VERTEBRATES: BACKBONE AND BRAIN
In tracing man's ancestry from fish upward we ought properly to
describe three or four fish, an amphibian, a reptile, and then take
up the series of mammalian ancestors. But we have not sufficient
time for so extended a study, and a simpler method may answer our
purpose fairly well. Let us fix our attention on the few organs
which still show the capacity of marked development, and follow each
one of these rapidly in its upward course.
We must remember that there are changes in the vegetative organs.
The digestive and excretory systems improve. But this improvement is
not for the sake of these vegetative functions. Brain and muscle
demand vastly more fuel, and produce vastly more waste which must be
removed. At almost the close of the series the reproductive system
undergoes a modification which is almost revolutionary in its
results. But we shall find that this modification is necessitated by
the smaller amount of material which can be spared for this
function; not by its increasing importance, still less its dominance
for its own worth. The vertebrate is like an old Roman; everything
is subordinated to mental and physical power. He is the world
conqueror.
The important changes from fish upward affect the following organs:
1. The skeleton. A light, solid framework must be developed for the
body. 2. The appendages start as fins, and end as the legs and arms
of man. 3. The circulatory and respiratory systems developed so as
to carry with the utmost rapidity and certainty fuel and oxygen to
the muscular and nervous high-pressure engines. Or, to change the
figure, they are the roads along which supplies and munitions can be
carried to the army suddenly mobilized at any point on the frontier.
4. Above all, the brain, especially the cerebrum, the crown and goal
of vertebrate structure. The improvement is now practically
altogether in the animal organs of locomotion and thought. Still,
among these animal organs, the lower systems will lead in point of
time. The brain must to a certain extent wait for the skeleton.
1. The skeleton. The axial skeleton consists, in the lowest fish, of
the notochord, a cylindrical unsegmented rod of cartilage running
nearly the length of the body. This is surrounded by a sheath of
connective tissue, at first merely membranous, later becoming
cartilaginous or gristly. Pieces of cartilage extend upward over the
spinal marrow, and downward around the great aortic artery, forming
the neural and hæmal arches. These unite with the masses of
cartilage surrounding the notochord to form cartilaginous vertebræ,
which may be stiffened by an infiltration of carbonate of lime. The
vertebral column of sharks has reached this stage. Then the
cartilaginous vertebræ ossify and form a true backbone. I have
described the process as if it were very simple. But only the
student of comparative osteology can have any conception of the
number of experiments which were tried in different groups before
the definite mode of forming a bony vertebra was attained. At the
same time the skull was developing in a somewhat similar manner. But
the skull is far more complex in origin and undergoes far more
numerous and important changes than the simpler vertebral column.
Into its history we have no time to enter.
And what shall we say of bone itself as a mere material or tissue,
with its admirable lightness, compactness, and flawlessness. And
every bone in our body is a triumph of engineering architecture. No
engineer could better recognize the direction of strain and stress,
and arrange his rods and columns, arches and buttresses, to suitably
meet them, than these problems are solved in the long bone of our
thigh. And they must be lengthened while the child is leaping upon
them. An engineer is justly proud if he can rebuild or lengthen a
bridge without delaying the passage of a single train. But what
would he say if you asked him to rebuild a locomotive, while it was
running even twenty miles an hour? And yet a similar problem had to
be solved in our bodies.
But the vertebral column is not perfected by fish. The vertebræ with
few exceptions are hollow in front and behind, biconcave; and
between each two vertebræ there is a large cavity still occupied by
the notochord. Thus these vertebræ join one another by their edges,
like two shallow wine-glasses placed rim to rim. Only gradually is
the notochord crowded out so that the vertebræ join by their whole
adjacent surfaces. Even in highest forms, for the sake of mobility,
they are united by washer-like disks of cartilage. Biconcave
vertebræ persisted through the oldest amphibia, reptiles, and
birds. But finally a firm backbone and skull were attained.
2. The appendages. Of these we can say but little. The fish has
oar-like fins, attached to the body by a joint, but themselves
unjointed. By the amphibia legs, with the same regions as our own
and with five toes, have already appeared. The development of the
leg out of the fin is one of the most difficult and least understood
problems of vertebrate comparative anatomy. The legs are at first
weak and scarcely capable of supporting the body. Only gradually do
they strengthen into the fore- and hind-legs of mammals, or into the
legs and wings of birds and old flying reptiles.
3. Changes in the circulatory and respiratory systems. The fish
lives altogether in the water and breathes by gills, but the dipnoi
among fishes breathes by lungs as well as gills. As long as
respiration takes place by gills alone, the circulation is simple;
the blood flows from the heart to the gills, and thence directly all
over the body; the oxygenated blood from the gills does not return
directly to the heart. But the blood from the lungs does return to
the heart; and there at first mixes in the ventricle with the impure
blood which has returned from the rest of the body. Gradually a
partition arises in the ventricle, dividing it into a right and left
half. Thus the two circulations of the venous blood to the lungs,
and of the oxygenated blood over the body, are more and more
separated until, in higher reptiles, they become entirely distinct.
As the animal came on land and breathed the air, more completely
oxygenated blood was carried to the organs, and their activity was
greatly heightened. As more and more heat was produced by the
combustion in muscular and nervous tissues, and less was lost by
conduction, the temperature of the body rose, and in birds and
mammals becomes constant several degrees above the highest summer
temperature of the surrounding air.
