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Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




I have already stated to you that the inquiry respecting the causes of the
phenomena of organic nature resolves itself into two problems--the first
being the question of the origination of living or organic beings; and the
second being the totally distinct problem of the modification and
perpetuation of organic beings when they have already come into existence.
The first question Mr. Darwin does not touch; he does not deal with it at
all; but he says:--"Given the origin of organic matter--supposing its
creation to have already taken place, my object is to show in consequence
of what laws and what demonstrable properties of organic matter, and of its
environments, such states of organic nature as those with which we are
acquainted must have come about." This, you will observe, is a perfectly
legitimate proposition; every person has a right to define the limits of
the inquiry which he sets before himself; and yet it is a most singular
thing that in all the multifarious, and, not unfrequently, ignorant attacks
which have been made upon the "Origin of Species," there is nothing which
has been more speciously criticised than this particular limitation. If
people have nothing else to urge against the book, they say--"Well, after
all, you see Mr. Darwin's explanation of the 'Origin of Species' is not
good for much, because, in the long run, he admits that he does not know
how organic matter began to exist. But if you admit any special creation
for the first particle of organic matter you may just as well admit it for
all the rest; five hundred or five thousand distinct creations are just as
intelligible, and just as little difficult to understand, as one." The
answer to these cavils is two-fold. In the first place, all human inquiry
must stop somewhere; all our knowledge and all our investigation cannot
take us beyond the limits set by the finite and restricted character of our
faculties, or destroy the endless unknown, which accompanies, like its
shadow, the endless procession of phenomena. So far as I can venture to
offer an opinion on such a matter, the purpose of our being in existence,
the highest object that human beings can set before themselves, is not the
pursuit of any such chimera as the annihilation of the unknown; but it is
simply the unwearied endeavour to remove its boundaries a little further
from our little sphere of action.

I wonder if any historian would for a moment admit the objection, that it
is preposterous to trouble ourselves about the history of the Roman Empire,
because we do not know anything positive about the origin and first
building of the city of Rome! Would it be a fair objection to urge,
respecting the sublime discoveries of a Newton, or a Kepler, those great
philosophers, whose discoveries have been of the profoundest benefit and
service to all men--to say to them--"After all that you have told us as to
how the planets revolve, and how they are maintained in their orbits, you
cannot tell us what is the cause of the origin of the sun, moon, and stars.
So what is the use of what you have done?" Yet these objections would not
be one whit more preposterous than the objections which have been made to
the "Origin of Species." Mr. Darwin, then, had a perfect right to limit his
inquiry as he pleased, and the only question for us--the inquiry being so
limited--is to ascertain whether the method of his inquiry is sound or
unsound; whether he has obeyed the canons which must guide and govern all
investigation, or whether he has broken them; and it was because our
inquiry this evening is essentially limited to that question, that I spent
a good deal of time in a former lecture (which, perhaps some of you thought
might have been better employed), in endeavouring to illustrate the method
and nature of scientific inquiry in general. We shall now have to put in
practice the principles that I then laid down.

I stated to you in substance, if not in words, that wherever there are
complex masses of phenomena to be inquired into, whether they be phenomena
of the affairs of daily life, or whether they belong to the more abstruse
and difficult problems laid before the philosopher, our course of
proceeding in unravelling that complex chain of phenomena with a view to
get at its cause, is always the same; in all cases we must invent an
hypothesis; we must place before ourselves some more or less likely
supposition respecting that cause; and then, having assumed an hypothesis,
having supposed a cause for the phenomena in question, we must endeavour,
on the one hand, to demonstrate our hypothesis, or, on the other, to upset
and reject it altogether, by testing it in three ways. We must, in the
first place, be prepared to prove that the supposed causes of the phenomena
exist in nature; that they are what the logicians call _vera
causæ_--true causes;--in the next place, we should be prepared to show
that the assumed causes of the phenomena are competent to produce such
phenomena as those which we wish to explain by them; and in the last place,
we ought to be able to show that no other known causes are competent to
produce these phenomena. If we can succeed in satisfying these three
conditions we shall have demonstrated our hypothesis; or rather I ought to
say we shall have proved it as far as certainty is possible for us; for,
after all, there is no one of our surest convictions which may not be
upset, or at any rate modified by a further accession of knowledge. It was
because it satisfied these conditions that we accepted the hypothesis as to
the disappearance of the tea-pot and spoons in the case I supposed in a
previous lecture; we found that our hypothesis on that subject was tenable
and valid, because the supposed cause existed in nature, because it was
competent to account for the phenomena, and because no other known cause
was competent to account for them; and it is upon similar grounds that any
hypothesis you choose to name is accepted in science as tenable and valid.

