Darwiniana
by
Thomas Henry Huxley

Part 5 out of 6

form exceedingly large deposits. You are probably aware that before the
attempt was made to lay the Atlantic telegraphic cable, the Government
employed vessels in making a series of very careful observations and
soundings of the bottom of the Atlantic; and although, as we must all
regret, that up to the present time that project has not succeeded, we have
the satisfaction of knowing that it yielded some most remarkable results to
science. The Atlantic Ocean had to be sounded right across, to depths of
several miles in some places, and the nature of its bottom was carefully
ascertained. Well, now, a space of about 1,000 miles wide from east to
west, and I do not exactly know how many from north to south, but at any
rate 600 or 700 miles, was carefully examined, and it was found that over
the whole of that immense area an excessively fine chalky mud is being
deposited; and this deposit is entirely made up of animals whose hard parts
are deposited in this part of the ocean, and are doubtless gradually
acquiring solidity and becoming metamorphosed into a chalky limestone.
Thus, you see, it is quite possible in this way to preserve unmistakable
records of animal and vegetable life. Whenever the sea-bottom, by some of
those undulations of the earth's crust that I have referred to, becomes
up-heaved, and sections or borings are made, or pits are dug, then we
become able to examine the contents and constituents of these ancient
sea-bottoms, and find out what manner of animals lived at that period.

Now it is a very important consideration in its bearing on the completeness
of the record, to inquire how far the remains contained in these
fossiliferous limestones are able to convey anything like an accurate or
complete account of the animals which were in existence at the time of its
formation. Upon that point we can form a very clear judgment, and one in
which there is no possible room for any mistake. There are of course a
great number of animals--such as jellyfishes, and other animals--without
any hard parts, of which we cannot reasonably expect to find any traces
whatever: there is nothing of them to preserve. Within a very short time,
you will have noticed, after they are removed from the water, they dry up
to a mere nothing; certainly they are not of a nature to leave any very
visible traces of their existence on such bodies as chalk or mud. Then
again, look at land animals; it is, as I have said, a very uncommon thing
to find a land animal entire after death. Insects and other carnivorous
animals very speedily pull them to pieces, putrefaction takes place, and
so, out of the hundreds of thousands that are known to die every year, it
is the rarest thing in the world to see one imbedded in such a way that its
remains would be preserved for a lengthened period. Not only is this the
case, but even when animal remains have been safely imbedded, certain
natural agents may wholly destroy and remove them.

Almost all the hard parts of animals--the bones and so on--are composed
chiefly of phosphate of lime and carbonate of lime. Some years ago, I had
to make an inquiry into the nature of some very curious fossils sent to me
from the North of Scotland. Fossils are usually hard bony structures that
have become imbedded in the way I have described, and have gradually
acquired the nature and solidity of the body with which they are
associated; but in this case I had a series of _holes_ in some pieces
of rock, and nothing else. Those holes, however, had a certain definite
shape about them, and when I got a skilful workman to make castings of the
interior of these holes, I found that they were the impressions of the
joints of a backbone and of the armour of a great reptile, twelve or more
feet long. This great beast had died and got buried in the sand; the sand
had gradually hardened over the bones, but remained porous. Water had
trickled through it, and that water being probably charged with a
superfluity of carbonic acid, had dissolved all the phosphate and carbonate
of lime, and the bones themselves had thus decayed and entirely
disappeared; but as the sandstone happened to have consolidated by that
time, the precise shape of the bones was retained. If that sandstone had
remained soft a little longer, we should have known nothing whatsoever of
the existence of the reptile whose bones it had encased.

How certain it is that a vast number of animals which have existed at one
period on this earth have entirely perished, and left no trace whatever of
their forms, may be proved to you by other considerations. There are large
tracts of sandstone in various parts of the world, in which nobody has yet
found anything but footsteps. Not a bone of any description, but an
enormous number of traces of footsteps. There is no question about them.
There is a whole valley in Connecticut covered with these footsteps, and
not a single fragment of the animals which made them have yet been found.
Let me mention another case while upon that matter, which is even more
surprising than those to which I have yet referred. There is a limestone
formation near Oxford, at a place called Stonesfield, which has yielded the
remains of certain very interesting mammalian animals, and up to this time,
if I recollect rightly, there have been found seven specimens of its lower
jaws, and not a bit of anything else, neither limb-bones nor skull, nor any
part whatever; not a fragment of the whole system! Of course, it would be
preposterous to imagine that the beasts had nothing else but a lower jaw!
The probability is, as Dr. Buckland showed, as the result of his
observations on dead dogs in the river Thames, that the lower jaw, not
being secured by very firm ligaments to the bones of the head, and being a
weighty affair, would easily be knocked off, or might drop away from the
body as it floated in water in a state of decomposition. The jaw would thus
be deposited immediately, while the rest of the body would float and drift
away altogether, ultimately reaching the sea, and perhaps becoming
destroyed. The jaw becomes covered up and preserved in the river silt, and
thus it comes that we have such a curious circumstance as that of the lower
jaws in the Stonesfield slates. So that, you see, faulty as these layers of
stone in the earth's crust are, defective as they necessarily are as a
record, the account of contemporaneous vital phenomena presented by them
is, by the necessity of the case, infinitely more defective and
fragmentary.

It was necessary that I should put all this very strongly before you,
because, otherwise, you might have been led to think differently of the
completeness of our knowledge by the next facts I shall state to you.

The researches of the last three-quarters of a century have, in truth,
revealed a wonderful richness of organic life in those rocks. Certainly not
fewer than thirty or forty thousand different species of fossils have been
discovered. You have no more ground for doubting that these creatures
really lived and died at or near the places in which we find them than you
have for like scepticism about a shell on the sea-shore. The evidence is as
good in the one case as in the other.

Our next business is to look at the general character of these fossil
remains, and it is a subject which will be requisite to consider carefully;
and the first point for us is to examine how much the extinct _Flora_
and _Fauna_ as a _whole_--disregarding altogether the
_succession_ of their constituents, of which I shall speak
afterwards--differ from the _Flora_ and _Fauna_ of the present
day;--how far they differ in what we _do_ know about them, leaving
altogether out of consideration speculations based upon what we _do
not_ know.

I strongly imagine that if it were not for the peculiar appearance that
fossilised animals have, any of you might readily walk through a museum
which contains fossil remains mixed up with those of the present forms of
life, and I doubt very much whether your uninstructed eyes would lead you
to see any vast or wonderful difference between the two. If you looked
closely, you would notice, in the first place, a great many things very
like animals with which you are acquainted now: you would see differences
of shape and proportion, but on the whole a close similarity.

I explained what I meant by ORDERS the other day, when I described the
animal kingdom as being divided into sub-kingdoms, classes and orders. If
you divide the animal kingdom into orders you will find that there are
above one hundred and twenty. The number may vary on one side or the other,
but this is a fair estimate. That is the sum total of the orders of all the
animals which we know now, and which have been known in past times, and
left remains behind.

Now, how many of those are absolutely extinct? That is to say, how many of
these orders of animals have lived at a former period of the world's
history but have at present no representatives? That is the sense in which
I meant to use the word "extinct." I mean that those animals did live on
this earth at one time, but have left no one of their kind with us at the
present moment. So that estimating the number of extinct animals is a sort
of way of comparing the past creation as a whole with the present as a
whole. Among the mammalia and birds there are none extinct; but when we
come to the reptiles there is a most wonderful thing: out of the eight
orders, or thereabouts, which you can make among reptiles, one-half are
extinct. These diagrams of the plesiosaurus, the ichthyosaurus, the
pterodactyle, give you a notion of some of these extinct reptiles. And here
is a cast of the pterodactyle and bones of the ichthyosaurus and the
plesiosaurus, just as fresh-looking as if it had been recently dug up in a
churchyard. Thus, in the reptile class, there are no less than half of the
orders which are absolutely extinct. If we turn to the _Amphibia_,
there was one extinct order, the Labyrinthodonts, typified by the large
salamander-like beast shown in this diagram.

No order of fishes is known to be extinct. Every fish that we find in the
strata--to which I have been referring--can be identified and placed in one
of the orders which exist at the present day. There is not known to be a
single ordinal form of insect extinct. There are only two orders extinct
among the _Crustacea_. There is not known to be an extinct order of
these creatures, the parasitic and other worms; but there are two, not to
say three, absolutely extinct orders of this class, the
_Echinodermata_; out of all the orders of the _Coelenterata_ and
_Protozoa_ only one, the Rugose Corals.

So that, you see, out of somewhere about 120 orders of animals, taking them
altogether, you will not, at the outside estimate, find above ten or a
dozen extinct. Summing up all the order of animals which have left remains
behind them, you will not find above ten or a dozen which cannot be
arranged with those of the present day; that is to say, that the difference
does not amount to much more than ten per cent.: and the proportion of
extinct orders of plants is still smaller. I think that that is a very
astounding a most astonishing fact: seeing the enormous epochs of time
which have elapsed during the constitution of the surface of the earth as
it at present exists, it is, indeed, a most astounding thing that the
proportion of extinct ordinal types should be so exceedingly small.

But now, there is another point of view in which we must look at this past
creation. Suppose that we were to sink a vertical pit through the floor
beneath us, and that I could succeed in making a section right through in
the direction of New Zealand, I should find in each of the different beds
through which I passed the remains of animals which I should find in that
stratum and not in the others. First, I should come upon beds of gravel or
drift containing the bones of large animals, such as the elephant,
rhinoceros, and cave tiger. Rather curious things to fall across in
Piccadilly! If I should dig lower still, I should come upon a bed of what
we call the London clay, and in this, as you will see in our galleries up
stairs, are found remains of strange cattle, remains of turtles, palms, and
large tropical fruits; with shell-fish such as you see the like of now only
in tropical regions. If I went below that, I should come upon the chalk,
and there I should find something altogether different, the remains of
ichthyosauria and pterodactyles, and ammonites, and so forth.

I do not know what Mr. Godwin Austin would say comes next, but probably
rocks containing more ammonites, and more ichthyosauria and plesiosauria,
with a vast number of other things; and under that I should meet with yet
older rocks containing numbers of strange shells and fishes; and in thus
passing from the surface to the lowest depths of the earth's crust, the
forms of animal life and vegetable life which I should meet with in the
successive beds would, looking at them broadly, be the more different the
further that I went down. Or, in other words, inasmuch as we started with
the clear principle, that in a series of naturally-disposed mud beds the
lowest are the oldest, we should come to this result, that the further we
go back in time the more difference exists between the animal and vegetable
life of an epoch and that which now exists. That was the conclusion to
which I wished to bring you at the end of this lecture.

III. THE METHOD BY WHICH THE CAUSES OF THE PRESENT AND PAST CONDITIONS OF
ORGANIC NATURE ARE TO BE DISCOVERED;--THE ORIGINATION OF LIVING BEINGS

In the two preceding lectures I have endeavoured to indicate to you the
extent of the subject-matter of the inquiry upon which we are engaged; and
having thus acquired some conception of the past and present phenomena of
organic nature, I must now turn to that which constitutes the great problem
which we have set before ourselves;--I mean, the question of what knowledge
we have of the causes of these phenomena of organic nature, and how such
knowledge is obtainable.

Here, on the threshold of the inquiry, an objection meets us. There are in
the world a number of extremely worthy, well-meaning persons, whose
judgments and opinions are entitled to the utmost respect on account of
their sincerity, who are of opinion that vital phenomena, and especially
all questions relating to the origin of vital phenomena, are questions
quite apart from the ordinary run of inquiry, and are, by their very
nature, placed out of our reach. They say that all these phenomena
originated miraculously, or in some way totally different from the ordinary
course of nature, and that therefore they conceive it to be futile, not to
say presumptuous, to attempt to inquire into them.

To such sincere and earnest persons, I would only say, that a question of
this kind is not to be shelved upon theoretical or speculative grounds. You
may remember the story of the Sophist who demonstrated to Diogenes in the
most complete and satisfactory manner that he could not walk; that, in
fact, all motion was an impossibility; and that Diogenes refuted him by
simply getting up and walking round his tub. So, in the same way, the man
of science replies to objections of this kind, by simply getting up and
walking onward, and showing what science has done and is doing---by
pointing to that immense mass of facts which have been ascertained as
systematised under the forms of the great doctrines of morphology, of
development, of distribution, and the like. He sees an enormous mass of
facts and laws relating to organic beings, which stand on the same good
sound foundation as every other natural law. With this mass of facts and
laws before us, therefore, seeing that, as far as organic matters have
hitherto been accessible and studied, they have shown themselves capable of
yielding to scientific investigation, we may accept this as proof that
order and law reign there as well as in the rest of Nature. The man of
science says nothing to objectors of this sort, but supposes that we can
and shall walk to a knowledge of the origin of organic nature, in the same
way that we have walked to a knowledge of the laws and principles of the
inorganic world.