The changes in the brain affect mainly the large and small brain.
The cerebellum increases with the greater locomotive powers of the
animal. But its development is evidently limited. The large brain,
or cerebrum, is in fish hardly as heavy as the mid-brain; in
amphibia the reverse is true. In higher recent reptiles the cerebrum
would somewhat outweigh all the other portions of the brain put
together. In mammals it extends upward and backward, has already in
lower forms overspread the mid-brain, and is beginning to cover the
small brain. But this was not so in the earliest mammals. Here the
cerebrum was small, more like that of reptiles. But during the
tertiary period the large brain began to increase with marvellous
rapidity. It was very late in arriving at the period of rapid
development, but it kept on after all the other organs of the body
had settled down into comparative rest, perhaps retrogression.
We have given thus a rapid sketch in outline of the changes in the
most characteristic systems between fish and mammals. Some of the
changes which took place in mammals were along the same lines, but
one at least is so new and unexpected that this highest class
demands more careful and detailed examination.
The mammal is a vertebrate. Hence all its organs are at their best.
But mammals stand, all things considered, at the head of
vertebrates. The skeleton is firm and compact. The muscles are
beautifully moulded and fitted to the skeleton so as to produce the
greatest effect with the least mass and weight of tissue. The
sense-organs are keen, and the eye and ear especially delicate, and
fitted for perception at long range. Yet in all these respects they
are surpassed by birds. As a mere anatomical machine the bird always
seems to me superior to the mammal. It is not easy to see why it
failed, as it has, to reach the goal of possibility of indefinite
development and dominance in the animal world. Why he stopped short
of the higher brain development I cannot tell. The fact remains that
the mammal is pre-eminent in brain power, and that this gave him the
supremacy.
But mammals came very late to the throne, and the probability of
their ever gaining it must for ages have appeared very doubtful.
They seem to have been a fairly old group with a very slow early
development. Reptiles especially, and even birds, were far more
precocious than these slower and weaker forms which crept along the
earth. But reptiles and birds, like many other precocious children,
soon reached the limit of their development. They had muscle, the
mammal brain and nerve; the mammal had the staying power and the
future. Bitter and discouraging must have been the struggle of these
feeble early mammals with their larger, swifter, and more powerful,
reptilian relatives. And yet, perhaps, by this very struggle the
mammal was trained to shrewdness and endurance.
The primitive mammals laid eggs like reptiles or birds. Only two
genera, echidna and platypus, survive to bear witness of these old
oviparous groups, and these only in New Zealand. These retain
several old reptilian characteristics. Their lower position is shown
also by the fact that the temperature of their bodies is, at least,
ten degrees Fahrenheit below that of higher mammals. One of these
carries the egg in a pouch on the ventral surface; the other, living
largely in water, deposits its eggs in a nest in a burrow in the
side of the bank of the stream.
After these came the marsupials. In these the eggs develop in a sort
of uterus; but there is no placenta, in the sense of an organic
connection between the embryo and the uterus of the mother. The
young are at birth exceedingly small and feeble. The adult giant
Kangaroo weighs over one hundred pounds; the young are at birth not
as large as your thumb. They are placed by the mother in a marsupial
pouch on her ventral surface, and here nourished till able to care
for themselves.
Pardon a moment's digression. The marsupials, except the opossum,
are confined to Australia, and the oviparous mammals, or monotremes,
to New Zealand. Formerly the marsupials, at least, ranged all over
Europe and Asia, for we have indisputable evidence in their fossil
remains. But they have survived only in this isolated area, and here
apparently only because their isolation preserved them from the
competition with higher forms. If the Australian continent had not
been thus early cut off from all the rest of the world, the only
trace of both these lower groups would have been the opossum in
America and certain peculiarities in the development of the egg in
higher mammals. This shows us how much weight should be assigned to
the formerly popular argument of the "missing links." The wonder is
not that so many links are missing, but that any of these primitive
forms have come down to us. For we see here another proof of the
fearful extermination of lower forms during the progress of life on
the globe. It seems as if the intermediate forms were less common
among these most recent animals than among the older types. This may
not be true, for it is not easy to compare the gap between two
mammals with that between two worms or insects, and mistakes are
very easily made. But it seems as if extermination had done its work
more ruthlessly among these highest forms than among their humbler
and lower ancestors. I would not lay much weight on such an opinion;
but, if true, it has a meaning and is worthy of study.
In higher, true, placental mammals the period of pregnancy is much
longer, and the young are born in a far higher stage of development,
or rather, growth. The stage of growth at which the young are born
differs markedly in different groups. A new-born kitten is a much
feebler, less developed being than a new-born calf. An embryonic
appendage, the allantois, used in reptiles and birds for
respiration, has here been turned to another purpose. It lays itself
against the walls of the uterus, uterine projections interlock with
those which it puts forth, and the blood of the mother circulates
through a host of capillaries separated from those of the blood
system of the embryo only by the thinnest membrane. This is the
placenta, developed, in part from the allantois of the embryo, in
part from the uterus of the mother. It is not a new organ, but an
old one turned to better and fuller use. In these closely
associated systems of blood-vessels, nutriment and oxygen diffuse
from the blood of the mother into that of the embryo, and thus rapid
growth is assured. The importance and far-reaching effect of this
new modification in the old reproductive system cannot be
over-estimated. The internal intra-uterine development of the young,
and the mammalian habit of suckling them, far more than any other
factors, have made man what he is. Some explanation must be sought
for such a fact.
