Our conclusions on the first of these heads must depend, of course, upon
the nature of the subject-matter of Biology; and I think a few
preliminary considerations will place before you in a clear light the
vast difference which exists between the living bodies with which
Physiological science is concerned, and the remainder of the
universe;--between the phænomena of Number and Space, of Physical and of
Chemical force, on the one hand, and those of Life on the other.
The mathematician, the physicist, and the chemist contemplate things in
a condition of rest; they look upon a state of equilibrium as that to
which all bodies normally tend.
The mathematician does not suppose that a quantity will alter, or that a
given point in space will change its direction with regard to another
point, spontaneously. And it is the same with the physicist. When Newton
saw the apple fall, he concluded at once that the act of falling was not
the result of any power inherent in the apple, but that it was the
result of the action of something else on the apple. In a similar
manner, all physical force is regarded as the disturbance of an
equilibrium to which things tended before its exertion,--to which they
will tend again after its cessation.
The chemist equally regards chemical change in a body, as the effect of
the action of something external to the body changed. A chemical
compound once formed would persist for ever, if no alteration took place
in surrounding conditions.
But to the student of Life the aspect of nature is reversed. Here,
incessant, and, so far as we know, spontaneous change is the rule, rest
the exception--the anomaly to be accounted for. Living things have no
inertia and tend to no equilibrium.
Permit me, however, to give more force and clearness to these somewhat
abstract considerations, by an illustration or two.
Imagine a vessel full of water, at the ordinary temperature, in an
atmosphere saturated with vapour. The _quantity_ and the _figure_ of
that water will not change, so far as we know, for ever.
Suppose a lump of gold be thrown into the vessel--motion and disturbance
of figure exactly proportional to the momentum of the gold will take
place. But after a time the effects of this disturbance will
subside--equilibrium will be restored, and the water will return to its
passive state.
Expose the water to cold--it will solidify--and in so doing its
particles will arrange themselves in definite crystalline shapes. But
once formed, these crystals change no further.
Again, substitute for the lump of gold some substance capable of
entering into chemical relations with the water:--say, a mass of that
substance which is called "protein"--the substance of flesh:--a very
considerable disturbance of equilibrium will take place--all sorts of
chemical compositions and decompositions will occur; but in the end, as
before, the result will be the resumption of a condition of rest.
Instead of such a mass of _dead_ protein, however, take a particle of
_living_ protein--one of those minute microscopic living things which
throng our pools, and are known as Infusoria--such a creature, for
instance, as an Euglena, and place it in our vessel of water. It is a
round mass provided with a long filament, and except in this peculiarity
of shape, presents no appreciable physical or chemical difference
whereby it might be distinguished from the particle of dead protein.
But the difference in the phænomena to which it will give rise is
immense: in the first place it will develope a vast quantity of physical
force--cleaving the water in all directions, with considerable rapidity,
by means of the vibrations of the long filament or cilium.
Nor is the amount of chemical energy which the little creature possesses
less striking. It is a perfect laboratory in itself, and it will act and
react upon the water and the matters contained therein; converting them
into new compounds resembling its own substance and, at the same time,
giving up portions of its own substance which have become effete.
Furthermore, the Euglena will increase in size; but this increase is by
no means unlimited, as the increase of a crystal might be. After it has
grown to a certain extent it divides, and each portion assumes the form
of the original and proceeds to repeat the process of growth and
division.
Nor is this all. For after a series of such divisions and subdivisions,
these minute points assume a totally new form, lose their long
tails--round themselves, and secrete a sort of envelope or box, in which
they remain shut up for a time, eventually to resume, directly or
indirectly, their primitive mode of existence.
Now, so far as we know, there is no natural limit to the existence of
the Euglena, or of any other living germ. A living species once launched
into existence tends to live for ever.
Consider how widely different this living particle is from the dead
atoms with which the physicist and chemist have to do!
