Feb. 24, 1870] 
NATURE 
439 
animals. For, in order that such evidence should be quite satis- 
factory, it is necessary that we should be acquainted with all the 
most important features of the organization of the animals which 
are supposed to be thus related ; and not merely with the frag- 
ments upon which the genera and species of the palzontologist 
are so often based. M. Gaudry has arranged the species of 
Hyenide, Proboscidea, Rhinocerotide, and LEquide in their 
order of filiation from their earliest appearance in the 
Miocene epoch to the present time, and Professor Riilimeyer 
has drawn up similar schemes for the Oxen—with what I am dis- 
posed to think is a fair and probable approximation to the order 
of Nature. Butas no one is better aware than these two learned, 
acute, and philosophical biologists, all such arrangements must 
be regarded as provisional, except in those cases in which, by a 
fortunate accident, large series of remains are obtainable from a 
thick and wide-spread series of deposits. It is easy to accumu- 
late probabilities—hard to make out some particular case in 
such a way that it will stand rigorous criticism. 
After much search, however, I think that such a case is to be 
made out in favour of the pedigree of the Horses. 
The genus Zyuus is represented as far back as the latter part 
of the Miocene epoch; but, in deposits belonging to the middle 
of that epoch, its place is taken by two other genera, Hipparion 
and Aipparitherium (or Anchitherium); and, in the lowest Miocene 
and upper Eocene only the last genus occurs. <A species of 
Hipparitherium was referred by Cuvier to the Paleotheria 
under the name of P. Aurelianense. The grinding teeth are in 
fact very similar in shape and in pattern, and in the absence of 
cement, to those of some species of Palotherium, especially 
Cuvier’s, Palawotherium minus, which has been formed into a 
separate genus, P/agiolophus, by Pomel. : 
But in the fact that there are six full-sized grinders, the first 
premolar being very small; that the anterior grinders are as 
large as, or rather larger than, the posterior ones; that the 
second premolar has an anterior prolongation ; and that the 
posterior molar of the lower jaw has, as Cuvier pointed out, a 
posterior lobe of much smaller size and different form, the 
dentition of Hifparitherium departs from the type of the 
Paleotherium, and approaches that of the horse. : 
Again, the skeleton of ipparitherium is extremely equine. M. 
Christol, who founded the genus, goes so far as to say that the 
descriptions of the bones of the horse, or the ass, current in 
veterinary works, would fit those of Aipparitherium. And, ina 
general way, this may be true enough, but there are some most 
important differences, which, indeed, are justly indicated by the 
same careful observer. Thus the ulna is complete throughout, 
and its shaft is not a mere rudiment, fused into one bone with the 
radius. There are three toes, one large in the middle, and one 
small on each side. The femur is quite like that of a horse, and 
has the characteristic fossa above the external condyle. In the 
British Museum, there is a most instructive specimen of the leg 
bones, showing that the fibula was represented by the external 
malleolus and by a flat tongue of bone, which extends up from 
it on the outer side of the tibia, and is closely ankylosed with the 
latter bone. The hind toes are three, like those of the fore 
leg ; and the middle metatarsal bone is much less compressed 
from side to side than in the horse. 
In the iffarion the teeth nearly resemble those of the Horses, 
though the crowns of the grinders are not so long ; like those of 
the Horses they are abundantly coated with cement. The shaft 
of the ulna is reduced to a mere style ankylosed throughout 
nearly its whole length with the radius, and appearing to be 
little more than a ridge on the surface of the latter bone until it 
is carefully examined. The front toes are still three, but the 
outer ones are more slender than in Avfparitherium, and their 
hoofs smaller in proportion to that of the middle toe. In the 
leg, the distal end of the fibula is so completely united with the 
tibia that it appears to be a mere process of the latter bone, as 
in the Horses. 
Inthe Horses, finally, the crowns of the grinding teeth become 
longer, and their patterns are slightly modified ; the middle of 
the shaft of the ulna vanishes, and its proximal and distal ends 
ankylose with the radius. The phalanges of the two outer toes 
in each foot disappear, their metacarpal and metatarsal bones 
being left as the ‘* splints.” 
The Hifparion has large depressions on the face in front of the 
orbits, like those for the ‘‘larmiers” of manysruminants ; but 
traces of these are to be seen in some of the fossil horses from 
the Sewalik Hills. 
