures
on the lower line are intended to exhibit occasional modifications of
the adult human ear, which approximate simian characters somewhat more
closely than does the normal type. It will be observed that in their
comparatively small lobes these ears resemble those of all the apes; and
that while the outer margin of one is not unlike that of the Barbary
ape, the outer margin of the other follows those of the chimpanzee and
orang. Of course it would be easy to select individual human ears which
present either of these characters in a more pronounced degree; but
these ears have been chosen as models because they present both
characters in conjunction. The upper row of figures likewise shows the
close similarity of hair-tracts, and the direction of growth on the part
of the hair itself, in cases where the human ear happens to be of an
abnormally hirsute character. But this particular instance (which I do
not think has been previously noticed) introduces us to the subject of
hair, and hair-growth, in general.
[Illustration: FIG. 22.--Vestigial characters of human ears. Drawn
from nature.]
(8) _Hair._--Adult man presents rudimentary hair over most parts of the
body. Wallace has sought to draw a refined distinction between this
vestigial coating and the useful coating of quadrumanous animals, in the
absence of the former from the human back. But even this refined
distinction does not hold. On the one hand, the comparatively hairless
chimpanzee which died last year in the Zoological Gardens (_T. calvus_)
was remarkably denuded over the back; and, on the other hand, men who
present a considerable development of hair over the rest of their bodies
present it also on their backs and shoulders. Again, in all men the
rudimentary hair on the upper and lower arm is directed towards the
elbow--a peculiarity which occurs nowhere else in the animal kingdom,
with the exception of the anthropoid apes and a few American monkeys,
where it presumably has to do with arboreal habits. For, when sitting in
trees, the orang, as observed by Mr. Wallace, places its hands above its
head with its elbows pointing downwards: the disposition of hair on
the arms and fore-arms then has the effect of thatch in turning the
rain. Again, I find that in all species of apes, monkeys, and baboons
which I have examined (and they have been numerous), the hair on the
backs of the hands and feet is continued as far as the first row of
phalanges; but becomes scanty, or disappears altogether, on the second
row; while it is invariably absent on the terminal row. I also find that
the same peculiarity occurs in man. We all have rudimentary hair on the
first row of phalanges, both of hands and feet: when present at all, it
is more scanty on the second row; and in no case have I been able to
find any on the terminal row. In all cases these peculiarities are
congenital, and the total absence or partial presence of hair on the
second phalanges is constant in different species of Quadrumana. For
instance, it is entirely absent in all the chimpanzees, which I have
examined, while scantily present in all the orangs. As in man, it occurs
in a patch midway between the joints.
[Illustration: FIG. 23.--Hair-tracts on the arms and hands of Man,
as compared with those on the arms and hands of Chimpanzee. Drawn
from life.]
Besides showing these two features with regard to the disposition of
hair on the human arm and hand, the above woodcut illustrates a third.
By looking closely at the arm of the very hairy man from whom the
drawing was taken, it could be seen that there was a strong tendency
towards a whorled arrangement of the hairs on the backs of the wrists.
This is likewise, as a general rule, a marked feature in the arrangement
of hair on the same places in the gorilla, orang, and chimpanzee. In the
specimen of the latter, however, from which the drawing was taken, this
characteristic was not well marked. The downward direction of the hair
on the backs of the hands is exactly the same in man as it is in all
the anthropoid apes. Again, with regard to hair, Darwin notices that
occasionally there appears in man a few hairs in the eyebrows much
longer than the others; and that they seem to be representative of
similarly long and scattered hairs which occur in the chimpanzee,
macacus, and baboons.
Lastly, it may be here more conveniently observed than in the next
chapter on Embryology, that at about the sixth month the human foetus
is often thickly coated with somewhat long dark hair over the entire
body, except the soles of the feet and palms of the hands, which are
likewise bare in all quadrumanous animals. This covering, which is
called the lanugo, and sometimes extends even to the whole forehead,
ears, and face, is shed before birth. So that it appears to be useless
for any purpose other than that of emphatically declaring man a child of
the monkey.
(9) _Teeth._--Darwin writes:--
It appears as if the posterior molar or wisdom-teeth were tending
to become rudimentary in the more civilized races of man. These
teeth are rather smaller than the other molars, as is likewise the
case with the corresponding teeth in the chimpanzee and orang; and
they have only two separate fangs.... They are also much more
liable to vary, both in structure and in the period of their
development, than the other teeth. In the Melanian races, on the
other hand, the wisdom-teeth are usually furnished with three
separate fangs, and are usually sound [i. e. not specially liable
to decay]; they also differ from the other molars in size, less
than in the Caucasian races.
Now, in addition to these there are other respects in which the
dwindling condition of wisdom-teeth is manifested--particularly with
regard to the pattern of their crowns. Indeed, in this respect it would
seem that even in the anthropoid apes there is the beginning of a
tendency to degeneration of the molar teeth from behind forwards. For if
we compare the three molars in the lower jaw of the gorilla, orang, and
chimpanzee, we find that the gorilla has five well-marked cusps on all
three of them; but that in the orang the cusps are not so pronounced,
while in the chimpanzee there are only four of them on the third molar.
Now in man it is only the first of these three teeth which normally
presents five cusps, both the others presenting only four. So that,
comparing all these genera together, it appears that the number of
cusps is being reduced from behind forwards; the chimpanzee having lost
one of them from the third molar, while man has not only lost this, but
also one from the second molar,--and, it may be added, likewise
partially (or even totally) from the first molar, as a frequent
variation among civilized races. But, on the other hand, variations are
often met with in the opposite direction, where the second or the third
molar of man presents five cusps--in the one case following the
chimpanzee, in the other the gorilla. These latter variations,
therefore, may fairly be regarded as reversionary. For these facts I am
indebted to the kindness of Mr. C. S. Tomes.
[Illustration: FIG. 24.--Molar teeth of lower jaw in Gorilla, Orang,
and Man. Drawn from nature, nat. size (_R. Mus. Coll. Surg._).]
(10) _Perforations of the humerus._--The peculiarities which we have to
notice under this heading are two in number. First, the supra condyloid
foramen is a normal feature in some of the lower Quadrumana (Fig. 25),
where it gives passage to the great nerve of the fore-arm, and often
also to the great artery. In man, however, it is not a normal feature.
Yet it occurs in a small percentage of cases--viz., according to Sir W.
Turner, in about one per cent., and therefore is regarded by Darwin as a
vestigial character. Secondly, there is inter-condyloid foramen, which
is also situated near the lower end of the humerus, but more in the
middle of the bone. This occurs, but not constantly, in apes, and also
in the human species. From the fact that it does so much more frequently
in the bones of ancient--and also of some savage--races of mankind (viz.
in 20 to 30 per cent. of cases), Darwin is disposed to regard it also as
a vestigial feature. On the other hand, Prof. Flower tells me that in
his opinion it is but an expression of impoverished nutrition during
the growth of the bone.
[Illustration: FIG. 25.--Perforation of the humerus (supra-condyloid
foramen) in three species of Quadrumana where it normally occurs,
and in Man, where it does not normally occur. Drawn from nature (_R.
Coll. Surg. Mus._).]
(11) _Flattening of tibia._--In some very ancient human skeletons, there
has also been found a lateral flattening of the tibia, which rarely
occurs in any existing human beings, but which appears to have been
usual among the earliest races of mankind hitherto discovered. According
to Broca, the measurements of these fossil human tibiae resemble those of
apes. Moreover, the bone is bent and strongly convex forwards, while its
angles are so rounded as to present the nearly oval section seen in
apes. It is in association with these ape-like human tibiae that
perforated humeri of man are found in greatest abundance.
On the other hand, however, there is reason to doubt whether this form
of tibia in man is really a survival from his quadrumanous ancestry.
For, as Boyd-Dawkins and Hartmann have pointed out, the degree of
flattening presented by some of these ancient human bones is _greater_
than that which occurs in any existing species of anthropoid ape. Of
course the possibility remains that the unknown species of ape from
which man descended may have had its tibia more flattened than is now
observable in any of the existing species. Nevertheless, as some doubt
attaches to this particular case, I do not press it--and, indeed, only
mention it at all in order that the doubt may be expressed.
Similarly, I will conclude by remarking that several other instances of
the survival of vestigial structures in man have been alleged, which are
of a still more doubtful character. Of such, for example, are the
supposed absence of the genial tubercle in the case of a very ancient
jaw-bone of man, and the disposition of valves in human veins. From the
former it was argued that the possessor of this very ancient jaw-bone
was probably speechless, inasmuch as the tubercle in existing man gives
attachment to muscles of the tongue. From the latter it has been argued
that all the valves in the veins of the human body have reference, in
their disposition, to the incidence of blood-pressure when the attitude
of the body is horizontal, or quadrupedal. Now, the former case has
already broken down, and I find that the latter does not hold. But we
can well afford to lose such doubtful and spurious cases, in view of all
the foregoing unquestionable and genuine cases of vestigial structures
which are to be met with even within the limits of our own
organization--and even when these limits are still further limited by
selecting only those instances which refer to the very latest chapter of
our long ancestral history.
CHAPTER IV.
EMBRYOLOGY.
We will next consider what of late years has become the most important
of the lines of evidence, not only in favour of the general fact of
evolution, but also of its history: I mean the evidence which has been
yielded by the newest of the sciences, the science of Embryology. But
here, as in the analogous case of adult morphology, in order to do
justice to the mass of evidence which has now been accumulated, a whole
volume would be necessary. As in that previous case, therefore, I must
restrict myself to giving an outline sketch of the main facts.
First I will display what in the language of Paley we may call "the
state of the argument."
It is an observable fact that there is often a close correspondence
between developmental changes as revealed by any chronological series of
fossils which may happen to have been preserved, and developmental
changes which may be observed during the life-history of now existing
individuals belonging to the same group of animals. For instance, the
successive development of prongs in the horns of deer-like animals,
which is so clearly shown in the geological history of this tribe, is
closely reproduced in the life-history of existing deer. Or, in other
words, the antlers of an existing deer furnish in their development a
kind of _resume_, or recapitulation, of the successive phases whereby
the primitive horn was gradually superseded by horns presenting a
greater and greater number of prongs in successive species of extinct
deer (Fig. 26). Now it must be obvious that such a recapitulation in the
life-history of an existing animal of developmental changes successively
distinctive of sundry allied, though now extinct species, speaks
strongly in favour of evolution. For as it is of the essence of this
theory that new forms arise from older forms by way of _hereditary_
descent, we should antecedently expect, if the theory is true, that the
phases of development presented by the individual organism would follow,
in their main outlines, those phases of development through which their
long line of ancestors had passed. The only alternative view is that as
species of deer, for instance, were separately created, additional
prongs were successively added to their antlers; and yet that, in order
to be so added to successive species every individual deer belonging to
later species was required to repeat in his own lifetime the process of
successive additions which had previously taken place in a remote series
of extinct species. Now I do not deny that this view is a possible view;
but I do deny that it is a probable one. According to the evolutionary
interpretation of such facts, we can see a very good _reason_ why the
life-history of the individual is thus a condensed _resume_ of the
life-history of its ancestral species. But according to the opposite
view no reason can be assigned why such should be the case. In a
previous chapter--the chapter on Classification--we have seen that if
each species were created separately, no reason can be assigned why they
should all have been turned out upon structural patterns so strongly
suggestive of hereditary descent with gradual modifications, or slow
divergence--the result being group subordinated to group, with the most
generalized (or least developed) forms at the bottom, and the highest
products of organization at the top. And now we see--or shall
immediately see--that this consideration admits of being greatly
fortified by a study of the developmental history of every individual
organism. If it would be an unaccountable fact that every separately
created species should have been created with close structural
resemblances to a certain limited number of other species, less close
resemblances to certain further species, and so backwards; assuredly it
would be a still more unaccountable fact that every individual of every
species should exhibit in its own person a history of developmental
change, every term of which corresponds with the structural
peculiarities of its now extinct predecessors--and this in the exact
historical order of their succession in geological time. The more that
we think about this antithesis between the naturalistic and the
non-naturalistic interpretations, the greater must we feel the contrast
in respect of rationality to become; and, therefore, I need not spend
time by saying anything further upon the antecedent standing of the two
theories in this respect. The evidence, then, which I am about to adduce
from the study of development in the life-histories of individual
organisms, will be regarded by me as so much unquestionable evidence in
favour of similar processes of development in the life-histories of
their respective species--in so far, I mean, as the two sets of changes
admit of being proved parallel.
