ON THE AFFINITIES AND GENEALOGY OF MAN.
Position of man in the animal series--The natural system
genealogical--Adaptive characters of slight value--Various
small points of resemblance between man and the
Quadrumana--Rank of man in the natural system--Birthplace and
antiquity of man--Absence of fossil connecting-links--Lower
stages in the genealogy of man, as inferred, firstly from his
affinities and secondly from his structure--Early androgynous
condition of the Vertebrata--Conclusion.
Even if it be granted that the difference between man and his nearest
allies is as great in corporeal structure as some naturalists maintain,
and although we must grant that the difference between them is immense
in mental power, yet the facts given in the previous chapters declare,
as it appears to me, in the plainest manner, that man is descended from
some lower form, notwithstanding that connecting-links have not hitherto
been discovered.
Man is liable to numerous, slight, and diversified variations, which are
induced by the same general causes, are governed and transmitted in
accordance with the same general laws, as in the lower animals. Man
tends to multiply at so rapid a rate that his offspring are necessarily
exposed to a struggle for existence, and consequently to natural
selection. He has given rise to many races, some of which are so
different that they have often been ranked by naturalists as distinct
species. His body is constructed on the same homological plan as that of
other mammals, independently of the uses to which the several parts may
be put. He passes through the same phases of embryological development.
He retains many rudimentary and useless structures, which no doubt were
once serviceable. Characters occasionally make their reappearance in
him, which we have every reason to believe were possessed by his early
progenitors. If the origin of man had been wholly different from that of
all other animals, these various appearances would be mere empty
deceptions; but such an admission is incredible. These appearances, on
the other hand, are intelligible, at least to a large extent, if man is
the co-descendant with other mammals of some unknown and lower form.
Some naturalists, from being deeply impressed with the mental and
spiritual powers of man, have divided the whole organic world into three
kingdoms, the Human, the Animal, and the Vegetable, thus giving to man a
separate kingdom.[255] Spiritual powers cannot be compared or classed by
the naturalist; but he may endeavour to shew, as I have done, that the
mental faculties of man and the lower animals do not differ in kind,
although immensely in degree. A difference in degree, however great,
does not justify us in placing man in a distinct kingdom, as will
perhaps be best illustrated by comparing the mental powers of two
insects, namely, a coccus or scale-insect and an ant, which undoubtedly
belong to the same class. The difference is here greater, though of a
somewhat different kind, than that between man and the highest mammal.
The female coccus, whilst young, attaches itself by its proboscis to a
plant; sucks the sap but never moves again; is fertilised and lays eggs;
and this is its whole history. On the other hand, to describe the habits
and mental powers of a female ant, would require, as Pierre Huber has
shewn, a large volume; I may, however, briefly specify a few points.
Ants communicate information to each other, and several unite for the
same work, or games of play. They recognise their fellow-ants after
months of absence. They build great edifices, keep them clean, close the
doors in the evening, and post sentries. They make roads, and even
tunnels under rivers. They collect food for the community, and when an
object, too large for entrance, is brought to the nest, they enlarge the
door, and afterwards build it up again.[256] They go out to battle in
regular bands, and freely sacrifice their lives for the common weal.
They emigrate in accordance with a preconcerted plan. They capture
slaves. They keep Aphides as milch-cows. They move the eggs of their
aphides, as well as their own eggs and cocoons, into warm parts of the
nest, in order that they may be quickly hatched; and endless similar
facts could be given. On the whole, the difference in mental power
between an ant and a coccus is immense; yet no one has ever dreamed of
placing them in distinct classes, much less in distinct kingdoms. No
doubt this interval is bridged over by the intermediate mental powers of
many other insects; and this is not the case with man and the higher
apes. But we have every reason to believe that breaks in the series are
simply the result of many forms having become extinct.
Professor Owen, relying chiefly on the structure of the brain, has
divided the mammalian series into four sub-classes. One of these he
devotes to man; in another he places both the marsupials and the
monotremata; so that he makes man as distinct from all other mammals as
are these two latter groups conjoined. This view has not been accepted,
as far as I am aware, by any naturalist capable of forming an
independent judgment, and therefore need not here be further considered.
