In the lecture which I delivered last Monday evening, I endeavoured to
sketch in a very brief manner, but as well as the time at my disposal would
permit, the present condition of organic nature, meaning by that large
title simply an indication of the great, broad, and general principles
which are to be discovered by those who look attentively at the phenomena
of organic nature as at present displayed. The general result of our
investigations might be summed up thus: we found that the multiplicity of
the forms of animal life, great as that may be, may be reduced to a
comparatively few primitive plans or types of construction; that a further
study of the development of those different forms revealed to us that they
were again reducible, until we at last brought the infinite diversity of
animal, and even vegetable life, down to the primordial form of a single
cell.
We found that our analysis of the organic world, whether animals or plants,
showed, in the long run, that they might both be reduced into, and were, in
fact, composed of, the same constituents. And we saw that the plant
obtained the materials constituting its substance by a peculiar combination
of matters belonging entirely to the inorganic world; that, then, the
animal was constantly appropriating the nitrogenous matters of the plant to
its own nourishment, and returning them back to the inorganic world, in
what we spoke of as its waste; and that finally, when the animal ceased to
exist, the constituents of its body were dissolved and transmitted to that
inorganic world whence they had been at first abstracted. Thus we saw in
both the blade of grass and the horse but the same elements differently
combined and arranged. We discovered a continual circulation going on,--the
plant drawing in the elements of inorganic nature and combining them into
food for the animal creation; the animal borrowing from the plant the
matter for its own support, giving off during its life products which
returned immediately to the inorganic world; and that, eventually, the
constituent materials of the whole structure of both animals and plants
were thus returned to their original source: there was a constant passage
from one state of existence to another, and a returning back again.
Lastly, when we endeavoured to form some notion of the nature of the forces
exercised by living beings, we discovered that they--if not capable of
being subjected to the same minute analysis as the constituents of those
beings themselves--that they were correlative with--that they were the
equivalents of the forces of inorganic nature--that they were, in the sense
in which the term is now used, convertible with them. That was our general
result.
And now, leaving the Present, I must endeavour in the same manner to put
before you the facts that are to be discovered in the Past history of the
living world, in the past conditions of organic nature. We have, to-night,
to deal with the facts of that history--a history involving periods of time
before which our mere human records sink into utter insignificance--a
history the variety and physical magnitude of whose events cannot even be
foreshadowed by the history of human life and human phenomena--a history of
the most varied and complex character.
We must deal with the history, then, in the first place, as we should deal
with all other histories. The historical student knows that his first
business should be to inquire into the validity of his evidence, and the
nature of the record in which the evidence is contained, that he may be
able to form a proper estimate of the correctness of the conclusions which
have been drawn from that evidence. So, here we must pass, in the first
place, to the consideration of a matter which may seem foreign to the
question under discussion. We must dwell upon the nature of the records,
and the credibility of the evidence they contain; we must look to the
completeness or incompleteness of those records themselves, before we turn
to that which they contain and reveal. The question of the credibility of
the history, happily for us, will not require much consideration, for, in
this history, unlike those of human origin, there can be no cavilling, no
differences as to the reality and truth of the facts of which it is made
up; the facts state themselves, and are laid out clearly before us.
But, although one of the greatest difficulties of the historical student is
cleared out of our path, there are other difficulties--difficulties in
rightly interpreting the facts as they are presented to us--which may be
compared with the greatest difficulties of any other kinds of historical
study.
What is this record of the past history of the globe, and what are the
questions which are involved in an inquiry into its completeness or
incompleteness? That record is composed of mud; and the question which we
have to investigate this evening resolves itself into a question of the
formation of mud. You may think, perhaps, that this is a vast step--of
almost from the sublime to the ridiculous--from the contemplation of the
history of the past ages of the world's existence to the consideration of
the history of the formation of mud! But, in Nature, there is nothing mean
and unworthy of attention; there is nothing ridiculous or contemptible in
any of her works; and this inquiry, you will soon see, I hope, takes us to
the very root and foundations of our subject.
How, then, is mud formed? Always, with some trifling exceptions, which I
need not consider now--always, as the result of the action of water,
wearing down and disintegrating the surface of the earth and rocks with
which it comes in contact--pounding and grinding it down, and carrying the
particles away to places where they cease to be disturbed by this
mechanical action, and where they can subside and rest. For the ocean,
urged by winds, washes, as we know, a long extent of coast, and every wave,
loaded as it is with particles of sand and gravel as it breaks upon the
shore, does something towards the disintegrating process. And thus, slowly
but surely, the hardest rocks are gradually ground down to a powdery
substance; and the mud thus formed, coarser or finer, as the case may be,
is carried by the rush of the tides, or currents, till it reaches the
comparatively deeper parts of the ocean, in which it can sink to the
bottom, that is, to parts where there is a depth of about fourteen or
fifteen fathoms, a depth at which the water is, usually, nearly motionless,
and in which, of course, the finer particles of this detritus, or mud as we
call it, sinks to the bottom.
