THE ARGUMENTS FROM EMBRYOLOGY.
§ 127a. Already I have emphasized the truth that Nature is always more
complex than we suppose (§ 74a)--that there are complexities within
complexities. Here we find illustrated this truth under another aspect.
When seeking to formulate the arguments from Embryology, we are shown that
the facts as presented in Nature are not to be expressed in the simple
generalizations we at first make.
While we recognize this truth we must also recognize the truth that only by
enunciation and acceptance of imperfect generalizations can we progress to
perfect ones. The order of Evolution is conformed to by ideas as by other
things. The advance is, and must be, from the indefinite to the definite.
It is impossible to express the totality of any natural phenomenon in a
single proposition. To the primary statement expressing that which is most
dominant have to be added secondary statements qualifying it. We see this
even in so simple a case as the flight of a projectile. The young artillery
officer is first taught that a cannon-shot describes a curve treated as a
parabola, though literally part of an extremely eccentric ellipse not
distinguishable from a parabola. Presently he learns that atmospheric
resistance, causing a continual decrease of velocity, entails a deviation
from that theoretical path which is calculated on the supposition that the
velocity is uniform; and this incorrectness he has to allow for. Then,
further, there comes the lateral deviation due to wind, which may be
appreciable if the wind is strong and the range great. To introduce him all
at once to the correct conception thus finally reached would be impossible:
it has to be reached through successive qualifications. And that which
holds even in this simple case necessarily holds more conspicuously in
complex cases.
The title of the chapter suggests a metaphor, which is, indeed, something
more than a metaphor. There is an embryology of conceptions. That this
statement is not wholly a figure of speech, we shall see on considering
that cerebral organization is a part of organization at large; and that the
evolving nervous plexus which is the correlative of an evolving conception,
must conform to the general law of change conformed to in the evolution of
the whole nervous structure as well as in the evolution of the whole bodily
structure. As the body has at first a rude form, very remotely suggesting
that which is presently developed by the superposing of modifications on
modifications; so the brain as a whole and its contained ideas together
make up an inner world answering with extreme indefiniteness to that outer
world to which it is brought by successive approximations into tolerable
correspondence; and so any nervous plexus and its associated hypothesis,
which refer to some external group of phenomena under investigation, have
to reach their final developments by successive corrections.
This being the course of discovery must also be the course of exposition.
In pursuance of this course we may therefore fitly contemplate that early
_formula_ of embryological development which we owe to von Baer.
§ 128. Already in § 52, where the generalization of von Baer respecting the
relations of embryos was set forth, there was given the warning, above
repeated with greater distinctness, that it is only an adumbration.
In the words of his translator, he "found that in its earliest stage, every
organism has the greatest number of characters in common with all other
organisms in their earliest stages; that at a stage somewhat later, its
structure is like the structures displayed at corresponding phases by a
less extensive multitude of organisms; that at each subsequent stage,
traits are acquired which successively distinguished the developing embryo
from groups of embryos that it previously resembled--thus step by step
diminishing the class of embryos which it still resembles; and that thus
the class of similar forms is finally narrowed to the species of which it
is a member."
Assuming for a moment that this generalization is true as it stands, or
rather, assuming that the qualifications needed are not such as destroy its
correspondence with the average facts, we shall see that it has profound
significance. For if we follow out in thought the implications--if we
conceive the germs of all kinds of organisms simultaneously developing, and
imagine that after taking their first step together, at the second step one
half of the vast multitude diverges from the other half; if, at the next
step, we mentally watch the parts of each great assemblage beginning to
take two or more routes of development; if we represent to ourselves such
bifurcations going on, stage after stage, in all the branches; we shall see
that there must result an aggregate analogous, in its arrangement of parts,
to a tree. If this vast genealogical tree be contemplated as a whole, made
up of trunk, main branches, secondary branches, and so on as far as the
terminal twigs; it will be perceived that all the various kinds of
organisms represented by these terminal twigs, forming the periphery of the
tree, will stand related to one another in small groups, which are united
into groups of groups, and so on. The embryological tree, expressing the
developmental relations of organisms, will be similar to the tree which
symbolizes their classificatory relations. That subordination of classes,
orders, genera, and species, to which naturalists have been gradually led,
is just that subordination which results from the divergence and
re-divergence of embryos, as they all unfold. On the hypothesis of
evolution this parallelism has a meaning--indicates that primordial kinship
of all organisms, and that progressive differentiation of them, which the
hypothesis alleges. But on any other hypothesis the parallelism is
meaningless; or rather, it raises a difficulty; since it implies either an
effect without a cause or a design without a purpose.