[Illustration: FIG. 26.--Antlers of Stag, showing successive
addition of branches in successive years. Drawn from nature (_Brit.
Mus._).]
In the only illustration hitherto adduced--viz. that of deers'
horns--the series of changes from a one-pronged horn to a fully
developed arborescent antler, is a series which takes place during the
adult life of the animal; for it is only when the breeding age has been
attained that horns are required to appear. But seeing that every animal
passes through most of the phases of its development, not only before
the breeding age has been attained, but even before the time of its own
birth, clearly the largest field for the study of individual development
is furnished by embryology. For instance, there is a salamander which
differs from most other salamanders in being exclusively terrestrial in
its habits. Now, the young of this salamander before their birth are
found to be furnished with gills, which, however, they are never
destined to use. Yet these gills are so perfectly formed, that if the
young salamanders be removed from the body of their mother shortly
before birth, and be then immediately placed in water, the little
animals show themselves quite capable of aquatic respiration, and will
merrily swim about in a medium which would quickly drown their own
parent. Here, then, we have both morphological and physiological
evidence pointing to the possession of gills by the ancestors of the
land salamander.
It would be easy to devote the whole of the present chapter to an
enumeration of special instances of the kinds thus chosen for purposes
of illustration; but as it is desirable to take a deeper, and therefore
a more general view of the whole subject, I will begin at the
foundation, and gradually work up from the earliest stages of
development to the latest. Before starting, however, I ask the reader to
bear in mind one consideration, which must reasonably prevent our
anticipating that in _every case_ the life-history of an individual
organism should present a _full_ recapitulation of the life-history of
its ancestral line of species. Supposing the theory of evolution to be
true, it must follow that in many cases it would have been more or less
disadvantageous to a developing type that it should have been obliged to
reproduce in its individual representatives all the phases of
development previously undergone by its ancestry--even within the limits
of the same family. We can easily understand, for example, that the
waste of material required for building up the useless gills of the
embryonic salamanders is a waste which, sooner or later, is likely to be
done away with; so that the fact of its occurring at all is in itself
enough to show that the change from aquatic to terrestrial habits on the
part of this species must have been one of comparatively recent
occurrence. Now, in as far as it is detrimental to a developing type
that it should pass through any particular ancestral phases of
development, we may be sure that natural selection--or whatever other
adjustive causes we may suppose to have been at work in the adaptation
of organisms to their surroundings--will constantly seek to get rid of
this necessity, with the result, when successful, of dropping out the
detrimental phases. Thus the foreshortening of developmental history
which takes place in the individual lifetime may be expected often to
take place, not only in the way of condensation, but also in the way of
excision. Many pages of ancestral history may be recapitulated in the
paragraphs of embryonic development, while others may not be so much as
mentioned. And that this is the true explanation of what embryologists
term "direct" development--or of a more or less sudden leap from one
phase to another, without any appearance of intermediate phases--is
proved by the fact that in some cases both direct and indirect
development occur within the same group of organisms, some genera or
families having dropped out the intermediate phases which other genera
or families retain.
* * * * *
The argument from embryology must be taken to begin with the first
beginning of individual life in the ovum. And, in order to understand
the bearings of the argument in this its first stage, we must consider
the phenomena of reproduction in the simplest form which these phenomena
are known to present.
The whole of the animal kingdom is divided into two great groups, which
are called the Protozoa and the Metazoa. Similarly, the whole of the
vegetable kingdom is divided into the Protophyta and the Metaphyta. The
characteristic feature of all the Protozoa and Protophyta is that the
organism consists of a single physiological cell, while the
characteristic of all the Metazoa and Metaphyta is that the organism
consists of a plurality of physiological cells, variously modified to
subserve different functions in the economy of the animal or plant, as
the case may be. For the sake of brevity, I shall hereafter deal only
with the case of animals (Protozoa and Metazoa); but it may throughout
be understood that everything which is said applies also to the case of
plants (Protophyta and Metaphyta).
A Protozooen (like a Protophyton) is a solitary cell, or a "unicellular
organism," while a Metazooen (like a Metaphyton) is a society of cells,
or a "multicellular organism." Now, it is only in the multicellular
organisms that there is any observable distinction of sex. In all the
unicellular organisms the phenomena of reproduction appear to be more or
less identical with those of growth. Nevertheless, as these phenomena
are here in some cases suggestively peculiar, I will consider them more
in detail.
A Protozooen is a single corpuscle of protoplasm which in different
species of Protozoa varies in size from more than one inch to less than
1/1000 of an inch in diameter. In some species there is an enveloping
cortical substance; in other species no such substance can be detected.
Again, in most species there is a nucleus, while in other species no
such differentiation of structure has hitherto been observed.
Nevertheless, from the fact that the nucleus occurs in the majority of
Protozoa, coupled with the fact that the demonstration of this body is
often a matter of extreme difficulty, not only in some of the Protozoa
where it has been but recently detected, but also in the case of certain
physiological cells elsewhere,--from these facts it is not unreasonable
to suppose that all the Protozoa possess a nucleus, whether or not it
admits of being rendered visible by histological methods thus far at our
disposal. If this is the case, we should be justified in saying, as I
have said, that a Protozooen is an isolated physiological cell, and, like
cells in general, multiplies by means of what Spencer and Haeckel have
aptly called a process of discontinuous growth. That is to say, when a
cell reaches maturity, further growth takes place in the direction of a
severance of its substance--the separated portion thus starting anew as
a distinct physiological unit. But, notwithstanding the complex changes
which have been more recently observed to take place in the nucleus of
some Protozoa prior to their division, the process of multiplication by
division may still be regarded as a process of growth, which differs
from the previous growth of the individual cell in being attended by a
severance of continuity. If we take a suspended drop of gum, and
gradually add to its size by allowing more and more gum to flow into it,
a point will eventually be reached at which the force of gravity will
overcome that of cohesion, and a portion of the drop will fall away from
the remainder. Here we have a rough physical simile, although of course
no true analogy. In virtue of a continuous assimilation of nutriment,
the protoplasm of a cell increases in mass, until it reaches the size at
which the forces of disruption overcome those of cohesion--or, in other
words, the point at which increase of size is no longer compatible with
continuity of substance. Nevertheless, it must not be supposed that the
process is thus merely a physical one. The phenomena which occur even in
the simplest--or so-called "direct"--cell-division, are of themselves
enough to prove that the process is vital, or physiological; and this in
a high degree of specialization. But so, likewise, are all processes of
growth in organic structures; and therefore the simile of the drop of
gum is not to be regarded as a true analogy: it serves only to indicate
the fact that when cell-growth proceeds beyond a certain point
cell-division ensues. The size to which cells may grow before they thus
divide is very variable in different kinds of cells; for while some may
normally attain a length of ten or twelve inches, others divide before
they measure 1/1000 of an inch. This, however, is a matter of detail,
and does not affect the general physiological principles on which we are
at present engaged.
Now, as we have seen, a Protozooen is a single cell; for even although in
some of the higher forms of protozoal life a colony of cells may be
bound together in organic connexion, each of these cells is in itself an
"individual," capable of self-nourishment, reproduction, and, generally,
of independent existence. Consequently, when the growth of a Protozooen
ends in a division of its substance, the two parts wander away from each
other as separate organisms. (Fig. 27.)
[Illustration: FIG. 27.--Fission of a Protozooen. In the left-hand
drawing the process is represented as having advanced sufficiently
far to have caused a division and segregation both of the nucleus
and the vesicle. In the right-hand drawing the process is
represented as complete. _n_, N, severed nucleus; _vc_, severed
vesicle; _ps_, pseudopodia; _f_, ingested food.]
The next point we have to observe is, that in all cases where a cell or
a Protozooen multiplies by way of fissiparous division, the process
begins in the nucleus. If the nucleus divides into two parts, the whole
cell will eventually divide into two parts, each of which retains a
portion of the original nucleus, as represented in the above figure. If
the nucleus divides into three, four, or even, as happens in the
development of some embryonic tissues, into as many as six parts, the
cell will subdivide into a corresponding number, each retaining a
portion of the nucleus. Therefore, in all cases of fissiparous division,
the seat or origin of the process is the nucleus.
Thus far, then, the phenomena of multiplication are identical in all the
lowest or unicellular organisms, and in the constituent cells of all the
higher or multicellular. And this is the first point which I desire to
make apparent. For where the object is to prove a continuity between the
phenomena of growth and reproduction, it is of primary importance to
show--1st, that there is such a continuity in the case of all the
unicellular organisms, and, 2nd, that there are all the above points of
resemblance between the multiplication of cells in the unicellular and
in the multicellular organisms.
It remains to consider the points of difference, and, if possible, to
show that these do not go to disprove the doctrine of continuity which
the points of resemblance so forcibly indicate.
The first point of difference obviously is, that in the case of all the
multicellular organisms the two or more "daughter-cells," which are
produced by division of the "mother-cell," do not wander away from one
another; but, as a rule, they continue to be held in more or less close
apposition by means of other cells and binding membranes,--with the
result of giving rise to those various "tissues," which in turn go to
constitute the material of "organs." I cannot suppose, however, that any
advocate of discontinuity will care to take his stand at this point.
But, if any one were so foolish as to do so, it would be easy to
dislodge him by describing the state of matters in some of the Protozoa
where a number of unicellular "individuals" are organically united so as
to form a "colony." These cases serve to bridge this distinction between
Protozoa and Metazoa, of which therefore we may now take leave.
