On the Relations of Man to the Lower Animals
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Thomas H. Huxley >> On the Relations of Man to the Lower Animals
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ON THE RELATIONS OF MAN TO THE LOWER ANIMALS
by Thomas H. Huxley
Multis videri poterit, majorem esso differentiam Simiae et Hominis, quam
diei et noctis; verum tamen hi, comparatione instituta inter summos
Europae Heroes et Hottentottos ad Caput bonae spei degentes,
difficillime sibi persuadebunt, has eosdem habere natales; vel si
virginem nobilem aulicam, maxime comtam et humanissimam, conferre
vellent cum homine sylvestri et sibi relicto, vix augurari possent,
hunc et illam ejusdem esse speciei.--'Linnaei Amoenitates Acad.
"Anthropomorpha."'
THE question of questions for mankind--the problem which underlies all
others, and is more deeply interesting than any other--is the
ascertainment of the place which Man occupies in nature and of his
relations to the universe of things. Whence our race has come; what are
the limits of our power over nature, and of nature's power over us; to
what goal we are tending; are the problems which present themselves
anew and with undiminished interest to every man born into the world.
Most of us, shrinking from the difficulties and dangers which beset the
seeker after original answers to these riddles, are contented to ignore
them altogether, or to smother the investigating spirit under the
featherbed of respected and respectable tradition. But, in every age,
one or two restless spirits, blessed with that constructive genius,
which can only build on a secure foundation, or cursed with the spirit
of mere scepticism, are unable to follow in the well-worn and
comfortable track of their forefathers and contemporaries, and
unmindful of thorns and stumbling-blocks, strike out into paths of their
own. The sceptics end in the infidelity which asserts the problem to
be insoluble, or in the atheism which denies the existence of any
orderly progress and governance of things: the men of genius propound
solutions which grow into systems of Theology or of Philosophy, or
veiled in musical language which suggests more than it asserts, take
the shape of the Poetry of an epoch.
Each such answer to the great question, invariably asserted by the
followers of its propounder, if not by himself, to be complete and
final, remains in high authority and esteem, it may be for one century,
or it may be for twenty: but, as invariably, Time proves each reply to
have been a mere approximation to the truth--tolerable chiefly on
account of the ignorance of those by whom it was accepted, and wholly
intolerable when tested by the larger knowledge of their successors.
In a well-worn metaphor, a parallel is drawn between the life of man and
the metamorphosis of the caterpillar into the butterfly; but the
comparison may be more just as well as more novel, if for its former
term we take the mental progress of the race. History shows that the
human mind, fed by constant accessions of knowledge, periodically grows
too large for its theoretical coverings, and bursts them asunder to
appear in new habiliments, as the feeding and growing grub, at
intervals, casts its too narrow skin and assumes another, itself but
temporary. Truly the imago state of Man seems to be terribly distant,
but every moult is a step gained, and of such there have been many.
Since the revival of learning, whereby the Western races of Europe were
enabled to enter upon that progress towards true knowledge, which was
commenced by the philosophers of Greece, but was almost arrested in
subsequent long ages of intellectual stagnation, or, at most, gyration,
the human larva has been feeding vigorously, and moulting in proportion.
A skin of some dimension was cast in the 16th century, and another
towards the end of the 18th, while, within the last fifty years, the
extraordinary growth of every department of physical science has spread
among us mental food of so nutritious and stimulating a character that a
new ecdysis seems imminent. But this is a process not unusually
accompanied by many throes and some sickness and debility, or, it may
be, by graver disturbances; so that every good citizen must feel bound
to facilitate the process, and even if he have nothing but a scalpel to
work withal, to ease the cracking integument to the best of his
ability.
In this duty lies my excuse for the publication of these essays. For it
will be admitted that some knowledge of man's position in the animate
world is an indispensable preliminary to the proper understanding of
his relations to the universe--and this again resolves itself, in the
long run, into an inquiry into the nature and the closeness of the ties
which connect him with those singular creatures whose history* has been
sketched in the preceding pages.
[footnote] * It will be understood that, in the preceding
Essay, I have selected for notice from the vast mass of
papers which have been written upon the man-like Apes, only
those which seem to me to be of special moment.
