ON SPECIES AND RACES, AND THEIR ORIGIN 389 
grouped. It is intimately allied in structure with three other members 
of the existing creation, the Hyrax, the Tapir, and the Rhinoceros ; 
and less strait, though still definite bonds of union connect it with 
every living thing. Going back in time, the Horse can be traced into 
the Pliocene formation, and perhaps it existed earlier still ; but in the 
newer Miocene of Germany it is replaced by the AHippothertum, an 
animal very like a true Eguvs, but having the two rudimental toes in 
each foot developed, though small. Further back in time, in the 
Eocene rocks, neither Eguus nor Hippotherium have been met with, 
nor Rhinoceros, Tapirus, or Hyrax, but instead of them, a singular 
animal, the Palgotherium, which exhibits certain points of resemblance 
with each of the four existing genera, is found. The speaker pointed 
out that these resemblances did not justify us in considering the 
Paleotherium as a more generalized type, any more than the resem- 
blance of a father to his four sons justifies us in considering him as of 
a more generalized type than theirs. 
The geographical distribution of the Eguzd@ was next considered ; 
and the anomalies and difficulties it offers were pointed out; and 
lastly the variations which horses offer in their feral and their domes- 
ticated condition, were discussed. 
The questions thus shown to be connected with the species Horse, 
are offered by all species whatever; and the next point of the discourse 
was the consideration of the general character of the problem of the 
origin of species of which they form a part, and the necessary con- 
ditions of its solution. 
So far as the logic of the matter goes, it was proved that this 
problem is of exactly the same character as multitudes of other 
physical problems, such as the origin of glaciers, or the origin of strata 
of marble: and a complete solution of it involves—1. The experi- 
mental determination of the conditions under which bodies having 
the characters of species are producible; 2. The proof that such 
conditions are actually operative in nature. 
Any doctrine of the origin of species which satisfies these require- 
ments must be regarded as a true theory of species ; while any which 
does not, is, so far, defective, and must be regarded only as a hypo- 
thesis whose value is greater or less, according to its approximation 
to this standard. 
It is Mr. Darwin’s peculiar merit to have apprehended these logical 
necessities, and to have. endeavoured to comply with them. The 
Pigeons called Pouters, Tumblers, Fantails, &c., which the audience 
had an opportunity of examining, are, in his view, the result of so 
many long-continued experiments on the manufacture of species ; and 