In the second place, there is the no less obvious distinction that the
result of cell-division in the Metazoa is not merely to multiply cells
all of the same kind: on the contrary, the process here gives rise to as
many different kinds of cells as there are different kinds of tissue
composing the adult organism. But no one, I should think, is likely to
oppose the doctrine of continuity on the ground of this distinction. For
the distinction is clearly one which must necessarily arise, if the
doctrine of continuity between unicellular and multicellular organisms
be true. In other words, it is a distinction which the theory of
evolution itself must necessarily pre-suppose, and therefore it is no
objection to the theory that its pre-supposition is realized. Moreover,
as we shall see better presently, there is no difficulty in
understanding why this distinction should have arisen, so soon as it
became necessary (or desirable) that individual cells, when composing a
"colony," should conform to the economic principle of the division of
labour--a principle, indeed, which is already foreshadowed in the
constituent parts of a single cell, since the nucleus has one set of
functions and its surrounding protoplasm another.
But now, in the third place, we arrive at a more important distinction,
and one which lies at the root of the others still remaining to be
considered. I refer to sexual propagation. For it is a peculiarity of
the multicellular organisms that, although many of them may likewise
propagate themselves by other means (Fig. 28), they all propagate
themselves by means of sexual congress. Now, in its essence, sexual
congress consists in the fusion of two specialized cells (or, as now
seems almost certain, of the nuclei thereof), so that it is out of such
a combination that the new individual arises by means of successive
cell-divisions, which, beginning in the fertilized ovum, eventually
build up all the tissues and organs of the body.
[Illustration: FIG. 28.--_Hydra viridis_, partly in section. M,
mouth; O, ovary, or bud containing female reproductive cells; T,
testis, or bud containing male reproductive cells. In addition to
these buds containing germinal elements alone, there is another
which illustrates the process of "gemmation"--i. e. the direct
out-growth of a fully formed offspring.]
This process clearly indicates very high specialization on the part of
germ-cells. For we see by it that although these cells when young
resemble all other cells in being capable of self-multiplication by
binary division (thus reproducing cells exactly like themselves), when
older they lose this power; but, at the same time, they acquire an
entirely new and very remarkable power of giving rise to a vast
succession of many different kinds of cells, all of which are mutually
correlated as to their several functions, so as to constitute a
hierarchy of cells--or, to speak literally, a multicellular
_co-organization_. Here it is that we touch the really important
distinction between the Protozoa and the Metazoa; for although I have
said that some of the higher Protozoa foreshadow this state of matters
in forming cell-colonies, it must now be noted that the cells composing
such colonies are all of the same kind; and, therefore, that the
principle of producing different kinds of cells which, by mutual
co-adaptation of functions, shall be capable of constructing a
multicellular Metazooen,--this great principle of _co-organization_ is
but dimly nascent in the cell-colonies of Protozoa. And its marvellous
development in the Metazoa appears ultimately to depend upon the highly
specialized character of germ-cells. Even in cases where multicellular
organisms are capable of reproducing their kind without the need of any
preceding process of fertilization (parthenogenesis), and even in the
still more numerous cases where complete organisms are budded forth from
any part of their parent organism (gemmation, Fig. 28), there is now
very good reason to conclude that these powers of a-sexual reproduction
on the part of multicellular organisms are all ultimately due to the
specialized character of their germ-cells. For in all these cases the
tissues of the parent, from which the budding takes place, were
ultimately derived from germ-cells--no matter how many generations of
budded organisms may have intervened. And that propagation by budding,
&c, in multicellular organisms is thus ultimately due to their
propagation by sexual methods, seems to be further shown by certain
facts which will have to be discussed at some length in my next volume.
Here, therefore, I will mention only one of them--and this because it
furnishes what appears to be another important distinction between the
Protozoa and the Metazoa.
In nearly all cases where a Protozooen multiplies itself by fission, the
process begins by a simple division of the nucleus. But when a Metazooen
is developed from a germ-cell, although the process likewise begins by a
division of the nucleus, this division is not a simple or direct one; on
the contrary, it is inaugurated by a series of processes going on within
the nucleus, which are so enormously complex, and withal so beautifully
ordered, that to my mind they constitute the most wonderful--if not also
the most suggestive--which have ever been revealed by microscopical
research. It is needless to say that I refer to the phenomena of
karyokinesis. A few pages further on they will be described more fully.
For our present purposes it is sufficient to give merely a pictorial
illustration of their successive phases; for a glance at such a
representation serves to reveal the only point to which attention has
now to be drawn--namely, the immense complexity of the processes in
question, and therefore the contrast which they furnish to the simple
(or "direct") division of the nucleus preparatory to cell-division in
the unicellular organisms. Here, then (Fig. 29), we see the complex
processes of karyokinesis in the first two stages of egg-cell division.
But similar processes continue to repeat themselves in subsequent
stages; and this, there is now good reason to believe, throughout _all_
the stages of cell-division, whereby the original egg-cell eventually
constructs an entire organism. In other words, all the cells composing
all the tissues of a multicellular organism, at all stages of its
development, are probably originated by these complex processes, which
differ so much from the simple process of direct division in the
unicellular organisms[9]. In this important respect, therefore, it does
at first sight appear that we have a distinction between the Protozoa
and the Metazoa of so pronounced a character, as fairly to raise the
question whether cell-division is fundamentally identical in unicellular
and in multicellular organisms.
[9] I say "probably," because analogy points in this direction. As a
matter of fact, in many cases of tissue-formation karyokinesis has
not hitherto been detected. But even if in such cases it does not
occur--i. e. if failure to detect its occurrence be not due merely
to still remaining imperfections of our histological methods,--the
large number of cases in which it has been seen to occur in the
formation of sundry tissues are of themselves sufficient to indicate
some important difference between cells derived from ova (metazoal),
and cells which have not been so derived (protozoal). Which is the
point now under discussion.
[Illustration: FIG. 29.--Successive stages in the division of the
ovum, or egg-cell, of a worm. (After Strasburger.) _a_ to _d_ show
the changes taking place in the nucleus and surrounding
cell-contents, which result in the first segmentation of the ovum at
_e_; _f_ and _g_ show a repetition of these changes in each of the
two resulting cells, leading to the second segmentation stage at
_h_.]
Lastly, the only other distinction of a physiologically significant kind
between a single cell when it occurs as a Protozooen and when it does so
as the unfertilized ovum of a Metazooen is, that in the latter case the
nucleus discharges from its own substance two minute protoplasmic masses
("polar bodies"), which are then eliminated from the cell altogether.
This process, which will be more fully described later on, appears to be
of invariable occurrence in the case of all egg-cells, while nothing
resembling it has ever been observed in any of the Protozoa.
We must now consider these several points of difference _seriatim_.
First, with regard to sexual propagation, we have already seen that this
is by no means the only method of propagation among the multicellular
organisms; and it now remains to add that, on the other hand, there is,
to say the least, a suggestive foreshadowing of sexual propagation among
the unicellular organisms. For although simple binary fission is here
the more usual mode of multiplication, very frequently two (rarely three
or more) Protozoa of the same species come together, fuse into a single
mass, and thus become very literally "one flesh." This process of
"conjugation" is usually (though by no means invariably) followed by a
period of quiescent "encystation"; after which the contents of the cyst
escape in the form of a number of minute particles, or "spores," and
these severally develope into the parent type. Obviously this process of
conjugation, when it is thus a preliminary to multiplication, appears to
be in its essence the same as fertilization. And if it be objected that
encystation and spore-formation in the Protozoa are not always preceded
by conjugation, the answer would be that neither is oviparous
propagation in the Metazoa invariably preceded by fertilization.
Nevertheless, that there are great distinctions between true sexual
propagation and this foreshadowing of it in conjugation I do not deny.
The question, however, is whether they be so great as to justify any
argument against an historical continuity between them. What, then, are
these remaining distinctions? Briefly, as we have seen, they are the
extrusion from egg-cells of polar bodies, and the occurrence, both in
egg-cells and their products (tissue-cells), of the process of
karyokinesis. But, as regards the polar bodies, it is surely not
difficult to suppose that, whatever their significance may be, it is
probably in some way or another connected with the high specialization
of the functions which an egg-cell has to discharge. Nor is there any
difficulty in further supposing that, whatever purpose is served by
getting rid of polar bodies, the process whereby they are got rid of was
originally one of utilitarian development--i. e. a process which at its
commencement did not betoken any difference of kind, or breach of
continuity, between egg-cells and cells of simpler constitution.
Lastly, with respect to karyokinesis, although it is true that the
microscope has in comparatively recent years displayed this apparently
important distinction between unicellular and multicellular organisms,
two considerations have here to be supplied. The first is, that in some
of the Protozoa processes very much resembling those of karyokinesis
have already been observed taking place in the nucleus preparatory to
its division. And although such processes do not present quite the same
appearances as are to be met with in egg-cells, neither do the
karyokinetic processes in tissue-cells, which in their sundry kinds
exhibit great variations in this respect. Moreover, even if such were
not the case, the bare fact that nuclear division is not invariably of
the simple or direct character in the case of all Protozoa, is
sufficient to show that the distinction now before us--like the one last
dealt with--is by no means absolute. As in the case of sexual
propagation, so in that of karyokinesis, processes which are common to
all the Metazoa are not wholly without their foreshadowings in the
Protozoa. And seeing how greatly exalted is the office of egg-cells--and
even of tissue-cells--as compared with that of their supposed ancestry
in protozoal cells, it seems to me scarcely to be wondered at if their
specializations of function should be associated with corresponding
peculiarities of structure--a general fact which would in no way
militate against the doctrine of evolution. Could we know the whole
truth, we should probably find that in order to endow the most primitive
of egg-cells with its powers of marshalling its products into a living
army of cell-battalions, such an egg-cell must have been passed through
a course of developmental specialization of so elaborate a kind, that
even the complex processes of karyokinesis are but a very inadequate
expression thereof.
Probably I have now said enough to show that, remarkable and altogether
exceptional as the properties of germ-cells of the multicellular
organisms unquestionably show themselves to be, yet when these
properties are traced back to their simplest beginnings in the
unicellular organisms, they may fairly be regarded as fundamentally
identical with the properties of living cells in general. Thus viewed,
no line of real demarcation can be drawn between growth and
reproduction, even of the sexual kind. The one process is, so to speak,
physiologically continuous with the other; and hence, so far as the
pre-embryonic stage of life-history is concerned, the facts cannot
fairly be regarded as out of keeping with the theory of evolution.
I will now pass on to consider the embryogeny of the Metazoa, beginning
at its earliest stage in the fertilization of the ovum. And here it is
that the constructive argument in favour of evolution which is derived
from embryology may be said properly to commence. For it is surely in
itself a most suggestive fact that all the Metazoa begin their life in
the same way, or under the same form and conditions. _Omne vivum ex
ovo._ This is a formula which has now been found to apply throughout the
whole range of the multicellular organisms. And seeing, as we have just
seen, that the ovum is everywhere a single cell, the formula amounts to
saying that, physiologically speaking, every Metazooen begins its life as
a Protozooen, and every Metaphyton as a Protophyton[10].
[10] Even when propagated by budding, a multicellular organism has
been ultimately derived from a germ-cell.