What is Mr. Darwin's hypothesis? As I apprehend it--for I have put it into
a shape more convenient for common purposes than I could find
_verbatim_ in his book--as I apprehend it, I say, it is, that all the
phenomena of organic nature, past and present, result from, or are caused
by, the inter-action of those properties of organic matter, which we have
called ATAVISM and VARIABILITY, with the CONDITIONS OF EXISTENCE, or, in
other words,--given the existence of organic matter, its tendency to
transmit its properties, and its tendency occasionally to vary; and,
lastly, given the conditions of existence by which organic matter is
surrounded--that these put together are the causes of the Present and of
the Past conditions of ORGANIC NATURE.

Such is the hypothesis as I understand it. Now let us see how it will stand
the various tests which I laid down just now. In the first place, do these
supposed causes of the phenomena exist in nature? Is it the fact that, in
nature, these properties of organic matter--atavism and variability--and
those phenomena which we have called the conditions of existence,--is it
true that they exist? Well, of course, if they do not exist, all that I
have told you in the last three or four lectures must be incorrect, because
I have been attempting to prove that they do exist, and I take it that
there is abundant evidence that they do exist; so far, therefore, the
hypothesis does not break down.

But in the next place comes a much more difficult inquiry:--Are the causes
indicated competent to give rise to the phenomena of organic nature? I
suspect that this is indubitable to a certain extent. It is demonstrable, I
think, as I have endeavoured to show you, that they are perfectly competent
to give rise to all the phenomena which are exhibited by RACES in nature.
Furthermore, I believe that they are quite competent to account for all
that we may call purely structural phenomena which are exhibited by SPECIES
in nature. On that point also I have already enlarged somewhat. Again, I
think that the causes assumed are competent to account for most of the
physiological characteristics of species, and I not only think that they
are competent to account for them, but I think that they account for many
things which otherwise remain wholly unaccountable and inexplicable, and I
may say incomprehensible. For a full exposition of the grounds on which
this conviction is based, I must refer you to Mr. Darwin's work; all that I
can do now is to illustrate what I have said by two or three cases taken
almost at random.

I drew your attention, on a previous evening, to the facts which are
embodied in our systems of Classification, which are the results of the
examination and comparison of the different members of the animal kingdom
one with another. I mentioned that the whole of the animal kingdom is
divisible into five sub-kingdoms; that each of these sub-kingdoms is again
divisible into provinces; that each province may be divided into classes,
and the classes into the successively smaller groups, orders, families,
genera, and species.

Now, in each of these groups the resemblance in structure among the members
of the group is closer in proportion as the group is smaller. Thus, a man
and a worm are members of the animal kingdom in virtue of certain
apparently slight though really fundamental resemblances which they
present. But a man and a fish are members of the same sub-kingdom
_Vertebrata_, because they are much more like one another than either
of them is to a worm, or a snail, or any member of the other sub-kingdoms.
For similar reasons men and horses are arranged as members of the same
Class, _Mammalia_; men and apes as members of the same Order,
_Primates_; and if there were any animals more like men than they were
like any of the apes, and yet different from men in important and constant
particulars of their organisation, we should rank them as members of the
same Family, or of the same Genus, but as of distinct Species.