When we consider these facts, and the further circumstance 
that the Hipparions, the remains of which have been collected 
in immense numbers, were subject, as M. Gaudry and others 
have pointed out, to a great range of variation, it appears to me 
impossible to resist the conclusion that the types of the Appart- 
therium, ofthe Hipparion, and of the ancient Horses constitute the 
lineage of the modern Horses, the Hipparion being the intermediate 
stage between the other two, and answering to B in my former 
illustration. 
The nature of the process by which the Aipparitherium has been 
converted into the horse is one of specialisation or of more and 
more complete deviation from what might be called the average 
form of an ungulate mammal. In the Horses, the reduction of 
some parts of the limbs, together with the special modification 
of those which are left, is carried to a greater extent than in 
any other hoofedmammals. The reduction is less, and the 
specialisation is lessin the igfarion, and still less in the Hippari- 
therium ; but yet as compared with other mammals, the reduction 
and specialisation of parts in the Aipparitheriwm remains great. 
Is it not probable, then, that, just as in the Miocene epoch 
we find an ancestral equine form less modified than the horse, so, 
if we go back to the Eocene epoch we shall find some quadruped 
related to the Aipparitherium, as Hipparion is related to Lguus, 
and consequently departing less from the average form ? 
I think that this desideratum is very nearly, if not quite, sup- 
plied by Plagiolophus, remains of which occur abundantly in 
some parts of the upper and middle Eocene formations. The 
patterns of the grinding teeth of Plagiolophus are similar to 
those of Hipparitherium, and they are similarly deficient in 
cement; but the grinders diminish in size forwards, and the 
last lower molar has a large hind lobe, convex outwards and con- 
cave inwards, as in Paleotherium. The ulna is complete and 
much larger than in any of the Zguéde, while it is more slender 
than in most of the Palwotheria. It is fixedly united, but not 
ankylosed with the radius. There are three toes in the fore- 
limb, the outer ones being slender, but less attenuated than in 
the guide. The femur is more like that of the Falwotheria 
than that of the horse, and has only a small depression above 
its outer condyle in the place of the great fossa which is so 
obvious in the Zguide. The fibula is distinct, but very slender, 
and its distal end is ankylosed with the tibia. There are three 
toes on the hind-foot haying similar proportions to those on the 
fore-foot. The principal metacarpal and metatarsal bones are 
flatter than they are in any of the Zgu:de. 
In its general form, Plagiolophus resembles a very small and 
slender horse, and totally unlike the reluctant, pig-like creature 
depicted in Cuvier’s restoration of his Palwotherium minus in the 
Ossemeus Fossiles. 
It would be hazardous to say that Plagiolophus is the exact 
radical form of the Equine quadrupeds ; but 1 do not think there 
can be any reasonable doubt the latter animals have resulted 
from the modification of some quadruped similar to Plagiolophus. 
We have thus arrived at the Middle Eocene formation, and 
yet have traced back the Horses only to a three-toed stock. But 
these three-toed forms, no less than the Equine quadrupeds them- 
selves, present rudiments of the two other toes which appertain 
to what [have termed the ‘‘average” quadruped. If the expecta- 
tion raised by the splints of the horse that, insome ancestor of the 
horse, these splints would be found to be complete digits, has 
been verified, we are furnished with very strong reasons for 
looking for a no less complete verification of the expectation that 
the three-toed Plagiolophus-like ‘‘avus” of the horse must have 
had a five-toed ‘‘atayus”’ at some earlier period, 
No such five-toed ‘‘atavus,” however, has yet made its appear- 
ance among the few middle and older Eocene Mammalia which 
are known. 
Another series of closely-affiliated forms, though the evidence 
they afford is perhaps less complete than that of the Equine 
series, is presented to us by the Dichobune of the Eocene epoch, 
the Cainotherium of the Miocene, and the 7ragudide, or so-called 
“Muskdeer ” of the present day. ; 
The Zragudide have no incisors in the upper jaw, and only 
six grinding teeth on each side of each jaw, while the canine is 
moved up to the outer incisor, and there is a diastema in the 
lower jaw. There are four complete toes on the hind-foot, but 
the middle metatarsals usually become, sooner or later, ankylosed 
into acannon bone. ‘The navicular and the cuboid unite, and 
the distal end of the fibula is ankylosed with the tibia. 
In Cainotherium and Dichibune the upper incisors are fully 
developed. There are seven grinders ; the teeth form a con- 
tinuous series without diastema. The metatarsals, the nayi- 