But there are objectors who say the same from ignorance and ill-will. To
such I would reply that the objection comes ill from them, and that the
real presumption, I may almost say the real blasphemy, in this matter, is
in the attempt to limit that inquiry into the causes of phenomena, which is
the source of all human blessings, and from which has sprung all human
prosperity and progress; for, after all, we can accomplish comparatively
little; the limited range of our own faculties bounds us on every
side,--the field of our powers of observation is small enough, and he who
endeavours to narrow the sphere of our inquiries is only pursuing a course
that is likely to produce the greatest harm to his fellow-men.

But now, assuming, as we all do, I hope, that these phenomena are properly
accessible to inquiry, and setting out upon our search into the causes of
the phenomena of organic nature, or at any rate, setting out to discover
how much we at present know upon these abstruse matters, the question
arises as to what is to be our course of proceeding, and what method we
must lay down for our guidance. I reply to that question, that our method
must be exactly the same as that which is pursued in any other scientific
inquiry, the method of scientific investigation being the same for all
orders of facts and phenomena whatsoever.

I must dwell a little on this point, for I wish you to leave this room with
a very clear conviction that scientific investigation is not, as many
people seem to suppose, some kind of modern black art. I say that you might
easily gather this impression from the manner in which many persons speak
of scientific inquiry, or talk about inductive and deductive philosophy, or
the principles of the "Baconian philosophy." I do protest that, of the vast
number of cants in this world, there are none, to my mind, so contemptible
as the pseudo-scientific cant which is talked about the "Baconian
philosophy."

To hear people talk about the great Chancellor--and a very great man he
certainly was,--you would think that it was he who had invented science,
and that there was no such thing as sound reasoning before the time of
Queen Elizabeth! Of course you say, that cannot possibly be true; you
perceive, on a moment's reflection, that such an idea is absurdly wrong,
and yet, so firmly rooted is this sort of impression,--I cannot call it an
idea, or conception,--the thing is too absurd to be entertained,--but so
completely does it exist at the bottom of most men's minds, that this has
been a matter of observation with me for many years past. There are many
men who, though knowing absolutely nothing of the subject with which they
may be dealing, wish, nevertheless, to damage the author of some view with
which they think fit to disagree. What they do, then, is not to go and
learn something about the subject, which one would naturally think the best
way of fairly dealing with it; but they abuse the originator of the view
they question, in a general manner, and wind up by saying that, "After all,
you know, the principles and method of this author are totally opposed to
the canons of the Baconian philosophy." Then everybody applauds, as a
matter of course, and agrees that it must be so. But if you were to stop
them all in the middle of their applause, you would probably find that
neither the speaker nor his applauders could tell you how or in what way it
was so; neither the one nor the other having the slightest idea of what
they mean when they speak of the "Baconian philosophy."

You will understand, I hope, that I have not the slightest desire to join
in the outcry against either the morals, the intellect, or the great genius
of Lord Chancellor Bacon. He was undoubtedly a very great man, let people
say what they will of him; but notwithstanding all that he did for
philosophy, it would be entirely wrong to suppose that the methods of
modern scientific inquiry originated with him, or with his age; they
originated with the first man, whoever he was; and indeed existed long
before him, for many of the essential processes of reasoning are exerted by
the higher order of brutes as completely and effectively as by ourselves.
We see in many of the brute creation the exercise of one, at least, of the
same powers of reasoning as that which we ourselves employ.

The method of scientific investigation is nothing but the expression of the
necessary mode of working of the human mind. It is simply the mode at which
all phenomena are reasoned about, rendered precise and exact. There is no
more difference, but there is just the same kind of difference, between the
mental operations of a man of science and those of an ordinary person, as
there is between the operations and methods of a baker or of a butcher
weighing out his goods in common scales, and the operations of a chemist in
performing a difficult and complex analysis by means of his balance and
finely-graduated weights. It is not that the action of the scales in the
one case, and the balance in the other, differ in the principles of their
construction or manner of working; but the beam of one is set on an
infinitely finer axis than the other, and of course turns by the addition
of a much smaller weight.

You will understand this better, perhaps, if I give you some familiar
example. You have all heard it repeated, I dare say, that men of science
work by means of induction and deduction, and that by the help of these
operations, they, in a sort of sense, wring from Nature certain other
things, which are called natural laws, and causes, and that out of these,
by some cunning skill of their own, they build up hypotheses and theories.
And it is imagined by many, that the operations of the common mind can be
by no means compared with these processes, and that they have to be
acquired by a sort of special apprenticeship to the craft. To hear all
these large words, you would think that the mind of a man of science must
be constituted differently from that of his fellow men; but if you will not
be frightened by terms, you will discover that you are quite wrong, and
that all these terrible apparatus are being used by yourselves every day
and every hour of your lives.

There is a well-known incident in one of Moli�re's plays, where the author
makes the hero express unbounded delight on being told that he had been
talking prose during the whole of his life. In the same way, I trust, that
you will take comfort, and be delighted with yourselves, on the discovery
that you have been acting on the principles of inductive and deductive
philosophy during the same period. Probably there is not one here who has
not in the course of the day had occasion to set in motion a complex train
of reasoning, of the very same kind, though differing of course in degree,
as that which a scientific man goes through in tracing the causes of
natural phenomena.

A very trivial circumstance will serve to exemplify this. Suppose you go
into a fruiterer's shop, wanting an apple,--you take up one, and, on biting
it, you find it is sour; you look at it, and see that it is hard and green.
You take up another one, and that too is hard, green, and sour. The shopman
offers you a third; but, before biting it, you examine it, and find that it
is hard and green, and you immediately say that you will not have it, as it
must be sour, like those that you have already tried.

Nothing can be more simple than that, you think; but if you will take the
trouble to analyse and trace out into its logical elements what has been
done by the mind, you will be greatly surprised. In the first place, you
have performed the operation of induction. You found that, in two
experiences, hardness and greenness in apples went together with sourness.
It was so in the first case, and it was confirmed by the second. True, it
is a very small basis, but still it is enough to make an induction from;
you generalise the facts, and you expect to find sourness in apples where
you get hardness and greenness. You found upon that a general law, that all
hard and green apples are sour; and that, so far as it goes, is a perfect
induction. Well, having got your natural law in this way, when you are
offered another apple which you find is hard and green, you say, "All hard
and green apples are sour; this apple is hard and green, therefore this
apple is sour." That train of reasoning is what logicians call a syllogism,
and has all its various parts and terms,--its major premiss, its minor
premiss, and its conclusion. And, by the help of further reasoning, which,
if drawn out, would have to be exhibited in two or three other syllogisms,
you arrive at your final determination, "I will not have that apple." So
that, you see, you have, in the first place, established a law by
induction, and upon that you have founded a deduction, and reasoned out the
special conclusion of the particular case. Well now, suppose, having got
your law, that at some time afterwards, you are discussing the qualities of
apples with a friend: you will say to him, "It is a very curious
thing,--but I find that all hard and green apples are sour!" Your friend
says to you, "But how do you know that?" You at once reply, "Oh, because I
have tried them over and over again, and have always found them to be so."
Well, if we were talking science instead of common sense, we should call
that an experimental verification. And, if still opposed, you go further,
and say, "I have heard from the people in Somersetshire and Devonshire,
where a large number of apples are grown, that they have observed the same
thing. It is also found to be the case in Normandy, and in North America.
In short, I find it to be the universal experience of mankind wherever
attention has been directed to the subject." Whereupon, your friend, unless
he is a very unreasonable man, agrees with you, and is convinced that you
are quite right in the conclusion you have drawn. He believes, although
perhaps he does not know he believes it, that the more extensive
verifications are,--that the more frequently experiments have been made,
and results of the same kind arrived at,--that the more varied the
conditions under which the same results are attained, the more certain is
the ultimate conclusion, and he disputes the question no further. He sees
that the experiment has been tried under all sorts of conditions, as to
time, place, and people, with the same result; and he says with you,
therefore, that the law you have laid down must be a good one, and he must
believe it.

In science we do the same thing;--the philosopher exercises precisely the
same faculties, though in a much more delicate manner. In scientific
inquiry it becomes a matter of duty to expose a supposed law to every
possible kind of verification, and to take care, moreover, that this is
done intentionally, and not left to a mere accident, as in the case of the
apples. And in science, as in common life, our confidence in a law is in
exact proportion to the absence, of variation in the result of our
experimental verifications. For instance, if you let go your grasp of an
article you may have in your hand, it will immediately fall to the ground.
That is a very common verification of one of the best established laws of
nature--that of gravitation. The method by which men of science establish
the existence of that law is exactly the same as that by which we have
established the trivial proposition about the sourness of hard and green
apples. But we believe it in such an extensive, thorough, and unhesitating
manner because the universal experience of mankind verifies it, and we can
verify it ourselves at any time; and that is the strongest possible
foundation on which any natural law can rest.

So much, then, by way of proof that the method of establishing laws in
science is exactly the same as that pursued in common life. Let us now turn
to another matter (though really it is but another phase of the same
question), and that is, the method by which, from the relations of certain
phenomena, we prove that some stand in the position of causes towards the
others.

I want to put the case clearly before you, and I will therefore show you
what I mean by another familiar example. I will suppose that one of you, on
coming down in the morning to the parlour of your house, finds that a
tea-pot and some spoons which had been left in the room on the previous
evening are gone,--the window is open, and you observe the mark of a dirty
hand on the window-frame, and perhaps, in addition to that, you notice the
impress of a hob-nailed shoe on the gravel outside. All these phenomena
have struck your attention instantly, and before two seconds have passed
you say, "Oh, somebody has broken open the window, entered the room, and
run off with the spoons and the tea-pot!" That speech is out of your mouth
in a moment. And you will probably add, "I know there has; I am quite sure
of it!" You mean to say exactly what you know; but in reality you are
giving expression to what is, in all essential particulars, an hypothesis.
You do not _know_ it at all; it is nothing but an hypothesis rapidly
framed in your own mind. And it is an hypothesis founded on a long train of
inductions and deductions.

What are those inductions and deductions, and how have you got at this
hypothesis? You have observed, in the first place, that the window is open;
but by a train of reasoning involving many inductions and deductions, you
have probably arrived long before at the general law--and a very good one
it is--that windows do not open of themselves; and you therefore conclude
that something has opened the window. A second general law that you have
arrived at in the same way is, that tea-pots and spoons do not go out of a
window spontaneously, and you are satisfied that, as they are not now where
you left them, they have been removed. In the third place, you look at the
marks on the window-sill, and the shoe-marks outside, and you say that in
all previous experience the former kind of mark has never been produced by
anything else but the hand of a human being; and the same experience shows
that no other animal but man at present wears shoes with hob-nails in them
such as would produce the marks in the gravel. I do not know, even if we
could discover any of those "missing links" that are talked about, that
they would help us to any other conclusion! At any rate the law which
states our present experience is strong enough for my present purpose. You
next reach the conclusion, that as these kinds of marks have not been left
by any other animals than men, or are liable to be formed in any other way
than by a man's hand and shoe, the marks in question have been formed by a
man in that way. You have, further, a general law, founded on observation
and experience, and that, too, is, I am sorry to say, a very universal and
unimpeachable one,--that some men are thieves; and you assume at once from
all these premisses--and that is what constitutes your hypothesis--that the
man who made the marks outside and on the window-sill, opened the window,
got into the room, and stole your tea-pot and spoons. You have now arrived
at a _vera causa_;--you have assumed a cause which, it is plain, is
competent to produce all the phenomena you have observed. You can explain
all these phenomena only by the hypothesis of a thief. But that is a
hypothetical conclusion, of the justice of which you have no absolute proof
at all; it is only rendered highly probable by a series of inductive and
deductive reasonings.

I suppose your first action, assuming that you are a man of ordinary common
sense, and that you have established this hypothesis to your own
satisfaction, will very likely be to go off for the police, and set them on
the track of the burglar, with the view to the recovery of your property.
But just as you are starting with this object, some person comes in, and on
learning what you are about, says, "My good friend, you are going on a
great deal too fast. How do you know that the man who really made the marks
took the spoons? It might have been a monkey that took them, and the man
may have merely looked in afterwards." You would probably reply, "Well,
that is all very well, but you see it is contrary to all experience of the
way tea-pots and spoons are abstracted; so that, at any rate, your
hypothesis is less probable than mine." While you are talking the thing
over in this way, another friend arrives, one of that good kind of people
that I was talking of a little while ago. And he might say, "Oh, my dear
sir, you are certainly going on a great deal too fast. You are most
presumptuous. You admit that all these occurrences took place when you were
fast asleep, at a time when you could not possibly have known anything
about what was taking place. How do you know that the laws of Nature are
not suspended during the night? It may be that there has been some kind of
supernatural interference in this case." In point of fact, he declares that
your hypothesis is one of which you cannot at all demonstrate the truth,
and that you are by no means sure that the laws of Nature are the same when
you are asleep as when you are awake.