We have already seen that any animal devotes to reproduction the
balance between income and expenditure of nutriment. Now, the
digestive system is here well developed, and the income is large.
But we have already noticed that, as animals grow larger, the ratio
between the digestive surface and the mass to be supported grows
continually smaller. On account of size alone the mammal has but a
small balance. But the amount of expenditure is proportional to the
mass and activity of the muscular and nervous systems. And the
mammal is, and from the beginning had to be, an exceedingly active,
energetic, and nervous animal. The income has increased, but the
expenses have far outrun the increase. The mammal can devote but
little to reproduction.
Moreover, it requires a large amount of material to form a mammalian
egg, such as that of the monotreme. It requires indefinitely more
nutriment to build a mammal than a worm, for the former is not only
larger and more perfect at birth; it is also vastly more
complicated. The embryonic journey has, so to speak, lengthened out
immensely. One monotreme egg represents more economy and saving than
a thousand eggs of a worm. Moreover, where the individuals are
longer lived and the generations follow one another at longer
intervals, the number of favorable variations and the possibility of
conformity to environment through these is greatly lessened. In such
a group it is of the utmost importance that every egg should
develop; the destruction of a single one is a real and important
loss to the species. It is not enough to produce such an egg; it
must be most scrupulously guarded. Even the egg of the platypus is
deposited in a nest in a hole in the bank, and the female Echidna
carries the egg in a marsupial pouch until it develops.
Notice further that among certain species of fish, amphibia, and
reptiles, the females carry the eggs in the body until the embryos
or young are fairly developed. Viviparous forms are unknown by
birds, probably because this mode of development is incompatible
with flight, their dominant characteristic. Putting these facts
together, what more probable than that certain primitive egg-laying
mammals should have carried the eggs as long as possible in the
uterus. The embryo under these conditions would be better nourished
by a secretion of the uterine glands than by a very large amount of
yolk. The yolk would diminish and the egg decrease in size, and thus
the marsupial mode of development would have resulted. And, given
the marsupial mode of development and an embryo possessing an
allantois, it is almost a physiological necessity that in some forms
at least a placenta should develop. That the placenta has resulted
from some such process of evolution is proven by its different
stages of development in different orders of mammals. And even the
feeblest attachment of the allantois of the embryo to the wall of
the uterus would be of the greatest advantage to the species.
This is not the whole explanation; other factors still undiscovered
were undoubtedly concerned. But even this shows us that the internal
development of the young and the habit of suckling them was a
logical result of mammalian structure and position. The grand
results of this change we shall trace farther on.
The changes from the lower true mammals to the apes are of great
interest, but we can notice only one or two of the more important.
The prosimii, or "half apes," including the lemurs, are nearly all
arboreal forms. Perhaps they were driven to this life by their more
powerful competitors. The arboreal life developed the fingers and
toes, and most of these end, not with a claw, but with a nail. The
little group has much diversity of structure, and at present finds
its home mainly in Madagascar; though in earlier times apparently
occurring all over the globe. The brain is more highly developed
than in the average mammal, but far inferior to that of the apes.
They have a fairly opposable thumb.
The highest mammals are the primates. Their characteristics are the
following: Fingers and toes all armed with nails, the eyes
comparatively near together and fully enclosed in a bony case. The
cerebrum with well-developed furrows covers the other portions of
the brain. There is but one pair of milk-glands, and these on the
breast. The differences between hand and foot become most strongly
marked by the "anthropoid" apes. These have become accustomed to an
upright gait in their climbing; hence the feet are used for
supporting the body and the hands for grasping. Both thumb and
great toe are opposable; but the foot is a true foot, and the hand a
true hand, in anatomical structure. The face, hands, and feet have
mainly lost the covering of hair. They have no tail, or rather its
rudiments are concealed beneath the skin. These include the gibbon,
the orang, the gorilla, and the chimpanzee.
We can sum up the few attainments of mammals in a line. The lower
forms attained the placental mode of embryonic development; the
higher attained upright gait, hands and feet, and a great increase
of brain. Anatomically considered these were but trifles, but the
addition of these trifles revolutionized life on the globe. The
principal anatomical differences between man and the anthropoid ape
are the following: Man is a strictly erect animal. The foot of the
ape is less fitted for walking on the ground, where he usually "goes
on all fours." The skull is almost balanced on the condyles by which
it articulates with the neck, and has but slight tendency to tip
forward. The facial portion, nose and jaws, is less developed and
retracted beneath the larger cranium or brain-case. This has greatly
changed the appearance of the head. Protruding jaws and chin, even
when combined with large cranium and brain, always give man the
appearance of brutality and low intelligence.
The pelvis is broad and comparatively shallow. The legs, especially
the thighs, are long. The foot is long and strong, and rests its
lower surface, not merely the outer margin as in apes, on the
ground. The elastic arch of the instep must be excepted in the above
description, and adds lightness and swiftness to his otherwise slow
gait. The great toe is short and generally not opposable. The
muscles of the leg are heavy and the knee-joint has a very broad
articulating surface. But the great result of man's erect posture is
that the hand is set free from the work of locomotion, and has
become a delicate tactile and tool-using organ. The importance of
this change we cannot over-estimate. The hand was the servant of the
brain for trying all experiments. Had not our arboreal ancestors
developed the hand for us we could never have invented tools nor
used them if invented. And its reflex influence in developing the
brain has been enormous. The arm is shorter and the hand smaller.