§ 129. This conception of a tree, symbolizing the relationships of types
and a species derived from the same root, has a concomitant conception. The
implication is that each organism, setting out from the simple nucleated
cell, must in the course of its development follow the line of the trunk,
some main branch, some sub-branch, some sub-sub-branch, &c., of this
embryological tree; and so on till it reaches that ultimate twig
representing the species of which it is a member. It must in a general way
go through the particular line of forms which preceded it in all past
times: there must be what has been aptly called a "recapitulation" of the
successive ancestral structures. This, at least, is the conclusion
necessitated by the generalization we are considering under its original
crude form.
Von Baer lived in the days when the Development Hypothesis was mentioned
only to be ridiculed, and he joined in the ridicule. What he conceived to
be the meaning of these groupings of organisms and these relations among
their embryological histories, is not obvious. The only alternative to the
hypothesis of Evolution is the hypothesis of Special Creation; and as he
did not accept the one it is inferable that he accepted the other. But if
he did this he must in the first place have found no answer to the inquiry
why organisms specially created should have the embryological kinships he
described. And in the second place, after discovering that his alleged law
was traversed by many and various nonconformities, he would have been
without any explanation of these. Observe the positions which were open to
him and the reasons which show them to be untenable.
If it be said that the conditions of the case necessitated the derivation
of all organisms from simple germs, and therefore necessitated a
morphological unity in their primitive states; there arises the obvious
answer, that the morphological unity thus implied, is not the only
morphological unity to be accounted for. Were this the only unity, the
various kinds of organisms, setting out from a common primordial form,
should all begin from the first to diverge individually, as so many radii
from a centre; which they do not. If, otherwise, it be said that organisms
were framed upon certain types, and that those of the same type continue
developing together in the same direction, until it is time for them to
begin putting on their specialities of structure; then the answer is, that
when they do finally diverge they ought severally to develop in direct
lines towards their final forms. No reason can be assigned why, having
parted company, some should progress towards their final forms by irregular
or circuitous routes. On the hypothesis of design such deviations are
inexplicable.
The hypothesis of evolution, however, while it pre-supposes those kinships
among embryos in their early forms which are found to exist, also leads us
to expect nonconformities in their courses of development. If, as any
rational theory of evolution implies, the progressive differentiations of
types from one another during past times, have resulted from the direct and
indirect effects of external conditions--if races of organisms have become
different, either by immediate adaptations to unlike habits of life, or by
the mediate adaptations resulting from preservation of the individuals most
fitted for such habits of life, or by both; and if most embryonic changes
are significant of changes that were undergone by ancestral races; then
these irregularities must be anticipated. For the successive changes in
modes of life pursued by successive ancestral races, can have had no
regularity of sequence. In some cases they must have been more numerous
than in others; in some cases they must have been greater in degree than in
others; in some cases they must have been to simpler modes, in some cases
to more complex modes, and in some cases to modes neither higher nor lower.
Of two cognate races which diverged in the remote past, the one may have
had descendants that have remained tolerably constant in their habits,
while the other may have had descendants that have passed through
widely-aberrant modes of life; and yet some of these last may have
eventually taken to modes of life like those of the other races derived
from the same stock. And if the metamorphoses of embryos indicate, in a
general way, the changes of structure undergone by ancestors; then, the
later embryologic changes of such two allied races will be somewhat
different, though they may end in very similar forms. An illustration will
make this clear. Mr. Darwin says: "Petrels are the most aërial and oceanic
of birds, but in the quiet sounds of Tierra del Fuego, the _Puffinuria
berardi_, in its general habits, in its astonishing power of diving, its
manner of swimming, and of flying when unwillingly it takes flight, would
be mistaken by any one for an auk or grebe; nevertheless, it is essentially
a petrel, but with many parts of its organization profoundly modified." Now
if we suppose these grebe-like habits to be continued through a long epoch,
the petrel-form to be still more obscured, and the approximation to the
grebe-form still closer; it is manifest that while the chicks of the grebe
and the _Puffinuria_ will, during their early stages of development,
display that likeness involved by their common derivation from some early
type of bird, the chick of the _Puffinuria_ will eventually begin to show
deviations, representative of the ancestral petrel-structure, and will
afterwards begin to lose these distinctions and assume the grebe-structure.
