EVOLUTION OF THE HORSE 



307 



almost the only link in the pedigree of birds which has so far 

 been discovered, and it teaches us nothing as to the origin of 

 those characteristic avian structures, the feathers, which it 

 possesses already in a fully developed condition. 



It will be observed that Archseopteryx occupies a position 

 between reptiles and typical birds exactly comparable with that 

 of the Monotremata between reptiles and typical mammals (see 

 Chapter XVII), the only difference being that the Monotremata 

 still survive side by side with mammals of the most highly 

 advanced type, while Archaeopteryx has long since become extinct. 



One of the most complete fossil pedigrees as yet known to us 



FIG. 152. Skeleton of Phenacodus, a five-toed Eocene Ungulate. (From 

 Lankester's "Extinct Animals.") 



is that of the Equidas or horse family. As we have already seen 

 in Chapter XVII, the study of comparative anatomy indicates very 

 clearly that the highly specialized single-toed limbs of the horse 

 (Fig. 97) must have arisen from some primitive pentadactyl 

 type by gradual suppression of all the digits except the middle 

 one. Amongst the fossil remains of horse-like animals which 

 abound in various tertiary formations of Europe and America we 

 find a very complete series of stages in the evolution of the 

 modern horse, which entirely confirms this conclusion. Our 

 knowledge of this extremely interesting phylogenetic series is due 

 largely to the late Professor 0. C. Marsh and has been admirably 

 summarized by Mr. B. S. Lull in the American Journal of Science. 1 



1 Series IV, Vol. XXIII, 1907. 



x 2 