The brain is absolutely and relatively large, and its surface
greatly convoluted. This gives place for a large amount of "gray
matter," whose functions are perception, thought, and will. For this
gray matter forms a layer on the outside of the brain.
Thus, even anatomically, man differs from the anthropoid apes. His
whole structure is moulded to and by the higher mental powers, so
that he is the "Anthropos" of the old Greek philosophers, the being
who "turns his face upward." Yet in all these anatomical respects
some of the apes differ less from him than from the lower apes or
"half apes." And every one of these can easily be explained as the
result of progressive development and modification. Whoever will
deny the possibility or probability of man's development from some
lower form must argue on psychological, not on anatomical, grounds;
and it grows clearer every day that even the former but poorly
justify such a denial.
But it is interesting to note that no one ape most closely
approaches man in all anatomical respects. Thus among the
anthropoids the orang is perhaps most similar to man in cerebral
structure, the chimpanzee in form of skull, the gorilla in feet and
hands. No evolutionist would claim that any existing ape represents
the ancestor of man. The anthropoids represent very probably the
culmination of at least three distinct lines of development. But we
must remember that in early tertiary times apes occurred all over
Europe, and probably Asia, many degrees farther north than now. In
those days, as later, the fauna and flora of northern climates were
superior in vigor and height of development to that of Africa or
Australia. It is thus, to say the least, not at all improbable that
there existed in those times apes considerably, if not far, superior
to any surviving forms. Whether the palæontologist will find for us
remains of such anthropoids is still to be seen.
But you will naturally ask, "Is there not, after all, a vast
difference between the brain of man and that of the ape?" Let us
examine this question as fully as our very brief time will allow.
Considerable emphasis used to be laid on the facial angle between a
line drawn parallel to the base of the skull and one obliquely
vertical touching the teeth and most prominent portion of the
forehead. Now this angle is in man very large--from seventy-five to
eighty-five degrees, or even more, and rarely falling below
sixty-five degrees. But this angle depends largely on the protrusion
of the jaws, and varies greatly in species of animals showing much
the same grade of intelligence. In some not especially intelligent
South American monkeys the facial angle amounts to about sixty-five
degrees. In this respect the skull of a chimpanzee reminds us of a
human skull of small cranial capacity and large jaws, in which the
cranium has been pressed back and the jaws crowded forward and
slightly upward.
The weight of the brain in proportion to that of the body has been
considered as of great importance, and within certain limits this is
undoubtedly correct. Thus, according to Leuret, the weight of the
brain is to that of the whole body: In fish, 1:5,668; in reptiles,
1:1,320; in birds, 1:212; in mammals, 1:186. These figures give the
averages of large numbers of observations and have a certain
amount of value. But within the same class the ratio varies
extraordinarily. Thus the weight of the brain is to that of the
whole body: In the elephant, 1:500; in the largest dogs, 1:305; in
the cat, 1:156; in the rat, 1:76; in the chimpanzee, 1:50; in man,
1:36; in the field-mouse, 1:31; in the goldfinch, 1:24.
From this series it is evident that the relative weight of the brain
is no index of the intelligence of the animal. Indeed if the brain
were purely an organ of mind, there is no reason that it should be
any larger in an elephant than in a mouse, provided they had the
same mental capacity. As animals grow larger the weight of the
brain, relatively to that of the body, decreases, and considering
the size of man it is remarkable that it should form so large a
fraction of his weight. Still the fraction in the chimpanzee is not
so much smaller. It is still possible that this fraction is above
the normal for the chimpanzee, for some of the observations may have
been taken on animals which had died of consumption or some other
wasting disease. I have not been able to find whether this
possibility of error has been scrupulously avoided.
A fair idea of the size of the brain may be obtained by measuring
the cranial capacity. This varies in man from almost one-hundred
cubic inches to less than seventy. In the gorilla its average is
perhaps thirty, in the orang and chimpanzee rather less, about
twenty-eight. This is certainly a vast difference, especially when
we remember that the gorilla far exceeds man in weight.
Le Bon tells us that of a series of skulls forty-five per cent, of
the Australian had a cranial capacity of 1,200 to 1,300 c.c., while
46.7 per cent. of modern Parisian skulls showed a capacity of
between 1,500 and 1,600 c.c. The skull of the gorilla contains about
five hundred and seventy cubic centimetres. Broca found that the
cranial capacity of 115 Parisian skulls, of probably the higher
classes from the twelfth century, averaged about 1,426 cubic
centimetres, while ninety of those of the poorer classes of the
nineteenth century averaged about 1,484. His observations seemed to
prove that there has been a steady increase in Parisian cranial
capacity from the twelfth to the nineteenth century.
Turning to the actual weight of the brain, that of Cuvier weighed
64.5 ounces, and a few cases of weights exceeding 65 ounces have
been recorded. The lowest limit of weight in a normal human brain
has not yet been accurately determined. From 34 to 31 ounces have
been assigned by different writers. The brain of a Bush woman was
computed by Marshall at 31.5 ounces, and weights of even 31 ounces
have been recorded without any note to show that the possessors were
especially lacking in intelligence. As Professor Huxley says in his
"Man's Place in Nature," a little book which I cannot too highly
recommend to you all, "It may be doubted whether a healthy human
adult brain ever weighed less than 31 or 32 ounces, or that the
heaviest gorilla brain has ever exceeded 20 ounces. The difference
in weight of brain between the highest and the lowest men is far
greater, both relatively and absolutely, than that between the
lowest man and the highest ape. The latter, as has been seen, is
represented by 12 ounces of cerebral substance absolutely, or by
32:20 relatively. But as the largest recorded human brain weighed
between 65 and 66 ounces, the former difference is represented by 33
ounces absolutely, or by 65:32 relatively."
