按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
vice versa。 They thus constitute a grinding apparatus of
great efficiency； and one which is repaired as fast as it wears；
owing to the long…continued growth of the teeth。
Some other peculiarities of the dentition of the horse must be
noticed； as they bear upon what I shall have to say by and by。
Thus the crowns of the cutting teeth have a peculiar deep pit；
which gives rise to the well…known 〃mark〃 of the horse。 There is
a large space between the outer incisors and the front grinder。
In this space the adult male horse presents； near the incisors
on each side； above and below； a canine or 〃tush；〃 which is
commonly absent in mares。 In a young horse； moreover； there is
not unfrequently to be seen in front of the first grinder； a
very small tooth； which soon falls out。 If this small tooth be
counted as one； it will be found that there are seven teeth
behind the canine on each side； namely； the small tooth in
question； and the six great grinders； among which； by an unusual
peculiarity； the foremost tooth is rather larger than those
which follow it。
I have now enumerated those characteristic structures of the
horse which are of most importance for the purpose we have
in view。
To any one who is acquainted with the morphology of vertebrated
animals； they show that the horse deviates widely from the
general structure of mammals； and that the horse type is； in
many respects； an extreme modification of the general mammalian
plan。 The least modified mammals； in fact； have the radius and
ulna； the tibia and fibula； distinct and separate。 They have
five distinct and complete digits on each foot； and no one of
these digits is very much larger than the rest。 Moreover； in the
least modified mammals； the total number of the teeth is very
generally forty…four； while in horses； the usual number is
forty； and in the absence of the canines； it may be reduced to
thirty…six； the incisor teeth are devoid of the fold seen in
those of the horse： the grinders regularly diminish in size from
the middle of the series to its front end； while their crowns
are short； early attain their full length； and exhibit simple
ridges or tubercles； in place of the complex foldings of the
horse's grinders。
Hence the general principles of the hypothesis of evolution lead
to the conclusion that the horse must have been derived from
some quadruped which possessed five complete digits on each
foot； which had the bones of the fore…arm and of the leg
complete and separate； and which possessed forty…four teeth；
among which the crowns of the incisors and grinders had a simple
structure； while the latter gradually increased in size from
before backwards； at any rate in the anterior part of the
series； and had short crowns。
And if the horse has been thus evolved； and the remains of the
different stages of its evolution have been preserved； they
ought to present us with a series of forms in which the number
of the digits becomes reduced； the bones of the fore…arm and leg
gradually take on the equine condition； and the form and
arrangement of the teeth successively approximate to those which
obtain in existing horses。
Let us turn to the facts； and see how far they fulfil these
requirements of the doctrine of evolution。
In Europe abundant remains of horses are found in the Quaternary
and later Tertiary strata as far as the Pliocene formation。
But these horses； which are so common in the cave…deposits and
in the gravels of Europe； are in all essential respects like
existing horses。 And that is true of all the horses of the
latter part of the Pliocene epoch。 But； in deposits which belong
to the earlier Pliocene and later Miocene epochs； and which
occur in Britain； in France； in Germany； in Greece； in India； we
find animals which are extremely like horseswhich； in fact；
are so similar to horses； that you may follow descriptions given
in works upon the anatomy of the horse upon the skeletons of
these animalsbut which differ in some important particulars。
For example； the structure of their fore and hind limbs is
somewhat different。 The bones which； in the horse； are
represented by two splints； imperfect below； are as long as the
middle metacarpal and metatarsal bones； and； attached to the
extremity of each； is a digit with three joints of the same
general character as those of the middle digit； only very much
smaller。 These small digits are so disposed that they could have
had but very little functional importance； and they must have
been rather of the nature of the dew…claws； such as are to be
found in many ruminant animals。 The Hipparion； as the
extinct European three…toed horse is called； in fact； presents a
foot similar to that of the American Protohippus
（Fig。 9）； except that； in the Hipparion； the smaller
digits are situated farther back； and are of smaller
proportional size； than in the Protohippus。
The ulna is slightly more distinct than in the horse； and the
whole length of it； as a very slender shaft； intimately united
with the radius； is completely traceable。 The fibula appears to
be in the same condition as in the horse。 The teeth of the
Hipparion are essentially similar to those of the horse；
but the pattern of the grinders is in some respects a little
more complex； and there is a depression on the face of the skull
in front of the orbit； which is not seen in existing horses。
In the earlier Miocene； and perhaps the later Eocene deposits of
some parts of Europe； another extinct animal has been
discovered； which Cuvier； who first described some fragments of
it； considered to be a Palaeotherim。 But as further
discoveries threw new light upon its structure； it was
recognised as a distinct genus； under the name
of Anchitherium。
In its general characters； the skeleton of Anchitherium
is very similar to that of the horse。 In fact； Lartet and De
Blainville called it Pal？otherium equinum or
hippoides； and De Christol； in 1847； said that it
differed from Hipparion in little more than the
characters of its teeth； and gave it the name of
Hipparitherium。 Each foot possesses three complete toes；
while the lateral toes are much larger in proportion to the
middle toe than in Hipparion； and doubtless rested on the
ground in ordinary locomotion。
The ulna is complete and quite distinct from the radius； though
firmly united with the latter。 The fibula seems also to have
been complete。 Its lower end； though intimately united with that
of the tibia； is clearly marked off from the latter bone。
There are forty…four teeth。 The incisors have no strong pit。
The canines seem to have been well developed in both sexes。
The first of the seven grinders； which； as I have said； is
frequently absent； and； when it does exist； is small in the
horse； is a good…sized and permanent tooth； while the grinder
which follows it is but little larger than the hinder ones。
The crowns of the grinders are short； and though the fundamental
pattern of the horse…tooth is discernible； the front and back
ridges are less curved； the accessory pillars are wanting； and
the valleys； much shallower； are not filled up with cement。
Seven years ago； when I happened to be looking critically into
the bearing of palaentological facts upon the doctrine of
evolution； it appeared to me that the Anchitherium； the
Hipparion； and the modern horses； constitute a series in
which the modifications of structure coincide with the order of
chronological occurrence； in the manner in which they must
coincide； if the modern horses really are the result of the
gradual metamorphosis； in the course of the Tertiary epoch； of a
less specialised ancestral form。 And I found by correspondence
with the late eminent French anatomist and palaeontologist；
M。 Lartet； that he had arrived at the same conclusion from the
same data。
That the Anchitherium type had become metamorphosed into
the Hipparion type； and the latter into the Equine
type； in the course of that period of time which is represented
by the latter half of the Tertiary deposits； seemed to me to be
the only explanation of the facts for which there was even a
shadow of probability。
And； hence； I have ever since held that these facts afford
evidence of the occurrence of evolution； which； in the sense
already defined； may be termed demonstrative。
All who have occupied themselves with the structure of
Anchitherium； from Cuvier onwards； have acknowledged its
many points of likeness to a well…known genus of extinct Eocene
mammals； Palaeotherium。 Indeed； as we have seen； Cuvier
regarded his remains of Anchitherium as those of a
species of Palaeotherium。 Hence； in attempting to trace
the pedigree of the horse beyond the Miocene epoch and the
Anchitheroid for