SPECIAL FEATUEES IN STRUCTURE 



493 



belonged — in fact, it would be impossible for him to avoid doing so. In 

 like manner other characteristic structures and organs are in themselves 

 indisputable evidence of their origin, and to the naturalist the realization 

 of the form of an animal upon such evidence is a mere involuntary and 

 quite spontaneous mental process scarcely attended with any effort. 



To understand the value of the evidence afforded by the teeth and 

 certain bones of the skull of the horse as connecting the existing animal 

 with its remote ancestors, it will be necessary to consider some of the 

 most salient features of those 

 structures, premising that 

 no more than a cursory 

 view can be taken out of 

 respect for the patience of 

 the reader. 



If we compare the skull 

 of man with that of the 

 horse it will at once be 

 evident that the difference 

 of form is very marked, 

 as shown in the two illus- 

 trations (figs. 660 and 

 661) from Sir W. Flower's 

 book. 



The letters of reference in 

 the two figures are the same 

 in both, and indicate the 

 same bones. The remai'kable 

 difference in form of the two 

 skulls is due to the variation 

 in size and shape of the 



separate pieces of bone of which the cranial and facial divisions of the 

 skull are composed. 



Most noticeable is the vast difference in size of the cranium of man 

 as comjjared with that of the horse. There is no difficulty in recognizing 

 the fact that the facial division of the horse's skull, the part which is 

 mainly used for the mastication of the food, is developed enormously 

 out of proportion to the cranial division in which the brain is lodged — 

 the centre of whatever degree of intelligence the animal may possess, 

 and the source of some of the most important nerves. In man the 

 conditions are exactly the opposite. The cranium is of immense capacity 

 compared with the insignificant proportions of the facial bones, yet it 



i" /""■' 



Fig. 661.— Side View of the Skull of the Horse, with 

 the bone removed so as to show the whole of the 

 teeth and nasal bone 



n. Nasal bone; o, orbit; :, zygomatic arch; t, temporal fossa; 

 oc, occipital condyle ; e m, external auditory opening of glenoid 

 fossa for articulation of the lower jaw ; t^ i" i^, three incisor teeth ; 

 c, canine; pnt', the situation of the first rudimentary premolar, 

 which has been lost in the lower, but which is present in the 

 upper jaw; pni^ piifl pm^, the three fully-developed premolar 

 teeth ; vi^ tii- vi^, the three true molar teeth. 



VOL. III. 



97 