We can understand why a classification founded on any single character
or organ--even an organ so wonderfully complex and important as the
brain--or on the high development of the mental faculties, is almost
sure to prove unsatisfactory. This principle has indeed been tried with
hymenopterous insects; but when thus classed by their habits or
instincts, the arrangement proved thoroughly artificial.[257]
Classifications may, of course, be based on any character whatever, as
on size, colour, or the element inhabited; but naturalists have long
felt a profound conviction that there is a natural system. This system,
it is now generally admitted, must be, as far as possible, genealogical
in arrangement,--that is, the co-descendants of the same form must be
kept together in one group, separate from the co-descendants of any
other form; but if the parent-forms are related, so will be their
descendants, and the two groups together will form a larger group. The
amount of difference between the several groups--that is the amount of
modification which each has undergone--will be expressed by such terms
as genera, families, orders, and classes. As we have no record of the
lines of descent, these lines can be discovered only by observing the
degrees of resemblance between the beings which are to be classed. For
this object numerous points of resemblance are of much more importance
than the amount of similarity or dissimilarity in a few points. If two
languages were found to resemble each other in a multitude of words and
points of construction, they would be universally recognised as having
sprung from a common source, notwithstanding that they differed greatly
in some few words or points of construction. But with organic beings the
points of resemblance must not consist of adaptations to similar habits
of life: two animals may, for instance, have had their whole frames
modified for living in the water, and yet they will not be brought any
nearer to each other in the natural system. Hence we can see how it is
that resemblances in unimportant structures, in useless and rudimentary
organs, and in parts not as yet fully developed or functionally active,
are by far the most serviceable for classification; for they can hardly
be due to adaptations within a late period; and thus they reveal the old
lines of descent or of true affinity.
We can further see why a great amount of modification in some one
character ought not to lead us to separate widely any two organisms. A
part which already differs much from the same part in other allied forms
has already, according to the theory of evolution, varied much;
consequently it would (as long as the organism remained exposed to the
same exciting conditions) be liable to further variations of the same
kind; and these, if beneficial, would be preserved, and thus continually
augmented. In many cases the continued development of a part, for
instance, of the beak of a bird, or of the teeth of a mammal, would not
be advantageous to the species for gaining its food, or for any other
object; but with man we can see no definite limit, as far as advantage
is concerned, to the continued development of the brain and mental
faculties. Therefore in determining the position of man in the natural
or genealogical system, the extreme development of his brain ought not
to outweigh a multitude of resemblances in other less important or
quite unimportant points.
The greater number of naturalists who have taken into consideration the
whole structure of man, including his mental faculties, have followed
Blumenbach and Cuvier, and have placed man in a separate Order, under
the title of the Bimana, and therefore on an equality with the Orders of
the Quadrumana, Carnivora, &c. Recently many of our best naturalists
have recurred to the view first propounded by Linnaeus, so remarkable for
his sagacity, and have placed man in the same Order with the Quadrumana,
under the title of the Primates. The justice of this conclusion will be
admitted if, in the first place, we bear in mind the remarks just made
on the comparatively small importance for classification of the great
development of the brain in man; bearing, also, in mind that the
strongly-marked differences between the skulls of man and the Quadrumana
(lately insisted upon by Bischoff, Aeby, and others) apparently follow
from their differently developed brains. In the second place, we must
remember that nearly all the other and more important differences
between man and the Quadrumana are manifestly adaptive in their nature,
and relate chiefly to the erect position of man; such as the structure
of his hand, foot, and pelvis, the curvature of his spine, and the
position of his head. The family of seals offers a good illustration of
the small importance of adaptive characters for classification. These
animals differ from all other Carnivora in the form of their bodies and
in the structure of their limbs, far more than does man from the higher
apes; yet in every system, from that of Cuvier to the most recent one by
Mr. Flower,[258] seals are ranked as a mere family in the Order of the
Carnivora. If man had not been his own classifier, he would never have
thought of founding a separate order for his own reception.
It would be beyond my limits, and quite beyond my knowledge, even to
name the innumerable points of structure in which man agrees with the
other Primates. Our great anatomist and philosopher, Prof. Huxley, has
fully discussed this subject,[259] and has come to the conclusion that
man in all parts of his organisation differs less from the higher apes,
than these do from the lower members of the same group. Consequently
there "is no justification for placing man in a distinct order."
In an early part of this volume I brought forward various facts, shewing
how closely man agrees in constitution with the higher mammals; and this
agreement, no doubt, depends on our close similarity in minute structure
and chemical composition. I gave, as instances, our liability to the
same diseases, and to the attacks of allied parasites; our tastes in
common for the same stimulants, and the similar effects thus produced,
as well as by various drugs; and other such facts.
As small unimportant points of resemblance between man and the higher
apes are not commonly noticed in systematic works, and as, when
numerous, they clearly reveal our relationship, I will specify a few
such points. The relative position of the features are manifestly the
same in man and the Quadrumana; and the various emotions are displayed
by nearly similar movements of the muscles and skin, chiefly above the
eyebrows and round the mouth. Some few expressions are, indeed, almost
the same, as in the weeping of certain kinds of monkeys, and in the
laughing noise made by others, during which the corners of the mouth are
drawn backwards, and the lower eyelids wrinkled. The external ears are
curiously alike. In man the nose is much more prominent than in most
monkeys; but we may trace the commencement of an aquiline curvature in
the nose of the Hoolock Gibbon; and this in the _Semnopithecus nasica_
is carried to a ridiculous extreme.