Now, if the theory of evolution is true, what should we expect to happen
when these germ-cells are fertilized, and so enter upon their severally
distinct processes of development? Assuredly we should expect to find
that the higher organisms pass through the same phases of development as
the lower organisms, up to the time when their higher characters begin
to become apparent. If in the life-history of species these higher
characters were gained by gradual improvement upon lower characters, and
if the development of the higher individual is now a general
recapitulation of that of its ancestral species, in studying this
recapitulation we should expect to find the higher organism successively
unfolding its higher characters from the lower ones through which its
ancestral species had previously passed. And this is just what we do
find. Take, for example, the case of the highest organism, Man. Like
that of all other organisms, unicellular or multicellular, his
development starts from the nucleus of a single cell. Again, like that
of all the Metazoa and Metaphyta, his development starts from the
specially elaborated nucleus of an egg-cell, or a nucleus which has been
formed by the fusion of a male with a female element[11]. When his
animality becomes established, he exhibits the fundamental anatomical
qualities which characterize such lowly animals as polyps and
jelly-fish. And even when he is marked off as a Vertebrate, it cannot be
said whether he is to be a fish, a reptile, a bird, or a beast. Later on
it becomes evident that he is to be a Mammal; but not till later still
can it be said to which order of mammals he belongs.
[11] It has already been stated that both parthenogenesis and
gemmation are ultimately derived from sexual reproduction. It may
now be added, on the other hand, that the earlier stages of
parthenogenesis have been observed to occur sporadically in all
sub-kingdoms of the Metaxoa, including the Vertebrata, and even the
highest class, Mammalia. These earlier stages consist in
_spontaneous_ segmentations of the ovum; so that even if a virgin
has ever conceived and borne a son, and even if such a fact in the
human species has been unique, still it would not betoken any breach
of physiological continuity. Indeed, according to Weismann's not
improbable hypothesis touching the physiological meaning of polar
bodies, such a fact need betoken nothing more than a slight
disturbance of the complex machinery of ovulation, on account of
which the ovum failed to eliminate from its substance an almost
inconceivably minute portion of its nucleus.
Here, however, we must guard against an error which is frequently met
with in popular expositions of this subject. It is not true that the
embryonic phases in the development of a higher form always resemble so
many adult stages of lower forms. This may or may not be the case; but
what always is the case is, that the embryonic phases of the higher
form resemble the corresponding phases of the lower forms. Thus, for
example, it would be wrong to suppose that at any stage of his
development a man resembles a jelly-fish. What he does resemble at an
early stage of his development is the essential or groundplan of the
jelly-fish, which that animal presents in _its_ embryonic condition, or
before it begins to assume its more specialized characters fitting it
for its own particular sphere of life. The similarities, therefore,
which it is the function of comparative embryology to reveal are the
similarities of type or morphological plan: not similarities of specific
detail. Specific details may have been added to this, that, and the
other species for their own special requirements, after they had
severally branched off from the common ancestral stem; and so could not
be expected to recur in the life-history of an independent specific
branch. The comparison therefore must be a comparison of embryo with
embryo; not of embryos with adult forms.
* * * * *
In order to give a general idea of the results thus far yielded by a
study of comparative embryology in the present connexion, I will devote
the rest of this chapter to giving an outline sketch of the most
important and best established of these results.
Histologically the ovum, or egg-cell, is nearly identical in all
animals, whether vertebrate or invertebrate. Considered as a cell it is
of large size, but actually it is not more than 1/100, and may be less
than 1/200 of an inch in diameter. In man, as in most mammals, it is
about 1/120. It is a more or less spherical body, presenting a thin
transparent envelope, called the _zona pellucida_, which
contains--first, the protoplasmic cell-substance or "yolk," within which
lies, second, the nucleus or germinal vesicle, within which again lies,
third, the nucleolus or germinal spot. This description is true of the
egg-cells of all animals, if we add that in the case of the lowest
animals--such as sponges, &c.--there is no enveloping membrane: the
egg-cell is here a naked cell, and its constituent protoplasm, being
thus unconfined, is free to perform protoplasmic movements, which it
does after the manner, and with all the activity, of an amoeba. But
even with respect to this matter of an enveloping membrane, there is no
essential difference between an ovum of the lowest and an ovum of the
highest animals. For in their early stages of development within the
ovary the ova of the highest animals are likewise in the condition of
naked cells, exhibiting amoebiform movements; the enveloping membrane
of an ovum being the product of a later development. Moreover this
membrane, when present, is usually provided with one or more minute
apertures, through which the spermatozooen passes when fertilizing the
ovum. It is remarkable that the spermatozoa know, so to speak, of the
existence of these gate-ways,--their snake-like movements being directed
towards them, presumably by a stimulus due to some emanation
therefrom[12]. In the mammalian ovum, however, these apertures are
exceedingly minute, and distributed all round the circumference of the
pellucid envelope, as represented in this illustration (Fig. 32).
[12] The spermatozooids of certain plants can be strongly attracted
towards a pipette which is filled with malic acid--crowding around
and into it with avidity.
[Illustration: FIG. 30.--Ovarian ovum of a Mammal, (_a_) magnified
and viewed under pressure, (_b_) burst by increased pressure, with
yolk and nucleus escaping: (_c_) the nucleus more freed from
yolk-substance. (From _Quain's Anatomy_, after Allen Thomson.)]
[Illustration: FIG. 31.--Amoeboid movements of young egg-cells, _a_,
Amoeboid ovum of _Hydra_ (from Balfour, after Kleitnenberg); _b_,
early ovum of _Toxopneustes variegatus_, with pseudopodia-like
processes (from Balfour, after Selenka); _c_, ovum of _Toxopneustes
lividus_, more nearly ripe (from Balfour, Hertwig). A1 to A4, the
primitive egg-cell of a Chalk-Sponge (_Leuculmis echinus_), in four
successive conditions of motion. B1 to B8, ditto of a Hermit-Crab
(_Chondracanthus cornutus_), in eight successive stages (after E.
von Beneden). C1 to C5, ditto of a Cat, in five successive stages
(after Pflueger). D, ditto of Trout; E, of a Hen; F, of Man. The
first series is taken from the _Encycl. Brit._; the second from
Haeckel's _Evolution of Man_.]
[Illustration: FIG. 32.--Human ovum, mature and greatly magnified.
(After Haeckel.)]
In thus saying that the ova of all animals are, so far as microscopes
can reveal, _substantially_ similar, I am of course speaking of the
egg-cell proper, and not of what is popularly known as the egg. The egg
of a bird, for example, is the egg-cell, _plus_ an enormous aggregation
of nutritive material, an egg-shell, and sundry other structures suited
to the subsequent development of the egg-cell when separated from the
parent's body. But all these accessories are, from our present point of
view, accidental or adventitious. What we have now to understand by the
ovum, the egg, or the egg-cell, is the microscopical germ which I have
just described. So far then as this germ is concerned, we find that all
multicellular organisms begin their existence in the same kind of
structure, and that this structure is anatomically indistinguishable
from that of the permanent form presented by the lowest, or unicellular
organisms. But although anatomically indistinguishable, physiologically
they present the sundry peculiarities already mentioned.
Now I have endeavoured to show that none of these peculiarities are such
as to exclude--or even so much as to invalidate--the supposition of
developmental continuity between the lowest egg-cells and the highest
protozoal cells. It remains to show in this place, and on the other
hand, that there is no breach of continuity between the lowest and the
highest egg-cells; but, on the contrary, that the remarkable uniformity
of the complex processes whereby their peculiar characters are exhibited
to the histologist, is such as of itself to sustain the doctrine of
continuity in a singularly forcible manner. On this account, therefore,
and also because the facts will again have to be considered in another
connexion when we come to deal with Weismann's theory of heredity, I
will here briefly describe the processes in question.
We have already seen that the young egg-cell multiplies itself by simple
binary division, after the manner of unicellular organisms in
general--thereby indicating, as also by its amoebiform movements, its
fundamental identity with such organisms in kind. But, as we have
likewise seen, when the ovum ceases to resemble these organisms, by
taking on its higher degree of functional capacity, it is no longer able
to multiply itself in this manner. On the contrary, its cell-divisions
are now of an endogenous character, and result in the formation of many
different kinds of cells, in the order required for constructing the
multicellular organism to which the whole series of processes eventually
give rise. We have now to consider these processes _seriatim_.
[Illustration: FIG. 33.--Stages in the formation of the polar bodies
in the ovum of a star-fish. (After Hertwig.) _g.v._, germinal
vesicle transformed into a spindle-shaped system of fibres; _p.'_,
the first polar body becoming extruded; _p._, _p._, both polar
bodies fully extruded; _f. pn._, female pronucleus, or residue of
the germinal vesicle.]
First of all the nucleus discharges its polar bodies, as previously
mentioned, and in the manner here depicted on the previous page. (Fig.
33.) It will be observed that the nucleus of the ovum, or the germinal
vesicle as it is called, gets rid first of one and afterwards of the
other polar body by an "indirect," or karyokinetic, process of division.
(Fig. 33.) Extrusion of these bodies from the ovum (or it may be only
from the nucleus) having been accomplished, what remains of the nucleus
retires from the circumference of the ovum, and is called the female
pronucleus. (Fig. 33. _f. pn._) The ovum is now ready for fertilization.
A similar emission of nuclear substance is said by some good observers
to take place also from the male germ-cell, or spermatozooen, at or about
the close of _its_ development. The theories to which these facts have
given rise will be considered in future chapters on Heredity.
Turning now to the mechanism of fertilization, the diagrams (Figs. 34,
35) represent what happens in the case of star-fish.
[Illustration: FIG. 34.--Fertilization of the ovum of an echinoderm.
(From _Quain's Anatomy_, after Selenka.) S, spermatozooen; _m. pr._,
male pronucleus; _f. pr._, female pronucleus. 1 to 4 correspond to D
to G in the next figure.]
[Illustration: FIG. 35.--Fertilization of the ovum of a star-fish.
(From the _Encycl. Brit._ after Fol.) A, spermatozoa in the
mucilaginous coat of the ovum; a prominence is rising from the
surface of the ovum towards a spermatozooen; B, they have almost met;
C, they have met; D, the spermatozooen enters the ovum through a
distinct opening; H, the entire ovum, showing extruded polar bodies
on its upper surface, and the moving together of the male and female
pronuclei; E, F, G, meeting and coalescence of the pronuclei.]
The sperm-cell, or spermatozooen, is seen in the act of penetrating the
ovum. In the first figure it has already pierced the mucilaginous coat
of the ovum, the limit of which is represented by a line through which
the tail of the spermatozooen is passing: the head of the spermatozooen is
just entering the ovum proper. It may be noted that, in the case of many
animals, the general protoplasm of the ovum becomes aware, so to speak,
of the approach of a spermatozooen, and sends up a process to meet it.