Well, now, you cannot at the moment answer that kind of reasoning. You feel
that your worthy friend has you somewhat at a disadvantage. You will feel
perfectly convinced in your own mind, however, that you are quite right,
and you say to him, "My good friend, I can only be guided by the natural
probabilities of the case, and if you will be kind enough to stand aside
and permit me to pass, I will go and fetch the police." Well, we will
suppose that your journey is successful, and that by good luck you meet
with a policeman; that eventually the burglar is found with your property
on his person, and the marks correspond to his hand and to his boots.
Probably any jury would consider those facts a very good experimental
verification of your hypothesis, touching the cause of the abnormal
phenomena observed in your parlour, and would act accordingly.

Now, in this suppositious case, I have taken phenomena of a very common
kind, in order that you might see what are the different steps in an
ordinary process of reasoning, if you will only take the trouble to analyse
it carefully. All the operations I have described, you will see, are
involved in the mind of any man of sense in leading him to a conclusion as
to the course he should take in order to make good a robbery and punish the
offender. I say that you are led, in that case, to your conclusion by
exactly the same train of reasoning as that which a man of science pursues
when he is endeavouring to discover the origin and laws of the most occult
phenomena. The process is, and always must be, the same; and precisely the
same mode of reasoning was employed by Newton and Laplace in their
endeavours to discover and define the causes of the movements of the
heavenly bodies, as you, with your own common sense, would employ to detect
a burglar. The only difference is, that the nature of the inquiry being
more abstruse, every step has to be most carefully watched, so that there
may not be a single crack or flaw in your hypothesis. A flaw or crack in
many of the hypotheses of daily life may be of little or no moment as
affecting the general correctness of the conclusions at which we may
arrive; but, in a scientific inquiry, a fallacy, great or small, is always
of importance, and is sure to be in the long run constantly productive of
mischievous, if not fatal results.

Do not allow yourselves to be misled by the common notion that an
hypothesis is untrustworthy simply because it is an hypothesis. It is often
urged, in respect to some scientific conclusion, that, after all, it is
only an hypothesis. But what more have we to guide us in nine-tenths of the
most important affairs of daily life than hypotheses, and often very
ill-based ones? So that in science, where the evidence of an hypothesis is
subjected to the most rigid examination, we may rightly pursue the same
course. You may have hypotheses and hypotheses. A man may say, if he likes,
that the moon is made of green cheese: that is an hypothesis. But another
man, who has devoted a great deal of time and attention to the subject, and
availed himself of the most powerful telescopes and the results of the
observations of others, declares that in his opinion it is probably
composed of materials very similar to those of which our own earth is made
up: and that is also only an hypothesis. But I need not tell you that there
is an enormous difference in the value of the two hypotheses. That one
which is based on sound scientific knowledge is sure to have a
corresponding value; and that which is a mere hasty random guess is likely
to have but little value. Every great step in our progress in discovering
causes has been made in exactly the same way as that which I have detailed
to you. A person observing the occurrence of certain facts and phenomena
asks, naturally enough, what process, what kind of operation known to occur
in Nature applied to the particular case, will unravel and explain the
mystery? Hence you have the scientific hypothesis; and its value will be
proportionate to the care and completeness with which its basis had been
tested and verified. It is in these matters as in the commonest affairs of
practical life: the guess of the fool will be folly, while the guess of the
wise man will contain wisdom. In all cases, you see that the value of the
result depends on the patience and faithfulness with which the investigator
applies to his hypothesis every possible kind of verification.

I dare say I may have to return to this point by and by; but having dealt
thus far with our logical methods, I must now turn to something which,
perhaps, you may consider more interesting, or, at any rate, more tangible.
But in reality there are but few things that can be more important for you
to understand than the mental processes and the means by which we obtain
scientific conclusions and theories. [Footnote: Those who wish to study
fully the doctrines of which I have endeavoured to give some
rough-and-ready illustrations, must read Mr. John Stuart Mill's _System
of Logic_.] Having granted that the inquiry is a proper one, and having
determined on the nature of the methods we are to pursue and which only can
lead to success, I must now turn to the consideration of our knowledge of
the nature of the processes which have resulted in the present condition of
organic nature.

Here, let me say at once, lest some of you misunderstand me, that I have
extremely little to report. The question of how the present condition of
organic nature came about, resolves itself into two questions. The first
is: How has organic or living matter commenced its existence? And the
second is: How has it been perpetuated? On the second question I shall have
more to say hereafter. But on the first one, what I now have to say will be
for the most part of a negative character.

If you consider what kind of evidence we can have upon this matter, it will
resolve itself into two kinds. We may have historical evidence and we may
have experimental evidence. It is, for example, conceivable, that inasmuch
as the hardened mud which forms a considerable portion of the thickness of
the earth's crust contains faithful records of the past forms of life, and
inasmuch as these differ more and more as we go further down,--it is
possible and conceivable that we might come to some particular bed or
stratum which should contain the remains of those creatures with which
organic life began upon the earth. And if we did so, and if such forms of
organic life were preservable, we should have what I would call historical
evidence of the mode in which organic life began upon this planet. Many
persons will tell you, and indeed you will find it stated in many works on
geology, that this has been done, and that we really possess such a record;
there are some who imagine that the earliest forms of life of which we have
as yet discovered any record, are in truth the forms in which animal life
began upon the globe. The grounds on which they base that supposition are
these:--That if you go through the enormous thickness of the earth's crust
and get down to the older rocks, the higher vertebrate animals--the
quadrupeds, birds, and fishes--cease to be found; beneath them you find
only the invertebrate animals; and in the deepest and lowest rocks those
remains become scantier and scantier, not in any very gradual progression,
however, until, at length, in what are supposed to be the oldest rocks, the
animal remains which are found are almost always confined to four
forms--_Oldhamia_, whose precise nature is not known, whether plant or
animal; _Lingula_, a kind of mollusc; _Trilobites_, a crustacean
animal, having the same essential plan of construction, though differing in
many details from a lobster or crab; and _Hymenocaris_, which is also
a crustacean. So that you have all the _Fauna_ reduced, at this
period, to four forms: one a kind of animal or plant that we know nothing
about, and three undoubted animals--two crustaceans and one mollusc.

I think, considering the organisation of these mollusca and crustacea, and
looking at their very complex nature, that it does indeed require a very
strong imagination to conceive that these were the first created of all
living things. And you must take into consideration the fact that we have
not the slightest proof that these which we call the oldest beds are really
so: I repeat, we have not the slightest proof of it. When you find in some
places that in an enormous thickness of rocks there are but very scanty
traces of life, or absolutely none at all; and that in other parts of the
world rocks of the very same formation are crowded with the records of
living forms, I think it is impossible to place any reliance on the
supposition, or to feel one's self justified in supposing that these are
the forms in which life first commenced. I have not time here to enter upon
the technical grounds upon which I am led to this conclusion,--that could
hardly be done properly in half a dozen lectures on that part alone:--I
must content myself with saying that I do not at all believe that these are
the oldest forms of life.

I turn to the experimental side to see what evidence we have there. To
enable us to say that we know anything about the experimental origination
of organisation and life, the investigator ought to be able to take
inorganic matters, such as carbonic acid, ammonia, water, and salines, in
any sort of inorganic combination, and be able to build them up into
protein matter, and then that protein matter ought to begin to live in an
organic form. That, nobody has done as yet, and I suspect it will be a long
while before anybody does do it. But the thing is by no means so impossible
as it looks; for the researches of modern chemistry have shown us--I won't
say the road towards it, but, if I may so say, they have shown the
finger-post pointing to the road that may lead to it.

It is not many years ago--and you must recollect that Organic Chemistry is
a young science, not above a couple of generations old, you must not expect
too much of it,--it is not many years ago since it was said to be perfectly
impossible to fabricate any organic compound; that is to say, any
non-mineral compound which is to be found in an organised being. It
remained so for a very long period; but it is now a considerable number of
years since a distinguished foreign chemist contrived to fabricate urea, a
substance of a very complex character, which forms one of the waste
products of animal structures. And of late years a number of other
compounds, such as butyric acid, and others, have been added to the list. I
need not tell you that chemistry is an enormous distance from the goal I
indicate; all I wish to point out to you is, that it is by no means safe to
say that that goal may not be reached one day. It may be that it is
impossible for us to produce the conditions requisite to the origination of
life; but we must speak modestly about the matter, and recollect that
Science has put her foot upon the bottom round of the ladder. Truly he
would be a bold man who would venture to predict where she will be fifty
years hence.

There is another inquiry which bears indirectly upon this question, and
upon which I must say a few words. You are all of you aware of the
phenomena of what is called spontaneous generation. Our forefathers, down
to the seventeenth century, or thereabouts, all imagined, in perfectly good
faith, that certain vegetable and animal forms gave birth, in the process
of their decomposition, to insect life. Thus, if you put a piece of meat in
the sun, and allowed it to putrefy, they conceived that the grubs which
soon began to appear were the result of the action of a power of
spontaneous generation which the meat contained. And they could give you
receipts for making various animal and vegetable preparations which would
produce particular kinds of animals. A very distinguished Italian
naturalist, named Redi, took up the question, at a time when everybody
believed in it; among others our own great Harvey, the discoverer of the
circulation of the blood. You will constantly find his name quoted,
however, as an opponent of the doctrine of spontaneous generation; but the
fact is, and you will see it if you will take the trouble to look into his
works, Harvey believed it as profoundly as any man of his time; but he
happened to enunciate a very curious proposition--that every living thing
came from an _egg_; he did not mean to use the word in the sense in
which we now employ it, he only meant to say that every living thing
originated in a little rounded particle of organised substance; and it is
from this circumstance, probably, that the notion of Harvey having opposed
the doctrine originated. Then came Redi, and he proceeded to upset the
doctrine in a very simple manner. He merely covered the piece of meat with
some very fine gauze, and then he exposed it to the same conditions. The
result of this was that no grubs or insects were produced; he proved that
the grubs originated from the insects who came and deposited their eggs in
the meat, and that they were hatched by the heat of the sun. By this kind
of inquiry he thoroughly upset the doctrine of spontaneous generation, for
his time at least.

Then came the discovery and application of the microscope to scientific
inquiries, which showed to naturalists that besides the organisms which
they already knew as living beings and plants, there were an immense number
of minute things which could be obtained apparently almost at will from
decaying vegetable and animal forms. Thus, if you took some ordinary black
pepper or some hay, and steeped it in water, you would find in the course
of a few days that the water had become impregnated with an immense number
of animalcules swimming about in all directions. From facts of this kind
naturalists were led to revive the theory of spontaneous generation. They
were headed here by an English naturalist,--Needham,--and afterwards in
France by the learned Buffon. They said that these things were absolutely
begotten in the water of the decaying substances out of which the infusion
was made. It did not matter whether you took animal or vegetable matter,
you had only to steep it in water and expose it, and you would soon have
plenty of animalcules. They made an hypothesis about this which was a very
fair one. They said, this matter of the animal world, or of the higher
plants, appears to be dead, but in reality it has a sort of dim life about
it, which, if it is placed under fair conditions, will cause it to break up
into the forms of these little animalcules, and they will go through their
lives in the same way as the animal or plant of which they once formed a
part.

The question now became very hotly debated. Spallanzani, an Italian
naturalist, took up opposite views to those of Needham and Buffon, and by
means of certain experiments he showed that it was quite possible to stop
the process by boiling the water, and closing the vessel in which it was
contained. "Oh!" said his opponents; "but what do you know you may be doing
when you heat the air over the water in this way? You may be destroying
some property of the air requisite for the spontaneous generation of the
animalcules."

