Geological Evidence of Origin of Species. 211 



tions in which they are found, then we have an argument 

 for evolution which it will not be easy to meet. The dis- 

 covery of a species C intermediate between A and E would 

 be au instance of what I have already illustrated ; but if we 

 find other species, B and D intermediate respectively be 

 tween A and C and C and E, and if A B C D E occur in 

 regularly succeeding epochs, the case is altered. Prof. 

 Huxley, in his presidential address to the London Geolo- 

 gical Society in 1870, gives us the following example of ap- 

 parent descent. In the middle and upper eocene strata, 

 we have the genus Plagiolophus, which is connected with the 

 living horse by several intermediate forms. The gradation 

 is seen in many parts of the skeleton, but it will simplify 

 matters if I confine myself-to two particulars. Take, first, 

 the foreleg of the animal. In Plagiolophus the radius and 

 ulna are quite distinct from each other, but are firmly united. 

 In Anchitherium of the upper eocene and lower miocene, the 

 radius and ulna are still distinct, but the ulnais more slender 

 than in the older form. In Hipparion, of the middle and up- 

 per Miocene, the ulna is still more reduced in size, and is 

 anchylosed to the radius along its entire length. Lastly, in 

 the horse, which first occurs in the upper Miocene, the mid- 

 dle of the shaft of the ulna vanishes, and the two ends are 

 welded into one piece with the radius. Thus we have a 

 gradual transition from the Plagiolophus with distinct leg- 

 bones to the horse with only one bone. Again, Plagiolophus 

 has a foot with th ree toes. In Anchitherium the two side toes 

 are more slender, but still touch the ground. In Hipparion 

 they are still more attenuated, and do not touch the ground. 

 In the horse, they are reduced to the rudimentary form of 

 splints. Here we have a gradual transition from a three- 

 toed animal to one with a single toe. I might ask — on 

 what hypothesis save that of evolution can we account for 

 the useless side toes of the Hipparion and horse ? But the 

 subject of rudimentary organs, though of great importance 

 in our discussion, belongs more strictly to the comparative 