(Fig. 35, A, B, C.) Several--or even many--spermatozoa may thus enter
the coat of the ovum; but normally only one proceeds further, or right
into the substance of the ovum, for the purpose of effecting
fertilization. This spermatozooen, as soon as it enters the periphery of
the yolk, or cell-substance proper, sets up a series of remarkable
phenomena. First, its own head rapidly increases in size, and takes on
the appearance of a cell-nucleus: this is called the male pronucleus. At
the same time its tail begins to disappear, and the enlarged head
proceeds to make its way directly towards the nucleus of the ovum which,
as before stated, is now called the female pronucleus. The latter in its
turn moves towards the former, and when the two meet they fuse into one
mass, forming a new nucleus. Before the two actually meet, the
spermatozooen has lost its tail altogether; and it is noteworthy that
during its passage through the protoplasmic cell-contents of the ovum,
it appears to exercise upon this protoplasm an attractive influence; for
the granules of the latter in its vicinity dispose themselves around it
in radiating lines. All these various phenomena are depicted in the
above wood-cuts. (Figs. 34, 35.)
Fertilization having been thus effected by fusion of the male and female
pronuclei into a single (or new) nucleus, this latter body proceeds to
exhibit complicated processes of karyokinesis, which, as before shown,
are preliminary to nuclear division in the case of egg-cells. Indeed the
karyokinetic process may begin in both the pronuclei before their
junction is effected; and, even when their junction is effected, it does
not appear that complete fusion of the so-called chromatin elements of
the two pronuclei takes place. For the purpose of explaining what this
means, and still more for the purpose of giving a general idea of the
karyokinetic processes as a whole, I will quote the following
description of them, because, for terseness combined with lucidity, it
is unsurpassable.
[Illustration: FIG. 36.--Karyokinesis of a typical tissue-cell
(epithelium of Salamander). (After Flemming and Klein.) The series
from A to I represents the successive stages in the movement of the
chromatin fibres during division, excepting G, which represents the
"nucleus-spindle" of an egg-cell. A, resting nucleus; D,
wreath-form; E, single star, the loops of the wreath being broken;
F, separation of the star into two groups of U-shaped fibres; H,
diaster or double star; I, completion of the cell-division and
formation of two resting nuclei. In G the chromatin fibres are
marked _a_, and correspond to the "equatorial plate"; _b_,
achromatin fibres forming the nucleus-spindle; _c_, granules of the
cell-protoplasm forming a "polar star." Such a polar star is seen at
each end of the nucleus-spindle, and is not to be confused with the
diaster H, the two ends of which are composed of _chromatin_.]
Researches, chiefly due to Flemming, have shown that the nucleus in
very many tissues of higher plants and animals consists of a
capsule containing a plasma of "achromatin," not deeply stained by
re-agents, ramifying in which is a reticulum of "chromatin"
consisting of fibres which readily take a deep stain. (Fig. 36, A).
Further it is demonstrated that, when the cell is about to divide
into two, definite and very remarkable movements take place in the
nucleus, resulting in the disappearance of the capsule and in the
arrangement of its fibres first in the form of a wreath (D), and
subsequently (by the breaking of the loops formed by the fibres) in
the form of a star (E). A further movement within the nucleus leads
to an arrangement of the broken loops in two groups (F), the
position of the open ends of the broken loops being reversed as
compared with what previously obtained. Now the two groups diverge,
and in many cases a striated appearance of the achromatin substance
between the two groups of chromatin loops is observable (H). In
some cases (especially egg-cells) this striated arrangement of the
achromatin is then termed a "nucleus-spindle," and the group of
chromatin loops (G, _a_) is known as "the equatorial plate." At
each end of the nucleus-spindle in these cases there is often seen
a star consisting of granules belonging to the general protoplasm
of the cell (G, _c_). These are known as "polar stars." After the
separation of the two sets of loops (H) the protoplasm of the
general substance of the cell becomes constricted, and division
occurs, so as to include a group of chromatin loops in each of the
two fission products. Each of these then rearranges itself together
with the associated chromatin into a nucleus such as was present in
the mother cell to commence with (I)[13].
[13] Ray Lankester, _Encyclop. Brit._, 9th ed., Vol. XIX, pp. 832-3.
Since the above was published, however, further progress has been made.
In particular it has been found that the chromatin fibres pass from
phase D to phase F by a process of longitudinal splitting (Fig. 37 _g_,
_h_; Fig. 38, VI, VII)--which is a point of great importance for
Weismann's theory of heredity,--and that the protoplasm outside the
nucleus seems to take as important a part in the karyokinetic process as
does the nuclear substance. For the so-called "attraction-spheres" (Fig.
38 II _a_, III, III _a_, VIII to XII), which were at first supposed to
be of subordinate importance in the process as a whole, are now known to
take an exceedingly active part in it (see especially IX to XI). Lastly,
it may be added that there is a growing consensus of authoritative
opinion, that the chromatin fibres are the seats of the material of
heredity, or, in other words, that they contain those essential elements
of the cell which endow the daughter-cells with their distinctive
characters. Therefore, where the parent-cell is an ovum, it follows from
this view that all hereditary qualities of the future organism are
potentially present in the ultra-microscopical structure of the
chromatin fibres.
[Illustration: FIG. 37.--Study of successive changes taking place in
the nucleus of an epithelium cell, preparatory to division of the
cell. (From _Quain's Anatomy_, after Flemming.) _a_, resting cell,
showing the nuclear network; _b_, first stage of division, the
chromatoplasm transformed into a skein of closely contorted
filaments; _c_ to _f_, further stages in the growth and looping
arrangement of the filaments; _g_, stellate phase, or aster; _h_,
completion of the splitting of the filaments, already begun in _f_
and _g_; _i_, _j_, _k_, successive stages in separation of the
filaments into two groups; _l_, the final result of this (diaster);
_m_ to _q_, stages in the division of the whole cell into two,
showing increasing contortion of the filaments, until they reach the
resting stage at _q_].
[Illustration: FIG. 38.--Formation and conjugation of the pronuclei
in _Ascaris megalocephala_. (From _Quain's Anatomy_, after E. von
Beneden.) _f_, female pronucleus; _m_, male pronucleus; _p_, one of
the polar bodies.
I. The second polar body has just been extruded; both male and
female pronuclei contain two chromatin particles; those of the male
pronucleus are becoming transformed into a skein. II. The chromatin
in both pronuclei now forms into a skein.
II _a_. The skeins are more distinct. Two attraction (or
protoplasmic) spheres, each with a central particle united with a
small spindle of achromatic fibres, have made their appearance in
the general substance of the egg close to the mutually approaching
pronuclei. The male pronucleus has the remains of the body of the
spermatozooen adhering to it.
III. Only the female pronucleus is shown in this figure. The skein
is contracted and thickened. The attraction-spheres are near one
side of the ovum, and are connected with its periphery by a cone of
fibres forming a polar circle, _p.c._; _e.c._, equatorial circle.
III _a_. The pronuclei have come into contact, and the
spindle-system is now arranged across their common axis.
IV. Contraction of the skein, and formation of two U-or V-shaped
chromatin fibres in each pronucleus.
V. The V-shaped chromatin filaments are now quite distinct: the male
and female pronuclei are in close contact.]
[Illustration: (38 continued)
VI., VII. The V-shaped filaments are splitting longitudinally; their
structure of fine granules of chromatin is apparent in VII., which
is more highly magnified. The conjugation of the pronuclei is
apparently complete in VII. The attraction-spheres and achromatic
spindle, although present, are not depicted in IV., V., VI., and
VII.
VIII. Equatorial arrangement of the four chromatin loops in the
middle of the now segmenting ovum: the achromatic substance forming
a spindle-shaped system of granules with fibres radiating from the
poles of the spindle (attraction-spheres); the chromatin forms an
equatorial plate. (Compare Fig. 36 G.)
IX. Shows diagrammatically the commencing separation of the
chromatin fibres of the conjugated nuclei, and the system of fibres
radiating from the attraction-spheres. (Compare again Fig. 36 G.)
_p.c._, polar circle; _e.c._, equatorial circle; _c.c._, central
particle.
X. Further separation of the chromatin filaments. Each of the
central particles of the attraction-spheres has divided into two.
XI. The chromatin fibres are becoming developed into the skeins of
the two daughter-nuclei. These are still united by fibres of
achromatin. The general protoplasm of the ovum is becoming divided.
XII. The two daughter-nuclei exhibit a chromatin network. Each of
the attraction-spheres has divided into two, which are joined by
fibres of achromatin, and connected with the periphery of the cell
in the same way as in the original or parent sphere, III.]
As I shall have more to say about these processes in the next volume,
when we shall see the important part which they bear in Weismann's
theory of heredity, it is with a double purpose that I here introduce
these yet further illustrations of them upon a somewhat larger scale.
The present purpose is merely that of showing, more clearly than
hitherto, the great complexity of these processes on the one hand, and,
on the other, the general similarity which they display in egg-cells and
in tissue-cells. But as in relation to this purpose the illustrations
speak for themselves, I may now pass on at once to the history of
embryonic development, which follows fertilization of the ovum.
* * * * *
We have seen that when the new nucleus of the fertilized ovum (which is
formed by a coalescence of the male pronucleus with the female) has
completed its karyokinetic processes, it is divided into two equal
parts; that these are disposed at opposite poles of the ovum; and that
the whole contents of the ovum are thereupon likewise divided into two
equal parts, with the result that there are now two nucleated cells
within the spherical wall of the ovum where before there had only been
one. Moreover, we have also seen that a precisely similar series of
events repeat themselves in each of these two cells, thus giving rise to
four cells (see Fig. 29). It must now be added that such duplication is
continued time after time, as shown in the accompanying illustrations
(Figs. 39, 40).
[Illustration: FIG. 39.--Segmentation of ovum. (After Haeckel.)
Successive stages are marked by the letters A, B, C. D represents
several stages in advance of C.]
[Illustration: FIG. 40.--The contents of an ovum in an advanced
stage of segmentation, drawn in perspective. (After Haeckel.)]
All this, it will be noticed, is a case of cell-multiplication, which
differs from that which takes place in the unicellular organisms only in
its being _invariably_ preceded (as far as we know) by karyokinesis, and
in the resulting cells being all confined within a common envelope, and
so in not being free to separate. Nevertheless, from what has already
been said, it will also be noticed that this feature makes all the
difference between a Metazooen and a Protozooen; so that already the ovum
presents the distinguishing character of a Metazooen.
I have dealt thus at considerable length upon the processes whereby the
originally unicellular ovum and spermatozooen become converted into the
multicellular germ, because I do not know of any other exposition of the
argument from Embryology where this, the first stage of the argument,
has been adequately treated. Yet it is evident that the fact of all the
processes above described being so similar in the case of sexual (or
metazoal) reproduction among the innumerable organisms where it occurs,
constitutes in itself a strong argument in favour of evolution. For the
mechanism of fertilization, and all the processes which even thus far we
have seen to follow therefrom, are hereby shown to be not only highly
complex, but likewise highly specialized. Therefore, the remarkable
similarity which they present throughout the whole animal kingdom--not
to speak of the vegetable--is expressive of organic continuity, rather
than of absolute discontinuity in every case, as the theory of special
creation must necessarily suppose. And it is evident that this argument
is strong in proportion to the uniformity, the specialization, and the
complexity of the processes in question.