Hence, remembering the perpetual intrusions of organisms on one another's
modes of life, often widely different; and remembering that these
intrusions have been going on from the beginning; we shall be prepared to
find that the general law of embryonic parallelism is qualified by
irregularities which are mostly small, in many cases considerable, and
occasionally great. The hypothesis of evolution accounts for these: it does
more--it implies the necessity of them.
§ 130. The substitutions of organs and the suppressions of organs, are
among those secondary embryological phenomena which harmonize with the
belief in evolution but cannot be reconciled with any other belief. Some
embryos, during early stages of development, possess organs that afterwards
dwindle away, as there arise other organs to discharge the same functions.
And in other embryos organs make their appearance, grow to certain points,
have no functions to discharge, and disappear by absorption.
We have a remarkable instance of substitution in the temporary appliances
for respiration, which some embryos exhibit. During the first phase of its
development, the mammalian embryo possesses a system of blood-vessels
distributed over what is called the _area vasculosa_--a system of vessels
homologous with one which, among fishes, serves for aërating the blood
until the permanent respiratory organs come into play. Now since this
system of blood-vessels, not being in proximity to an oxygenated medium,
cannot be serviceable to the mammalian embryo during development of the
lungs, as it is serviceable in the embryo-fish during development of the
gills, this needless formation of it is unaccountable as a result of
design. But it is quite congruous with the supposition that the mammalian
type arose out of lower vertebrate types. For in such case the mammalian
embryo, passing through states representing in a general way those which
its remote ancestors had in common with the lower _Vertebrata_, develops
this system of vessels in like manner with them. An instance more
significant still is furnished by certain _Amphibia_. One of the facts
early made familiar to the natural-history student is that the tadpole
breathes by external branchiæ, and that these, needful during its aquatic
life, dwindle away as fast as it develops the lungs fitting it for
terrestrial life. But in one of the higher _Amphibia_, the viviparous
Salamander, these transformations ordinarily undergone during the free life
of the larva, are undergone by the embryo in the egg. The branchiæ are
developed though there is no use for them: lungs being substituted as
breathing appliances before the creature is born.
Even more striking than the substitutions of organs are the suppressions of
organs. Mr. Darwin names some cases as "extremely curious; for instance,
the presence of teeth in foetal whales, which when grown up have not a
tooth in their heads;... It has even been stated on good authority that
rudiments of teeth can be detected in the beaks of certain embryonic
birds." Irreconcilable with any teleological theory, these facts do not
even harmonize with the theory of fixed types which are maintained by the
development of all the typical parts, even where not wanted; seeing that
the disappearance of these incipient organs during foetal life spoils the
typical resemblance. But while to other hypotheses these facts are
stumbling-blocks, they yield strong support to the hypothesis of evolution.
Allied to these cases, are the cases of what has been called retrograde
development. Many parasitic creatures and creatures which, after leading
active lives for a time, become fixed, lose, in their adult states, the
limbs and senses they had when young. It may be alleged, however, that
these creatures could not secure the habitats needful for them, without
possessing, during their larval stages, eyes and swimming appendages which
eventually become useless; that though, by losing these, their organization
retrogresses in one direction, it progresses in another direction; and
that, therefore, they do not exhibit the needless development of a higher
type on the way to a lower type. Nevertheless there are instances of a
descent in organization, following an apparently-superfluous ascent. Mr.
Darwin says that in some genera of cirripedes, "the larvæ become developed
either into hermaphrodites having the ordinary structure, or into what I
have called complemental males, and in the latter, the development has
assuredly been retrograde; for the male is a mere sack, which lives for a
short time, and is destitute of mouth, stomach, or other organ of
importance, excepting for reproduction."
§ 130a. But now let us contemplate more closely the energies at work in the
unfolding embryo, or rather the energies which the facts appear to imply.
Whatever natures we ascribe to the hypothetical units proper to each kind
of organism, we must conclude that from the beginning of embryonic
development, they have a proclivity towards the structure of that organism.
Because of their phylogenetic origin, they must tend towards the form of
the primitive type; but the superposed modifications, conflicting with
their initial tendency, must cause a swerving towards each successively
higher type. To take an illustration:--If in the germ-plasm out of which
will come a vertebrate animal there is a proclivity towards the primitive
piscine form, there must, if the germ-plasm is derived from a mammal, be
also from the outset a proclivity towards the mammalian form. While the
initial type tends continually to establish itself the terminal type tends
also to establish itself. The intermediate structures must be influenced by
their conflict, as well as by the conflict of each with the proclivities
towards the amphibian and reptilian types. This complication of tendencies
is increased by the intervention of several other factors.