The faces of many monkeys are ornamented with beards, whiskers, or
moustaches. The hair on the head grows to a great length in some species
of Semnopithecus;[260] and in the Bonnet monkey (_Macacus radiatus_) it
radiates from a point on the crown, with a parting down the middle, as
in man. It is commonly said that the forehead gives to man his noble and
intellectual appearance; but the thick hair on the head of the Bonnet
monkey terminates abruptly downwards, and is succeeded by such short and
fine hair, or down, that at a little distance the forehead, with the
exception of the eyebrows, appears quite naked. It has been erroneously
asserted that eyebrows are not present in any monkey. In the species
just named the degree of nakedness of the forehead differs in different
individuals; and Eschricht states[261] that in our children the limit
between the hairy scalp and the naked forehead is sometimes not well
defined; so that here we seem to have a trifling case of reversion to a
progenitor, in whom the forehead had not as yet become quite naked.
It is well known that the hair on our arms tends to converge from above
and below to a point at the elbow. This curious arrangement, so unlike
that in most of the lower mammals, is common to the gorilla, chimpanzee,
orang, some species of Hylobates, and even to some few American monkeys.
But in _Hylobates agilis_ the hair on the fore-arm is directed
downwards or towards the wrist in the ordinary manner; and in _H. lar_
it is nearly erect, with only a very slight forward inclination; so that
in this latter species it is in a transitional state. It can hardly be
doubted that with most mammals the thickness of the hair and its
direction on the back is adapted to throw off the rain; even the
transverse hairs on the fore-legs of a dog may serve for this end when
he is coiled up asleep. Mr. Wallace remarks that the convergence of the
hair towards the elbow on the arms of the orang (whose habits he has so
carefully studied) serves to throw off the rain, when, as is the custom
of this animal, the arms are bent, with the hands clasped round a branch
or over its own head. We should, however, bear in mind that the attitude
of an animal may perhaps be in part determined by the direction of the
hair; and not the direction of the hair by the attitude. If the above
explanation is correct in the case of the orang, the hair on our
fore-arms offers a curious record of our former state; for no one
supposes that it is now of any use in throwing off the rain, nor in our
present erect condition is it properly directed for this purpose.
It would, however, be rash to trust too much to the principle of
adaptation in regard to the direction of the hair in man or his early
progenitors; for it is impossible to study the figures given by
Eschricht of the arrangement of the hair on the human foetus (this
being the same as in the adult) and not agree with this excellent
observer that other and more complex causes have intervened. The points
of convergence seem to stand in some relation to those points in the
embryo which are last closed in during development. There appears, also,
to exist some relation between the arrangement of the hair on the
limbs, and the course of the medullary arteries.[262]
It must not be supposed that the resemblances between man and certain
apes in the above and many other points--such as in having a naked
forehead, long tresses on the head, &c.--are all necessarily the result
of unbroken inheritance from a common progenitor thus characterised, or
of subsequent reversion. Many of these resemblances are more probably
due to analogous variation, which follows, as I have elsewhere attempted
to shew,[263] from co-descended organisms having a similar constitution
and having been acted on by similar causes inducing variability. With
respect to the similar direction of the hair on the fore-arms of man and
certain monkeys, as this character is common to almost all the
anthropomorphous apes, it may probably be attributed to inheritance; but
not certainly so, as some very distinct American monkeys are thus
characterised. The same remark is applicable to the tailless condition
of man; for the tail is absent in all the anthropomorphous apes.
Nevertheless this character cannot with certainty be attributed to
inheritance, as the tail, though not absent, is rudimentary in several
other Old World and in some New World species, and is quite absent in
several species belonging to the allied group of Lemurs.
Although, as we have now seen, man has no just right to form a separate
Order for his own reception, he may perhaps claim a distinct Sub-order
or Family. Prof. Huxley, in his last work,[264] divides the Primates
into three Sub-orders; namely, the Anthropidae with man alone, the
Simiadae including monkeys of all kinds, and the Lemuridae with the
diversified genera of lemurs. As far as differences in certain important
points of structure are concerned, man may no doubt rightly claim the
rank of a Sub-order; and this rank is too low, if we look chiefly to his
mental faculties. Nevertheless, under a genealogical point of view it
appears that this rank is too high, and that man ought to form merely a
Family, or possibly even only a Sub-family. If we imagine three lines of
descent proceeding from a common source, it is quite conceivable that
two of them might after the lapse of ages be so slightly changed as
still to remain as species of the same genus; whilst the third line
might become so greatly modified as to deserve to rank as a distinct
Sub-family, Family, or even Order. But in this case it is almost certain
that the third line would still retain through inheritance numerous
small points of resemblance with the other two lines. Here then would
occur the difficulty, at present insoluble, how much weight we ought to
assign in our classifications to strongly-marked differences in some few
points,--that is to the amount of modification undergone; and how much
to close resemblance in numerous unimportant points, as indicating the
lines of descent or genealogy. The former alternative is the most
obvious, and perhaps the safest, though the latter appears the most
correct as giving a truly natural classification.