Or, again, if you take a river, rushing down from its mountain sources,
brawling over the stones and rocks that intersect its path, loosening,
removing, and carrying with it in its downward course the pebbles and
lighter matters from its banks, it crushes and pounds down the rocks and
earths in precisely the same way as the wearing action of the sea waves.
The matters forming the deposit are torn from the mountain-side and whirled
impetuously into the valley, more slowly over the plain, thence into the
estuary, and from the estuary they are swept into the sea. The coarser and
heavier fragments are obviously deposited first, that is, as soon as the
current begins to lose its force by becoming amalgamated with the stiller
depths of the ocean, but the finer and lighter particles are carried
further on, and eventually deposited in a deeper and stiller portion of the
ocean.
It clearly follows from this that mud gives us a chronology; for it is
evident that supposing this, which I now sketch, to be the sea bottom, and
supposing this to be a coast-line; from the washing action of the sea upon
the rock, wearing and grinding it down into a sediment of mud, the mud will
be carried down, and, at length, deposited in the deeper parts of this sea
bottom, where it will form a layer; and then, while that first layer is
hardening, other mud which is coming from the same source will, of course,
be carried to the same place; and, as it is quite impossible for it to get
beneath the layer already there, it deposits itself above it, and forms
another layer, and in that way you gradually have layers of mud constantly
forming and hardening one above the other, and conveying a record of time.
It is a necessary result of the operation of the law of gravitation that
the uppermost layer shall be the youngest and the lowest the oldest, and
that the different beds shall be older at any particular point or spot in
exactly the ratio of their depth from the surface. So that if they were
upheaved afterwards, and you had a series of these different layers of mud,
converted into sandstone, or limestone, as the case might be, you might be
sure that the bottom layer was deposited first, and that the upper layers
were formed afterwards. Here, you see, is the first step in the
history--these layers of mud give us an idea of time.
The whole surface of the earth,--I speak broadly, and leave out minor
qualifications,--is made up of such layers of mud, so hard, the majority of
them, that we call them rock whether limestone or sandstone, or other
varieties of rock. And, seeing that every part of the crust of the earth is
made up in this way, you might think that the determination of the
chronology, the fixing of the time which it has taken to form this crust is
a comparatively simple matter. Take a broad average, ascertain how fast the
mud is deposited upon the bottom of the sea, or in the estuary of rivers;
take it to be an inch, or two, or three inches a year, or whatever you may
roughly estimate it at; then take the total thickness of the whole series
of stratified rocks, which geologists estimate at twelve or thirteen miles,
or about seventy thousand feet, make a sum in short division, divide the
total thickness by that of the quantity deposited in one year, and the
result will, of course, give you the number of years which the crust has
taken to form.
Truly, that looks a very simple process! It would be so except for certain
difficulties, the very first of which is that of finding how rapidly
sediments are deposited; but the main difficulty--a difficulty which
renders any certain calculations of such a matter out of the question--is
this, the sea-bottom on which the deposit takes place is continually
shifting.
Instead of the surface of the earth being that stable, fixed thing that it
is popularly believed to be, being, in common parlance, the very emblem of
fixity itself, it is incessantly moving, and is, in fact, as unstable as
the surface of the sea, except that its undulations are infinitely slower
and enormously higher and deeper.
Now, what is the effect of this oscillation? Take the case to which I have
previously referred. The finer or coarser sediments that are carried down
by the current of the river, will only be carried out a certain distance,
and eventually, as we have already seen, on reaching the stiller part of
the ocean, will be deposited at the bottom.
[Illustration: Fig. 4.]
Let C _y_ (Fig. 4) be the sea-bottom, _y_ D the shore, _x y_
the sea-level, then the coarser deposit will subside over the region B, the
finer over A, while beyond A there will be no deposit at all; and,
consequently, no record will be kept, simply because no deposit is going
on. Now, suppose that the whole land, C, D, which we have regarded as
stationary, goes down, as it does so, both A and B go further out from the
shore, which will be at _y1_; _x1_, _y1_, being the new
sea-level. The consequence will be that the layer of mud (A), being now,
for the most part, further than the force of the current is strong enough
to convey even the finest _débris_, will, of course, receive no more
deposits, and having attained a certain thickness will now grow no thicker.
We should be misled in taking the thickness of that layer, whenever it may
be exposed to our view, as a record of time in the manner in which we are
now regarding this subject, as it would give us only an imperfect and
partial record: it would seem to represent too short a period of time.