The importance of such an inquiry is indeed intuitively manifest
Brought face to face with these blurred copies of himself, the least
thoughtful of men is conscious of a certain shock, due perhaps, not so
much to disgust at the aspect of what looks like an insulting
caricature, as to the awakening of a sudden and profound mistrust of
time-honoured theories and strongly-rooted prejudices regarding his own
position in nature, and his relations to the under-world of life; while
that which remains a dim suspicion for the unthinking, becomes a vast
argument, fraught with the deepest consequences, for all who are
acquainted with the recent progress of the anatomical and physiological
sciences.
I now propose briefly to unfold that argument, and to set forth, in a
form intelligible to those who possess no special acquaintance with
anatomical science, the chief facts upon which all conclusions
respecting the nature and the extent of the bonds which connect man with
the brute world must be based: I shall then indicate the one immediate
conclusion which, in my judgment, is justified by those facts, and I
shall finally discuss the bearing of that conclusion upon the
hypotheses which have been entertained respecting the Origin of Man.
The facts to which I would first direct the reader's attention, though
ignored by many of the professed instructors of the public mind, are
easy of demonstration and are universally agreed to by men of science;
while their significance is so great, that whoso has duly pondered over
them will, I think, find little to startle him in the other revelations
of Biology. I refer to those facts which have been made known by the
study of Development.
It is a truth of very wide, if not of universal, application, that every
living creature commences its existence under a form different from,
and simpler than, that which it eventually attains.
FIG. 12.--A. Egg of the Dog, with the vitelline membrane burst, so as
to give exit to the yolk, the germinal vesicle (a), and its included
spot (b). B. C. D. E F. Successive changes of the yolk indicated in
the text. After Bischoff.
The oak is a more complex thing than the little rudimentary plant
contained in the acorn; the caterpillar is more complex than the egg;
the butterfly than the caterpillar; and each of these beings, in
passing from its rudimentary to its perfect condition, runs through a
series of changes, the sum of which is called its Development. In the
higher animals these changes are extremely complicated; but, within the
last half century, the labours of such men as Von Baer, Rathke,
Reichert, Bischof, and Remak, have almost completely unravelled them, so
that the successive stages of development which are exhibited by a Dog,
for example, are now as well known to the embryologist as are the steps
of the metamorphosis of the silkworm moth to the school-boy. It will
be useful to consider with attention the nature and the order of the
stages of canine development, as an example of the process in the
higher animals generally.
The Dog, like all animals, save the very lowest (and further inquiries
may not improbably remove the apparent exception), commences its
existence as an egg: as a body which is, in every sense, as much an egg
as that of a hen, but is devoid of that accumulation of nutritive
matter which confers upon the bird's egg its exceptional size and
domestic utility; and wants the shell, which would not only be useless
to an animal incubated within the body of its parent, but would cut it
off from access to the source of that nutriment which the young
creature requires, but which the minute egg of the mammal does not
contain within itself.
The Dog's egg is, in fact, a little spheroidal bag (Fig. 12), formed of
a delicate transparent membrane called the 'vitelline membrane', and
about 1/130 to 1/120th of an inch in diameter. It contains a mass of
viscid nutritive matter--the 'yelk'--within which is inclosed a second
much more delicate spheroidal bag, called the 'germinal vesicle' (a).
In this, lastly, lies a more solid rounded body, termed the 'germinal
spot' (b).
The egg, or 'Ovum,' is originally formed within a gland, from which, in
due season, it becomes detached, and passes into the living chamber
fitted for its protection and maintenance during the protracted process
of gestation. Here, when subjected to the required conditions, this
minute and apparently insignificant particle of living matter becomes
animated by a new and mysterious activity. The germinal vesicle and
spot cease to be discernible (their precise fate being one of the yet
unsolved problems of embryology), but the yelk becomes
circumferentially indented, as if an invisible knife had been drawn
round it, and thus appears divided into two hemispheres (Fig. 12, C).
By the repetition of this process in various planes, these hemispheres
become subdivided, so that four segments are produced (D); and these,
in like manner, divide and subdivide again, until the whole yelk is
converted into a mass of granules, each of which consists of a minute
spheroid of yelk-substance, inclosing a central particle, the so-called
'nucleus' (F). Nature, by this process, has attained much the same
result as that at which a human artificer arrives by his operations in
a brickfield. She takes the rough plastic material of the yelk and
breaks it up into well-shaped tolerably even-sized masses, handy for
building up into any part of the living edifice.