However, Spallanzani's views were supposed to be upon the right side, and
those of the others fell into discredit; although the fact was that
Spallanzani had not made good his views. Well, then, the subject continued
to be revived from time to time, and experiments were made by several
persons; but these experiments were not altogether satisfactory. It was
found that if you put an infusion in which animalcules would appear if it
were exposed to the air into a vessel and boiled it, and then sealed up the
mouth of the vessel, so that no air, save such as had been heated to 212�,
could reach its contents, that then no animalcules would be found; but if
you took the same vessel and exposed the infusion to the air, then you
would get animalcules. Furthermore, it was found that if you connected the
mouth of the vessel with a red-hot tube in such a way that the air would
have to pass through the tube before reaching the infusion, that then you
would get no animalcules. Yet another thing was noticed: if you took two
flasks containing the same kind of infusion, and left one entirely exposed
to the air, and in the mouth of the other placed a ball of cotton wool, so
that the air would have to filter itself through it before reaching the
infusion, that then, although you might have plenty of animalcules in the
first flask, you would certainly obtain none from the second.

These experiments, you see, all tended towards one conclusion--that the
infusoria were developed from little minute spores or eggs which were
constantly floating in the atmosphere, and which lose their power of
germination if subjected to heat. But one observer now made another
experiment, which seemed to go entirely the other way, and puzzled him
altogether. He took some of this boiled infusion that I have been speaking
of, and by the use of a mercurial bath--a kind of trough used in
laboratories--he deftly inverted a vessel containing the infusion into the
mercury, so that the latter reached a little beyond the level of the mouth
of the _inverted_ vessel. You see that he thus had a quantity of the
infusion shut off from any possible communication with the outer air by
being inverted upon a bed of mercury.

He then prepared some pure oxygen and nitrogen gases, and passed them by
means of a tube going from the outside of the vessel, up through the
mercury into the infusion; so that he thus had it exposed to a perfectly
pure atmosphere of the same constituents as the external air. Of course, he
expected he would get no infusorial animalcules at all in that infusion;
but, to his great dismay and discomfiture, he found he almost always did
get them.

Furthermore, it has been found that experiments made in the manner
described above answer well with most infusions; but that if you fill the
vessel with boiled milk, and then stop the neck with cotton-wool, you
_will_ have infusoria. So that you see there were two experiments that
brought you to one kind of conclusion, and three to another; which was a
most unsatisfactory state of things to arrive at in a scientific inquiry.

Some few years after this, the question began to be very hotly discussed in
France. There was M. Pouchet, a professor at Rouen, a very learned man, but
certainly not a very rigid experimentalist. He published a number of
experiments of his own, some of which were very ingenious, to show that if
you went to work in a proper way, there was a truth in the doctrine of
spontaneous generation. Well, it was one of the most fortunate things in
the world that M. Pouchet took up this question, because it induced a
distinguished French chemist, M. Pasteur, to take up the question on the
other side; and he has certainly worked it out in the most perfect manner.
I am glad to say, too, that he has published his researches in time to
enable me to give you an account of them. He verified all the experiments
which I have just mentioned to you--and then finding those extraordinary
anomalies, as in the case of the mercury bath and the milk, he set himself
to work to discover their nature. In the case of milk he found it to be a
question of temperature. Milk in a fresh state is slightly alkaline; and it
is a very curious circumstance, but this very slight degree of alkalinity
seems to have the effect of preserving the organisms which fall into it
from the air from being destroyed at a temperature of 212�, which is the
boiling point. But if you raise the temperature 10� when you boil it, the
milk behaves like everything else; and if the air with which it comes in
contact, after being boiled at this temperature, is passed through a
red-hot tube, you will not get a trace of organisms.

He then turned his attention to the mercury bath, and found on examination
that the surface of the mercury was almost always covered with a very fine
dust. He found that even the mercury itself was positively full of organic
matters; that from being constantly exposed to the air, it had collected an
immense number of these infusorial organisms from the air. Well, under
these circumstances he felt that the case was quite clear, and that the
mercury was not what it had appeared to M. Schwann to be,--a bar to the
admission of these organisms; but that, in reality, it acted as a reservoir
from which the infusion was immediately supplied with the large quantity
that had so puzzled him.

But not content with explaining the experiments of others, M. Pasteur went
to work to satisfy himself completely. He said to himself: "If my view is
right, and if, in point of fact, all these appearances of spontaneous
generation are altogether due to the falling of minute germs suspended in
the atmosphere,--why, I ought not only to be able to show the germs, but I
ought to be able to catch and sow them, and produce the resulting
organisms." He, accordingly, constructed a very ingenious apparatus to
enable him to accomplish the trapping of the "_germ dust_" in the air.
He fixed in the window of his room a glass tube, in the centre of which he
had placed a ball of gun-cotton, which, as you all know, is ordinary
cotton-wool, which, from having been steeped in strong acid, is converted
into a substance of great explosive power. It is also soluble in alcohol
and ether. One end of the glass tube was, of course, open to the external
air; and at the other end of it he placed an aspirator, a contrivance for
causing a current of the external air to pass through the tube. He kept
this apparatus going for four-and-twenty hours, and then removed the
_dusted_ gun-cotton, and dissolved it in alcohol and ether. He then
allowed this to stand for a few hours, and the result was, that a very fine
dust was gradually deposited at the bottom of it. That dust, on being
transferred to the stage of a microscope, was found to contain an enormous
number of starch grains. You know that the materials of our food and the
greater portion of plants are composed of starch, and we are constantly
making use of it in a variety of ways, so that there is always a quantity
of it suspended in the air. It is these starch grains which form many of
those bright specks that we see dancing in a ray of light sometimes. But
besides these, M. Pasteur found also an immense number of other organic
substances such as spores of fungi, which had been floating about in the
air and had got caged in this way.

He went farther, and said to himself, "If these really are the things that
give rise to the appearance of spontaneous generation, I ought to be able
to take a ball of this dusted gun-cotton and put it into one of my vessels,
containing that boiled infusion which has been kept away from the air, and
in which no infusoria are at present developed, and then, if I am right,
the introduction of this gun-cotton will give rise to organisms."

Accordingly, he took one of these vessels of infusion, which had been kept
eighteen months, without the least appearance of life in it, and by a most
ingenious contrivance, he managed to break it open and introduce such a
ball of gun-cotton, without allowing the infusion or the cotton ball to
come into contact with any air but that which had been subjected to a red
heat, and in twenty-four hours he had the satisfaction of finding all the
indications of what had been hitherto called spontaneous generation. He had
succeeded in catching the germs and developing organisms in the way ho had
anticipated.

It now struck him that the truth of his conclusions might be demonstrated
without all the apparatus he had employed. To do this, he took some
decaying animal or vegetable substance, such as urine, which is an
extremely decomposable substance, or the juice of yeast, or perhaps some
other artificial preparation, and filled a vessel having a long tubular
neck with it. He then boiled the liquid and bent that long neck into an S
shape or zig-zag, leaving it open at the end. The infusion then gave no
trace of any appearance of spontaneous generation, however long it might be
left, as all the germs in the air were deposited in the beginning of the
bent neck. He then cut the tube close to the vessel, and allowed the
ordinary air to have free and direct access; and the result of that was the
appearance of organisms in it, as soon as the infusion had been allowed to
stand long enough to allow of the growth of those it received from the air,
which was about forty-eight hours. The result of M. Pasteur's experiments
proved, therefore, in the most conclusive manner, that all the appearances
of spontaneous generation arose from nothing more than the deposition of
the germs of organisms which were constantly floating in the air.

To this conclusion, however, the objection was made, that if that were the
cause, then the air would contain such an enormous number of these germs,
that it would be a continual fog. But M. Pasteur replied that they are not
there in anything like the number we might suppose, and that an exaggerated
view has been held on that subject; he showed that the chances of animal or
vegetable life appearing in infusions, depend entirely on the conditions
under which they are exposed. If they are exposed to the ordinary
atmosphere around us, why, of course, you may have organisms appearing
early. But, on the other hand, if they are exposed to air at a great
height, or in some very quiet cellar, you will often not find a single
trace of life.

So that M. Pasteur arrived at last at the clear and definite result, that
all these appearances are like the case of the worms in the piece of meat,
which was refuted by Redi, simply germs carried by the air and deposited in
the liquids in which they afterwards appear. For my own part, I conceive
that, with the particulars of M. Pasteur's experiments before us, we cannot
fail to arrive at his conclusions; and that the doctrine of spontaneous
generation has received a final _coup de gr�ce_.

You, of course, understand that all this in no way interferes with the
_possibility_ of the fabrication of organic matters by the direct
method to which I have referred, remote as that possibility may be.

IV. THE PERPETUATION OF LIVING BEINGS, HEREDITARY TRANSMISSION AND
VARIATION

The inquiry which we undertook, at our last meeting, into the state of our
knowledge of the causes of the phenomena of organic nature,--of the past
and of the present,--resolved itself into two subsidiary inquiries: the
first was, whether we know anything, either historically or experimentally,
of the mode of origin of living beings; the second subsidiary inquiry was,
whether, granting the origin, we know anything about the perpetuation and
modifications of the forms of organic beings. The reply which I had to give
to the first question was altogether negative, and the chief result of my
last lecture was, that, neither historically nor experimentally, do we at
present know anything whatsoever about the origin of living forms. We saw
that, historically, we are not likely to know anything about it, although
we may perhaps learn something experimentally; but that at present we are
an enormous distance from the goal I indicated.

I now, then, take up the next question, What do we know of the
reproduction, the perpetuation, and the modifications of the forms of
living beings, supposing that we have put the question as to their
origination on one side, and have assumed that at present the causes of
their origination are beyond us, and that we know nothing about them? Upon
this question the state of our knowledge is extremely different; it is
exceedingly large: and, if not complete, our experience is certainly most
extensive. It would be impossible to lay it all before you, and the most I
can do, or need do to-night, is to take up the principal points and put
them before you with such prominence as may subserve the purposes of our
present argument.

The method of the perpetuation of organic beings is of two kinds,--the
non-sexual and the sexual. In the first the perpetuation takes place from
and by a particular act of an individual organism, which sometimes may not
be classed as belonging to any sex at all. In the second case, it is in
consequence of the mutual action and interaction of certain portions of the
organisms of usually two distinct individuals,--the male and the female.
The cases of non-sexual perpetuation are by no means so common as the cases
of sexual perpetuation; and they are by no means so common in the animal as
in the vegetable world. You are all probably familiar with the fact, as a
matter of experience, that you can propagate plants by means of what are
called "cuttings"; for example, that by taking a cutting from a geranium
plant, and rearing it properly, by supplying it with light and warmth and
nourishment from the earth, it grows up and takes the form of its parent,
having all the properties and peculiarities of the original plant.

Sometimes this process, which the gardener performs artificially, takes
place naturally; that is to say, a little bulb, or portion of the plant,
detaches itself, drops off, and becomes capable of growing as a separate
thing. That is the case with many bulbous plants, which throw off in this
way secondary bulbs, which are lodged in the ground and become developed
into plants. This is a non-sexual process, and from it results the
repetition or reproduction of the form of the original being from which the
bulb proceeds.

Among animals the same thing takes place. Among the lower forms of animal
life, the infusorial animalcul� we have already spoken of throw off certain
portions, or break themselves up in various directions, sometimes
transversely or sometimes longitudinally; or they may give off buds, which
detach themselves and develop into their proper forms. There is the common
fresh-water polype, for instance, which multiplies itself in this way. Just
in the same way as the gardener is able to multiply and reproduce the
peculiarities and characters of particular plants by means of cuttings, so
can the physiological experimentalist--as was shown by the Abb� Trembley
many years ago--so can he do the same thing with many of the lower forms of
animal life. M. de Trembley showed that you could take a polype and cut it
into two, or four, or many pieces, mutilating it in all directions, and the
pieces would still grow up and reproduce completely the original form of
the animal. These are all cases of non-sexual multiplication, and there are
other instances, and still more extraordinary ones, in which this process
takes place naturally, in a more hidden, a more recondite kind of way. You
are all of you familiar with that little green insect, the _Aphis_ or
blight, as it is called. These little animals, during a very considerable
part of their existence, multiply themselves by means of a kind of internal
budding, the buds being developed into essentially non-sexual animals,
which are neither male nor female; they become converted into young
_Aphides_, which repeat the process, and their offspring after them,
and so on again; you may go on for nine or ten, or even twenty or more
successions; and there is no very good reason to say how soon it might
terminate, or how long it might not go on if the proper conditions of
warmth and nourishment were kept up.