The particle of gold falls to the bottom and rests--the particle of dead
protein decomposes and disappears--it also rests: but the _living_
protein mass neither tends to exhaustion of its forces nor to any
permanency of form, but is essentially distinguished as a disturber of
equilibrium so far as force is concerned,--as undergoing continual
metamorphosis and change, in point of form.
Tendency to equilibrium of force, and to permanency of form then, are
the characters of that portion of the universe which does not live--the
domain of the chemist and physicist.
Tendency to disturb existing equilibrium,--to take on forms which
succeed one another in definite cycles, is the character of the living
world.
What is the cause of this wonderful difference between the dead particle
and the living particle of matter appearing in other respects identical?
that difference to which we give the name of Life?
I, for one, cannot tell you. It may be that, by and bye, philosophers
will discover some higher laws of which the facts of life are particular
cases--very possibly they will find out some bond between
physico-chemical phænomena on the one hand, and vital phænomena on the
other. At present, however, we assuredly know of none; and I think we
shall exercise a wise humility in confessing that, for us at least, this
successive assumption of different states--(external conditions
remaining the same)--this _spontaneity of action_--if I may use a term
which implies more than I would be answerable for--which constitutes so
vast and plain a practical distinction between living bodies and those
which do not live, is an ultimate fact; indicating as such, the
existence of a broad line of demarcation between the subject-matter of
Biological and that of all other sciences.
For I would have it understood that this simple Euglena is the type of
_all_ living things, so far as the distinction between these and inert
matter is concerned. That cycle of changes, which is constituted by
perhaps not more than two or three steps in the Euglena, is as clearly
manifested in the multitudinous stages through which the germ of an oak
or of a man passes. Whatever forms the Living Being may take on, whether
simple or complex,--_production_, _growth_, _reproduction_,--are the
phænomena which distinguish it from that which does not live.
If this be true, it is clear that the student, in passing from the
physico-chemical to the physiological sciences, enters upon a totally
new order of facts; and it will next be for us to consider how far these
new facts involve _new_ methods, or require a modification of those with
which he is already acquainted. Now a great deal is said about the
peculiarity of the scientific method in general, and of the different
methods which are pursued in the different sciences. The Mathematics are
said to have one special method; Physics another, Biology a third, and
so forth. For my own part, I must confess that I do not understand this
phraseology. So far as I can arrive at any clear comprehension of the
matter, Science is not, as many would seem to suppose, a modification of
the black art, suited to the tastes of the nineteenth century, and
flourishing mainly in consequence of the decay of the Inquisition.
Science is, I believe, nothing but _trained and organized common sense_,
differing from the latter only as a veteran may differ from a raw
recruit: and its methods differ from those of common sense only so far
as the guardsman's cut and thrust differ from the manner in which a
savage wields his club. The primary power is the same in each case, and
perhaps the untutored savage has the more brawny arm of the two. The
_real_ advantage lies in the point and polish of the swordsman's weapon;
in the trained eye quick to spy out the weakness of the adversary; in
the ready hand prompt to follow it on the instant. But after all, the
sword exercise is only the hewing and poking of the clubman developed
and perfected.
So, the vast results obtained by Science are won by no mystical
faculties, by no mental processes, other than those which are practised
by every one of us, in the humblest and meanest affairs of life. A
detective policeman discovers a burglar from the marks made by his shoe,
by a mental process identical with that by which Cuvier restored the
extinct animals of Montmartre from fragments of their bones. Nor does
that process of induction and deduction by which a lady, finding a stain
of a peculiar kind upon her dress, concludes that somebody has upset the
inkstand thereon, differ in any way, in kind, from that by which Adams
and Leverrier discovered a new planet.
The man of science, in fact, simply uses with scrupulous exactness, the
methods which we all, habitually and at every moment, use carelessly;
and the man of business must as much avail himself of the scientific
method--must be as truly a man of science--as the veriest book-worm of
us all; though I have no doubt that the man of business will find
himself out to be a philosopher with as much surprise as M. Jourdain
exhibited when he discovered that he had been all his life talking
prose. If, however, there be no real difference between the methods of
science and those of common life, it would seem on the face of the
matter highly improbable that there should be any difference between the
methods of the different sciences; nevertheless, it is constantly taken
for granted, that there is a very wide difference between the
Physiological and other sciences in point of method.