To form a judgment on this head, with reference to man we must glance at
the classification of the Simiadae. This family is divided by almost all
naturalists into the Catarhine group, or Old World monkeys, all of which
are characterised (as their name expresses) by the peculiar structure of
their nostrils and by having four premolars in each jaw; and into the
Platyrhine group or New World monkeys (including two very distinct
sub-groups), all of which are characterised by differently-constructed
nostrils and by having six premolars in each jaw. Some other small
differences might be mentioned. Now man unquestionably belongs in his
dentition, in the structure of his nostrils, and some other respects, to
the Catarhine or Old World division; nor does he resemble the
Platyrhines more closely than the Catarhines in any characters,
excepting in a few of not much importance and apparently of an adaptive
nature. Therefore it would be against all probability to suppose that
some ancient New World species had varied, and had thus produced a
man-like creature with all the distinctive characters proper to the Old
World division; losing at the same time all its own distinctive
characters. There can consequently hardly be a doubt that man is an
offshoot from the Old World Simian stem; and that under a genealogical
point of view, he must be classed with the Catarhine division.[265]
The anthropomorphous apes, namely the gorilla, chimpanzee, orang, and
hylobates, are separated as a distinct sub-group from the other Old
World monkeys by most naturalists. I am aware that Gratiolet, relying on
the structure of the brain, does not admit the existence of this
sub-group, and no doubt it is a broken one; thus the orang, as Mr. St.
G. Mivart remarks,[266] "is one of the most peculiar and aberrant forms
to be found in the Order." The remaining, non-anthropomorphous, Old
World monkeys, are again divided by some naturalists into two or three
smaller sub-groups; the genus Semnopithecus, with its peculiar
sacculated stomach, being the type of one such sub-group. But it appears
from M. Gaudry's wonderful discoveries in Attica, that during the
Miocene period a form existed there, which connected Semnopithecus and
Macacus; and this probably illustrates the manner in which the other and
higher groups were once blended together.
If the anthropomorphous apes be admitted to form a natural sub-group,
then as man agrees with them, not only in all those characters which he
possesses in common with the whole Catarhine group, but in other
peculiar characters, such as the absence of a tail and of callosities
and in general appearance, we may infer that some ancient member of the
anthropomorphous sub-group gave birth to man. It is not probable that a
member of one of the other lower sub-groups should, through the law of
analogous variation, have given rise to a man-like creature, resembling
the higher anthropomorphous apes in so many respects. No doubt man, in
comparison with most of his allies, has undergone an extraordinary
amount of modification, chiefly in consequence of his greatly developed
brain and erect position; nevertheless we should bear in mind that he
"is but one of several exceptional forms of Primates."[267]
Every naturalist, who believes in the principle of evolution, will
grant that the two main divisions of the Simiadae, namely the Catarhine
and Platyrhine monkeys, with their sub-groups, have all proceeded from
some one extremely ancient progenitor. The early descendants of this
progenitor, before they had diverged to any considerable extent from
each other, would still have formed a single natural group; but some of
the species or incipient genera would have already begun to indicate by
their diverging characters the future distinctive marks of the Catarhine
and Platyrhine divisions. Hence the members of this supposed ancient
group would not have been so uniform in their dentition or in the
structure of their nostrils, as are the existing Catarhine monkeys in
one way and the Platyrhines in another way, but would have resembled in
this respect the allied Lemuridae which differ greatly from each other in
the form of their muzzles,[268] and to an extraordinary degree in their
dentition.
The Catarhine and Platyrhine monkeys agree in a multitude of characters,
as is shewn by their unquestionably belonging to one and the same Order.
The many characters which they possess in common can hardly have been
independently acquired by so many distinct species; so that these
characters must have been inherited. But an ancient form which possessed
many characters common to the Catarhine and Platyrhine monkeys, and
others in an intermediate condition, and some few perhaps distinct from
those now present in either group, would undoubtedly have been ranked,
if seen by a naturalist, as an ape or monkey. And as man under a
genealogical point of view belongs to the Catarhine or Old World stock,
we must conclude, however much the conclusion may revolt our pride,
that our early progenitors would have been properly thus
designated.[269] But we must not fall into the error of supposing that
the early progenitor of the whole Simian stock, including man, was
identical with, or even closely resembled, any existing ape or monkey.