Sexual reproduction is quite a distinct matter. Here, in all these cases,
what is required is the detachment of two portions of the parental
organisms, which portions we know as the egg or the spermatozoon. In plants
it is the ovule and the pollen-grain, as in the flowering plants, or the
ovule and the antherozooid, as in the flowerless. Among all forms of animal
life, the spermatozoa proceed from the male sex, and the egg is the product
of the female. Now, what is remarkable about this mode of reproduction is
this, that the egg by itself, or the spermatozoa by themselves, are unable
to assume the parental form; but if they be brought into contact with one
another, the effect of the mixture of organic substances proceeding from
two sources appears to confer an altogether new vigour to the mixed
product. This process is brought about, as we all know, by the sexual
intercourse of the two sexes, and is called the act of impregnation. The
result of this act on the part of the male and female is, that the
formation of a new being is set up in the ovule or egg; this ovule or egg
soon begins to be divided and subdivided, and to be fashioned into various
complex organs, and eventually to develop into the form of one of its
parents, as I explained in the first lecture. These are the processes by
which the perpetuation of organic beings is secured. Why there should be
the two modes--why this re-invigoration should be required on the part of
the female element we do not know; but it is most assuredly the fact, and
it is presumable, that, however long the process of non-sexual
multiplication could be continued--I say there is good reason to believe
that it would come to an end if a new commencement were not obtained by a
conjunction of the two sexual elements.

That character which is common to these two distinct processes is this,
that, whether we consider the reproduction, or perpetuation, or
modification of organic beings as they take place non-sexually, or as they
may take place sexually--in either case, I say, the offspring has a
constant tendency to assume, speaking generally, the character of the
parent. As I said just now, if you take a slip of a plant, and tend it with
care, it will eventually grow up and develop into a plant like that from
which it had sprung; and this tendency is so strong that, as gardeners
know, this mode of multiplying by means of cuttings is the only secure mode
of propagating very many varieties of plants; the peculiarity of the
primitive stock seems to be better preserved if you propagate it by means
of a slip than if you resort to the sexual mode.

Again, in experiments upon the lower animals, such as the polype, to which
I have referred, it is most extraordinary that, although cut up into
various pieces, each particular piece will grow up into the form of the
primitive stock; the head, if separated, will reproduce the body and the
tail; and if you cut off the tail, you will find that that will reproduce
the body and all the rest of the members, without in any way deviating from
the plan of the organism from which these portions have been detached. And
so far does this go, that some experimentalists have carefully examined the
lower orders of animals,--among them the Abb� Spallanzani, who made a
number of experiments upon snails and salamanders,--and have found that
they might mutilate them to an incredible extent; that you might cut off
the jaw or the greater part of the head, or the leg or the tail, and repeat
the experiment several times, perhaps cutting off the same member again and
again; and yet each of those types would be reproduced according to the
primitive type: Nature making no mistake, never putting on a fresh kind of
leg, or head, or tail, but always tending to repeat and to return to the
primitive type.

It is the same in sexual reproduction: it is a matter of perfectly common
experience, that the tendency on the part of the offspring always is,
speaking broadly, to reproduce the form of the parents. The proverb has it
that the thistle does not bring forth grapes; so, among ourselves, there is
always a likeness, more or less marked and distinct, between children and
their parents. That is a matter of familiar and ordinary observation. We
notice the same thing occurring in the cases of the domestic animals--dogs,
for instance, and their offspring. In all these cases of propagation and
perpetuation, there seems to be a tendency in the offspring to take the
characters of the parental organisms. To that tendency a special name is
given--and as I may very often use it, I will write it up here on this
black-board that you may remember it--it is called _Atavism_; it
expresses this tendency to revert to the ancestral type, and comes from the
Latin word _atavus_, ancestor.

Well, this _Atavism_ which I shall speak of, is, as I said before, one
of the most marked and striking tendencies of organic beings; but, side by
side with this hereditary tendency there is an equally distinct and
remarkable tendency to variation. The tendency to reproduce the original
stock has, as it were, its limits, and side by side with it there is a
tendency to vary in certain directions, as if there were two opposing
powers working upon the organic being, one tending to take it in a straight
line, and the other tending to make it diverge from that straight line,
first to one side and then to the other.

So that you see these two tendencies need not precisely contradict one
another, as the ultimate result may not always be very remote from what
would have been the case if the line had been quite straight.

This tendency to variation is less marked in that mode of propagation which
takes place non-sexually; it is in that mode that the minor characters of
animal and vegetable structures are most completely preserved. Still, it
will happen sometimes, that the gardener, when he has planted a cutting of
some favourite plant, will find, contrary to his expectation, that the slip
grows up a little different from the primitive stock--that it produces
flowers of a different colour or make, or some deviation in one way or
another. This is what is called the "sporting" of plants.

In animals the phenomena of non-sexual propagation are so obscure, that at
present we cannot be said to know much about them; but if we turn to that
mode of perpetuation which results from the sexual process, then we find
variation a perfectly constant occurrence, to a certain extent; and,
indeed, I think that a certain amount of variation from the primitive stock
is the necessary result of the method of sexual propagation itself; for,
inasmuch as the thing propagated proceeds from two organisms of different
sexes and different makes and temperaments, and as the offspring is to be
either of one sex or the other, it is quite clear that it cannot be an
exact diagonal of the two, or it would be of no sex at all; it cannot be an
exact intermediate form between that of each of its parents--it must
deviate to one side or the other. You do not find that the male follows the
precise type of the male parent, nor does the female always inherit the
precise characteristics of the mother,--there is always a proportion of the
female character in the male offspring, and of the male character in the
female offspring. That must be quite plain to all of you who have looked at
all attentively on your own children or those of your neighbours; you will
have noticed how very often it may happen that the son shall exhibit the
maternal type of character, or the daughter possess the characteristics of
the father's family. There are all sorts of intermixtures and intermediate
conditions between the two, where complexion, or beauty, or fifty other
different peculiarities belonging to either side of the house, are
reproduced in other members of the same family. Indeed, it is sometimes to
be remarked in this kind of variation, that the variety belongs, strictly
speaking, to neither of the immediate parents; you will see a child in a
family who is not like either its father or its mother; but some old person
who knew its grandfather or grandmother, or, it may be, an uncle, or,
perhaps, even a more distant relative will see a great similarity between
the child and one of these. In this way it constantly happens that the
characteristic of some previous member of the family comes out and is
reproduced and recognised in the most unexpected manner.

But apart from that matter of general experience, there are some cases
which put that curious mixture in a very clear light. You are aware that
the offspring of the ass and the horse, or rather of the he-ass and the
mare, is what is called a mule; and, on the other hand, the offspring of
the stallion and the she-ass is what is called a hinny. It is a very rare
thing in this country to see a hinny. I never saw one myself; but they have
been very carefully studied. Now, the curious thing is this, that although
you have the same elements in the experiment in each case, the offspring is
entirely different in character, according as the male influence comes from
the ass or the horse. Where the ass is the male, as in the case of the
mule, you find that the head is like that of the ass, that the ears are
long, the tail is tufted at the end, the feet are small, and the voice is
an unmistakable bray; these are all points of similarity to the ass; but,
on the other hand, the barrel of the body and the cut of the neck are much
more like those of the mare. Then, if you look at the hinny,--the result of
the union of the stallion and the she-ass, then you find it is the horse
that has the predominance; that the head is more like that of the horse,
the ears are shorter, the legs coarser, and the type is altogether altered;
while the voice, instead of being a bray, is the ordinary neigh of the
horse. Here, you see, is a most curious thing: you take exactly the same
elements, ass and horse, but you combine the sexes in a different manner,
and the result is modified accordingly. You have in this case, however, a
result which is not general and universal--there is usually an important
preponderance, but not always on the same side.

Here, then, is one intelligible, and, perhaps, necessary cause of
variation: the fact, that there are two sexes sharing in the production of
the offspring, and that the share taken by each is different and variable,
not only for each combination, but also for different members of the same
family.

Secondly, there is a variation, to a certain extent--though, in all
probability, the influence of this cause has been very much
exaggerated--but there is no doubt that variation is produced, to a certain
extent, by what are commonly known as external conditions,--such as
temperature, food, warmth, and moisture. In the long run, every variation
depends, in some sense, upon external conditions, seeing that everything
has a cause of its own. I use the term "external conditions" now in the
sense in which it is ordinarily employed: certain it is, that external
conditions have a definite effect. You may take a plant which has single
flowers, and by dealing with the soil, and nourishment, and so on, you may
by and by convert single flowers into double flowers, and make thorns shoot
out into branches. You may thicken or make various modifications in the
shape of the fruit. In animals, too, you may produce analogous changes in
this way, as in the case of that deep bronze colour which persons rarely
lose after having passed any length of time in tropical countries. You may
also alter the development of the muscles very much, by dint of training;
all the world knows that exercise has a great effect in this way; we always
expect to find the arm of a blacksmith hard and wiry, and possessing a
large development of the brachial muscles. No doubt training, which is one
of the forms of external conditions, converts what are originally only
instructions, teachings, into habits, or, in other words, into
organisations, to a great extent; but this second cause of variation cannot
be considered to be by any means a large one. The third cause that I have
to mention, however, is a very extensive one. It is one that, for want of a
better name, has been called "spontaneous variation"; which means that when
we do not know anything about the cause of phenomena, we call it
spontaneous. In the orderly chain of causes and effects in this world,
there are very few things of which it can be said with truth that they are
spontaneous. Certainly not in these physical matters--in these there is
nothing of the kind--everything depends on previous conditions. But when we
cannot trace the cause of phenomena, we call them spontaneous.

Of these variations, multitudinous as they are, but little is known with
perfect accuracy. I will mention to you some two or three cases, because
they are very remarkable in themselves, and also because I shall want to
use them afterwards. R�aumur, a famous French naturalist, a great many
years ago, in an essay which he wrote upon the art of hatching
chickens--which was indeed a very curious essay--had occasion to speak of
variations and monstrosities. One very remarkable case had come under his
notice of a variation in the form of a human member, in the person of a
Maltese, of the name of Gratio Kelleia, who was born with six fingers upon
each hand, and the like number of toes to each of his feet. That was a case
of spontaneous variation. Nobody knows why he was born with that number of
fingers and toes, and as we don't know, we call it a case of "spontaneous"
variation. There is another remarkable case also. I select these, because
they happen to have been observed and noted very carefully at the time. It
frequently happens that a variation occurs, but the persons who notice it
do not take any care in noting down the particulars, until at length, when
inquiries come to be made, the exact circumstances are forgotten; and
hence, multitudinous as may be such "spontaneous" variations, it is
exceedingly difficult to get at the origin of them.

The second case is one of which you may find the whole details in the
"Philosophical Transactions" for the year 1813, in a paper communicated by
Colonel Humphrey to the President of the Royal Society--"On a new Variety
in the Breed of Sheep," giving an account of a very remarkable breed of
sheep, which at one time was well known in the northern states of America,
and which went by the name of the Ancon or the Otter breed of sheep. In the
year 1791, there was a farmer of the name of Seth Wright in Massachusetts,
who had a flock of sheep, consisting of a ram and, I think, of some twelve
or thirteen ewes. Of this flock of ewes, one at the breeding-time bore a
lamb which was very singularly formed; it had a very long body, very short
legs, and those legs were bowed. I will tell you by and by how this
singular variation in the breed of sheep came to be noted, and to have the
prominence that it now has. For the present, I mention only these two
cases; but the extent of variation in the breed of animals is perfectly
obvious to any one who has studied natural history with ordinary attention,
or to any person who compares animals with others of the same kind. It is
strictly true that there are never any two specimens which are exactly
alike; however similar, they will always differ in some certain particular.

Now let us go back to Atavism--to the hereditary tendency I spoke of. What
will come of a variation when you breed from it, when Atavism comes, if I
may say so, to intersect variation? The two cases of which I have mentioned
the history give a most excellent illustration of what occurs. Gratio
Kelleia, the Maltese, married when he was twenty-two years of age, and, as
I suppose there were no six-fingered ladies in Malta, he married an
ordinary five-fingered person. The result of that marriage was four
children; the first, who was christened Salvator, had six fingers and six
toes, like his father; the second was George, who had five fingers and
toes, but one of them was deformed, showing a tendency to variation; the
third was Andr�; he had five fingers and five toes, quite perfect; the
fourth was a girl, Marie; she had five fingers and five toes, but her
thumbs were deformed, showing a tendency toward the sixth.

These children grew up, and when they came to adult years, they all
married, and of course it happened that they all married five-fingered and
five-toed persons. Now let us see what were the results. Salvator had four
children; they were two boys, a girl, and another boy; the first two boys
and the girl were six-fingered and six-toed like their grandfather; the
fourth boy had only five fingers and five toes. George had only four
children; there were two girls with six fingers and six toes; there was one
girl with six fingers and five toes on the right side, and five fingers and
five toes on the left side, so that she was half and half. The last, a boy,
had five fingers and five toes. The third, Andr�, you will recollect, was
perfectly well-formed, and he had many children whose hands and feet were
all regularly developed. Marie, the last, who, of course, married a man who
had only five fingers, had four children; the first, a boy, was born with
six toes, but the other three were normal.