Having occupied so much space with supplying what appear to me the
deficiencies in previous expositions of the argument from Embryology, I
can now afford to take only a very general view of the more important
features of this argument as they are successively furnished by all the
later stages of individual development. But this is of little
consequence, seeing that from the point at which we have now arrived
previous expositions of the argument are both good and numerous. The
following then is to be regarded as a mere sketch of the evidences of
phyletic (or ancestral) evolution, which are so abundantly furnished by
all the subsequent phases of ontogenetic (or individual) evolution.
The multicellular body which is formed by the series of segmentations
above described is at first a sphere of cells (Fig. 40). Soon, however,
a watery fluid gathers in the centre, and progressively pushes the cells
towards the circumference, until they there constitute a single layer.
The ovum, therefore, is now in the form of a hollow sphere containing
fluid, confined within a continuous wall of cells (Fig. 41 A). The next
thing that happens is a pitting in of one portion of the sphere (B). The
pit becomes deeper and deeper, until there is a complete invagination of
this part of the sphere--the cells which constitute it being
progressively pushed inwards until they come into contact with those at
the opposite pole of the ovum. Consequently, instead of a hollow sphere
of cells, the ovum now becomes an open sac, the walls of which are
composed of a double layer of cells (C). The ovum is now what has been
called a gastrula; and it is of importance to observe that probably all
the Metazoa pass through this stage. At any rate it has been found to
occur in all the main divisions of the animal kingdom, as a glance at
the accompanying figures will serve to show (Fig. 42)[14]. Moreover many
of the lower kinds of Metazoa never pass beyond it; but are all their
lives nothing else than gastrulae, wherein the orifice becomes the mouth
of the animal, the internal or invaginated layer of cells the stomach,
and the outer layer the skin. So that if we take a child's india-rubber
ball, of the hollow kind with a hole in it, and push in one side with
our fingers till internal contact is established all round, by then
holding the indented side downwards we should get a very fair anatomical
model of a gastraea form, such as is presented by the adult condition of
many of the most primitive Metazoa--especially the lower
_Coelenterata_. The preceding figures represent two other such forms
in nature, the first locomotive and transitory, the second fixed and
permanent (Figs. 43, 44).
[14] In most vertebrated animals this process of gastrulation has
been more or less superseded by another, which is called
delamination; but it scarcely seems necessary for our present
purposes to describe the latter. For not only does it eventually
lead to the same result as gastrulation--i. e. the converting of the
ovum into a double-walled sac,--but there is good evidence among the
lower Vertebrata of its being preceded by gastrulation; so that,
even as to the higher Vertebrata, embryologists are pretty well
agreed that delamination has been but a later development of, or
possibly improvement upon, gastrulation.
[Illustration: FIG. 41.--Formation of the gastrula of _Amphioxus_.
(After Kowalevsky.) A, wall of the ovum, composed of a single layer
of cells; B, a stage in the process of gastrulation; C, completion
of the process; S, original or segmentation cavity of ovum; _al_,
alimentary cavity of gastrula; _ect_, outer layer of cells; _ent_,
inner layer of cells; _b_, orifice, constituting the mouth in
permanent forms.]
[Illustration: FIG. 42.--Gastrulation. A, Gastrula of a Zoophyte
(_Gastrophysema_). (After Haeckel.) B, Gastrula of a Worm
(_Sagitta_). (After Kowalevsky.) C, Gastrula of an Echinoderm
(_Uraster_). (After A. Agassiz.) D, Gastrula of an Arthropod
(_Nauplius_). (After Haeckel.) E, Gastrula of a Mollusk (_Limnaeus_).
(After Rabl.) F, Gastrula of a Vertebrate (_Amphioxus_). (After
Kowalevsky.) In all, _d_, indicates the intestinal cavity; _o_, the
primitive mouth; _s_, the cleavage-cavity; _i_, the endoderm, or
intestinal layer; _e_, the ectoderm or skin-layer.]
[Illustration: FIG. 43.--Gastrula of a Chalk Sponge. (After Haeckel.)
A, External view. B, Longitudinal section. _g_, digestive cavities;
_o_, mouth; _i_, endoderm; _e_, ectoderm.]
[Illustration: FIG. 44.--_Prophysema primordiale_, an extant
gastraea-form. (After Haeckel.) (A). External view of the whole
animal, attached by its foot to seaweed. (B). Longitudinal section
of the same. The digestive cavity (_d_) opens at its upper end in
the mouth (_m_). Among the cells of the endoderm (_g_) lie amoeboid
egg-cells of large size (_e_). The ectoderm (_h_) is encrusted with
grains of sand, above the sponge spicules.]
Here, then, we leave the lower forms of Metazoa in their condition of
permanent gastrulae. They differ from the transitory stage of other
Metazoa only in being enormously larger (owing to greatly further
_growth_, without any further _development_ as to matters of fundamental
importance), and in having sundry tentacles and other organs added later
on to meet their special requirements. The point to remember is, that in
all cases a gastrula is an open sac composed of two layers of cells--the
outer layer being called the ectoderm, and the inner the endoderm. They
have also been called the animal layer and the vegetative layer, because
it is the outer layer (ectoderm) that gives rise to all the organs of
sensation and movement--viz. the skin, the nervous system, and the
muscular system; while it is the inner layer (endoderm) that gives rise
to all the organs of nutrition and reproduction. It is desirable only
further to explain that gastrulation does not take place in all the
Metazoa after exactly the same plan. In different lines of descent
various and often considerable modifications of the original and most
simple plan have been introduced; but I will not burden the present
exposition by describing these modifications[15]. It is enough for us
that they always end in the formation of the two primary layers of
ectoderm and endoderm.
[15] The most extreme of them is that which is mentioned in the last
foot-note.
The next stage of differentiation is common to all the Metazoa, except
those lowest forms which, as we have just seen, remain permanently as
large gastrulae, with sundry specialized additions in the way of
tentacles, &c. This stage of differentiation consists in the formation
of either a pouch or an additional layer between the ectoderm and the
endoderm, which is called the mesoderm. It is probably in most cases
derived from the endoderm, but the exact mode of its derivation is still
somewhat obscure. Sometimes it has the appearance of itself constituting
two layers; but it is needless to go into these details; for in any case
the ultimate result is the same--viz. that of converting the Metazooen
into the form of a tube, the walls of which are composed of concentric
layers of cells. The outermost layer afterwards gives rise to the
epidermis with its various appendages, and also to the central nervous
system with its organs of special sense. The median layer gives rise to
the voluntary muscles, bones, cartilages, &c., the nutritive systems of
the blood, the chyle, the lymph, and the muscular tube of the intestine.
Lastly, the innermost layer developes into the epithelium lining of the
intestine, with its various appendages of liver, lungs, intestinal
glands, &c.
I have just said that this three or four layered stage is shared by all
the Metazoa, except those very lowest forms--such as sponges and
jelly-fish--which do not pass on to it. But from this point the
developmental histories of all the main branches of the Metazoa
diverge--the Vermes, the Echinodermata, the Mollusca, the Articulata,
and the Vertebrata, each taking a different road in their subsequent
evolution. I will therefore confine attention to only one of these
several roads or methods, namely, that which is followed by the
Vertebrata--observing merely that, if space permitted, the same
principles of progressive though diverging histories of evolution would
equally well admit of being traced in all the other sub-kingdoms which
have just been named.
In order to trace these principles in the case of the Vertebrata, it is
desirable first of all to obtain an idea of the anatomical features
which most essentially distinguish the sub-kingdom as a whole. The
following, then, is what may be termed the ideal plan of vertebrate
organization, as given by Prof. Haeckel. First, occupying the major axis
of body we perceive the primitive vertebral column. The parts lying
above this axis are those which have been developed from the ectoderm
and mesoderm--viz. voluntary muscles, central nervous system, and organs
of special sense. The parts lying below this axis are for the most part
those which have been developed from the endoderm--namely, the
digestive tract with its glandular appendages, the circulating system
and the respiratory system. In transverse section, therefore, the ideal
vertebrate consists of a solid axis, with a small tube occupied by the
nervous system above, and a large tube, or body-cavity, below. This
body-cavity contains the viscera, breathing organs, and heart, with its
prolongations into the main blood-vessels of the organism. Lastly, on
either side of the central axis are to be found large masses of
muscle--two on the dorsal and two on the ventral. As yet, however, there
are no limbs, nor even any bony skeleton, for the primitive vertebral
column is hitherto unossified cartilage. This ideal animal, therefore,
is to all appearance as much like a worm as a fish, and swims by means
of a lateral undulation of its whole body, assisted, perhaps, by a
dorsal fin formed out of skin.
[Illustration: FIG. 45.--Ideal primitive vertebrate, seen from the
left side. (After Haeckel.) _na_, nose; _au_, eye; _g_, ear; _md_,
mouth; _ks_, gill-openings; _x_, notochord; _mr_, spinal tube; _kg_,
gill-vessels; _k_, gill-intestine; _hz_, heart; _ms_, muscles; _ma_,
stomach; _v_, intestinal vein; _c_, body-cavity; _a_, aorta; _l_,
liver; _d_, small intestine; _e_, ovary; _h_, testes; _n_, kidney
canal; _af_, anus; _lh_, true or leather-skin; _oh_, outer-skin
(epidermis); _f_, skin-fold, acting as a fin.]
[Illustration: FIG. 46.--The same in transverse section through the
ovaries; lettering as in the preceding Fig.]
[Illustration: FIG. 47.--_Amphioxus lanceolatus_. (After Haeckel.)
_a_, anus; _au_, eye; _b_, ventral muscles; _c_, body-cavity; _ch_,
notochord; _d_, intestine; _do_ and _du_, dorsal and ventral walls
of intestine; _f_, fin-seam; _h_, skin; _k_, gills; _ka_,
gill-artery; _lb_, liver; _lv_, liver-vein; _m 1_, brain-bladder; _m
2_, spinal marrow; _mg_, stomach; _o_, mouth; _p_, ventral pore;
_r_, dorsal muscle; _s_, tail-fin; _t_, aorta; _v_, intestinal vein;
_x_, boundary between gill-intestine and stomach-intestine; _y_,
hypobranchial groove.]