_On the Birthplace and Antiquity of Man._--We are naturally led to
enquire where was the birthplace of man at that stage of descent when
our progenitors diverged from the Catarhine stock. The fact that they
belonged to this stock clearly shews that they inhabited the Old World;
but not Australia nor any oceanic island, as we may infer from the laws
of geographical distribution. In each great region of the world the
living mammals are closely related to the extinct species of the same
region. It is therefore probable that Africa was formerly inhabited by
extinct apes closely allied to the gorilla and chimpanzee; and as these
two species are now man's nearest allies, it is somewhat more probable
that our early progenitors lived on the African continent than
elsewhere. But it is useless to speculate on this subject, for an ape
nearly as large as a man, namely the Dryopithecus of Lartet, which was
closely allied to the anthropomorphous Hylobates, existed in Europe
during the Upper Miocene period; and since so remote a period the earth
has certainly undergone many great revolutions, and there has been ample
time for migration on the largest scale.
At the period and place, whenever and wherever it may have been, when
man first lost his hairy covering, he probably inhabited a hot country;
and this would have been favourable for a frugiferous diet, on which,
judging from analogy, he subsisted. We are far from knowing how long ago
it was when man first diverged from the Catarhine stock; but this may
have occurred at an epoch as remote as the Eocene period; for the higher
apes had diverged from the lower apes as early as the Upper Miocene
period, as shewn by the existence of the Dryopithecus. We are also quite
ignorant at how rapid a rate organisms, whether high or low in the
scale, may under favourable circumstances be modified: we know, however,
that some have retained the same form during an enormous lapse of time.
From what we see going on under domestication, we learn that within the
same period some of the co-descendants of the same species may be not at
all changed, some a little, and some greatly changed. Thus it may have
been with man, who has undergone a great amount of modification in
certain characters in comparison with the higher apes.
The great break in the organic chain between man and his nearest allies,
which cannot be bridged over by any extinct or living species, has often
been advanced as a grave objection to the belief that man is descended
from some lower form; but this objection will not appear of much weight
to those who, convinced by general reasons, believe in the general
principle of evolution. Breaks incessantly occur in all parts of the
series, some being wide, sharp and defined, others less so in various
degrees; as between the orang and its nearest allies--between the
Tarsius and the other Lemuridae--between the elephant and in a more
striking manner between the Ornithorhynchus or Echidna, and other
mammals. But all these breaks depend merely on the number of related
forms which have become extinct. At some future period, not very distant
as measured by centuries, the civilised races of man will almost
certainly exterminate and replace throughout the world the savage races.
At the same time the anthropomorphous apes, as Professor Schaaffhausen
has remarked,[270] will no doubt be exterminated. The break will then be
rendered wider, for it will intervene between man in a more civilised
state, as we may hope, than the Caucasian, and some ape as low as a
baboon, instead of as at present between the negro or Australian and the
gorilla.
With respect to the absence of fossil remains, serving to connect man
with his ape-like progenitors, no one will lay much stress on this fact,
who will read Sir C. Lyell's discussion,[271] in which he shews that in
all the vertebrate classes the discovery of fossil remains has been an
extremely slow and fortuitous process. Nor should it be forgotten that
those regions which are the most likely to afford remains connecting man
with some extinct ape-like creature, have not as yet been searched by
geologists.
_Lower Stages in the Genealogy of Man._--We have seen that man appears
to have diverged from the Catarhine or Old World division of the
Simiadae, after these had diverged from the New World division. We will
now endeavour to follow the more remote traces of his genealogy,
trusting in the first place to the mutual affinities between the various
classes and orders, with some slight aid from the periods, as far as
ascertained, of their successive appearance on the earth. The Lemuridae
stand below and close to the Simiadae, constituting a very distinct
family of the Primates, or, according to Haeckel, a distinct Order. This
group is diversified and broken to an extraordinary degree, and includes
many aberrant forms. It has, therefore, probably suffered much
extinction. Most of the remnants survive on islands, namely in
Madagascar and in the islands of the Malayan archipelago, where they
have not been exposed to such severe competition as they would have been
on well-stocked continents. This group likewise presents many
gradations, leading, as Huxley remarks,[272] "insensibly from the crown
and summit of the animal creation down to creatures from which there is
but a step, as it seems, to the lowest, smallest, and least intelligent
of the placental mammalia." From these various considerations it is
probable that the Simiadae were originally developed from the progenitors
of the existing Lemuridae; and these in their turn from forms standing
very low in the mammalian series.