Now observe what very extraordinary phenomena are presented here. You have
an accidental variation giving rise to what you may call a monstrosity; you
have that monstrosity or variation diluted in the first instance by an
admixture with a female of normal construction, and you would naturally
expect that, in the results of such an union, the monstrosity, if repeated,
would be in equal proportion with the normal type; that is to say, that the
children would be half and half, some taking the peculiarity of the father,
and the others being of the purely normal type of the mother; but you see
we have a great preponderance of the abnormal type. Well, this comes to be
mixed once more with the pure, the normal type, and the abnormal is again
produced in large proportion, notwithstanding the second dilution. Now what
would have happened if these abnormal types had intermarried with each
other; that is to say, suppose the two boys of Salvator had taken it into
their heads to marry their first cousins, the two first girls of George,
their uncle? You will remember that these are all of the abnormal type of
their grandfather. The result would probably have been, that their
offspring would have been in every case a further development of that
abnormal type. You see it is only in the fourth, in the person of Marie,
that the tendency, when it appears but slightly in the second generation,
is washed out in the third, while the progeny of Andr�, who escaped in the
first instance, escape altogether.

We have in this case a good example of nature's tendency to the
perpetuation of a variation. Here it is certainly a variation which earned
with it no use or benefit; and yet you see the tendency to perpetuation may
be so strong, that, notwithstanding a great admixture of pure blood, the
variety continues itself up to the third generation, which is largely
marked with it. In this case, as I have said, there was no means of the
second generation intermarrying with any but five-fingered persons, and the
question naturally suggests itself, What would have been the result of such
marriage? R�aumur narrates this case only as far as the third generation.
Certainly it would have been an exceedingly curious thing if we could have
traced this matter any further; had the cousins intermarried, a
six-fingered variety of the human race might have been set up.

To show you that this supposition is by no means an unreasonable one, let
me now point out what took place in the case of Seth Wright's sheep, where
it happened to be a matter of moment to him to obtain a breed or raise a
flock of sheep like that accidental variety that I have described--and I
will tell you why. In that part of Massachusetts where Seth Wright was
living, the fields were separated by fences, and the sheep, which were very
active and robust, would roam abroad, and without much difficulty jump over
these fences into other people's farms. As a matter of course, this
exuberant activity on the part of the sheep constantly gave rise to all
sorts of quarrels, bickerings, and contentions among the farmers of the
neighbourhood; so it occurred to Seth Wright, who was, like his successors,
more or less 'cute, that if he could get a stock of sheep like those with
the bandy legs, they would not be able to jump over the fences so readily;
and he acted upon that idea. He killed his old ram, and as soon as the
young one arrived at maturity, he bred altogether from it. The result was
even more striking than in the human experiment which I mentioned just now.
Colonel Humphreys testifies that it always happened that the offspring were
either pure Ancons or pure ordinary sheep; that in no case was there any
mixing of the Ancons with the others. In consequence of this, in the course
of a very few years, the farmer was able to get a very considerable flock
of this variety, and a large number of them were spread throughout
Massachusetts. Most unfortunately, however--I suppose it was because they
were so common--nobody took enough notice of them to preserve their
skeletons; and although Colonel Humphreys states that he sent a skeleton to
the President of the Royal Society at the same time that he forwarded his
paper, I am afraid that the variety has entirely disappeared; for a short
time after these sheep had become prevalent in that district, the Merino
sheep were introduced; and as their wool was much more valuable, and as
they were a quiet race of sheep, and showed no tendency to trespasser jump
over fences, the Otter breed of sheep, the wool of which was inferior to
that of the Merino, was gradually allowed to die out.

You see that these facts illustrate perfectly well what may be done if you
take care to breed from stocks that are similar to each other. After having
got a variation, if, by crossing a variation with the original stock, you
multiply that variation, and then take care to keep that variation distinct
from the original stock, and make them breed together,--then you may almost
certainly produce a race whose tendency to continue the variation is
exceedingly strong.

This is what is called "selection"; and it is by exactly the same process
as that by which Seth Wright bred his Ancon sheep, that our breeds of
cattle, dogs, and fowls are obtained. There are some possibilities of
exception, but still, speaking broadly, I may say that this is the way in
which all our varied races of domestic animals have arisen; and you must
understand that it is not one peculiarity or one characteristic alone in
which animals may vary. There is not a single peculiarity or characteristic
of any kind, bodily or mental, in which offspring may not vary to a certain
extent from the parent and other animals.

Among ourselves this is well known. The simplest physical peculiarity is
mostly reproduced. I know a case of a woman who has the lobe of one of her
ears a little flattened. An ordinary observer might scarcely notice it, and
yet every one of her children has an approximation to the same peculiarity
to some extent. If you look at the other extreme, too, the gravest
diseases, such as gout, scrofula, and consumption, may be handed down with
just the same certainty and persistence as we noticed in the perpetuation
of the bandy legs of the Ancon sheep.

However, these facts are best illustrated in animals, and the extent of the
variation, as is well known, is very remarkable in dogs. For example, there
are some dogs very much smaller than others; indeed, the variation is so
enormous that probably the smallest dog would be about the size of the head
of the largest; there are very great variations in the structural forms not
only of the skeleton but also in the shape of the skull, and in the
proportions of the face and the disposition of the teeth.

The Pointer, the Retriever, Bulldog, and the Terrier differ very greatly,
and yet there is every reason to believe that every one of these races has
arisen from the same source,--that all the most important races have arisen
by this selective breeding from accidental variation.

A still more striking case of what may be done by selective breeding, and
it is a better case, because there is no chance of that partial infusion of
error to which I alluded, has been studied very carefully by Mr.
Darwin,--the case of the domestic pigeons. I dare say there may be some
among you who may be pigeon _fanciers_, and I wish you to understand
that in approaching the subject, I would speak with all humility and
hesitation, as I regret to say that I am not a pigeon fancier. I know it is
a great art and mystery, and a thing upon which a man must not speak
lightly; but I shall endeavour, as far as my understanding goes, to give
you a summary of the published and unpublished information which I have
gained from Mr. Darwin.

Among the enormous variety,--I believe there are somewhere about a hundred
and fifty kinds of pigeons,--there are four kinds which may be selected as
representing the extremest divergences of one kind from another. Their
names are the Carrier, the Pouter, the Fantail, and the Tumbler. In these
large diagrams that I have here they are each represented in their relative
sizes to each other. This first one is the Carrier; you will notice this
large excrescence on its beak; it has a comparatively small head; there is
a bare space round the eyes; it has a long neck, a very long beak, very
strong legs, large feet, long wings, and so on. The second one is the
Pouter, a very large bird, with very long legs and beak. It is called the
Pouter because it is in the habit of causing its gullet to swell up by
inflating it with air. I should tell you that all pigeons have a tendency
to do this at times, but in the Pouter it is carried to an enormous extent.
The birds appear to be quite proud of their power of swelling and puffing
themselves out in this way; and I think it is about as droll a sight as you
can well see to look at a cage full of these pigeons puffing and blowing
themselves out in this ridiculous manner.

This diagram is a representation of the third kind I mentioned--the
Fantail. It is, you see, a small bird, with exceedingly small legs and a
very small beak. It is most curiously distinguished by the size and extent
of its tail, which, instead of containing twelve feathers, may have many
more,--say thirty, or even more--I believe there are some with as many as
forty-two. This bird has a curious habit of spreading out the feathers of
its tail in such a way that they reach forward and touch its head; and if
this can be accomplished, I believe it is looked upon as a point of great
beauty.

But here is the last great variety,--the Tumbler; and of that great
variety, one of the principal kinds, and one most prized, is the specimen
represented here--the short-faced Tumbler. Its beak, you see, is reduced to
a mere nothing. Just compare the beak of this one and that of the first
one, the Carrier--I believe the orthodox comparison of the head and beak of
a thoroughly well-bred Tumbler is to stick an oat into a cherry, and that
will give you the proper relative proportions of the beak and head. The
feet and legs are exceedingly small, and the bird appears to be quite a
dwarf when placed side by side with this great Carrier.

These are differences enough in regard to their external appearance; but
these differences are by no means the whole or even the most important of
the differences which obtain between these birds. There is hardly a single
point of their structure which has not become more or less altered; and to
give you an idea of how extensive these alterations are, I have here some
very good skeletons, for which I am indebted to my friend, Mr. Tegetmeier,
a great authority in these matters; by means of which, if you examine them
by and by, you will be able to see the enormous difference in their bony
structures.

I had the privilege, some time ago, of access to some important MSS. of Mr.
Darwin, who, I may tell you, has taken very great pains and spent much
valuable time and attention on the investigation of these variations, and
getting together all the facts that bear upon them. I obtained from these
MSS. the following summary of the differences between the domestic breeds
of pigeons; that is to say, a notification of the various points in which
their organisation differs. In the first place, the back of the skull may
differ a good deal, and the development of the bones of the face may vary a
great deal; the back varies a good deal; the shape of the lower jaw varies;
the tongue varies very greatly, not only in correlation to the length and
size of the beak, but it seems also to have a kind of independent variation
of its own. Then the amount of naked skin round the eyes, and at the base
of the beak, may vary enormously; so may the length of the eyelids, the
shape of the nostrils, and the length of the neck. I have already noticed
the habit of blowing out the gullet, so remarkable in the Pouter, and
comparatively so in the others. There are great differences, too, in the
size of the female and the male, the shape of the body, the number and
width of the processes of the ribs, the development of the ribs, and the
size, shape, and development of the breastbone. We may notice, too--and I
mention the fact because it has been disputed by what is assumed to be high
authority,--the variation in the number of the sacral vertebrae. The number
of these varies from eleven to fourteen, and that without any diminution in
the number of the vertebrae of the back or of the tail. Then the number and
position of the tail-feathers may vary enormously, and so may the number of
the primary and secondary feathers of the wings. Again, the length of the
feet and of the beak,--although they have no relation to each other, yet
appear to go together,--that is, you have a long beak wherever you have
long feet. There are differences also in the periods of the acquirement of
the perfect plumage--the size and shape of the eggs--the nature of flight,
and the powers of flight--so-called _"homing"_ birds having enormous
flying powers; [Footnote: The _"Carrier,"_ I learn from Mr.
Tegetmeier, does not _carry_; a high-bred bird of this breed being but
a poor flier. The birds which fly long distances, and come home--"homing"
birds-and are consequently used as carriers, are not "carriers" in the
fancy sense.] while, on the other hand, the little Tumbler is so called
because of its extraordinary faculty of turning head over heels in the air,
instead of pursuing a direct course. And, lastly, the dispositions and
voices of the birds may vary. Thus the case of the pigeons shows you that
there is hardly a single particular--whether of instinct, or habit, or bony
structure, or of plumage--of either the internal economy or the external
shape, in which some variation or change may not take place, which, by
selective breeding, may become perpetuated, and form the foundation of, and
give rise to, a new race.

If you carry in your mind's eye these four varieties of pigeons, you will
bear with you as good a notion as you can have, perhaps, of the enormous
extent to which a deviation from a primitive type may be carried by means
of this process of selective breeding.

V. THE CONDITIONS OF EXISTENCE AS AFFECTING THE PERPETUATION OF LIVING
BEINGS

In the last Lecture I endeavoured to prove to you that, while, as a general
rule, organic beings tend to reproduce their kind, there is in them, also,
a constantly recurring tendency to vary--to vary to a greater or to a less
extent. Such a variety, I pointed out to you, might arise from causes which
we do not understand; we therefore called it spontaneous; and it might come
into existence as a definite and marked thing, without any gradations
between itself and the form which preceded it. I further pointed out, that
such a variety having once arisen, might be perpetuated to some extent, and
indeed to a very marked extent, without any direct interference, or without
any exercise of that process which we called selection. And then I stated
further, that by such selection, when exercised artificially--if you took
care to breed only from those forms which presented the same peculiarities
of any variety which had arisen in this manner--the variation might be
perpetuated, as far as we can see, indefinitely.

The next question, and it is an important one for us, is this: Is there any
limit to the amount of variation from the primitive stock which can be
produced by this process of selective breeding? In considering this
question, it will be useful to class the characteristics, in respect of
which organic beings vary, under two heads: we may consider structural
characteristics, and we may consider physiological characteristics.