That it is possible to arrange all the varied forms of animals into groups,
having this sort of singular subordination one to the other, is a very
remarkable circumstance; but, as Mr. Darwin remarks, this is a result which
is quite to be expected, if the principles which he lays down be correct.
Take the case of the races which are known to be produced by the operation
of atavism and variability, and the conditions of existence which check and
modify these tendencies. Take the case of the pigeons that I brought before
you: there it was shown that they might be all classed as belonging to some
one of five principal divisions, and that within these divisions other
subordinate groups might be formed. The members of these groups are related
to one another in just the same way as the genera of a family, and the
groups themselves as the families of an order, or the orders of a class;
while all have the same sort of structural relations with the wild
rock-pigeon, as the members of any great natural group have with a real or
imaginary typical form. Now, we know that all varieties of pigeons of every
kind have arisen by a process of selective breeding from a common stock,
the rock-pigeon; hence, you see, that if all species of animals have
proceeded from some common stock, the general character of their structural
relations, and of our systems of classification, which express those
relations, would be just what we find them to be. In other words, the
hypothetical cause is, so far, competent to produce effects similar to
those of the real cause.

Take, again, another set of very remarkable facts,--the existence of what
are called rudimentary organs, organs for which we can find no obvious use,
in the particular animal economy in which they are found, and yet which are
there.

Such are the splint-like bones in the leg of the horse, which I here show
you, and which correspond with bones which belong to certain toes and
fingers in the human hand and foot. In the horse you see they are quite
rudimentary, and bear neither toes nor fingers; so that the horse has only
one "finger" in his fore-foot and one "toe" in his hind-foot. But it is a
very curious thing that the animals closely allied to the horse show more
toes than he; as the rhinoceros, for instance: he has these extra toes well
formed, and anatomical facts show very clearly that he is very closely
related to the horse indeed. So we may say that animals, in an anatomical
sense nearly related to the horse, have those parts which are rudimentary
in him fully developed.

Again, the sheep and the cow have no cutting-teeth, but only a hard pad in
the upper jaw. That is the common characteristic of ruminants in general.
But the calf has in its upper jaw some rudiments of teeth which never are
developed, and never play the part of teeth at all. Well, if you go back in
time, you find some of the older, now extinct, allies of the ruminants have
well-developed teeth in their upper jaws; and at the present day the pig
(which is in structure closely connected with ruminants) has well-developed
teeth in its upper jaw; so that here is another instance of organs
well-developed and very useful, in one animal, represented by rudimentary
organs, for which we can discover no purpose whatsoever in another closely
allied animal. The whalebone whale, again, has horny "whalebone" plates in
its mouth, and no teeth; but the young foetal whale before it is born has
teeth in its jaws; they, however, are never used, and they never come to
anything. But other members of the group to which the whale belongs have
well-developed teeth in both jaws.

Upon any hypothesis of special creation, facts of this kind appear to me to
be entirely unaccountable and inexplicable, but they cease to be so if you
accept Mr. Darwin's hypothesis, and see reason for believing that the
whalebone whale and the whale with teeth in its mouth both sprang from a
whale that had teeth, and that the teeth of the foetal whale are merely
remnants--recollections, if we may so say--of the extinct whale. So in the
case of the horse and the rhinoceros: suppose that both have descended by
modification from some earlier form which had the normal number of toes,
and the persistence of the rudimentary bones which no longer support toes
in the horse becomes comprehensible.

In the language that we speak in England, and in the language of the
Greeks, there are identical verbal roots, or elements entering into the
composition of words. That fact remains unintelligible so long as we
suppose English and Greek to be independently created tongues; but when it
is shown that both languages are descended from one original, we give an
explanation of that resemblance. In the same way the existence of identical
structural roots, if I may so term them, entering into the composition of
widely different animals, is striking evidence in favour of the descent of
those animals from a common original.

To turn to another kind of illustration:--If you regard the whole series of
stratified rocks--that enormous thickness of sixty or seventy thousand feet
that I have mentioned before, constituting the only record we have of a
most prodigious lapse of time, that time being, in all probability, but a
fraction of that of which we have no record;--if you observe in these
successive strata of rocks successive groups of animals arising and dying
out, a constant succession, giving you the same kind of impression, as you
travel from one group of strata to another, as you would have in travelling
from one country to another;--when you find this constant succession of
forms, their traces obliterated except to the man of science--when you look
at this wonderful history, and ask what it means, it is only a paltering
with words if you are offered the reply--"They were so created."

