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 organization； 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 remnantsrecollections； if we may so sayof 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； the
Sanscrit； 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 rocksthat 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 organized 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 meaningupon 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
Faunae and Florae； whose remains are preserved on the rocks： you never
find any great and enormous difference between the immediately
successive Faunae and Florae； 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； hor