FIG. 13.--Earliest rudiment of the Dog. B. Rudiment further advanced,
showing the foundations of the head, tail, and vertebral column. C.
The very young puppy, with attached ends of the yelk-sac and allantois,
and invested in the amnion.
Next, the mass of organic bricks, or 'cells' as they are technically
called, thus formed, acquires an orderly arrangement, becoming
converted into a hollow spheroid with double walls. Then, upon one
side of this spheroid, appears a thickening, and, by and bye, in the
centre of the area of thickening, a straight shallow groove (Fig. 13,
A) marks the central line of the edifice which is to be raised, or, in
other words, indicates the position of the middle line of the body of
the future dog. The substance bounding the groove on each side next
rises up into a fold, the rudiment of the side wall of that long
cavity, which will eventually lodge the spinal marrow and the brain;
and in the floor of this chamber appears a solid cellular cord, the
so-called 'notochord.' One end of the inclosed cavity dilates to form
the head (Fig. 13, B), the other remains narrow, and eventually becomes
the tail; the side walls of the body are fashioned out of the downward
continuation of the walls of the groove; and from them, by and bye,
grow out little buds which, by degrees, assume the shape of limbs.
Watching the fashioning process stage by stage, one is forcibly
reminded of the modeller in clay. Every part, every organ, is at
first, as it were, pinched up rudely, and sketched out in the rough;
then shaped more accurately; and only, at last, receives the touches
which stamp its final character.
Thus, at length, the young puppy assumes such a form as is shown in Fig.
13, C. In this condition it has a disproportionately large head, as
dissimilar to that of a dog as the bud-like limbs are unlike his legs.
The remains of the yelk, which have not yet been applied to the
nutrition and growth of the young animal, are contained in a sac
attached to the rudimentary intestine, and termed the yelk sac, or
'umbilical vesicle.' Two membranous bags, intended to subserve
respectively the protection and nutrition of the young creature, have
been developed from the skin and from the under and hinder surface of
the body; the former, the so-called 'amnion,' is a sac filled with
fluid, which invests the whole body of the embryo, and plays the part of
a sort of water-bed for it; the other, termed the 'allantois,' grows
out, loaded with blood-vessels, from the ventral region, and eventually
applying itself to the walls of the cavity, in which the developing
organism is contained, enables these vessels to become the channel by
which the stream of nutriment, required to supply the wants of the
offspring, is furnished to it by the parent.
The structure which is developed by the interlacement of the vessels of
the offspring with those of the parent, and by means of which the
former is enabled to receive nourishment and to get rid of effete
matters, is termed the 'Placenta.'
It would be tedious, and it is unnecessary for my present purpose, to
trace the process of development further; suffice it to say, that, by a
long and gradual series of changes, the rudiment here depicted and
described becomes a puppy, is born, and then, by still slower and less
perceptible steps, passes into the adult Dog.
There is not much apparent resemblance between a barndoor Fowl and the
Dog who protects the farm-yard. Nevertheless the student of
development finds, not only that the chick commences its existence as
an egg, primarily identical, in all essential respects, with that of
the Dog, but that the yelk of this egg undergoes division--that the
primitive groove arises, and that the contiguous parts of the germ are
fashioned, by precisely similar methods, into a young chick, which, at
one stage of its existence, is so like the nascent Dog, that ordinary
inspection would hardly distinguish the two.
The history of the development of any other vertebrate animal, Lizard,
Snake, Frog, or Fish, tells the same story. There is always, to begin
with, an egg having the same essential structure as that of the
Dog:--the yelk of that egg always undergoes division, or 'segmentation'
as it is often called: the ultimate products of that segmentation
constitute the building materials for the body of the young animal; and
this is built up round a primitive groove, in the floor of which a
notochord is developed. Furthermore, there is a period in which the
young of all these animals resemble one another, not merely in outward
form, but in all essentials of structure, so closely, that the
differences between them are inconsiderable, while, in their subsequent
course, they diverge more and more widely from one another. And it is a
general law, that, the more closely any animals resemble one another in
adult structure, the longer and the more intimately do their embryos
resemble one another: so that, for example, the embryos of a Snake and
of a Lizard remain like one another longer than do those of a Snake and
of a Bird; and the embryo of a Dog and of a Cat remain like one another
for a far longer period than do those of a Dog and a Bird; or of a Dog
and an Opossum; or even than those of a Dog and a Monkey.