But if, on the other hand, you look on all forms of organised beings as the
results of the gradual modification of a primitive type, the facts receive
a meaning, and you see that these older conditions are the necessary
predecessors of the present. Viewed in this light the facts of
palaeontology receive a meaning--upon any other hypothesis I am unable to
see, in the slightest degree, what knowledge or signification we are to
draw out of them. Again, note as bearing upon the same point, the singular
likeness which obtains between the successive Faunæ and Floræ, whose
remains are preserved on the rocks: you never find any great and enormous
difference between the immediately successive Faunæ and Floræ, unless you
have reason to believe there has also been a great lapse of time or a great
change of conditions. The animals, for instance, of the newest tertiary
rocks, in any part of the world, are always, and without exception, found
to be closely allied with those which now live in that part of the world.
For example, in Europe, Asia, and Africa, the large mammals are at present
rhinoceroses, hippopotamuses, elephants, lions, tigers, oxen, horses, &c.;
and if you examine the newest tertiary deposits, which contain the animals
and plants which immediately preceded those which now exist in the same
country, you do not find gigantic specimens of ant-eaters and kangaroos,
but you find rhinoceroses, elephants, lions, tigers, &c.,--of different
species to those now living--but still their close allies. If you turn to
South America, where, at the present day, we have great sloths and
armadilloes and creatures of that kind, what do you find in the newest
tertiaries? You find the great sloth-like creature, the _Megatherium_,
and the great armadillo, the _Glyptodon_, and so on. And if you go to
Australia you find the same law holds good, namely, that that condition of
organic nature which has preceded the one which now exists, presents
differences perhaps of species, and of genera, but that the great types of
organic structure are the same as those which now flourish.

What meaning has this fact upon any other hypothesis or supposition than
one of successive modification? But if the population of the world, in any
age, is the result of the gradual modification of the forms which peopled
it in the preceding age--if that has been the case, it is intelligible
enough; because we may expect that the creature that results from the
modification of an elephantine mammal shall be something like an elephant,
and the creature which is produced by the modification of an armadillo-like
mammal shall be like an armadillo. Upon that supposition, I say, the facts
are intelligible; upon any other, that I am aware of, they are not.

So far, the facts of palæontology are consistent with almost any form of
the doctrine of progressive modification; they would not be absolutely
inconsistent with the wild speculations of De Maillet, or with the less
objectionable hypothesis of Lamarck. But Mr. Darwin's views have one
peculiar merit; and that is, that they are perfectly consistent with an
array of facts which are utterly inconsistent with, and fatal to, any other
hypothesis of progressive modification which has yet been advanced. It is
one remarkable peculiarity of Mr. Darwin's hypothesis that it involves no
necessary progression or incessant modification, and that it is perfectly
consistent with the persistence for any length of time of a given primitive
stock, contemporaneously with its modifications. To return to the case of
the domestic breeds of pigeons, for example; you have the dove-cot pigeon,
which closely resembles the rock pigeon, from which they all started,
existing at the same time with the others. And if species are developed in
the same way in nature, a primitive stock and its modifications may,
occasionally, all find the conditions fitted for their existence; and
though they come into competition, to a certain extent, with one another,
the derivative species may not necessarily extirpate the primitive one, or
_vice versa_.

Now palæontology shows us many facts which are perfectly harmonious with
these observed effects of the process by which Mr. Darwin supposes species
to have originated, but which appear to me to be totally inconsistent with
any other hypothesis which has been proposed. There are some groups of
animals and plants, in the fossil world, which have been said to belong to
"persistent types," because they have persisted, with very little change
indeed, through a very great range of time, while everything about them has
changed largely. There are families of fishes whose type of construction
has persisted all the way from the carboniferous strata right up to the
cretaceous; and others which have lasted through almost the whole range of
the secondary rocks, and from the lias to the older tertiaries. It is
something stupendous this--to consider a genus lasting without essential
modifications through all this enormous lapse of time while almost
everything else was changed and modified.

