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forms, if Mr. Darwin^s theory be correct; because in many 
cases we should have the actual common progenitor and the 
resulting species. If, for example, we suppose that the two living 
species the Horse and the Ass are descended by evolution from 
a common progenitor, it may be, and is doubtless, true that we 
should find no links directly uniting the one with the other. 
But, looking into “the dark backward and abysm of time,” we 
may perchance find this common progenitor, and then the 
element “ unknown” is eliminated, and we may reasonably ask 
for tbe directly intermediate forms which unite each species 
with the now known progenitor. In the present instance, 
most evolutionists would admit Hipparion to be the required 
common progenitor. No directly intermediate links, however, 
have yet been discovered between Hipparion and Equus. Or, 
if, in order to evade this difficulty, it were supposed that Equus 
and Hipparion constituted two distinct and diverging lines of 
descent from a still older common progenitor, such as Anchi- 
therium, it would still remain to find directly intermediate forms 
between each of these and the latter ; and no such transitional 
links have as yet been discovered. The general view, no doubt, 
is to regard Anchitherium as being the at present oldest known 
common progenitor of the Horse and Ass, and to consider that 
Hipparion is the required directly intermediate form, or rather 
one of such forms. This view, however, disregards the fact 
that the requirements of the case necessitate the bringing for- 
ward of directly intermediate forms between two existing 
species and the nearest common progenitor that can be found. 
If Equus has been developed from Anchitherium, and Hipparion 
has constituted an intermediate stage between the two, then Hip- 
parion is the nearest common progenitor at present known of the 
existing species of Equus, and we have the right to expect the 
production of forms directly intermediate between them. Simi- 
larly, we should expect to find forms directly intermediate 
between Hipparion and Anchitherium. In neither case, how- 
ever, are any such intermediate links at present known.* It 
* The new and remarkable forms of Equidas discovered by Leidy and 
Marsh in the Tertiary formations of North America, do not supply the 
desired links between Hipparion and Equus, or between Hipparion and 
Anchitherium. Thus Orohippus, though closely related to Anchitherium, 
has four digits in the manus and no antorbital fossa. Miohippus may be 
regarded as linking Orohippus to Anchitherium, since it has only three 
digits to the manus, but it also has no antorbital fossa; whilst Piiohippus, 
though resembling Equus in its digits, differs in the important characters of 
possessing a large antorbital fossa and an additional upper prsemolar. 
Hence all these forms, though perhaps indicating the occurrence of some 
kind of evolution, arc so distinct and isolated in their characters that 