Thus the study of development affords a clear test of closeness of
structural affinity, and one turns with impatience to inquire what
results are yielded by the study of the development of Man. Is he
something apart? Does he originate in a totally different way from Dog,
Bird, Frog, and Fish, thus justifying those who assert him to have no
place in nature and no real affinity with the lower world of animal
life? Or does he originate in a similar germ, pass through the same
slow and gradually progressive modifications,--depend on the same
contrivances for protection and nutrition, and finally enter the world
by the help of the same mechanism? The reply is not doubtful for a
moment, and has not been doubtful any time these thirty years. Without
question, the mode of origin and the early stages of the development of
man are identical with those of the animals immediately below him in the
scale:--without a doubt, in these respects, he is far nearer the Apes,
than the Apes are to the Dog.
The Human ovum is about l/125 of an inch in diameter, and might be
described in the same terms as that of the Dog, so that I need only
refer to the figure illustrative (14 A) of its structure. It leaves
the organ in which it is formed in a similar fashion and enters the
organic chamber prepared for its reception in the same way, the
conditions of its development being in all respects the same. It has
not yet been possible (and only by some rare chance can it ever be
possible) to study the human ovum in so early a developmental stage as
that of yelk division, but there is every reason to conclude that the
changes it undergoes are identical with those exhibited by the ova of
other vertebrated animals; for the formative materials of which the
rudimentary human body is composed, in the earliest conditions in which
it has been observed, are the same as those of other animals. Some of
these earliest stages are figured below, and, as will be seen, they are
strictly comparable to the very early states of the Dog; the marvellous
correspondence between the two which is kept up, even for some time, as
development advances, becoming apparent by the simple comparison of the
figures with those on page 249.
Fig. 14.--A. Human ovum (after Kolliker). a. germinal vesicle. b.
germinal spot. B. A very early condition of Man, with yelk-sac,
allantois, and amnion (original). C. A more advanced stage (after
Kolliker), compare Fig. 13, C.
Indeed, it is very long before the body of the young human being can be
readily discriminated from that of the young puppy; but, at a tolerably
early period, the two become distinguishable by the different form of
their adjuncts, the yelk-sac and the allantois. The former, in the
Dog, becomes long and spindle-shaped, while in Man it remains
spherical; the latter, in the Dog, attains an extremely large size, and
the vascular processes which are developed from it and eventually give
rise to the formation of the placenta (taking root, as it were, in the
parental organism, so as to draw nourishment therefrom, as the root of
a tree extracts it from the soil) are arranged in an encircling zone,
while in Man, the allantois remains comparatively small, and its
vascular rootlets are eventually restricted to one disk-like spot.
Hence, while the placenta of the Dog is like a girdle, that of Man has
the cake-like form, indicated by the name of the organ.
But, exactly in those respects in which the developing Man differs from
the Dog, he resembles the ape, which, like man, has a spheroidal
yelk-sac and a discoidal--sometimes partially lobed--placenta. So that
it is only quite in the later stages of development that the young
human being presents marked differences from the young ape, while the
latter departs as much from the dog in its development, as the man
does.
Startling as the last assertion may appear to be, it is demonstrably
true, and it alone appears to me sufficient to place beyond all doubt
the structural unity of man with the rest of the animal world, and more
particularly and closely with the apes.
Thus, identical in the physical processes by which he
originates--identical in the early stages of his formation--identical
in the mode of his nutrition before and after birth, with the animals
which lie immediately below him in the scale--Man, if his adult and
perfect structure be compared with theirs, exhibits, as might be
expected, a marvellous likeness of organization. He resembles them as
they resemble one another--he differs from them as they differ from one
another.--And, though these differences and resemblances cannot be
weighed and measured, their value may be readily estimated; the scale
or standard of judgment, touching that value, being afforded and
expressed by the system of classification of animals now current among
zoologists.