Suppose, on the other hand, that the land (C D) had gone on rising slowly
and gradually--say an inch or two inches in the course of a century,--what
would be the practical effect of that movement? Why, that the sediment A
and B which has been already deposited, would eventually be brought nearer
to the shore-level and again subjected to the wear and tear of the sea; and
directly the sea begins to act upon it, it would of course soon cut up and
carry it way, to a greater or less extent, to be re-deposited further out.
Well, as there is, in all probability, not one single spot on the whole
surface of the earth, which has not been up and down in this way a great
many times, it follows that the thickness of the deposits formed at any
particular spot cannot be taken (even supposing we had at first obtained
correct data as to the rate at which they took place), as affording
reliable information as to the period of time occupied in its deposit. So
that you see it is absolutely necessary from these facts, seeing that our
record entirely consists of accumulations of mud, superimposed one on the
other; seeing in the next place that any particular spots on which
accumulations have occurred, have been constantly moving up and down, and
sometimes out of the reach of a deposit, and at other times its own deposit
broken up and carried away, it follows that our record must be in the
highest degree imperfect, and we have hardly a trace left of thick
deposits, or any definite knowledge of the area that they occupied, in a
great many cases. And mark this! That supposing even that the whole surface
of the earth had been accessible to the geologist,--that man had had access
to every part of the earth, and had made sections of the whole, and put
them all together,--even then his record must of necessity be imperfect.
But to how much has man really access? If you will look at this map you
will see that it represents the proportion of the sea to the earth: this
coloured part indicates all the dry land, and this other portion is the
water. You will notice at once that the water covers three-fifths of the
whole surface of the globe, and has covered it in the same manner ever
since man has kept any record of his own observations, to say nothing of
the minute period during which he has cultivated geological inquiry. So
that three-fifths of the surface of the earth is shut out from us because
it is under the sea. Let us look at the other two-fifths, and see what are
the countries in which anything that may be termed searching geological
inquiry has been carried out: a good deal of France, Germany, and Great
Britain and Ireland, bits of Spain, of Italy, and of Russia, have been
examined, but of the whole great mass of Africa, except parts of the
southern extremity, we know next to nothing; little bits of India, but of
the greater part of the Asiatic continent nothing; bits of the Northern
American States and of Canada, but of the greater part of the continent of
North America, and in still larger proportion, of South America, nothing!
Under these circumstances, it follows that even with reference to that kind
of imperfect information which we can possess, it is only of about the
ten-thousandth part of the accessible parts of the earth that has been
examined properly. Therefore, it is with justice that the most thoughtful
of those who are concerned in these inquiries insist continually upon the
imperfection of the geological record; for, I repeat, it is absolutely
necessary, from the nature of things, that that record should be of the
most fragmentary and imperfect character. Unfortunately this circumstance
has been constantly forgotten. Men of science, like young colts in a fresh
pasture, are apt to be exhilarated on being turned into a new field of
inquiry, to go off at a hand-gallop, in total disregard of hedges and
ditches, to lose sight of the real limitation of their inquiries, and to
forget the extreme imperfection of what is really known. Geologists have
imagined that they could tell us what was going on at all parts of the
earth's surface during a given epoch; they have talked of this deposit
being contemporaneous with that deposit, until, from our little local
histories of the changes at limited spots of the earth's surface, they have
constructed a universal history of the globe as full of wonders and
portents as any other story of antiquity.
But what does this attempt to construct a universal history of the globe
imply? It implies that we shall not only have a precise knowledge of the
events which have occurred at any particular point, but that we shall be
able to say what events, at any one spot, took place at the same time with
those at other spots.
Let us see how far that is in the nature of things practicable. Suppose
that here I make a section of the Lake of Killarney, and here the section
of another lake--that of Loch Lomond in Scotland for instance. The rivers
that flow into them are constantly carrying down deposits of mud, and beds,
or strata, are being as constantly formed, one above the other, at the
bottom of those lakes. Now, there is not a shadow of doubt that in these
two lakes the lower beds are all older than the upper--there is no doubt
about that; but what does _this_ tell us about the age of any given
bed in Loch Lomond, as compared with that of any given bed in the Lake of
Killarney? It is, indeed, obvious that if any two sets of deposits are
separated and discontinuous, there is absolutely no means whatever given
you by the nature of the deposit of saying whether one is much younger or
older than the other; but you may say, as many have said and think, that
the case is very much altered if the beds which we are comparing are
continuous. Suppose two beds of mud hardened into rock,--A and B--are seen
in section. (Fig. 5.)
[Illustration: Fig. 5.]
Well, you say, it is admitted that the lowermost bed is always the older.