Now I should not have presented this ideal representation of a primitive
vertebrate--for I have very little faith in the "scientific use of the
imagination" where it aspires to discharge the functions of a Creator in
the manufacture of archetypal forms--I say I should not have presented
this ideal representative of a primitive vertebrate, were it not that
the ideal is actually realized in a still existing animal. For there
still survives what must be an immensely archaic form of vertebrate,
whose anatomy is almost identical with that of the imaginary type which
has just been described. I allude, of course, to _Amphioxus_, which is
by far the most primitive or generalized type of vertebrated animal
hitherto discovered. Indeed, we may say that this remarkable creature is
almost as nearly allied to a worm as it is to a fish. For it has no
specialized head, and therefore no skull, brain, or jaws: it is
destitute alike of limbs, of a centralized heart, of developed liver,
kidneys, and, in short, of most of the organs which belong to the other
Vertebrata. It presents, however, a rudimentary backbone, in the form of
what is called a notochord. Now a primitive dorsal axis of this kind
occurs at a very early period of embryonic life in all vertebrated
animals; but, with the exception of _Amphioxus_, in all other existing
Vertebrata this structure is not itself destined to become the permanent
or bony vertebral column. On the contrary, it gives way to, or is
replaced by, this permanent bony structure at a later stage of
development. Consequently, it is very suggestive that so distinctively
embryonic a structure as this temporary cartilaginous axis of all the
other known Vertebrata should be found actually persisting to the
present day as the permanent axis of _Amphioxus_. In many other
respects, likewise, the early embryonic history of other Vertebrata
refers us to the permanent condition of _Amphioxus_. In particular, we
must notice that the wall of the neck is always perforated by what in
_Amphioxus_ are the gill-openings, and that the blood-vessels as they
proceed from the heart are always distributed in the form of what are
called gill-arches, adapted to convey the blood round or through the
gills for the purpose of aeration. In all existing fish and other
gill-breathing Vertebrata, this arrangement is permanent. It is
likewise met with in a peculiar kind of worm, called _Balanoglossus_--a
creature so peculiar, indeed, that it has been constituted by Gegenbaur
a class all by itself. We can see by the wood-cuts that it presents a
series of gill-slits, like the homologous parts of the fishes with which
it is compared--i. e. fishes of a comparatively low type of
organization, which dates from a time before the development of external
gills. (Figs. 48, 49, 50.) Now, as I have already said, these
gill-_slits_ are supported internally by the gill-_arches_, or the
blood-vessels which convey the blood to be oxygenized in the branchial
apparatus (see below, Figs. 51, 52, 53); and the whole arrangement is
developed from the anterior part of the intestine--as is likewise the
respiratory mechanism of all the gill-breathing Vertebrata. That so
close a parallel to this peculiar mechanism should be met with in a
worm, is a strong additional piece of evidence pointing to the
derivation of the Vertebrata from the Vermes.
[Illustration: FIG. 48.--_Balanoglossus_. (After A. Agassiz.) _r_,
proboscis; _h_, collar; _k_, gill-slits; _d_, digestive posterior
intestine; _v_, intestinal vessel; _a_, anus.]
[Illustration: FIG. 49.--A large Sea-lamprey (_Petromyzon marinus_),
much reduced in size. (After Cuvier and Haeckel.) A series of seven
gill-slits are visible.]
[Illustration: FIG. 50--Adult Shark (_Carcharias melanopterus_).
(After Cuvier and Haeckel.)]
[Illustration: FIG. 51.--Diagram of heart and gill-arches of a fish.
(After Owen.)]
[Illustration: FIG. 52.--One gill-arch, with branchial fringe
attached. (After Owen.) H, Heart.]
[Illustration: FIG. 53.--Diagram of heart and gill-arches in a
lizard. (After Owen.) The gill-arches, _a a' a''_, and _b b' b''_,
are called aortic arches in air-breathing vertebrata.]
Well, I have just said that in all the gill-breathing Vertebrata, this
mechanism of gill-slits and vascular gill-arches in the front part of
the intestinal tract is permanent. But in the air-breathing Vertebrata
such an arrangement would obviously be of no use. Consequently, the
gill-slits in the sides of the neck (see Figs. 16 and 57, 58), and the
gill-arches of the large blood-vessels (Figs. 54, 55, 56), are here
exhibited only as transitory phases of development. But as such they
occur in all air-breathing Vertebrata. And, as if to make the homologies
as striking as possible, at the time when the gill-slits and the
gill-arches are developed in the embryonic young of air-breathing
Vertebrata, the heart is constructed upon the fish-like type. That is to
say, it is placed far forwards, and, from having been a simple tube as
in Worms, is now divided into two chambers, as in Fish. Later on it
becomes progressively pushed further back between the developing lungs,
while it progressively acquires the three cavities distinctive of
Amphibia, and finally the four cavities belonging only to the complete
double circulation of Birds and Mammals. Moreover, it has now been
satisfactorily shown that the lungs of air-breathing Vertebrata, which
are thus destined to supersede the function of gills, are themselves the
modified swim-bladder or float, which belongs to Fish. Consequently, all
these progressive modifications in the important organs of circulation
and respiration in the air-breathing Vertebrata, together make up as
complete a history of their aquatic pedigree as it would be possible for
the most exacting critic to require.
[Illustration: FIG. 54.--Ideal diagram, of primitive gill-or
aortic-arches. (After Rathke.) H, outline of heart. The arrows show
the course of the blood.]
[Illustration: FIG. 55.--The same, modified for a bird. (After Le
Conte.) The dark lines show the aortic arches which persist. A,
aorta; _p_, pulmonary arches; SC, S'C', sub-clavian; C, C',
carotids.]
[Illustration: FIG. 56.--The same, modified for a mammal. (After Le
Conte.)]
[Illustration: FIG. 57.--A series of embryos at three comparable and
progressive stages of development (marked I, II, III), representing
each of the classes of vertebrated animals below the Mammalia (After
Haeckel.)]
[Illustration: FIG. 58.--Another series of embryos, also at three
comparable and progressive stages of development (marked I, II,
III), representing four different divisions of the class Mammalia.
(After Haeckel.)]
If space permitted, it would be easy to present abundance of additional
evidence to the same effect from the development of the skeleton, the
skull, the brain, the sense-organs, and, in short, of every constituent
part of the vertebrate organization. Even without any anatomical
dissection, the similarity of all vertebrated embryos at comparable
stages of development admits of being strikingly shown, if we merely
place the embryos one beside the other. Here, for instance, are the
embryos of a fish, a salamander, a tortoise, a bird, and four different
mammals. In each case three comparable stages of development are
represented. Now, if we read the series horizontally, we can see that
there is very little difference between the eight animals at the
earliest of the three stages represented--all having fish-like tails,
gill-slits, and so on. In the next stage further differentiation has
taken place, but it will be observed that the limbs are still so
rudimentary that even in the case of Man they are considerably shorter
than the tail. But in the third stage the distinctive characters are
well marked.
* * * * *
So much then for an outline sketch of the main features in the embryonic
history of the Vertebrata. But it must be remembered that the science of
comparative embryology extends to each of the other three great branches
of the tree of life, where these take their origin, through the worms,
from the still lower, or gastraea, forms. And in each of these three
great branches--namely, the Echinodermata, the Mollusca, and the
Arthropoda--we have a repetition of just the same kind of evidence in
favour of continuous descent, with adaptive modification in sundry
lines, as that which I have thus briefly sketched in the case of the
Vertebrata. The roads are different, but the method of travelling is the
same. Moreover, when the embryology of the Worms is closely studied, the
origin of these different roads admits of being clearly traced. So that
when all this mass of evidence is taken together, we cannot wonder that
evolutionists should now regard the science of comparative embryology as
the principal witness to their theory.
CHAPTER V.
PALAEONTOLOGY.
The present Chapter will be devoted to a consideration of the evidence
of organic evolution which has been furnished by the researches of
geologists. On account of its direct or historical nature, this branch
of evidence is popularly regarded as the most important--so much so,
indeed, that in the opinion of most educated persons the whole doctrine
of organic evolution must stand or fall according to the so-called
"testimony of the rocks." Now, without at all denying the peculiar
importance of this line of evidence, I must begin by remarking that it
does not present the denominating importance which popular judgment
assigns to it. For although popular judgment is right in regarding the
testimony of the rocks as of the nature of a history, this judgment, as
a rule, is very inadequately acquainted with the great imperfections of
that history. Knowing in a general way what magnificent advances the
science of geology has made during the present century, the public mind
is more or less imbued with the notion, that because we now possess a
tolerably complete record of the chronological succession of geological
formations, we must therefore possess a correspondingly complete record
of the chronological succession of the forms of life which from time to
time have peopled the globe. Now in one sense this notion is partly
true, but in another sense it is profoundly false. It is partly true if
we have regard only to those larger divisions of the vegetable or animal
kingdoms which naturalists designate by the terms classes and orders.
But the notion becomes progressively more untrue when it is applied to
families and genera, while it is most of all untrue when applied to
species. That this must be so may be rendered apparent by two
considerations.
In the first place, it does not follow that because we have a tolerably
complete record of the succession of geological formations, we have
therefore any correspondingly complete record of their fossiliferous
contents. The work of determining the relative ages of the rocks does
not require that every cubic mile of the earth's surface should be
separately examined, in order to find all the different fossils which it
may contain. Were this the case, we should hitherto have made but very
small progress in our reading of the testimony of the rocks. The
relative ages of the rocks are determined by broad comparative surveys
over extensive areas; and although the identification of widely
separated deposits is often greatly assisted by a study of their
fossiliferous contents, the mere pricking of a continent here and there
is all that is required for this purpose. Hence, the accuracy of our
information touching the relative ages of geological strata does not
depend upon--and, therefore, does not betoken--any equivalent accuracy
of knowledge touching the fossiliferous material which these strata may
at the present time actually contain. And, as we well know, the
opportunities which the geologist has of discovering fossils are
extremely limited, if we consider these opportunities in relation to the
area of geological formations. The larger portion of the earth's surface
is buried beneath the sea; and much the larger portion of the
fossiliferous deposits on shore are no less hopelessly buried beneath
the land. Therefore it is only upon the fractional portion of the
earth's surface which at the present time happens to be actually exposed
to his view that the geologist is able to prosecute his search for
fossils. But even here how miserably inadequate this search has hitherto
been! With the exception of a scratch or two in the continents of Asia
and America, together with a somewhat larger number of similar scratches
over the continent of Europe, even that comparatively small portion of
the earth's surface which is available for the purpose has been hitherto
quite unexplored by the palaeontologist. How enormously rich a store of
material remains to be unearthed by the future scratchings of this
surface, we may dimly surmise from the astonishing world of bygone life
which is now being revealed in the newly discovered fossiliferous
deposits on the continent of America.
But, besides all this, we must remember, in the second place, that all
the fossiliferous deposits in the world, even if they could be
thoroughly explored, would still prove highly imperfect, considered as a
history of extinct forms of life. In order that many of these forms
should have been preserved as fossils, it is necessary that they should
have died upon a surface neither too hard nor too soft to admit of their
leaving an impression; that this surface should afterwards have
hardened sufficiently to retain the impression; that it should then have
been protected from the erosion of water, as well as from the
disintegrating influence of the air; and yet that it should not have
sunk far enough beneath the surface to have come within the no less
disintegrating influence of subterranean heat. Remembering thus, as a
general rule, how many conditions require to have met before a fossil
can have been both formed and preserved, we must conclude that the
geological record is probably as imperfect in itself as are our
opportunities of reading even the little that has been recorded. If we
speak of it as a history of the succession of life upon the planet, we
must allow, on the one hand, that it is a history which merits the name
of a "chapter of accidents"; and, on the other hand, that during the
whole course of its compilation pages were being destroyed as fast as
others were being formed, while even of those that remain it is only a
word, a line, or at most a short paragraph here and there, that we are
permitted to see. With so fragmentary a record as this to study, I do
not think it is too much to say that no conclusions can be fairly based
upon it, merely from the absence of testimony. Only if the testimony
were positively opposed to the theory of descent, could any argument be
fairly raised against that theory on the grounds of this testimony. In
other words, if any of the fossils hitherto discovered prove the order
of succession to have been incompatible with the theory of genetic
descent, then the record may fairly be adduced in argument, because we
should then be in possession of definite information of a positive
kind, instead of a mere absence of information of any kind. But if the
adverse argument reaches only to the extent of maintaining that the
geological record does not furnish us with so complete a series of
"connecting links" as we might have expected, then, I think, the
argument is futile. Even in the case of human histories, written with
the intentional purpose of conveying information, it is an unsafe thing
to infer the non-occurrence of an event from a mere silence of the
historian--and this especially in matters of comparatively small detail,
such as would correspond (in the present analogy) to the occurrence of
_species_ and _genera_ as connecting links. And, of course, if the
history had only come down to us in fragments, no one would attach any
importance at all to what might have been only the _apparent_ silence of
the historian.