The Marsupials stand in many important characters below the placental
mammals. They appeared at an earlier geological period, and their range
was formerly much more extensive than what it now is. Hence the
Placentata are generally supposed to have been derived from the
Implacentata or Marsupials; not, however, from forms closely like the
existing Marsupials, but from their early progenitors. The Monotremata
are plainly allied to the Marsupials; forming a third and still lower
division in the great mammalian series. They are represented at the
present day solely by the Ornithorhynchus and Echidna; and these two
forms may be safely considered as relics of a much larger group which
have been preserved in Australia through some favourable concurrence of
circumstances. The Monotremata are eminently interesting, as in several
important points of structure they lead towards the class of reptiles.
In attempting to trace the genealogy of the Mammalia, and therefore of
man, lower down in the series, we become involved in greater and greater
obscurity. He who wishes to see what ingenuity and knowledge can effect,
may consult Prof. Haeckel's works.[273] I will content myself with a few
general remarks. Every evolutionist will admit that the five great
vertebrate classes, namely, mammals, birds, reptiles, amphibians, and
fishes, are all descended from some one prototype; for they have much in
common, especially during their embryonic state. As the class of fishes
is the most lowly organised and appeared before the others, we may
conclude that all the members of the vertebrate kingdom are derived from
some fish-like animal, less highly organised than any as yet found in
the lowest known formations. The belief that animals so distinct as a
monkey or elephant and a humming-bird, a snake, frog, and fish, &c.,
could all have sprung from the same parents, will appear monstrous to
those who have not attended to the recent progress of natural history.
For this belief implies the former existence of links closely binding
together all these forms, now so utterly unlike.
Nevertheless it is certain that groups of animals have existed, or do
now exist, which serve to connect more or less closely the several great
vertebrate classes. We have seen that the Ornithorhynchus graduates
towards reptiles; and Prof. Huxley has made the remarkable discovery,
confirmed by Mr. Cope and others, that the old Dinosaurians are
intermediate in many important respects between certain reptiles and
certain birds--the latter consisting of the ostrich-tribe (itself
evidently a widely-diffused remnant of a larger group) and of the
Archeopteryx, that strange Secondary bird having a long tail like that
of the lizard. Again, according to Prof. Owen,[274] the
Ichthyosaurians--great sea-lizards furnished with paddles--present many
affinities with fishes, or rather, according to Huxley, with amphibians.
This latter class (including in its highest division frogs and toads) is
plainly allied to the Ganoid fishes. These latter fishes swarmed during
the earlier geological periods, and were constructed on what is called a
highly generalised type, that is they presented diversified affinities
with other groups of organisms. The amphibians and fishes are also so
closely united by the Lepidosiren, that naturalists long disputed in
which of these two classes it ought to be placed. The Lepidosiren and
some few Ganoid fishes have been preserved from utter extinction by
inhabiting our rivers, which are harbours of refuge, bearing the same
relation to the great waters of the ocean that islands bear to
continents.
Lastly, one single member of the immense and diversified class of
fishes, namely the lancelet or amphioxus, is so different from all other
fishes, that Haeckel maintains that it ought to form a distinct class in
the vertebrate kingdom. This fish is remarkable for its negative
characters; it can hardly be said to possess a brain, vertebral column,
or heart, &c.; so that it was classed by the older naturalists amongst
the worms. Many years ago Prof. Goodsir perceived that the lancelet
presented some affinities with the Ascidians, which are invertebrate,
hermaphrodite, marine creatures permanently attached to a support. They
hardly appear like animals, and consist of a simple, tough, leathery
sack, with two small projecting orifices. They belong to the Molluscoida
of Huxley--a lower division of the great kingdom of the Mollusca; but
they have recently been placed by some naturalists amongst the Vermes or
worms. Their larvae somewhat resemble tadpoles in shape,[275] and have
the power of swimming freely about. Some observations lately made by M.
Kowalevsky,[276] since confirmed by Prof. Kuppfer, will form a discovery
of extraordinary interest, if still further extended, as I hear from M.
Kowalevsky in Naples he has now effected. The discovery is that the
larvae of Ascidians are related to the Vertebrata, in their manner of
development, in the relative position of the nervous system, and in
possessing a structure closely like the _chorda dorsalis_ of vertebrate
animals. It thus appears, if we may rely on embryology, which has always
proved the safest guide in classification, that we have at last gained a
clue to the source whence the Vertebrata have been derived. We should
thus be justified in believing that at an extremely remote period a
group of animals existed, resembling in many respects the larvae of our
present Ascidians, which diverged into two great branches--the one
retrograding in development and producing the present class of
Ascidians, the other rising to the crown and summit of the animal
kingdom by giving birth to the Vertebrata.