Very well; B, therefore, is older than A. No doubt, _as a whole_, it
is so; or if any parts of the two beds which are in the same vertical line
are compared, it is so. But suppose you take what seems a very natural step
further, and say that the part _a_ of the bed A is younger than the
part _b_ of the bed B. Is this sound reasoning? If you find any record
of changes taking place at _b_, did they occur before any events which
took place while _a_ was being deposited? It looks all very plain
sailing, indeed, to say that they did; and yet there is no proof of
anything of the kind. As the former Director of this Institution, Sir H. De
la Beche, long ago showed, this reasoning may involve an entire fallacy. It
is extremely possible that _a_ may have been deposited ages before
_b_. It is very easy to understand how that can be. To return to Fig.
4; when A and B were deposited, they were _substantially_
contemporaneous; A being simply the finer deposit, and B the coarser of the
same detritus or waste of land. Now suppose that that sea-bottom goes down
(as shown in Fig. 4), so that the first deposit is carried no farther than
_a_, forming the bed A1, and the coarse no farther than _b_,
forming the bed B1, the result will be the formation of two continuous
beds, one of fine sediment (A A1) over-lapping another of coarse sediment
(B B1). Now suppose the whole sea-bottom is raised up, and a section
exposed about the point A1; no doubt, _at this spot_, the upper bed is
younger than the lower. But we should obviously greatly err if we concluded
that the mass of the upper bed at A was younger than the lower bed at B;
for we have just seen that they are contemporaneous deposits. Still more
should we be in error if we supposed the upper bed at A to be younger than
the continuation of the lower bed at B1; for A was deposited long before
B1. In fine, if, instead of comparing immediately adjacent parts of two
beds, one of which lies upon another, we compare distant parts, it is quite
possible that the upper may be any number of years older than the under,
and the under any number of years younger than the upper.
Now you must not suppose that I put this before you for the purpose of
raising a paradoxical difficulty; the fact is, that the great mass of
deposits have taken place in sea-bottoms which are gradually sinking, and
have been formed under the very conditions I am here supposing.
Do not run away with the notion that this subverts the principle I laid
down at first. The error lies in extending a principle which is perfectly
applicable to deposits in the same vertical line to deposits which are not
in that relation to one another.
It is in consequence of circumstances of this kind, and of others that I
might mention to you, that our conclusions on and interpretations of the
record are really and strictly only valid so long as we confine ourselves
to one vertical section. I do not mean to tell you that there are no
qualifying circumstances, so that, even in very considerable areas, we may
safely speak of conformably superimposed beds being older or younger than
others at many different points. But we can never be quite sure in coming
to that conclusion, and especially we cannot be sure if there is any break
in their continuity, or any very great distance between the points to be
compared.
Well now, so much for the record itself,--so much for its
imperfections,--so much for the conditions to be observed in interpreting
it, and its chronological indications, the moment we pass beyond the limits
of a vertical linear section.
Now let us pass from the record to that which it contains,--from the book
itself to the writing and the figures on its pages. This writing and these
figures consist of remains of animals and plants which, in the great
majority of cases, have lived and died in the very spot in which we now
find them, or at least in the immediate vicinity. You must all of you be
aware--and I referred to the fact in my last lecture--that there are vast
numbers of creatures living at the bottom of the sea. These creatures, like
all others, sooner or later die, and their shells and hard parts lie at the
bottom; and then the fine mud which is being constantly brought down by
rivers and the action of the wear and tear of the sea, covers them over and
protects them from any further change or alteration; and, of course, as in
process of time the mud becomes hardened and solidified, the shells of
these animals are preserved and firmly imbedded in the limestone or
sandstone which is being thus formed. You may see in the galleries of the
Museum up stairs specimens of limestones in which such fossil remains of
existing animals are imbedded. There are some specimens in which turtles'
eggs have been imbedded in calcareous sand, and before the sun had hatched
the young turtles, they became covered over with calcareous mud, and thus
have been preserved and fossilised.
Not only does this process of imbedding and fossilisation occur with marine
and other aquatic animals and plants, but it affects those land animals and
plants which are drifted away to sea, or become buried in bogs or morasses;
and the animals which have been trodden down by their fellows and crushed
in the mud at the river's bank, as the herd have come to drink. In any of
these cases, the organisms may be crushed or be mutilated, before or after
putrefaction, in such a manner that perhaps only a part will be left in the
form in which it reaches us. It is, indeed, a most remarkable fact, that it
is quite an exceptional case to find a skeleton of any one of all the
thousands of wild land animals that we know are constantly being killed, or
dying in the course of nature: they are preyed on and devoured by other
animals, or die in places where their bodies are not afterwards protected
by mud. There are other animals existing on the sea, the shells of which
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 Molière'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 grâce_.
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 animalculæ 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 Abbé 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 Abbé 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.