In the first place it is said--and I take this point first, because the
imputation is too frequently admitted by Physiologists themselves--that
Biology differs from the Physico-chemical and Mathematical sciences, in
being "inexact."
Now, this phrase "inexact" must refer either to the _methods_ or to the
_results_ of Physiological science.
It cannot be correct to apply it to the methods; for, as I hope to show
you by and bye, these are identical in all sciences, and whatever is
true of Physiological method is true of Physical and Mathematical
method.
Is it then the _results_ of Biological science which are "inexact"? I
think not. If I say that respiration is performed by the lungs; that
digestion is effected in the stomach; that the eye is the organ of
sight; that the jaws of a vertebrated animal never open sideways, but
always up and down; while those of an annulose animal always open
sideways, and never up and down--I am enumerating propositions which are
as exact as anything in Euclid. How then has this notion of the
inexactness of Biological science come about? I believe from two causes:
first, because, in consequence of the great complexity of the science
and the multitude of interfering conditions, we are very often only
enabled to predict approximately what will occur under given
circumstances; and secondly, because, on account of the comparative
youth of the Physiological sciences, a great many of their laws are
still imperfectly worked out. But in an educational point of view, it is
most important to distinguish between the essence of a science and the
accidents which surround it; and essentially, the methods and results of
Physiology are as exact as those of Physics or Mathematics.
It is said that the Physiological method is especially
_comparative_[56]; and this dictum also finds favour in the eyes of
many. I should be sorry to suggest that the speculators on scientific
classification have been misled by the accident of the name of one
leading branch of Biology--_Comparative Anatomy_; but I would ask
whether _comparison_, and that classification which is the result of
comparison, are not the essence of every science whatsoever? How is it
possible to discover a relation of cause and effect of _any_ kind
without comparing a series of cases together in which the supposed cause
and effect occur singly, or combined? So far from comparison being in
any way peculiar to Biological science, it is, I think, the essence of
every science.
A speculative philosopher again tells us that the Biological sciences
are distinguished by being sciences of observation and not of
experiment![57]
Of all the strange assertions into which speculation without practical
acquaintance with a subject may lead even an able man, I think this is
the very strangest. Physiology not an experimental science! Why, there
is not a function of a single organ in the body which has not been
determined wholly and solely by experiment? How did Harvey determine the
nature of the circulation, except by experiment? How did Sir Charles
Bell determine the functions of the roots of the spinal nerves, save by
experiment? How do we know the use of a nerve at all, except by
experiment? Nay, how do you know even that your eye is your seeing
apparatus, unless you make the experiment of shutting it; or that your
ear is your hearing apparatus, unless you close it up and thereby
discover that you become deaf?
It would really be much more true to say that Physiology is _the_
experimental science _par excellence_ of all sciences; that in which
there is least to be learnt by mere observation, and that which affords
the greatest field for the exercise of those faculties which
characterize the experimental philosopher. I confess, if any one were to
ask me for a model application of the logic of experiment, I should know
no better work to put into his hands than Bernard's late Researches on
the Functions of the Liver.[58]
Not to give this lecture a too controversial tone however, I must only
advert to one more doctrine, held by a thinker of our own age and
country, whose opinions are worthy of all respect. It is, that the
Biological sciences differ from all others, inasmuch as in _them_,
classification takes place by type and not by definition.[59]
It is said, in short, that a natural-history class is not capable of
being defined--that the class Rosaceæ, for instance, or the class of
Fishes, is not accurately and absolutely definable, inasmuch as its
members will present exceptions to every possible definition; and that
the members of the class are united together only by the circumstance
that they are all more like some imaginary average rose or average fish,
than they resemble anything else.
