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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 itfor I have put it
into a shape more convenient for common purposes than I could find
'verbatim' in his bookas 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 surroundedthat 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 matteratavism
and variabilityand 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 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 w