A careful study of the resemblances and differences presented by animals
has, in fact, led naturalists to arrange them into groups, or
assemblages, all the members of each group presenting a certain amount
of definable resemblance, and the number of points of similarity being
smaller as the group is larger and 'vice versa'. Thus, all creatures
which agree only in presenting the few distinctive marks of animality
form the 'Kingdom' ANIMALIA. The numerous animals which agree only in
possessing the special characters of Vertebrates form one 'Sub-Kingdom'
of this Kingdom. Then the Sub-kingdom VERTEBRATA is subdivided into
the five 'Classes,' Fishes, Amphibians, Reptiles, Birds, and Mammals,
and these into smaller groups called 'Orders'; these into 'Families'
and 'Genera'; while the last are finally broken up into the smallest
assemblages, which are distinguished by the possession of constant,
not-sexual, characters. These ultimate groups are Species.
Every year tends to bring about a greater uniformity of opinion
throughout the zoological world as to the limits and characters of
these groups, great and small. At present, for example, no one has the
least doubt regarding the characters of the classes Mammalia, Aves, or
Reptilia; nor does the question arise whether any thoroughly well-known
animal should be placed in one class or the other. Again, there is a
very general agreement respecting the characters and limits of the
orders of Mammals, and as to the animals which are structurally
necessitated to take a place in one or another order.
No one doubts, for example, that the Sloth and the Ant-eater, the
Kangaroo and the Opossum, the Tiger and the Badger, the Tapir and the
Rhinoceros, are respectively members of the same orders. These
successive pairs of animals may, and some do, differ from one another
immensely, in such matters as the proportions and structure of their
limbs; the number of their dorsal and lumbar vertebrae; the adaptation
of their frames to climbing, leaping, or running; the number and form
of their teeth; and the characters of their skulls and of the contained
brain. But, with all these differences, they are so closely connected
in all the more important and fundamental characters of their
organization, and so distinctly separated by these same characters from
other animals, that zoologists find it necessary to group them together
as members of one order. And if any new animal were discovered, and
were found to present no greater difference from the Kangaroo and the
Opossum, for example, than these animals do from one another, the
zoologist would not only be logically compelled to rank it in the same
order with these, but he would not think of doing otherwise.
Bearing this obvious course of zoological reasoning in mind, let us
endeavour for a moment to disconnect our thinking selves from the mask
of humanity; let us imagine ourselves scientific Saturnians, if you
will, fairly acquainted with such animals as now inhabit the Earth, and
employed in discussing the relations they bear to a new and singular
'erect and featherless biped,' which some enterprising traveller,
overcoming the difficulties of space and gravitation, has brought from
that distant planet for our inspection, well preserved, may be, in a
cask of rum. We should all, at once, agree upon placing him among the
mammalian vertebrates; and his lower jaw, his molars, and his brain,
would leave no room for doubting the systematic position of the new
genus among those mammals, whose young are nourished during gestation
by means of a placenta, or what are called the 'placental mammals.'
Further, the most superficial study would at once convince us that,
among the orders of placental mammals, neither the Whales, nor the
hoofed creatures, nor the Sloths and Ant-eaters, nor the carnivorous
Cats, Dogs, and Bears, still less the Rodent Rats and Rabbits, or the
Insectivorous Moles and Hedgehogs, or the Bats, could claim our 'Homo',
as one of themselves.
There would remain then, but one order for comparison, that of the Apes
(using that word in its broadest sense), and the question for
discussion would narrow itself to this--is Man so different from any of
these Apes that he must form an order by himself? Or does he differ
less from them than they differ from one another, and hence must take
his place in the same order with them?
Being happily free from all real, or imaginary, personal interest in the
results of the inquiry thus set afoot, we should proceed to weigh the
arguments on one side and on the other, with as much judicial calmness
as if the question related to a new Opossum. We should endeavour to
ascertain, without seeking either to magnify or diminish them, all the
characters by which our new Mammal differed from the Apes; and if we
found that these were of less structural value, than those which
distinguish certain members of the Ape order from others universally
admitted to be of the same order, we should undoubtedly place the newly
discovered tellurian genus with them.