440 
NATURE 
[ Feb. 24, 1870 
cular and cuboid, and the distal end of the fibula, remain free. 
The Cainotherium, also the second metacarpal, is developed, but 
is much shorter than the third, while the fifth is absent or rudi- 
mentary. In this respect it resembles Anoflotherium secundarium. 
This circumstance, and the peculiar pattern of the upper molars 
in Cainotherium, lead me to hesitate in considering it as the 
actual ancestor of the modern 7yagulide. If Dichobune has a four- 
toed front foot (though I am inclined to suspect it resembles 
Cainotherium) it will be a better representative of the oldest form 
of the Traguline series. But Dichobune occurs in the middle 
Eocene, and is, in fact, the oldest known artiodactyle mammal. 
Where, then, must we look for its five-toed ancestor ? 
Tf we follow down otherlines of recent and tertiary Ungw/ata, 
the same question presents itself. The pigs are traceable back 
through the Miocene epoch to the upper Eocene, where they 
appear in the two well-marked forms of Hyofetamus and Chero- 
potamus. But Hyopotamus appears to have had only two toes. 
Again, all the great groups of the Ruminants, the Bowide, 
Antédopide, Camelcopardalide, and Cervide, are represented in 
the Miocene epoch, and so are the camels. The upper Eocene 
Anoplotherium, which is intercalary between the pigs and the 
Tragulide, has only two or, at most, three toes. Among the 
scanty mammals of the lower Eocene formation we have the peris- 
sodactyle Ungulata represented by Coryphodon, Hyracotherium, 
and Pliolophus. Suppose for a moment, for the sake of follow- 
ing out the argument, that Po/ophus represents the primary stock 
of the perissodactyles, and Dichobune that of the Artiodactyles 
(though I am far from saying that such is the case) then, we find in 
the earliest fauna of the Eocene epoch, to which our inyestiga- 
tions carry us, the two divisions of the Ugu/ata completely 
differentiated, and no trace of any common stock of both or five- 
toed predecessors to either. With the case of the horse before us, 
justifying a belief in the production of new animal forms by 
modification of old ones, I see no escape from the necessity 
of seeking for these ancestors of the (/gudata beyond the limits 
of the tertiary formations. 
I could as soon admit special creation, at once, as suppose that 
the perissodactyles and artiodactyles had no five-toed ancestors. 
And when we consider how large a portion of the tertiary period 
elapsed before Hifparitherium was converted into Zgzzs, it is 
difhcult to escape the conclusion that a large proportion of time 
anterior to the tertiary must have been expended in converting 
the common stockof the Ungz/afa into perissodactyles and 
artiodactyles. 
The same moral is inculcated by the study of every other order 
of tertiary monodelphius AZammalia, Each of these orders is 
represented in the Miocene epoch:—the Eocene formation, 
as I have already said, contains Cheiropiera, Jnsectivora, 
Rodentia, Ungulata, Carnivora, and Cetacea, But the Chet- 
roptera are extreme modifications of the Zzzsectivora; just as 
the Cetacea are extreme modifications of the Carnivorous type; 
and therefore it is to my mind incredible that monodelphous 
Tusectivera and Carnivora should not have been abundantly 
developed along with Uzgw/ata in the Mesozoic epoch. But, 
if this be the case, how much farther back must we go to 
find the common stock of the monodelphous A/ammalia? As 
to the Didelphia, if we may trust the evidence which seems 
to be afforded by their very scanty remains, that a Hypsi- 
prymnoid form existed at the epoch of the Trias, side by side 
with a carnivorous form. At the epoch of the Trias, there- 
fore, the AZarsupialia must have already existed long enough 
to have become differentiated into carniyorous and herbivorous 
forms. But the A/onotremata are lower forms than the Didelphia, 
which last are intercalary between the Ornithodelphia and the 
Monodedphia, To what point of the palzozoic epoch then 
must we, upon any rational estimate, relegate the origin of the 
Monotremata ? 
The investigation of the occurrence of the classes and of the 
orders of the Sawrofsida in time, points in exactly the same 
direction. If, as there is great reason to believe, true Birds 
existed in the Triassic epoch, the ornithoscelidous forms by 
which Reptiles passed into Birds must have preceded them. 
In fact, there is even, at present, considerable ground for sus- 
pecting the existence of Dézoesauria in the Permian formations. 