In the first place, as regards structural characteristics, I endeavoured to
show you, by the skeletons which I had upon the table, and by reference to
a great many well-ascertained facts, that the different breeds of Pigeons,
the Carriers, Pouters, and Tumblers, might vary in any of their internal
and important structural characters to a very great degree; not only might
there be changes in the proportions of the skull, and the characters of the
feet and beaks, and so on; but that there might be an absolute difference
in the number of the vertebrae of the back, as in the sacral vertebras of
the Pouter; and so great is the extent of the variation in these and
similar characters that I pointed out to you, by reference to the skeletons
and the diagrams, that these extreme varieties may absolutely differ more
from one another in their structural characters than do what naturalists
call distinct SPECIES of pigeons; that is to say, that they differ so much
in structure that there is a greater difference between the Pouter and the
Tumbler than there is between such wild and distinct forms as the Rock
Pigeon or the Ring Pigeon, or the Ring Pigeon and the Stock Dove; and
indeed the differences are of greater value than this, for the structural
differences between these domesticated pigeons are such as would be
admitted by a naturalist, supposing he knew nothing at all about their
origin, to entitle them to constitute even distinct genera.

As I have used this term SPECIES, and shall probably use it a good deal, I
had better perhaps devote a word or two to explaining what I mean by it.

Animals and plants are divided into groups, which become gradually smaller,
beginning with a KINGDOM, which is divided into SUB-KINGDOMS; then come the
smaller divisions called PROVINCES; and so on from a PROVINCE to a CLASS,
from a CLASS to an ORDER, from ORDERS to FAMILIES, and from these to
GENERA, until we come at length to the smallest groups of animals which can
be defined one from the other by constant characters, which are not sexual;
and these are what naturalists call SPECIES in practice, whatever they may
do in theory.

If, in a state of nature, you find any two groups of living beings, which
are separated one from the other by some constantly-recurring
characteristic, I don't care how slight and trivial, so long as it is
defined and constant, and does not depend on sexual peculiarities, then all
naturalists agree in calling them two species; that is what is meant by the
use of the word species--that is to say, it is, for the practical
naturalist, a mere question of structural differences. [Footnote: I lay
stress here on the _practical_ signification of "Species." Whether a
physiological test between species exist or not, it is hardly ever
applicable by the practical naturalist.] We have seen now--to repeat this
point once more, and it is very essential that we should rightly understand
it--we have seen that breeds, known to have been derived from a common
stock by selection, may be as different in their structure from the
original stock as species may be distinct from each other.

But is the like true of the physiological characteristics of animals? Do
the physiological differences of varieties amount in degree to those
observed between forms which naturalists call distinct species? This is a
most important point for us to consider.

As regards the great majority of physiological characteristics, there is no
doubt that they are capable of being developed, increased, and modified by
selection.

There is no doubt that breeds may be made as different as species in many
physiological characters. I have already pointed out to you very briefly
the different habits of the breeds of Pigeons, all of which depend upon
their physiological peculiarities--as the peculiar habit of tumbling, in
the Tumbler--the peculiarities of flight, in the "homing" birds--the
strange habit of spreading out the tail, and walking in a peculiar fashion,
in the Fantail--and, lastly, the habit of blowing out the gullet, so
characteristic of the Pouter. These are all due to physiological
modifications, and in all these respects these birds differ as much from
each other as any two ordinary species do.

So with Dogs in their habits and instincts. It is a physiological
peculiarity which leads the Greyhound to chase its prey by sight--that
enables the Beagle to track it by the scent--that impels the Terrier to its
rat-hunting propensity--and that leads the Retriever to its habit of
retrieving. These habits and instincts are all the results of physiological
differences and peculiarities, which have been developed from a common
stock, at least there is every reason to believe so. But it is a most
singular circumstance, that while you may run through almost the whole
series of physiological processes, without finding a check to your
argument, you come at last to a point where you do find a check, and that
is in the reproductive processes. For there is a most singular circumstance
in respect to natural species--at least about some of them--and it would be
sufficient for the purposes of this argument if it were true of only one of
them, but there is, in fact, a great number of such cases--and that is,
that, similar as they may appear to be to mere races or breeds, they
present a marked peculiarity in the reproductive process. If you breed from
the male and female of the same race, you of course have offspring of the
like kind, and if you make the offspring breed together, you obtain the
same result, and if you breed from these again, you will still have the
same kind of offspring; there is no check. But if you take members of two
distinct species, however similar they may be to each other, and make them
breed together, you will find a check, with some modifications and
exceptions, however, which I shall speak of presently. If you cross two
such species with each other, then--although you may get offspring in the
case of the first cross, yet, if you attempt to breed from the products of
that crossing, which are what are called HYBRIDS--that is, if you couple a
male and a female hybrid--then the result is that in ninety-nine cases out
of a hundred you will get no offspring at all; there will be no result
whatsoever.

The reason of this is quite obvious in some cases; the male hybrids,
although possessing all the external appearances and characteristics of
perfect animals, are physiologically imperfect and deficient in the
structural parts of the reproductive elements necessary to generation. It
is said to be invariably the case with the male mule, the cross between the
Ass and the Mare; and hence it is, that, although crossing the Horse with
the Ass is easy enough, and is constantly done, as far as I am aware, if
you take two mules, a male and a female, and endeavour to breed from them,
you get no offspring whatever; no generation will take place. This is what
is called the sterility of the hybrids between two distinct species.

You see that this is a very extraordinary circumstance; one does not see
why it should be. The common teleological explanation is, that it is to
prevent the impurity of the blood resulting from the crossing of one
species with another, but you see it does not in reality do anything of the
kind. There is nothing in this fact that hybrids cannot breed with each
other, to establish such a theory; there is nothing to prevent the Horse
breeding with the Ass, or the Ass with the Horse. So that this explanation
breaks down, as a great many explanations of this kind do, that are only
founded on mere assumptions.

Thus you see that there is a great difference between "mongrels," which are
crosses between distinct races, and "hybrids," which are crosses between
distinct species. The mongrels are, so far as we know, fertile with one
another. But between species, in many cases, you cannot succeed in
obtaining even the first cross; at any rate it is quite certain that the
hybrids are often absolutely infertile one with another.

Here is a feature, then, great or small as it may be, which distinguishes
natural species of animals. Can we find any approximation to this in the
different races known to be produced by selective breeding from a common
stock? Up to the present time the answer to that question is absolutely a
negative one. As far as we know at present, there is nothing approximating
to this check. In crossing the breeds between the Fantail and the Pouter,
the Carrier and the Tumbler, or any other variety or race you may name--so
far as we know at present--there is no difficulty in breeding together the
mongrels. Take the Carrier and the Fantail, for instance, and let them
represent the Horse and the Ass in the case of distinct species; then you
have, as the result of their breeding, the Carrier-Fantail mongrel,--we
will say the male and female mongrel,--and, as far as we know, these two
when crossed would not be less fertile than the original cross, or than
Carrier with Carrier. Here, you see, is a physiological contrast between
the races produced by selective modification and natural species. I shall
inquire into the value of this fact, and of some modifying circumstances by
and by; for the present I merely put it broadly before you.

But while considering this question of the limitations of species, a word
must be said about what is called RECURRENCE--the tendency of races which
have been developed by selective breeding from varieties to return to their
primitive type. This is supposed by many to put an absolute limit to the
extent of selective and all other variations. People say, "It is all very
well to talk about producing these different races, but you know very well
that if you turned all these birds wild, these Pouters, and Carriers, and
so on, they would all return to their primitive stock." This is very
commonly assumed to be a fact, and it is an argument that is commonly
brought forward as conclusive; but if you will take the trouble to inquire
into it rather closely, I think you will find that it is not worth very
much. The first question of course is, Do they thus return to the primitive
stock? And commonly as the thing is assumed and accepted, it is extremely
difficult to get anything like good evidence of it. It is constantly said,
for example, that if domesticated Horses are turned wild, as they have been
in some parts of Asia Minor and South America, that they return at once to
the primitive stock from which they were bred. But the first answer that
you make to this assumption is, to ask who knows what the primitive stock
was; and the second answer is, that in that case the wild Horses of Asia
Minor ought to be exactly like the wild Horses of South America. If they
are both like the same thing, they ought manifestly to be like each other!
The best authorities, however, tell you that it is quite different. The
wild Horse of Asia is said to be of a dun colour, with a largish head, and
a great many other peculiarities; while the best authorities on the wild
Horses of South America tell you that there is no similarity between their
wild Horses and those of Asia Minor; the cut of their heads is very
different, and they are commonly chestnut or bay-coloured. It is quite
clear, therefore, that as by these facts there ought to have been two
primitive stocks, they go for nothing in support of the assumption that
races recur to one primitive stock, and so far as this evidence is
concerned, it falls to the ground.

Suppose for a moment that it were so, and that domesticated races, when
turned wild, did return to some common condition, I cannot see that this
would prove much more than that similar conditions are likely to produce
similar results; and that when you take back domesticated animals into what
we call natural conditions, you do exactly the same thing as if you
carefully undid all the work you had gone through, for the purpose of
bringing the animal from its wild to its domesticated state. I do not see
anything very wonderful in the fact, if it took all that trouble to get it
from a wild state, that it should go back into its original state as soon
as you removed the conditions which produced the variation to the
domesticated form. There is an important fact, however, forcibly brought
forward by Mr. Darwin, which has been noticed in connection with the
breeding of domesticated pigeons; and it is, that however different these
breeds of pigeons may be from each other, and we have already noticed the
great differences in these breeds, that if, among any of those variations,
you chance to have a blue pigeon turn up, it will be sure to have the black
bars across the wings, which are characteristic of the original wild stock,
the Rock Pigeon.

Now, this is certainly a very remarkable circumstance; but I do not see
myself how it tells very strongly either one way or the other. I think, in
fact, that this argument in favour of recurrence to the primitive type
might prove a great deal too much for those who so constantly bring it
forward. For example, Mr. Darwin has very forcibly urged, that nothing is
commoner than if you examine a dun horse--and I had an opportunity of
verifying this illustration lately while in the islands of the West
Highlands, where there are a great many dun horses--to find that horse
exhibit a long black stripe down his back, very often stripes on his
shoulder, and very often stripes on his legs. I, myself, saw a pony of this
description a short time ago, in a baker's cart, near Rothesay, in Bute: it
had the long stripe down the back, and stripes on the shoulders and legs,
just like those of the Ass, the Quagga, and the Zebra. Now, if we interpret
the theory of recurrence as applied to this case, might it not be said that
here was a case of a variation exhibiting the characters and conditions of
an animal occupying something like an intermediate position between the
Horse, the Ass, the Quagga, and the Zebra, and from which these had been
developed? In the same way with regard even to Man. Every anatomist will
tell you that there is nothing commoner, in dissecting the human body, than
to meet with what are called muscular variations--that is, if you dissect
two bodies very carefully, you will probably find that the modes of
attachment and insertion of the muscles are not exactly the same in both,
there being great peculiarities in the mode in which the muscles are
arranged; and it is very singular, that in some dissections of the human
body you will come upon arrangements of the muscles very similar indeed to
the same parts in the Apes. Is the conclusion in that case to be, that this
is like the black bars in the case of the Pigeon, and that it indicates a
recurrence to the primitive type from which the animals have been probably
developed? Truly, I think that the opponents of modification and variation
had better leave the argument of recurrence alone, or it may prove
altogether too strong for them.

To sum up,--the evidence as far as we have gone is against the argument as
to any limit to divergences, so far as structure is concerned; and in
favour of a physiological limitation. By selective breeding we can produce
structural divergences as great as those of species, but we cannot produce
equal physiological divergences. For the present I leave the question
there.

Now, the next problem that lies before us--and it is an extremely important
one--is this: Does this selective breeding occur in nature? Because, if
there is no proof of it, all that I have been telling you goes for nothing
in accounting for the origin of species. Are natural causes competent to
play the part of selection in perpetuating varieties? Here we labour under
very great difficulties. In the last lecture I had occasion to point out to
you the extreme difficulty of obtaining evidence even of the first origin
of those varieties which we know to have occurred in domesticated animals.
I told you, that almost always the origin of these varieties is overlooked,
so that I could only produce two or three cases, as that of Gratio Kelleia
and of the Ancon sheep. People forget, or do not take notice of them until
they come to have a prominence; and if that is true of artificial cases,
under our own eyes, and in animals in our own care, how much more difficult
it must be to have at first hand good evidence of the origin of varieties
in nature! Indeed, I do not know that it is possible by direct evidence to
prove the origin of a variety in nature, or to prove selective breeding;
but I will tell you what we can prove--and this comes to the same
thing--that varieties exist in nature within the limits of species, and,
what is more, that when a variety has come into existence in nature, there
are natural causes and conditions, which are amply competent to play the
part of a selective breeder; and although that is not quite the evidence
that one would like to have--though it is not direct testimony--yet it is
exceeding good and exceedingly powerful evidence in its way.