But there is another characteristic of the brain which seems to bear
a close relation to the degree of intelligence. The surface of the
human brain is not smooth but covered with convolutions, with
alternating grooves or sulci, which vastly increase its surface and
thus make room for more gray matter. Says Gratiolett: "On comparing
a series of human and simian brains we are immediately struck with
the analogy exhibited in the cerebral forms in all these creatures.
There is a cerebral form peculiar to man and the apes; and so in the
cerebral convolutions, wherever they appear, there is a general
unity of arrangement, a plan, the type of which is common to all
these creatures." Professor Huxley says: "It is most remarkable
that, as soon as all the principal sulci appear, the pattern
according to which they are arranged is identical with the
corresponding sulci in man. The surface of the brain of the monkey
exhibits a sort of skeleton map of man's, and in the man-like apes
the details become more and more filled in, until it is only in
minor characters that the chimpanzee's or orang's brain can be
structurally distinguished from man's."
The facts of anatomy, at least, are all against us. Struggle as we
may, be as snobbish as we will, we cannot shake off these poor
relations of ours. Our adult anatomy at once betrays our ancestry,
if we attempt to deny it. Read the first chapter of that remarkable
book by Professor Drummond on the "Ascent of Man," the chapter on
the ascent of the body, and the second chapter on the scaffolding
left in the body. The tips of our ears and our rudimentary ear
muscles, the hair on hand and arm, and the little plica semilunaris,
or rudimentary third eyelid in the inner angle of our eyes, the
vermiform appendage of the intestine, the coracoid process on our
shoulder-blades, the atlas vertebra of our necks--to say nothing of
the coccyx at the other end of the backbone--many malformations, and
a host of minor characteristics all refute our denial.
If we appeal from adult anatomy to embryology the case becomes all
the worse for us. Our ear is lodged in the gill-slit of a fish, our
jaws are branchial arches, our hyoid bone the rudiment of this
system of bones supporting the gills. Our circulation begins as a
veritable fish circulation; our earliest skeleton is a notochord;
Meckel's cartilage, from which our lower jaw and the bones of our
middle ear develop, is a whole genealogical tree of disagreeable
ancestors. Our glandula thyreoidea has, according to good
authorities, an origin so slimy that it should never be mentioned in
polite society. The origin of our kidneys appears decidedly vermian.
Time fails me to read merely the name of the witnesses which could
be summoned from our own bodies to witness against us.
Even if the testimony of some of these witnesses is not as strong
as many think, and we have misunderstood several of them, they are
too numerous and their stories hang too well together not to impress
an intelligent and impartial jury. But what if it is all true? What
if, as some think, our millionth cousin, the tiger or cat, is
anatomically a better mammal than I? His teeth and claws and
magnificent muscles are of small value compared with man's mental
power.
What a comedy that man should work so hard to prove that his chief
glory is his opposable thumb, or a few ounces of brain matter! Man's
glory is his mind and will, his reason and moral powers, his vision
of, and communion with, God. And supposing it be true, as I believe
it is true, that the animal has the germ of these also, does that
cloud my mind or obscure my vision or weaken my action? It bids me
only strive the harder to be worthy of the noble ancestors who have
raised me to my higher level and on whose buried shoulders I stand.
Whatever may have been our origin, whoever our ancestors, we are
men. Then let us play the man. If we will but play our part as well
as our old ancestors played theirs, if we will but walk and act
according to our light one-half as heroically and well as they
groped in the darkness, we need not worry about the future. That
will be assured.
Says Professor Huxley: "Man now stands as on a mountain-top far
above the level of his humble fellows, and transfigured from his
grosser nature by reflecting here and there a ray from the infinite
source of truth. And thoughtful man, once escaped from the blinding
influences of traditional prejudice, will find in the lowly stock
whence man has sprung the best evidence of the splendor of his
capacities, and will discern in his long progress through the past a
reasonable ground of faith in his attainment of a nobler future."
We have sketched hastily and in rude outline the anatomical
structure of the successive stages of man's ancestry; let us now, in
a very brief recapitulation, condense this chronicle into a
historical record of progress.
We began with the amoeba. This could not have been the beginning.
In all its structure it tells us of something earlier and far
simpler, but what this earlier ancestor was we do not know. Rather
more highly organized relatives of the amoeba, the flagellata,
have produced a membrane, and swim by means of vibratile,
whiplash-like flagella. We must emphasize that these little animals
correspond in all essential respects to the cells of our bodies;
they are unicellular animals. And the cell once developed remains
essentially the same structure, modified only in details, throughout
higher animals. And these unicellular animals have the rudiments of
all our functions. Their protoplasm and functions seem to differ
from those of higher animals only in degree, not in kind. And the
more we consider both these facts the more remarkable and suggestive
do they become.