But in that case Lizards must be of still earlier date. And if 
the very small differences which are observable between the 
Crocodilia of the olde: mesozoic formations and those of the 
present day, furnish any sort of approximation towards an esti- 
mate of the average rate of change among the Sgurofsida ; it is 
almost appalling to reflect how far back in paleozoic times we 
must go, before we can hope to arrive at that common stock 
from which the Crocodilia, Lacertilia, Ornithoscelida, and Plesio- 
sania, which had attained so great a development in the Triassic 
epoch must have been derived. 
The Amphibia and Pisces tell the same story. There is not a 
single class of vertebrated animals, which, when it first appears, is 
represented by analogues of the lowest known members of the 
same class. Therefore, if there is any truth in the doctrine of 
evolution, every class must be vastly older than the first record 
of its appearance upon the surface of the globe. But if 
considerations of this kind compel us to place the origin 
of vertebrated animals at a period sufficiently distant from 
the upper Silurian, in which the first Elasmobranchs and Ganoids 
occur, to allow of the evolution of such fishes as these from a 
Vertebrate as simple as the Amp/hioxus ; I can only repeat that it 
is appalling to speculate upon the extent to which that origin 
must have preceded the epoch of the first recorded appearance of 
vertebrate life. 
Such is the further commentary which I have to offer upon 
the statement of the chief results of paleeontology, which I 
formerly ventured to lay before you. 
But the growth of knowledge in the interval makes me 
conscious of an omission of considerable moment in that state- 
ment, inasmuch as it contains no reference to the bearings of 
paleontology upon the theory of the distribution of life ; or takes 
note of the remarkable manner in which the facts of distribution, 
in present and past times, accord with the doctrine of evolution 
—especially in regard to land animals. 
That connection between palzontology and geology on the 
one hand, and the present distribution of terrestrial animals, 
which so strikingly impressed Mr, Darwin thirty years ago, as 
to lead him to speak of a ‘‘law of succession of types”; and of 
the wonderful relationship on the same continent between the 
dead and the living, has recently received much elucidation from 
the researches of Gaudry, of Riitimeyer, of Leidig, and of 
Alphonse Milne-Edwards, taken in connection with the earlier 
labours of our lamented colleague Falconer. And it has been 
instructively discussed in the thoughtful and ingenious work 
of Mr. Andrew Murray ‘‘On the geographical distribution of 
mammals.” 
I propose to lay before you, as briefly as I can, the ideas to which 
a long consideration of the subject has given rise in my own 
mind. 
If the doctrine of evolution is sound, one of its immediate 
consequences clearly is, that the present distribution of life upon 
the globe is the product of two factors: the one being the 
distribution which obtained in the immediately preceding epoch; 
and the other, the character and the extent of the changes which 
have taken place in physical geography between the one 
epoch and the other. Or, to put the matter in another 
way—the Fauna and Flora of any given area, in any given 
epoch, can consist only of such forms of life as are directly 
descended from those which constituted the Fauna and Flora 
of the same area, in the immediately preceding epoch ; unless the 
physical geography (under which I include climatal conditions) 
of the area has been so altered as to give rise to immigration 
of living forms from some other area. 
The eyolutionist therefore is bound to grapple with the 
following problem wheneyer it is clearly put before him :— 
Here are the Faunz of the same area during successive epochs. 
Show good cause for believing, either that these Faunze have been 
derived from one another by gradual modification, or that the 
Faune have reached the area in question by migration from 
some area in which they have mieone their development. 
I propose to attempt to deal with this problem so far as it 
is exemplified by the distribution of the terrestrial Vertebrata, 
and I shall endeavour to show you that it is capable of solution 
in a sense entirely favourable to the doctrine of evolution. 
Ihave, elsewhere, * stated, at length, the reasons which lead me 
to recognise four primary distributional provinces for the 
terrestrial Vertebrata in the present world ; namely, firstly, the 
Novozelanian, or New Zealand province: secondly, the Australian 
province, including Australia, Tasmania, and the Negrito Islands: 
thirdly, Azstro-Columbia, or South America plus North America 
as far as Mexico; and fourthly, the rest of the world or 
Arctogea, mm which proyince America, north of Mexico, con- 
stitutes one sub-province ; Africa, south of the Sahara, a second ; 
Tindostan a third ; and the remainder of the old world a fourth. 
* “On the classification and distribution of the Alectoromorpha." Pro- 
ceedings of the Zoological Society, 1868, 