But here, as before, I think the distinction has arisen entirely from
confusing a transitory imperfection with an essential character. So long
as our information concerning them is imperfect, we class all objects
together according to resemblances which we _feel_, but cannot _define_:
we group them round _types_, in short. Thus, if you ask an ordinary
person what kinds of animals there are, he will probably say, beasts,
birds, reptiles, fishes, insects, &c. Ask him to define a beast from a
reptile, and he cannot do it; but he says, things like a cow or a horse
are beasts, and things like a frog or a lizard are reptiles. You see _he
does_ class by type, and not by definition. But how does this
classification differ from that of the scientific Zoologist? How does
the meaning of the scientific class-name of "Mammalia" differ from the
unscientific of "Beasts"?
Why, exactly because the former depends on a definition, the latter on a
type. The class Mammalia is scientifically defined as "all animals which
have a vertebrated skeleton and suckle their young." Here is no
reference to type, but a definition rigorous enough for a geometrician.
And such is the character which every scientific naturalist recognizes
as that to which his classes must aspire--knowing, as he does, that
classification by type is simply an acknowledgment of ignorance and a
temporary device.
So much in the way of negative argument as against the reputed
differences between Biological and other methods. No such differences, I
believe, really exist. The subject-matter of Biological science is
different from that of other sciences, but the methods of all are
identical; and these methods are--
1. _Observation_ of facts--including under this head that _artificial
observation_ which is called _experiment_.
2. That process of tying up similar facts into bundles, ticketed and
ready for use, which is called _Comparison_ and _Classification_,--the
results of the process, the ticketed bundles, being named _General
propositions_.
3. _Deduction_, which takes us from the general proposition to facts
again--teaches us, if I may so say, to anticipate from the ticket what
is inside the bundle. And finally--
4. _Verification_, which is the process of ascertaining whether, in
point of fact, our anticipation is a correct one.
Such are the methods of all science whatsoever; but perhaps you will
permit me to give you an illustration of their employment in the science
of Life; and I will take as a special case, the establishment of the
doctrine of the _Circulation of the Blood_.
In this case, _simple observation_ yields us a knowledge of the
existence of the blood from some accidental hæmorrhage, we will say: we
may even grant that it informs us of the localisation of this blood in
particular vessels, the heart, &c., from some accidental cut or the
like. It teaches also the existence of a pulse in various parts of the
body, and acquaints us with the structure of the heart and vessels.
Here, however, _simple observation_ stops, and we must have recourse to
_experiment_.
You tie a vein, and you find that the blood accumulates on the side of
the ligature opposite the heart. You tie an artery, and you find that
the blood accumulates on the side near the heart. Open the chest, and
you see the heart contracting with great force. Make openings into its
principal cavities, and you will find that all the blood flows out, and
no more pressure is exerted on either side of the arterial or venous
ligature.
Now all these facts, taken together, constitute the evidence that the
blood is propelled by the heart through the arteries, and returns by the
veins--that, in short, the blood circulates.
Suppose our experiments and observations have been made on horses, then
we group and ticket them into a general proposition, thus:--_all horses
have a circulation of their blood_.
Henceforward a horse is a sort of indication or label, telling us where
we shall find a peculiar series of phænomena called the circulation of
the blood.
Here is our _general proposition_ then.
How and when are we justified in making our next step--a _deduction_
from it?
Suppose our physiologist, whose experience is limited to horses, meets
with a zebra for the first time,--will he suppose that his
generalization holds good for zebras also?
That depends very much on his turn of mind. But we will suppose him to
be a bold man. He will say, "The zebra is certainly not a horse, but it
is very like one,--so like, that it must be the 'ticket' or mark of a
blood-circulation also; and, I conclude that the zebra has a
circulation."
That is a deduction, a very fair deduction, but by no means to be
considered scientifically secure. This last quality in fact can only be
given by _verification_--that is, by making a zebra the subject of all
the experiments performed on the horse. Of course in the present case
the _deduction_ would be _confirmed_ by this process of verification,
and the result would be, not merely a positive widening of knowledge,
but a fair increase of confidence in the truth of one's generalizations
in other cases.