As to the first point, of varieties existing among natural species, I might
appeal to the universal experience of every naturalist, and of any person
who has ever turned any attention at all to the characteristics of plants
and animals in a state of nature; but I may as well take a few definite
cases, and I will begin with Man himself.

I am one of those who believe that, at present, there is no evidence
whatever for saying, that mankind sprang originally from any more than a
single pair; I must say, that I cannot see any good ground whatever, or
even any tenable sort of evidence, for believing that there is more than
one species of Man. Nevertheless, as you know, just as there are numbers of
varieties in animals, so there are remarkable varieties of men. I speak not
merely of those broad and distinct variations which you see at a glance.
Everybody, of course, knows the difference between a Negro and a white man,
and can tell a Chinaman from an Englishman. They each have peculiar
characteristics of colour and physiognomy; but you must recollect that the
characters of these races go very far deeper--they extend to the bony
structure, and to the characters of that most important of all organs to
us--the brain; so that, among men belonging to different races, or even
within the same race, one man shall have a brain a third, or half, or even
seventy per cent, bigger than another; and if you take the whole range of
human brains, you will find a variation in some cases of a hundred per
cent. Apart from these variations in the size of the brain, the characters
of the skull vary. Thus if I draw the figures of a Mongol and of a Negro
head on the blackboard, in the case of the last the breadth would be about
seven-tenths, and in the other it would be nine-tenths of the total length.
So that you see there is abundant evidence of variation among men in their
natural condition. And if you turn to other animals there is just the same
thing. The fox, for example, which has a very large geographical
distribution all over Europe, and parts of Asia, and on the American
Continent, varies greatly. There are mostly large foxes in the North, and
smaller ones in the South. In Germany alone the foresters reckon some eight
different sorts.

Of the tiger, no one supposes that there is more than one species; they
extend from the hottest parts of Bengal, into the dry, cold, bitter steppes
of Siberia, into a latitude of 50�,--so that they may even prey upon the
reindeer. These tigers have exceedingly different characteristics, but
still they all keep their general features, so that there is no doubt as to
their being tigers. The Siberian tiger has a thick fur, a small mane, and a
longitudinal stripe down the back, while the tigers of Java and Sumatra
differ in many important respects from the tigers of Northern Asia. So
lions vary; so birds vary; and so, if you go further back and lower down in
creation, you find that fishes vary. In different streams, in the same
country even, you will find the trout to be quite different to each other
and easily recognisable by those who fish in the particular streams. There
is the same differences in leeches; leech collectors can easily point out
to you the differences and the peculiarities which you yourself would
probably pass by; so with fresh-water mussels; so, in fact, with every
animal you can mention.

In plants there is the same kind of variation. Take such a case even as the
common bramble. The botanists are all at war about it; some of them wanting
to make out that there are many species of it, and others maintaining that
they are but many varieties of one species; and they cannot settle to this
day which is a species and which is a variety!

So that there can be no doubt whatsoever that any plant and any animal may
vary in nature; that varieties may arise in the way I have described--as
spontaneous varieties--and that those varieties may be perpetuated in the
same way that I have shown you spontaneous varieties are perpetuated; I
say, therefore, that there can be no doubt as to the origin and
perpetuation of varieties in nature.

But the question now is:--Does selection take place in nature? Is there
anything like the operation of man in exercising selective breeding, taking
place in nature? You will observe that, at present, I say nothing about
species; I wish to confine myself to the consideration of the production of
those natural races which everybody admits to exist. The question is,
whether in nature there are causes competent to produce races, just in the
same way as man is able to produce by selection, such races of animals as
we have already noticed.

When a variety has arisen, the CONDITIONS OF EXISTENCE are such as to
exercise an influence which is exactly comparable to that of artificial
selection. By Conditions of Existence I mean two things--there are
conditions which are furnished by the physical, the inorganic world, and
there are conditions of existence which are furnished by the organic world.
There is, in the first place, CLIMATE; under that head I include only
temperature and the varied amount of moisture of particular places. In the
next place there is what is technically called STATION, which means--given
the climate, the particular kind of place in which an animal or a plant
lives or grows; for example, the station of a fish is in the water, of a
fresh-water fish in fresh water; the station of a marine fish is in the
sea, and a marine animal may have a station higher or deeper. So again with
land animals: the differences in their stations are those of different
soils and neighbourhoods; some being best adapted to a calcareous, and
others to an arenaceous soil. The third condition of existence is FOOD, by
which I mean food in the broadest sense, the supply of the materials
necessary to the existence of an organic being; in the case of a plant the
inorganic matters, such as carbonic acid, water, ammonia, and the earthy
salts or salines; in the case of the animal the inorganic and organic
matters, which we have seen they require; then these are all, at least the
first two, what we may call the inorganic or physical conditions of
existence. Food takes a mid-place, and then come the organic conditions; by
which I mean the conditions which depend upon the state of the rest of the
organic creation, upon the number and kind of living beings, with which an
animal is surrounded. You may class these under two heads: there are
organic beings, which operate as _opponents_, and there are organic
beings which operate as _helpers_ to any given organic creature. The
opponents may be of two kinds: there are the _indirect opponents_,
which are what we may call _rivals_; and there are the _direct
opponents_, those which strive to destroy the creature; and these we
call _enemies_. By rivals I mean, of course, in the case of plants,
those which require for their support the same kind of soil and station,
and, among animals, those which require the same kind of station, or food,
or climate; those are the indirect opponents; the direct opponents are, of
course, those which prey upon an animal or vegetable. The _helpers_
may also be regarded as direct and indirect: in the case of a carnivorous
animal, for example, a particular herbaceous plant may, in multiplying, be
an indirect helper, by enabling the herbivora on which the carnivore preys
to get more food, and thus to nourish the carnivore more abundantly; the
direct helper may be best illustrated by reference to some parasitic
creature, such as the tape-worm. The tape-worm exists in the human
intestines, so that the fewer there are of men the fewer there will be of
tape-worms, other things being alike. It is a humiliating reflection,
perhaps, that we may be classed as direct helpers to the tape-worm, but the
fact is so: we can all see that if there were no men there would be no
tape-worms.

It is extremely difficult to estimate, in a proper way, the importance and
the working of the Conditions of Existence. I do not think there were any
of us who had the remotest notion of properly estimating them until the
publication of Mr. Darwin's work, which has placed them before us with
remarkable clearness; and I must endeavour, as far as I can in my own
fashion, to give you some notion of how they work. We shall find it easiest
to take a simple case, and one as free as possible from every kind of
complication.

I will suppose, therefore, that all the habitable part of this globe--the
dry land, amounting to about 51,000,000 square miles--I will suppose that
the whole of that dry land has the same climate, and that it is composed of
the same kind of rock or soil, so that there will be the same station
everywhere; we thus get rid of the peculiar influence of different climates
and stations. I will then imagine that there shall be but one organic being
in the world, and that shall be a plant. In this we start fair. Its food is
to be carbonic acid, water and ammonia, and the saline matters in the soil,
which are, by the supposition, everywhere alike. We take one single plant,
with no opponents, no helpers, and no rivals; it is to be a "fair field,
and no favour." Now, I will ask you to imagine further that it shall be a
plant which shall produce every year fifty seeds, which is a very moderate
number for a plant to produce; and that, by the action of the winds and
currents, these seeds shall be equally and gradually distributed over the
whole surface of the land. I want you now to trace out what will occur, and
you will observe that I am not talking fallaciously any more than a
mathematician does when he expounds his problem. If you show that the
conditions of your problem are such as may actually occur in Nature and do
not transgress any of the known laws of Nature in working out your
proposition, then you are as safe in the conclusion you arrive at as is the
mathematician in arriving at the solution of his problem. In science, the
only way of getting rid of the complications with which a subject of this
kind is environed, is to work in this deductive method. What will be the
result, then? I will suppose that every plant requires one square foot of
ground to live upon; and the result will be that, in the course of nine
years, the plant will have occupied every single available spot in the
whole globe! I have chalked upon the blackboard the figures by which I
arrive at the result:--

Plants. Plants.

1 x 50 in 1st year = 50
50 x 50 " 2nd " = 2,500
2,500 x 50 " 3rd " = 125,000
125,000 x 50 " 4th " = 6,250,000
6,250,000 x 50 " 5th " = 312,500,000
312,500,000 x 50 " 6th " = 15,625,000,000
15,625,000,000 x 50 " 7th " = 781,250,000,000
781,250,000,000 x 50 " 8th " = 39,062,500,000,000
39,062,500,000,000 x 50 " 9th " = 1,953,125,000,000,000

51,000,000 square miles--the )
dry surface of the earth x )
27,878,400--the number of ) = sq. ft. 1,421,798,400,000,000
sq. ft. in 1 sq. mile ) ---------------------
being 531,326,600,000,000
square feet less than would be required at the end of the ninth
year.

You will see from this that, at the end of the first year the single plant
will have produced fifty more of its kind; by the end of the second year
these will have increased to 2,500; and so on, in succeeding years, you get
beyond even trillions; and I am not at all sure that I could tell you what
the proper arithmetical denomination of the total number really is; but, at
any rate, you will understand the meaning of all those noughts. Then you
see that, at the bottom, I have taken the 51,000,000 of square miles,
constituting the surface of the dry land; and as the number of square feet
are placed under and subtracted from the number of seeds that would be
produced in the ninth year, you can see at once that there would be an
immense number more of plants than there would be square feet of ground for
their accommodation. This is certainly quite enough to prove my point; that
between the eighth and ninth year after being planted the single plant
would have stocked the whole available surface of the earth.

This is a thing which is hardly conceivable--it seems hardly
imaginable--yet it is so. It is indeed simply the law of Malthus
exemplified. Mr. Malthus was a clergyman, who worked out this subject most
minutely and truthfully some years ago; he showed quite clearly--and
although he was much abused for his conclusions at the time, they have
never yet been disproved and never will be--he showed that in consequence
of the increase in the number of organic beings in a geometrical ratio,
while the means of existence cannot be made to increase in the same ratio,
that there must come a time when the number of organic beings will be in
excess of the power of production of nutriment, and that thus some check
must arise to the further increase of those organic beings. At the end of
the ninth year we have seen that each plant would not be able to get its
full square foot of ground, and at the end of another year it would have to
share that space with fifty others the produce of the seeds which it would
give off.

What, then, takes place? Every plant grows up, flourishes, occupies its
square foot of ground, and gives off its fifty seeds; but notice this, that
out of this number only one can come to anything; there is thus, as it
were, forty-nine chances to one against its growing up; it depends upon the
most fortuitous circumstances whether any one of these fifty seeds shall
grow up and flourish, or whether it shall die and perish. This is what Mr.
Darwin has drawn attention to, and called the "STRUGGLE FOR EXISTENCE"; and
I have taken this simple case of a plant because some people imagine that
the phrase seems to imply a sort of fight.

I have taken this plant and shown you that this is the result of the ratio
of the increase, the necessary result of the arrival of a time coming for
every species when exactly as many members must be destroyed as are born;
that is the inevitable ultimate result of the rate of production. Now, what
is the result of all this? I have said that there are forty-nine struggling
against every one; and it amounts to this, that the smallest possible start
given to any one seed may give it an advantage which will enable it to get
ahead of all the others; anything that will enable any one of these seeds
to germinate six hours before any of the others will, other things being
alike, enable it to choke them out altogether. I have shown you that there
is no particular in which plants will not vary from each other; it is quite
possible that one of our imaginary plants may vary in such a character as
the thickness of the integument of its seeds; it might happen that one of
the plants might produce seeds having a thinner integument, and that would
enable the seeds of that plant to germinate a little quicker than those of
any of the others, and those seeds would most inevitably extinguish the
forty-nine times as many that were struggling with them.

I have put it in this way, but you see the practical result of the process
is the same as if some person had nurtured the one and destroyed the other
seeds. It does not matter how the variation is produced, so long as it is
once allowed to occur. The variation in the plant once fairly started tends
to become hereditary and reproduce itself; the seeds would spread
themselves in the same way and take part in the struggle with the
forty-nine hundred, or forty-nine thousand, with which they might be
exposed. Thus, by degrees, this variety with some slight organic change or
modification, must spread itself over the whole surface of the habitable
globe, and extirpate or replace the other kinds. That is what is meant by
NATURAL SELECTION; that is the kind of argument by which it is perfectly

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DarwinianabyThomas Henry Huxley

Part 5 out of 6

Darwiniana
by
Thomas Henry Huxley