There is the factor of economy. An embryo in which the transformations have
absorbed the smallest amount of energy and wasted the smallest amount of
matter, will have an advantage over embryos the transformations of which
have cost more in energy and matter: the young animal will set out with a
greater surplus of vitality, and will be more likely than others to live
and propagate. Again, in the embryos of its descendants, inheriting the
tendency to economical transformation, those which evolve at the least cost
will thrive more than the rest and be more likely to have posterity. Thus
will result a continual shortening of the processes. We can see alike that
this must take place and that it does take place. If the whole series of
phylogenetic changes had to be repeated--if the embryo mammal had to become
a complete fish, and then a complete amphibian, and then a complete
reptile, there would be an immense amount of superfluous building up and
pulling down, entailing great waste of time and of materials. Evidently
these abridgments which economy entails, necessitate that unfolding embryos
bear but rude resemblances to lower types ancestrally passed
through--vaguely represent their dominant traits only.
From this principle of economy arise several derivative principles, which
may be best dealt with separately.
§ 130b. In some cases the substitution of an abridged for an unabridged
course of evolution causes the entire disappearance of certain intermediate
forms. Structural arrangements once passed through during the unfolding are
dropped out of the series.
In the evolution of these embryos with which there is not laid up a large
amount of food-yolk there occurs at the outset a striking omission of this
kind. When, by successive fissions, the fertilized cell has given rise to a
cluster of cells constituting a hollow sphere, known as a _blastula_, the
next change under its original form is the introversion of one side, so as
to produce two layers in place of one. An idea of the change may be
obtained by taking an india-rubber ball (having a hole through which the
air may escape) and thrusting in one side until its anterior surface
touches the interior surface of the other side. If the cup-shaped structure
resulting be supposed to have its wide opening gradually narrowed, until it
becomes the mouth of an internal chamber, it will represent what is known
as a _gastrula_--a double layer of cells, of which the outer is called
epiblast and the inner hypoblast (answering to ectoderm and endoderm)
inclosing a cavity known as the _archenteron_, or primitive digestive sac.
But now in place of this original mode of forming the _gastrula_, there
occurs a mode known as delamination. Throughout its whole extent the single
layer splits so as to become a double layer--one sphere of cells inclosing
the other; and after this direct formation of the double layer there is a
direct formation of an opening through it into the internal cavity. There
is thus a shortening of the primitive process: a number of changes are left
out.
Often a kindred passing over of stages at later periods of development may
be observed. In certain of the _Mollusca_, as the _Patella chiton_, the egg
gives origin to a free-swimming larva known as a trochosphere, from which
presently comes the ordinary molluscous organization. In the highest
division of the Molluscs, however, the Cephalopods, no trochosphere is
formed. The nutritive matter laid up in the egg is used in building up the
young animal without any indication of an ancestral larva.
§ 130c. Among principles derived from the principle of economy is the
principle of pre-adaptation--a name which we may appropriately coin to
indicate an adaptation made in advance of the time at which it could have
arisen in the course of phylogenetic history.
How pre-adaptation may result from economy will be shown by an illustration
which human methods of construction furnish. Let us assume that building
houses of a certain type has become an established habit, and that, as a
part of each house, there is a staircase of given size. And suppose that in
consequence of changed conditions--say the walling in of the town, limiting
the internal space and increasing ground-rents--it becomes the policy to
build houses of many stories, let out in flats to different tenants. For
the increased passing up and down, a staircase wider at its lower part will
be required. If now the builder, when putting up the ground floor, follows
the old dimensions, then after all the stories are built, the lower part of
the staircase, if it is to yield equal facilities for passage, must be
reconstructed. Instead of a staircase adapted to those few stories which
the original type of house had, economy will dictate a pre-adaptation of
the staircase to the additional stories.
On carrying this idea with us, we shall see that if from some type of
organism there is evolved a type in which enlargement of a certain part is
needed to meet increased functions, the greater size of this part will
begin to show itself during early stages of unfolding. That unbuilding and
rebuilding which would be needful were it laid down of its original size,
will be made needless if from the beginning it is laid down of a larger
size. Hence, in successive generations, the greater prosperity and
multiplication of individuals in which this part is at the outset somewhat
larger than usual, must eventually establish a marked excess in its
development at an early stage. The facts agree with this inference.