Thus, having settled the point in the zebra and horse, our philosopher
would have great confidence in the existence of a circulation in the
ass. Nay, I fancy most persons would excuse him, if in this case he did
not take the trouble to go through the process of verification at all;
and it would not be without a parallel in the history of the human mind,
if our imaginary physiologist now maintained that he was acquainted with
asinine circulation _à priori_.
However, if I might impress any caution upon your minds, it is, the
utterly conditional nature of all our knowledge,--the danger of
neglecting the process of verification under any circumstances; and the
film upon which we rest, the moment our deductions carry us beyond the
reach of this great process of verification. There is no better instance
of this than is afforded by the history of our knowledge of the
circulation of the blood in the animal kingdom until the year 1824. In
every animal possessing a circulation at all, which had been observed up
to that time, the current of the blood was known to take one definite
and invariable direction. Now, there is a class of animals called
_Ascidians_, which possess a heart and a circulation, and up to the
period of which I speak, no one would have dreamt of questioning the
propriety of the deduction, that these creatures have a circulation in
one direction; nor would any one have thought it worth while to verify
the point. But, in that year, M. von Hasselt happening to examine a
transparent animal of this class, found to his infinite surprise, that
after the heart had beat a certain number of times, it stopped, and then
began beating the opposite way--so as to reverse the course of the
current, which returned by and bye to its original direction.
I have myself timed the heart of these little animals. I found it as
regular as possible in its periods of reversal: and I know no spectacle
in the animal kingdom more wonderful than that which it presents--all
the more wonderful that to this day it remains an unique fact, peculiar
to this class among the whole animated world. At the same time I know of
no more striking case of the necessity of the _verification_ of even
those deductions which seem founded on the widest and safest inductions.
Such are the methods of Biology--methods which are obviously identical
with those of all other sciences, and therefore wholly incompetent to
form the ground of any distinction between it and them.[60]
But I shall be asked at once, do you mean to say that there is no
difference between the habit of mind of a mathematician and that of a
naturalist? Do you imagine that Laplace might have been put into the
Jardin des Plantes, and Cuvier into the Observatory, with equal
advantage to the progress of the sciences they professed?
To which I would reply, that nothing could be further from my thoughts.
But different habits and various special tendencies of two sciences do
not imply different methods. The mountaineer and the man of the plains
have very different habits of progression, and each would be at a loss
in the other's place; but the method of progression, by putting one leg
before the other, is the same in each case. Every step of each is a
combination of a lift and a push; but the mountaineer lifts more and the
lowlander pushes more. And I think the case of two sciences resembles
this.
I do not question for a moment, that while the Mathematician is busy
with deductions _from_ general propositions, the Biologist is more
especially occupied with observation, comparison, and those processes
which lead _to_ general propositions. All I wish to insist upon is,
that this difference depends not on any fundamental distinction in the
sciences themselves, but on the accidents of their subject-matter, of
their relative complexity, and consequent relative perfection.
The Mathematician deals with two properties of objects only, number and
extension, and all the inductions he wants have been formed and finished
ages ago. He is occupied now with nothing but deduction and
verification.
The biologist deals with a vast number of properties of objects, and his
inductions will not be completed, I fear, for ages to come; but when
they are, his science will be as deductive and as exact as the
Mathematics themselves.
Such is the relation of Biology to those sciences which deal with
objects having fewer properties than itself. But as the student in
reaching Biology looks back upon sciences of a less complex and
therefore more perfect nature, so on the other hand does he look forward
to other more complex and less perfect branches of knowledge. Biology
deals only with living beings as isolated things--treats only of the
life of the individual: but there is a higher division of science still,
which considers living beings as aggregates--which deals with the
relation of living beings one to another--the science which _observes_
men--whose _experiments_ are made by nations one upon another, in
battle-fields--whose _general propositions_ are embodied in history,
morality, and religion--whose _deductions_ lead to our happiness or our
misery,--and whose _verifications_ so often come too late, and serve
only
"To point a moral or adorn a tale"--