We have thus far endeavoured rudely to trace the genealogy of the
Vertebrata by the aid of their mutual affinities. We will now look to
man as he exists; and we shall, I think, be able partially to restore
during successive periods, but not in due order of time, the structure
of our early progenitors. This can be effected by means of the rudiments
which man still retains, by the characters which occasionally make their
appearance in him through reversion, and by the aid of the principles of
morphology and embryology. The various facts, to which I shall here
allude, have been given in the previous chapters. The early progenitors
of man were no doubt once covered with hair, both sexes having beards;
their ears were pointed and capable of movement; and their bodies were
provided with a tail, having the proper muscles. Their limbs and bodies
were also acted on by many muscles which now only occasionally reappear,
but are normally present in the Quadrumana. The great artery and nerve
of the humerus ran through a supra-condyloid foramen. At this or some
earlier period, the intestine gave forth a much larger diverticulum or
caecum than that now existing. The foot, judging from the condition of
the great toe in the foetus, was then prehensile; and our progenitors,
no doubt, were arboreal in their habits, frequenting some warm,
forest-clad land. The males were provided with great canine teeth,
which served them as formidable weapons.
At a much earlier period the uterus was double; the excreta were voided
through a cloaca; and the eye was protected by a third eyelid or
nictitating membrane. At a still earlier period the progenitors of man
must have been aquatic in their habits; for morphology plainly tells us
that our lungs consist of a modified swim-bladder, which once served as
a float. The clefts on the neck in the embryo of man show where the
branchiae once existed. At about this period the true kidneys were
replaced by the corpora Wolffiana. The heart existed as a simple
pulsating vessel; and the chorda dorsalis took the place of a vertebral
column. These early predecessors of man, thus seen in the dim recesses
of time, must have been as lowly organised as the lancelet or amphioxus,
or even still more lowly organised.
There is one other point deserving a fuller notice. It has long been
known that in the vertebrate kingdom one sex bears rudiments of various
accessory parts, appertaining to the reproductive system, which properly
belong to the opposite sex; and it has now been ascertained that at a
very early embryonic period both sexes possess true male and female
glands. Hence some extremely remote progenitor of the whole vertebrate
kingdom appears to have been hermaphrodite or androgynous.[277] But here
we encounter a singular difficulty. In the mammalian class the males
possess in their vesiculae prostaticae rudiments of a uterus with the
adjacent passage; they bear also rudiments of mammae, and some male
marsupials have rudiments of a marsupial sack.[278] Other analogous
facts could be added. Are we, then, to suppose that some extremely
ancient mammal possessed organs proper to both sexes, that is, continued
androgynous after it had acquired the chief distinctions of its proper
class, and therefore after it had diverged from the lower classes of the
vertebrate kingdom? This seems improbable in the highest degree; for had
this been the case, we might have expected that some few members of the
two lower classes, namely fishes[279] and amphibians, would still have
remained androgynous. We must, on the contrary, believe that when the
five vertebrate classes diverged from their common progenitor the sexes
had already become separated. To account, however, for male mammals
possessing rudiments of the accessory female organs, and for female
mammals possessing rudiments of the masculine organs, we need not
suppose that their early progenitors were still androgynous after they
had assumed their chief mammalian characters. It is quite possible that
as the one sex gradually acquired the accessory organs proper to it,
some of the successive steps or modifications were transmitted to the
opposite sex. When we treat of sexual selection, we shall meet with
innumerable instances of this form of transmission,--as in the case of
the spurs, plumes, and brilliant colours, acquired by male birds for
battle or ornament, and transferred to the females in an imperfect or
rudimentary condition.
The possession by male mammals of functionally imperfect mammary organs
is, in some respects, especially curious. The Monotremata have the
proper milk-secreting glands with orifices, but no nipples; and as these
animals stand at the very base of the mammalian series, it is probable
that the progenitors of the class possessed, in like manner, the
milk-secreting glands, but no nipples. This conclusion is supported by
what is known of their manner of development; for Professor Turner
informs me, on the authority of Koelliker and Lauger, that in the embryo
the mammary glands can be distinctly traced before the nipples are in
the least visible; and it should be borne in mind that the development
of successive parts in the individual generally seems to represent and
accord with the development of successive beings in the same line of
descent. The Marsupials differ from the Monotremata by possessing
nipples; so that these organs were probably first acquired by the
Marsupials after they had diverged from, and risen above, the
Monotremata, and were then transmitted to the placental mammals. No one
will suppose that after the Marsupials had approximately acquired their
present structure, and therefore at a rather late period in the
development of the mammalian series, any of its members still remained
androgynous. We seem, therefore, compelled to recur to the foregoing
view, and to conclude that the nipples were first developed in the
females of some very early marsupial form, and were then, in accordance
with a common law of inheritance, transferred in a functionally
imperfect condition to the males.