Thus I have no doubt that Mr. Darwin's hypothesis will be found competent
to explain the majority of the phenomena exhibited by species in nature;
but in an earlier lecture I spoke cautiously with respect to its power of
explaining all the physiological peculiarities of species.

There is, in fact, one set of these peculiarities which the theory of
selective modification, as it stands at present, is not wholly competent to
explain, and that is the group of phenomena which I mentioned to you under
the name of Hybridism, and which I explained to consist in the sterility of
the offspring of certain species when crossed one with another. It matters
not one whit whether this sterility is universal, or whether it exists only
in a single case. Every hypothesis is bound to explain, or, at any rate,
not be inconsistent with, the whole of the facts which it professes to
account for; and if there is a single one of these facts which can be shown
to be inconsistent with (I do not merely mean inexplicable by, but contrary
to) the hypothesis, the hypothesis falls to the ground,--it is worth
nothing. One fact with which it is positively inconsistent is worth as
much, and as powerful in negativing the hypothesis, as five hundred. If I
am right in thus defining the obligations of an hypothesis, Mr. Darwin, in
order to place his views beyond the reach of all possible assault, ought to
be able to demonstrate the possibility of developing from a particular
stock by selective breeding, two forms, which should either be unable to
cross one with another, or whose cross-bred offspring should be infertile
with one another.

For, you see, if you have not done that you have not strictly fulfilled all
the conditions of the problem; you have not shown that you can produce, by
the cause assumed, all the phenomena which you have in nature. Here are the
phenomena of Hybridism staring you in the face, and you cannot say, "I can,
by selective modification, produce these same results." Now, it is admitted
on all hands that, at present, so far as experiments have gone, it has not
been found possible to produce this complete physiological divergence by
selective breeding. I stated this very clearly before, and I now refer to
the point, because, if it could be proved, not only that this _has_
not been done, but that it _cannot_ be done; if it could be
demonstrated that it is impossible to breed selectively, from any stock, a
form which shall not breed with another, produced from the same stock; and
if we were shown that this must be the necessary and inevitable results of
all experiments, I hold that Mr. Darwin's hypothesis would be utterly
shattered.

But has this been done? or what is really the state of the case? It is
simply that, so far as we have gone yet with our breeding, we have not
produced from a common stock two breeds which are not more or less fertile
with one another.

I do not know that there is a single fact which would justify any one in
saying that any degree of sterility has been observed between breeds
absolutely known to have been produced by selective breeding from a common
stock. On the other hand, I do not know that there is a single fact which
can justify any one in asserting that such sterility cannot be produced by
proper experimentation. For my own part, I see every reason to believe that
it may, and will be so produced. For, as Mr. Darwin has very properly
urged, when we consider the phenomena of sterility, we find they are most
capricious; we do not know what it is that the sterility depends on. There
are some animals which will not breed in captivity; whether it arises from
the simple fact of their being shut up and deprived of their liberty, or
not, we do not know, but they certainly will not breed. What an astounding
thing this is, to find one of the most important of all functions
annihilated by mere imprisonment!

So, again, there are cases known of animals which have been thought by
naturalists to be undoubted species, which have yielded perfectly fertile
hybrids; while there are other species which present what everybody
believes to be varieties [Footnote: And as I conceive with very good
reason; but if any objector urges that we cannot prove that they have been
produced by artificial or natural selection, the objection must be
admitted--ultra-sceptical as it is. But in science, scepticism is a duty.]
which are more or less infertile with one another. There are other cases
which are truly extraordinary; there is one, for example, which has been
carefully examined,--of two kinds of sea-weed, of which the male element of
the one, which we may call A, fertilises the female element of the other,
B; while the male element of B will not fertilise the female element of A;
so that, while the former experiment seems to show us that they are
_varieties_, the latter leads to the conviction that they are
_species_.

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