Cells with membranes can unite in colonies capable of division of
labor and differentiation. And magosphæra is just such a little
spheroidal colony. But the cells are still all alike, each one
performs all functions equally well. But in volvox division of labor
and differentiation of structure have taken place. Certain cells
have become purely reproductive, while the rest gather nutriment for
these, but are at the same time sensitive and locomotive, excretory
and respiratory. The first function to have cells specially devoted
to it is the reproductive; this is a function absolutely necessary
for the maintenance of the species. For the nutritive cells die when
they have brought the reproductive cells to their full development.
These few nutritive cells represent the body of all higher animals
in contrast with the reproductive elements. And with the development
of a body, death, as a normal process, enters the world. The
dominant function is here evidently the reproductive, and the whole
body is subservient to this.
In hydra the union and differentiation of cells is carried further.
But the cells are still much alike and only slowly lose their own
individuality in that of the whole animal. This is shown in the fact
that each entodermal cell digests its own particles of food,
although the nutriment once digested diffuses to all parts of the
body. Also almost any part of the animal containing both ectoderm
and entoderm can be cut off and will develop into a new animal.
But beside the reproductive cells and tissues hydra has developed a
very simple digestive system, in which the newly caught food at
least macerates and begins to be dissolved. This is the second
essential function. The animal can, and the plant as a rule does,
exist with only the lowest rudiments of anything like nervous or
muscular power; but no species can exist without good powers of
digestion and reproduction. These essential organs must first
develop and the higher must wait. And the inner, digestive, layer of
cells persists in our bodies as the lining of the mid-intestine. We
compared hydra therefore to a little patch of the lining of our
intestine covered with a flake of epidermis; only these layers in
hydra possess powers lost to the corresponding cells of our bodies
in the process of differentiation. Notice, please, that when cell or
organ has once been developed it persists, as a rule, modified, but
not lost. Nature's experiments are not in vain; her progress is very
slow but sure. But hydra has also the promise of better things,
traces of muscular and nervous tissue. There are still no compact
muscles, like our own, much less ganglion or brain or nerve-centre
of individuality. The tissues are diffuse, but they are the
materials out of which the organs of higher animals will
crystallize, so to speak. Notice also that these higher muscles and
nerves are here entirely subservient to, and exist for, digestion
and reproduction.
In the turbellaria the reproductive system has reached a very high
grade of development. It is a complex and beautifully constructed
organ. The digestive system has also vastly improved; it has its own
muscular layers, and often some means of grasping food. But it is
slower in reaching its full development than the reproductive
system. But all the muscles are no longer attached to the stomach;
they are beginning to assert their independence, and, in a rude way,
to build a body-wall. But they are in many layers, and run in almost
all directions. Some of these layers will disappear, but the most
important ones, consisting of longitudinal and transverse fibres,
will persist in higher forms. Locomotion by means of these muscles
is slowly coming into prominence. They are no longer merely slaves
of digestion.
But a muscular fibril contracts only under the stimulus of a nervous
impulse. More nerve-cells are necessary to control these more
numerous muscular fibrils. The animal now moves with one end
foremost, and that end first comes in contact with food, hindrances,
or injurious surroundings. Here the sensory cells of feeling and
their nerve fibrils multiply. Remember that these neuro-epithelial
sensory cells are suited to respond not merely to pressure, but to a
variety of the stimuli, chemical, molecular, and of vibration, which
excite our organs of smell, taste, and hearing. Such organs and the
directive eyes appear mainly at this anterior end. But a ganglion
cell sends an impulse to a muscle because it has received one along
a sensory nerve from one or more of these sensory cells. Hence the
ganglion cells will increase in number. The old cobweb-like plexus
condenses into a little knot, the supra-oesophageal ganglion. This
ganglion cannot do much, if any, thinking; it is rather a steering
organ to control the muscles and guide the animal. It is the servant
of the locomotive system. Yet it is the beginning of the brain of
higher animals, and probably still persists as an infinitesimal
portion of our human brain. And all this is the prophecy of a head
soon to be developed. An excretory system has appeared to carry off
the waste of the muscles and nerves.
In the schematic worm and annelid the reproductive system is
simpler, though perhaps equally effective. It takes the excess of
nutriment of the body. The muscular system has taken the form of a
sack composed of longitudinal and transverse fibres. The
perivisceral cavity, formed perhaps by cutting off and enlarging the
lateral pouches of the turbellarian digestive system, serves as a
very simple but serviceable circulatory system. But in the annelid
and all higher forms a special system of tubes has developed to
carry the nutriment, and usually oxygen also, needed to keep up the
combustion required to furnish the energy in these active organs.
The digestive system has attained its definite form with the
appearance of an anal opening and the accompanying division of labor
and differentiation into fore-, mid-, and hind-intestine.
The digestive and reproductive systems have thus nearly attained
their final form. From the higher worms upward the digestive system
will improve greatly. Its lining will fold and flex and vastly
increase the digestive and absorptive surfaces. The layer of cells
which now secrete the digestive fluids will in part be replaced by
massive glands. Far better means of grasping food than the horny
teeth of annelids will yet appear. But all these changes are
inconsiderable compared with the vast advance made by the muscular
and nervous systems. Reproduction and digestion are losing their
supremacy in the animal body. Their advance and improvement will
require but little further attention.