Nevertheless a suspicion has sometimes crossed my mind that long after
the progenitors of the whole mammalian class had ceased to be
androgynous, both sexes might have yielded milk and thus nourished their
young; and in the case of the Marsupials, that both sexes might have
carried their young in marsupial sacks. This will not appear utterly
incredible, if we reflect that the males of syngnathous fishes receive
the eggs of the females in their abdominal pouches, hatch them, and
afterwards, as some believe, nourish the young;[280]--that certain other
male fishes hatch the eggs within their mouths or branchial
cavities;--that certain male toads take the chaplets of eggs from the
females and wind them round their own thighs, keeping them there until
the tadpoles are born;--that certain male birds undertake the whole duty
of incubation, and that male pigeons, as well as the females, feed their
nestlings with a secretion from their crops. But the above suspicion
first occurred to me from the mammary glands in male mammals being
developed so much more perfectly than the rudiments of those other
accessory reproductive parts, which are found in the one sex though
proper to the other. The mammary glands and nipples, as they exist in
male mammals, can indeed hardly be called rudimentary; they are simply
not fully developed and not functionally active. They are
sympathetically affected under the influence of certain diseases, like
the same organs in the female. At birth they often secrete a few drops
of milk; and they have been known occasionally in man and other mammals
to become well developed, and to yield a fair supply of milk. Now if we
suppose that during a former prolonged period male mammals aided the
females in nursing their offspring, and that afterwards from some cause,
as from a smaller number of young being produced, the males ceased
giving this aid, disuse of the organs during maturity would lead to
their becoming inactive; and from two well-known principles of
inheritance this state of inactivity would probably be transmitted to
the males at the corresponding age of maturity. But at all earlier ages
these organs would be left unaffected, so that they would be equally
well developed in the young of both sexes.
_Conclusion._--The best definition of advancement or progress in the
organic scale ever given, is that by Von Baer; and this rests on the
amount of differentiation and specialisation of the several parts of the
same being, when arrived, as I should be inclined to add, at maturity.
Now as organisms have become slowly adapted by means of natural
selection for diversified lines of life, their parts will have become,
from the advantage gained by the division of physiological labour, more
and more differentiated and specialised for various functions. The same
part appears often to have been modified first for one purpose, and then
long afterwards for some other and quite distinct purpose; and thus all
the parts are rendered more and more complex. But each organism will
still retain the general type of structure of the progenitor from which
it was aboriginally derived. In accordance with this view it seems, if
we turn to geological evidence, that organisation on the whole has
advanced throughout the world by slow and interrupted steps. In the
great kingdom of the Vertebrata it has culminated in man. It must not,
however, be supposed that groups of organic beings are always supplanted
and disappear as soon as they have given birth to other and more perfect
groups. The latter, though victorious over their predecessors, may not
have become better adapted for all places in the economy of nature. Some
old forms appear to have survived from inhabiting protected sites, where
they have not been exposed to very severe competition; and these often
aid us in constructing our genealogies, by giving us a fair idea of
former and lost populations. But we must not fall into the error of
looking at the existing members of any lowly-organised group as perfect
representatives of their ancient predecessors.
The most ancient progenitors in the kingdom of the Vertebrata, at which
we are able to obtain an obscure glance, apparently consisted of a group
of marine animals,[281] resembling the larvae of existing Ascidians.
These animals probably gave rise to a group of fishes, as lowly
organised as the lancelet; and from these the Ganoids, and other fishes
like the Lepidosiren, must have been developed. From such fish a very
small advance would carry us on to the amphibians. We have seen that
birds and reptiles were once intimately connected together; and the
Monotremata now, in a slight degree, connect mammals with reptiles. But
no one can at present say by what line of descent the three higher and
related classes, namely, mammals, birds, and reptiles, were derived from
either of the two lower vertebrate classes, namely amphibians and
fishes. In the class of mammals the steps are not difficult to conceive
which led from the ancient Monotremata to the ancient Marsupials; and
from these to the early progenitors of the placental mammals. We may
thus ascend to the Lemuridae; and the interval is not wide from these to
the Simiadae. The Simiadae then branched off into two great stems, the New
World and Old World monkeys; and from the latter, at a remote period,
Man, the wonder and glory of the Universe, proceeded.
Thus we have given to man a pedigree of prodigious length, but not, it
may be said, of noble quality. The world, it has often been remarked,
appears as if it had long been preparing for the advent of man; and
this, in one sense is strictly true, for he owes his birth to a long
line of progenitors. If any single link in this chain had never existed,
man would not have been exactly what he now is. Unless we wilfully close
our eyes, we may, with our present knowledge, approximately recognise
our parentage; nor need we feel ashamed of it. The most humble organism
is something much higher than the inorganic dust under our feet; and no
one with an unbiassed mind can study any living creature, however
humble, without being struck with enthusiasm at its marvellous structure
and properties.