Referring to the contrasts between embryos, Mr. Adam Sedgwick says that "a
species is distinct and distinguishable from its allies from the very
earliest stages." Whereas, according to the law of von Baer, "animals so
closely allied as the fowl and duck would be indistinguishable in the early
stages of development," "yet I can distinguish a fowl and a duck embryo on
the second day by the inspection of a single transverse section through the
trunk." This experience harmonizes with the statement of the late Prof.
Agassiz, that in some cases traits characterizing the species appear at an
earlier period than traits characterizing the genus.
Similar in their implications are the facts recently published by Dr. E.
Mehnert, concerning the feet of pentadactyle vertebrates. A leading example
is furnished by the foot in the struthious birds. Out of the original five
digits the two which eventually become large while the others disappear,
soon give sign of their future predominance: their early sizes being in
excess of those required for the usual functional requirements in birds,
and preparing the way for their special requirements in the struthious
birds. Dr. Mehnert shows that a like lesson is given by the relative
developments of legs and wings in these birds. Ordinarily in vertebrates
the fore limbs grow more rapidly than the hind limbs; but in the ostrich,
in which the hind limbs or legs have to become so large while the wings are
but little wanted, the leg development goes in advance of the
wing-development in early embryonic stages: there is a pre-adaptation.
Much more striking are examples furnished by creatures whose modes of
existence require that they shall have enormous fertility--require that the
generative system shall be very large. Ordinarily the organs devoted to
maintenance of the race develop later than the organs devoted to
maintenance of the individual. But this order is inverted in certain
_Entozoa_. To these creatures, imbedded in nutritive matters,
self-maintenance cost nothing, and the structures devoted to it are
relatively of less importance than the structures devoted to
race-maintenance, which, to make up for the small chance any one germ has
of getting into a fit habitat, have to produce immense numbers of germs.
Here the rudiments of the generative systems are the first to become
visible--here, in virtue of the principle of pre-adaptation, a structure
belonging to the terminal form asserts itself so early in the developmental
process as almost to obliterate the structure of the initial form.
It may be that in some cases where the growth of certain organs goes in
advance of the normal order, the element of time comes into play--the
greater time required for construction. To elucidate this let us revert to
our simile. Suppose that the staircase above instanced, or at any rate its
lower part, is required to be of marble with balusters finely carved. If
this piece of work is not promptly commenced and pushed on fast, it will
not be completed when the rest of the house is ready: workmen and tools
will still block it up at a time when it should be available. Similarly
among the parts of an unfolding embryo, those in which there is a great
deal of constructive work must early take such shape as will allow of this.
Now of all the tissues the nervous tissue is that which takes longest to
repair when injured; and it seems a not improbable inference that it is a
tissue which is slower in its histological development than others. If this
be so, we may see why, in the embryos of the higher vertebrates, the
central nervous system quickly grows large in comparison to the other
systems--why by pre-adaptation the brain of a chick develops in advance of
other organs so much more than the brain of a fish.
§ 130d. Yet another complication has to be noted. From the principle of
economy, it seems inferable that decrease and disappearance of organs which
were useful in ancestral types but have ceased to be useful, should take
place uniformly; but they do not. In the words of Mr. Adam Sedgwick, "some
ancestral organs persist in the embryo in a functionless rudimentary
(vestigial) condition and at the same time without any reference to adult
structures, while other ancestral organs have disappeared without leaving a
trace."[46] This anomaly is rendered more striking when joined with the
fact that some of the structures which remain conspicuous are relatively
ancient, while some which have been obliterated are relatively modern--_e.
g._, "gill slits [which date back to the fish-ancestor], have been retained
in embryology, whereas other organs which have much more recently
disappeared, _e. g._ teeth of birds, fore-limbs of snakes [dating back to
the reptile ancestor], have been entirely lost."[47] Mr. Sedgwick ascribes
these anomalies to the difference between larval development and embryonic
development, and expresses his general belief thus:--
"The conclusion here reached is that, whereas larval development must
retain traces (it may be very faint) of ancestral stages of structure
because they are built out of ancestral stages, embryonic development
need not necessarily do so, and very often does not; that embryonic
development in so far as it is a record at all, is a record of structural
features of previous larval stages. Characters which disappear during
free life disappear also in the embryo, but characters which though lost
by the adult are retained in the larva may ultimately be absorbed into
the embryonic phase and leave their traces in embryonic development."[48]
To set forth the evidence justifying this view would encumber too much the
general argument. Towards elucidation of such irregularities let me name
two factors which should I think be taken into account.