In view, then, of the unfortunate imperfection of the geological record
_per se_, as well as of the no less unfortunate limitation of our means
of reading even so much of the record as has come down to us, I conclude
that this record can only be fairly used in two ways. It may fairly be
examined for positive testimony against the theory of descent, or for
proof of the presence of organic remains of a high order of development
in a low level of strata. And it may be fairly examined for negative
testimony, or for the absence of connecting links, if the search be
confined to the larger taxonomic divisions of the fauna and flora of the
world. The more minute these divisions, the more restricted must have
been the areas of their origin, and hence the less likelihood of their
having been preserved in the fossil state, or of our finding them even
if they have been. Therefore, if the theory of evolution is true, we
ought not to expect from the geological record a full history of
_specific_ changes in any but at most a comparatively small number of
instances, where local circumstances happen to have been favourable for
the writing and preservation of such a history. But we might reasonably
expect to find a general concurrence of geological testimony to the
larger fact--namely, of there having been throughout all geological time
a uniform progression as regards the larger taxonomic divisions. And, as
I will next proceed to show, this is, in a general way, what we do find,
although not altogether without some important exceptions, with which I
shall deal in an Appendix.
There is no _positive_ proof _against_ the theory of descent to be drawn
from a study of palaeontology, or proof of the presence of any kind of
fossils in strata where the fact of their presence is incompatible with
the theory of evolution. On the other hand, there is an enormous body of
uniform evidence to prove two general facts of the highest importance in
the present connexion. The first of these general facts is, that an
increase in the diversity of types both of plants and animals has been
constant and progressive from the earliest to the latest times, as we
should anticipate that it must have been on the theory of descent in
ever-ramifying lines of pedigree. And the second general fact is, that
through all these branching lines of ever-multiplying types, from the
first appearance of each of them to their latest known conditions, there
is overwhelming evidence of one great law of organic nature--the law of
gradual advance from the general to the special, from the low to the
high, from the simple to the complex.
Now, the importance of these large and general facts in the present
connexion must be at once apparent; but it may perhaps be rendered more
so if we try to imagine how the case would have stood supposing
geological investigation to have yielded in this matter an opposite
result, or even so much as an equivocal result. If it had yielded an
opposite result, if the lower geological formations were found to
contain as many, as diverse, and as highly organized types as the later
geological formations, clearly there would have been no room at all for
any theory of progressive evolution. And, by parity of reasoning, in
whatever degree such a state of matters were found to prevail, in that
degree would the theory in question have been discredited. But seeing
that these opposite principles do not prevail in any (relatively
speaking) considerable degree[16], we have so far positive testimony of
the largest and most massive character in favour of this theory. For
while all these large and general facts are very much what they ought to
be according to this theory, they cannot be held to lend any support at
all to the rival theory. In other words, it is clearly no essential part
of the theory of special creation that species should everywhere exhibit
this gradual multiplication as to number, coupled with a gradual
diversification and general elevation of types, in all the growing
branches of the tree of life. No one could adopt seriously the jocular
lines of Burns, to the effect that the Creator required to practise his
prentice hand on lower types before advancing to the formation of
higher. Yet, without some such assumption, it would be impossible to
explain, on the theory of independent creations, why there should have
been this gradual advance from the few to the many, from the general to
the special, from the low to the high.
[16] For objections which may be brought against this and similar
statements, see the Appendix.
+---+--------------------------+---------------------------------------
| |_Epochs and Formations._ |_Faunal Characters._
|C | |
|a |--------------------------+---------------------------------------
|i |POST-PLIOCENE. |Man. Mammalia principally of living
|n | Glacial Period. | species. Mollusca exclusively recent.
|o | +---------------------------------------
|z |PLIOCENE, 3,000 feet. |Mammalia principally of recent genera
|o | | --living species rare. Mollusca very
|i | | modern.
|c | +---------------------------------------
| |MIOCENE, 4,000 ft. |Mammalia principally of living
|o | | families; extinct genera numerous;
|r | | species all extinct. Mollusca largely
| |OLIGOCENE, 8,000 ft. | of recent species.
|T | +---------------------------------------
|e |EOCENE, 10,000 ft. |Mammalia with numerous extinct families
|r | | and orders; all the species and
|i | | most of the genera extinct. Modern
|a | | type Shell-Fish.
|r | |
|y | |
+---|--------------------------+---------------------------------------
| |LARAMIE, 4,000 ft. |Passage beds.
| |--------------------------+---------------------------------------
|M |CRETACEOUS, 12,000 ft. |Dinosaurian (bird-like) Reptiles;
|e | Chalk. | Pterodactyls (flying Reptiles);
|s | | toothed Birds; earliest Snake; bony
|o | | Fishes; Crocodiles; Turtles;
|z | | Ammonites.
|o | +---------------------------------------
|i |JURASSIC, 6,000 ft. |Earliest Birds; giant Reptiles
|c | Oolite. | (Ichthyosaurs, Dinosaurs,
| | Lias. | Pterodactyls); Ammonites; Clam- and
|o | | Snail-Shells very abundant; decline
|r | | of Brachiopods; Butterfly.
| | +---------------------------------------
|S |TRIAS, 5,000 ft. |First Mammalian (Marsupial); 2-gilled
|e | New Red Sandstone. | Cephalopods (Cuttle-Fishes,
|c | | Belemnites); reptilian Foot-Prints.
|o | |
|n | |
|d | |
|a | |
|r | |
|y | |
|---|--------------------------+---------------------------------------
|P |PERMIAN, 5,000 ft. |Earliest true Reptiles.
|a | +---------------------------------------
|l |CARBONIFEROUS, 26,000 ft. |Earliest Amphibian (Labyrinthodont);
|e | | extinction of Trilobites; first
|o | Coal. | Cray-fish; Beetles; Cockroaches;
|z | | Centipedes; Spiders.
|o | +---------------------------------------
|i |DEVONIAN, 18,000 ft. |Cartilaginous and Ganoid Fishes;
|c | Old Red Sandstone. | earliest and (snail) and freshwater
| | | Shells; Shell-Fish abundant; decline
|o | | of Trilobites; May-flies; Crab.
|r | +---------------------------------------
| |SILURIAN, 33,000 ft. |Earliest Fish; the first Air-Breathers
|P | | (Insect, Scorpion); Brachiopods and
|r | | 4-gilled Cephalopods very abundant;
|i | | Trilobites; Corals; Graptolites.
|m | +---------------------------------------
|a |CAMBRIAN, 24,000 ft. |Trilobites; Brachiopod Mollusks.
|r | |
|y | |
|---|--------------------------+---------------------------------------
|A |ARCHAEEAN, 30,000 ft. |
|z | Huronian. |Eozooen, (probably not a fossil).
|o | Laurentian. |
|i |--------------------------+---------------------------------------
|c |PRIMEVAL. |Non-sedimentary.
+------------------------------+--------------------------------------+
I submit, then, that so far as the largest and most general principles
in the matter of palaeontology are concerned, we have about as strong and
massive a body of evidence as we could reasonably expect this branch of
science to yield; for it is at once enormous in amount and positive in
character. Therefore, if I do not further enlarge upon the evidence
which we here have, as it were _en masse_, it is only because I do not
feel that any words could add to its obvious significance. It may best
be allowed to speak for itself in the millions of facts which are
condensed in this tabular statement of the order of succession of all
the known forms of animal life, as presented by the eminent
palaeontologist, Professor Cope[17].
[17] For difficulties and objections, see Appendix.
Or, taking a still more general survey, this tabular statement may be
still further condensed, and presented in a diagrammatic form, as it has
been by another eminent American palaeontologist, Prof. Le Conte, in his
excellent little treatise on _Evolution and its Relations to Religious
Thought_. The following is his diagrammatic representation, with his
remarks thereon.
When each ruling class declined in importance, it did not perish,
but continued in a subordinate position. Thus, the whole organic
kingdom became not only higher and higher in its highest forms, but
also more and more complex in its structure and in the interaction
of its correlated parts. The whole process and its result is
roughly represented in the accompanying diagram, in which A B
represents the course of geological time, and the curve, the rise,
culmination, and decline of successive dominate classes.
[Illustration: FIG. 59.--Diagram of Geological Succession of the
Classes of the Animal Kingdom. (After Le Conte.)]
I will here leave the evidence which is thus yielded by the most general
principles that have been established by the science of palaeontology;
and I will devote the rest of this chapter to a detailed consideration
of a few highly special lines of evidence. By thus suddenly passing from
one extreme to the other, I hope to convey the best idea that can be
conveyed within a brief compass of the minuteness, as well as the
extent, of the testimony which is furnished by the rocks.
* * * * *
When Darwin first published his _Origin of Species_, adverse critics
fastened upon the "missing-link" argument as the strongest that they
could bring against the theory of descent. Although Darwin had himself
strongly insisted on the imperfection of the geological record, and the
consequent precariousness of any negative conclusions raised upon it,
these critics maintained that he was making too great a demand upon the
argument from ignorance--that, even allowing for the imperfection of the
record, they would certainly have expected at least a few cases of
testimony to _specific_ transmutation. For, they urged in effect,
looking to the enormous profusion of the extinct species on the one
hand, and to the immense number of known fossils on the other, it was
incredible that no satisfactory instances of specific transmutation
should ever have been brought to light, if such transmutation had ever
occurred in the universal manner which the theory was bound to suppose.
But since Darwin first published his great work palaeontologists have
been very active in discovering and exploring fossiliferous beds in
sundry parts of the world; and the result of their labours has been to
supply so many of the previously missing links that the voice of
competent criticism in this matter has now been well-nigh silenced.
Indeed, the material thus furnished to an advocate of evolution at the
present time is so abundant that his principal difficulty is to select
his samples. I think, however, that the most satisfactory result will be
gained if I restrict my exposition to a minute account of some few
series of connecting links, rather than if I were to take a more general
survey of a larger number. I will, therefore, confine the survey to the
animal kingdom, and there mention only some of the cases which have
yielded well-detailed proof of continuous differentiation.
It is obvious that the parts of animals most likely to have been
preserved in such a continuous series of fossils as the present line of
evidence requires, would have been the hard parts. These are horns,
bones, teeth, and shells. Therefore I will consider each of these four
classes of structures separately.