In the annelid especially, and to some extent in the schematic worm,
the supra-oesophageal ganglion is relieved in part of the direct
control of the muscular fibrils and has become an organ of
perception and the seat of government of lower nervous centres. In
all higher forms it innervates directly only the principal
sense-organs of the head. And at this stage the light-perceiving
directive eye has developed into a form-perceiving, eidoscopic
organ. The eye was short of range and its images were perhaps rude
and imperfect, but it was a visual eye and had vast possibilities.
The animal is taking cognizance of ever more subtle elements in its
environment. Perhaps it is not too much to say that the eidoscopic
eye first awakened the slumbering animal mind, for its reflex effect
upon the supra-oesophageal ganglion cannot be over-estimated. The
animal will very soon begin to think.
Between the turbellarian and the annelid many aberrant lines
diverged. Some of these attained a comparatively high level and then
seemed to meet insuperable obstacles, while others came to an end or
turned downward very early. Three of these demanded attention, those
leading to mollusks, insects, and vertebrates. And it is interesting
to notice that the fundamental difference between these three lines
was the skeleton, or perhaps we ought to say it was the habit of
life which led to the development of such a skeleton.
The mollusk took to a sluggish, creeping mode of life, under an
external purely protective skeleton; the insect to a creeping mode
of life, with an external but almost purely locomotive skeleton; the
vertebrate kept on swimming and developed an internal locomotive
skeleton. And it must already have become clear to you that the
destiny of these different lines was fixed not so much directly by
the skeleton itself as by its reflex effect in moulding the
muscular, and ultimately the nervous, system.
The insects formed their skeleton by thickening the horny cuticle of
the annelid. They transformed the annelid parapodia into legs and
developed wings. They attained life in the air. They devoted the
muscles of the body largely to the extremities and gained swift
locomotion. They have a fair circulatory and an excellent
respiratory system. Best of all, they developed a head and a brain
by fusing the three anterior ganglia of the body. The insect could
and does think. Such a structure ought to lead to great and high
results. But actually their possibilities were very limited. They
have not progressed markedly during the last geological period.
Their external skeleton was easily attained and brought speedy
advantages, which for a time placed them far above all competitors.
But it limited their size and length of life and opportunities, and
finally their intelligence. They remained largely the slaves of
instinct. They followed an attractive and exceedingly promising
path, but it led to the bottom of a cliff, not to the summit.
The mollusks, clams, and snails took an easier, down-hill road. They
formed a shell, and it developed large enough to cover them. It
hampered and almost destroyed locomotion and reduced nerve to a
minimum. But nerves are nothing but a nuisance anyhow. And why
should they move? Food was plenty down in the mud, and if danger
threatened, they withdrew into the shell. They stayed down in the
mud and let the world go its way. If grievously afflicted by a
parasite they produced a pearl--to save themselves from further
discomfort. They developed just enough muscle and nervous system to
close the shell or drag it a little way; that was all. Digestion and
reproduction retained the supremacy. They were fruitful and
multiplied, and produced hosts of other clams and snails. The
present was enough for them and they had that.
For if the winner in the struggle for existence is the one who gains
the most food, the most entire protection against discomfort, danger
from enemies or unfavorable surroundings, and the most fruitful and
rapid reproduction--and these are all good--then the clam is the
highest product of evolution. It never has been surpassed--I venture
to say it never can be--except possibly by the tape-worms. I can
never help thinking with what contempt these primitive oysters, if
they had had brains enough, would have looked down upon the toiling,
struggling, discontented, fighting, aspiring primitive vertebrates.
How they would have wondered why God allowed such disagreeable,
disturbing, unconventional creatures to exist, and thanked him that
he had made the world for them, and heaven too, if there be such a
place for mollusks. Their road led to the Slough of Contentment.
But even in molluscan history there was a tragic chapter. The squids
and cuttle-fishes regained the swimming life, and in their latest
forms gave up the protective shell. But its former presence had so
modified their structure that any great advance was impossible. It
was too late. The sins of the fathers were visited upon the children
in the thousandth generation.
The vertebrate developed an internal skeleton. This was necessarily
a slow growth, and the type came late to supremacy. The longitudinal
muscles are arranged in heavy bands on each side of the back, and
the animal swims rapidly. The sense-organs are keen. The brain
contains the ganglia of several or many segments and is highly
differentiated. It has a special centre of perception, thought, and
will; it is an organ of mind. The vertebrate has the physical and
mental advantages of large size.
First the definite form and mode of developing a vertebra is
attained. Then the vertebral column is perfected. The fins are
modified into legs. The lungs increase in size and the heart becomes
double. The animal emerges on land; and, with a better supply of
oxygen and less loss of heat, all the functions are performed with
the highest possible efficiency. First, apparently, amphibia, then
reptiles, and finally mammals of enormous size and strength
appeared. It looked as if the earth were to be an arena where
gigantic beasts fought a never-ending battle of brute force. But
these great brutes reproduced slowly, had therefore little power of
adaptation, were fitted to special conditions, and when the
conditions changed they disappeared. The bird tried once more the
experiment of developing the locomotive powers to the highest
possible extent. It became a flying machine, and every organ was
moulded to suit this life. Every ounce of spare weight was thrown
aside, the muscles were wonderfully arranged and of the highest
possible efficiency. The body temperature is higher than that of
mammals. The whole organization is a physiological high-pressure
engine. The sense-organs are perhaps the finest and keenest in the
whole animal kingdom. The brain is inferior only to that of mammals.