Abridgment of embryonic stages cannot go on uniformly with all disused
organs. Where an organ is of such size that progressive diminution of it
will appreciably profit the young animal, by leaving it a larger surplus of
unused material, we may expect progressive diminution to occur.
Contrariwise, if the organ is relatively so small that each decrease will
not, by sensibly increasing the reserve of nutriment, give the young animal
an advantage over others, decrease must not be looked for: there may be a
survival of it even though of very ancient origin.
Again, the reduction of a superfluous part can take place only on condition
that the economy resulting from each descending variation of it, is of
greater importance than are the effects of variations simultaneously
occurring in other parts. If by increase or decrease of any other parts of
the embryo, survival of the animal is furthered in a greater degree than by
decrease of this superfluous part, then such decrease is unlikely; since it
is illegitimate to count upon the repeated concurrence of favourable
variations in two or more parts which are independent. So that if changes
of an advantageous kind are going on elsewhere in the embryo a useless part
may remain long undiminished.
Yet another cause operates, and perhaps cooperates. Embryonic survival of
an organ which has become functionless, may readily happen if, during
subsequent stages of development, parts of it are utilized as parts of
other organs. In the words of Mr. J. T. Cunningham:--
"It seems to be a general fact that a structure which in metamorphosis
disappears completely may easily be omitted altogether in embryonic
development, while one which is modified into something else continues to
pass more or less through its original larval condition." (_Science
Progress_, July, 1897, p. 488.)
One more factor of considerable importance should be taken into account. A
disused organ which entails evil because construction of it involves
needless cost, may entail further evil by being in the way. This, it seems
to me, is the reason why the fore-limbs of snakes have disappeared from
their embryos. When the long-bodied lizard out of which the ophidian type
evolved, crept through stiff herbage, and moved its head from side to side
to find openings, there resulted alternate bends of its body, which were
the beginnings of lateral undulations; and we may easily see that in
proportion as it thus progressed by insinuating itself through interstices,
the fore-limbs, less and less used for walking, would be more and more in
the way; and the lengthening of the body, increasing the undulatory motion
and decreasing the use of the fore-limbs, would eventually make them
absolute impediments. Hence besides the benefit in economy of construction
gained by embryos in which the fore-limbs were in early stages a little
less developed than usual, they would gain an advantage by having, when
mature, smaller fore-limbs than usual, leading to greater facility of
locomotion. There would be a double set of influences causing, through
selection, a comparatively rapid decrease of these appendages. And we may I
think see also, on contemplating the kind of movement, that the fore-limbs
would be more in the way than the hind limbs, which would consequently
dwindle with such smaller rapidity as to make continuance of the rudiments
of them comprehensible.
§ 131-132. So that while the embryonic law enunciated by von Baer is in
harmony with the hypothesis of evolution, and is, indeed, a law which this
hypothesis implies, the nonconformities to the law are also interpretable
by this hypothesis.
Parallelism between the courses of development in species allied by remote
ancestry, is liable to be variously modified in correspondence with the
later ancestral forms passed through after divergence of such species. The
substitution of a direct for an indirect process of formation, which we
have reason to believe will show itself, must obscure the embryonic
history. And the principle of economy which leads to this substitution
produces effects that are very irregular and uncertain in consequence of
the endlessly varied conditions. Thus several causes conspire to produce
deviations from the general law.
Let it be remarked, finally, that the ability to trace out embryologic
kinships and the inability to do this, occur just where, according to the
hypothesis of Evolution, they should occur. We saw in § 100a that
zoologists are agreed in grouping animals into some 17 phyla--_Mollusca_,
_Arthropoda_, _Echinodermata_, &c.--each of which includes a number of
classes severally sub-divided into orders, genera, species. All the members
of each phylum are so related embryologically, that the existence of a
common ancestor of them in the remote past is considered certain. But when
it comes to the relations among the archaic ancestors, opinion is
unsettled. Whether, for instance, the primitive _Chordata_, out of which
the _Vertebrata_ emerged, have molluscan affinities or annelidan
affinities, is still a matter in dispute. With regard to the origins of
various other types no settled conclusions are held. Now it is clear that
on tracing down each branch of the great genealogical tree, kinships would
be much more manifest among the recently-differentiated forms than among
those forms which diverged from one another in the earliest stages of
organic life, and had separated widely before any of the types we now know
had come into existence.