196 Professor T, H. Huxley, [Feb. 10, 



larynx, its short ears, and broad hoofs : under the latter head, the fact, 

 that the offspring of the horse with any of the allied species is a 

 liybrid, incapable of propagation with another mule, was particularly 

 mentioned. 



Leaving open the question whether the physiological distinction 

 just mentioned is, or is not, a universal character of species, it is in- 

 dubitable that it obtains between many species, and therefore has to be 

 accounted for by any theory of their origin. 



The species Equus cahallus, thus separated from all others, is the 

 centre round which a number of other remarkable phenomena are 

 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 Hippotherium, an 

 animal very like a true Equus, but having the two rudimental toes in 

 each foot developed, though small. Further back in time, in the 

 Eocene rocks, neither Equus nor Hippotherium have been met with, 

 nor Rhinoceros, Tapirus, or Hyrax ; but instead of them, a singular 

 animal, the Palceotherium, 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 

 Palceotherium 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 Equidce 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 domesticated 

 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 pro- 

 blem is of exactly the same character as multitudes of other physical 

 problems, such as the origin of glaciers, or the origin of strata of mar- 

 ble ; and a complete solution of it involves — 1. The experimental 

 determination of the conditions under which bodies having the 

 characters of species are producible ; 2. The proof that such condi- 

 tions 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 hypothesis 

 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 