The experiment could not have been tried under more favorable
conditions; it was not a failure, it certainly was not a success
when compared with that of mammals.
The possibilities of every system except one had been practically
exhausted. Only brain development remained as the last hope of
success. Here was an untried line, and the mammals followed it.
During the short tertiary period the brain in many of their genera
seems to have increased tenfold. By the arboreal life of the highest
forms the hand is developed as the instrument of the thinking brain.
The battle is beginning to become one of wits, and the crown will
soon pass from the strongest to the shrewdest. Mind, not muscle,
much less digestion or reproduction, is the goal of the animal
kingdom. And we shall see later that the mammalian mode of
reproduction and of care of the young led to an almost purely mental
and moral advance. For these could have but one logical outcome,
family life. And the family is the foundation of society. And family
and social life have been the school in which man has been compelled
to learn the moral lessons, the application of which has made him
what he is.
You must all, I think, have noticed that the different systems of
organs succeed one another in a certain definite order; and that
each stage from the lowest to the highest is characterized by the
predominance of a certain function or group of functions. This
sequence of functions is not a deduction but a fact. Place side by
side all possible genealogical trees of the animal kingdom, whether
founded on comparative anatomy, embryology, palæontology, or all
combined. They will all disclose this sequence of functions arranged
in the same order. Let me call your attention to the fact that this
order is not due to chance, but rests upon a physiological basis. We
might almost claim that if the evolution of man from the single cell
be granted, no other order of their occurrence is possible.
The protozoa are mostly, though not purely, nutritive and
reproductive. These functions are essential to the existence of the
species. Naturally in the early protozoan colonies, and in forms
like hydra, these functions predominated. But mere digestive tissue
is not enough for digestion. Muscles are needed to draw the food to
the mouth, to keep the digestive sack in contact with it, and for
other purposes. A little higher they are used to enable the animal
to go in search of its food. They are still, however, more or less
entirely subservient to digestion. But in the highest worms we are
beginning to see signs that muscles are predominating in the body;
and we feel that, while mutually helpful, the digestive system
exists for the muscles, and these latter are becoming the aim of
development. From worms upward there is a marked advance in physical
activity and strength. The muscles thicken and are arranged in
heavier bands. Skeleton and locomotive appendages and jaws follow in
insects and vertebrates. The direct battle of animal against animal,
and of strength opposed to strength or activity, becomes ever
sharper. The strongest and most active are selected and survive.
And yet this is not the whole truth. Some power of perception is
possessed by every animal. But until muscles had developed the
nervous system could be of but little practical value. Knowledge of
even a great emergency is of little use, if I can do nothing about
it. But when the muscles appeared, nerves and ganglion cells were
necessary to stimulate and control them. And this highest system
holds for a long time a position subordinate to that of the lower
muscular organ. Its development seems at first sight extraordinarily
slow. Only in insects and vertebrates has it become a centre of
instinct and thought. Through the sense-organs it is gaining an ever
clearer, deeper, and wider knowledge of its environment. First it is
affected only by the lower stimuli of touch, taste, and smell. Then
with the development of ear and eye it takes cognizance of ever
subtler forces and movements. Memory comes into activity very early.
The animal begins to learn by experience. The brain is becoming not
merely a steering but a thinking organ. More and more nervous
material is crowded into it and detailed for its work. Wits and
shrewdness are beginning to count for something in the battle. Not
only the animal with the strongest muscles, but the one with the
best brain survives. And thus at last the brain began to develop
with a rapidity as remarkable as its long delay. Thus each higher
function is called into activity by the next lower, serves this at
first, and only later attains its supremacy.
And yet the advance of the different functions is not altogether
successive. Muscle and nerve do not wait for digestion and
reproduction to show signs of halting before they begin to advance.
They all advance at once. But the progress of reproduction and
digestion is most rapid at first, and it appears as if they would
outrun the others. But in the ascending series the others follow
after, and soon overtake and pass by them. And these lower
functions, when out-marched, do not lag behind, but keep in touch
with the others, forming the rear-guard and supply-train of the
army. And notice that each organ holds the predominance about as
long as it shows the power of rapid improvement. The length of its
reign is pretty closely proportional to its capacity of development.
The digestive system reaches that limit early, the muscular system
is capable of indefinitely higher complexity, as we see in our hand.
But the muscular system has nearly or quite reached its limit. The
body had seen its day of dominance before man arrived on the globe.
But where is the limit to man's mental or moral powers? Every
upward step in knowledge, wisdom, and righteousness only opens our
eyes to greater heights, before unperceived and still to be
attained. These capacities, even to our dim vision, are evidently
capable of an indefinite, perhaps infinite, development. What, as
yet only partially developed, faculty remains to supersede them? As
being capable of an endless development and without a rival, may we
not, _must_ we not, consider them as ends in themselves? They are
evidently what we are here for. Everything points to a spiritual end
in animal evolution. The line of development is from the
predominantly material to the predominance of the non-material. Not
that the material is to be crowded out. It is to reach its highest
development in the service of the mind. The body must be sustained
and perfected, but it is not the end. The goal is mind, the body is
of subordinate importance.
But if this is true, we must study carefully the development of mind
in the animal. The question presses upon us; if there is a sequence
of physical functions in animal development, is there not perhaps
also a sequence in the development of the mental faculties? What is
the crowning faculty of the human mind and how is its fuller
development to be attained? Let us pass therefore to the question of
mind in the animal kingdom.