EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 11 



marked (pp. 23 and 43) that the second and fourth digits of the mod- 

 ern horse have disappeared except for their metacarpals, which are pres- 

 ent as the rudimentary sphnt bones. It will be recalled that the metacarpal 

 of the third digit forms the large cannon bone, to which the phalanges of 

 that digit articulate. Occasionally a horse is born with one of the splint 

 bones also bearing phalanges. These phalanges are small but sometimes 

 the terminal one bears a greatly re- 

 duced hoof (Fig. 4.19). The author's 

 introduction to this phenomenon oc- 

 curred when as a boy he was enticed 

 by a lurid poster into a side show at a 

 county fair to see an "eight-footed 

 horse." Once inside he beheld a 

 healthy horse quietly eating hay. The 

 horse had the usual four hooves, but 

 beside each one there dangled a tiny 

 extra hoof, just as illustrated in the 

 figure. The similarity of this abnormal 

 structure to the structure normal for 

 the foot of such a prehistoric horse as 

 Merychippus (Fig. 10.5, p. 202) is 

 evident. 



-IV 



\-iii 



k> 



Conclusion 



FIG. 4.19. Forefoot (a) and hindfoot 

 (b) of a multitoed modern horse. (From 

 Lull, "The evolution of the horse family, 

 as illustrated in the Yale collections," 

 American Journal of Science, Vol. 23, 

 1907, p. 166.) 



In this chapter similarities of embry- 

 onic development have been stressed. 

 We have noted that all animals above 

 single-celled protozoa are similar in 

 the early stages of development, and 

 that in general there is direct relation- 

 ship between similarity of adult structure and the proportion of embryonic 

 development which is similar in different animals. Thus, dissimilar animals 

 are found to follow like paths of development for a time and then to di- 

 verge, each going its own way. The more dissimilar the animals the shorter 

 the period of embryonic development which they share in common. Why 

 do dissimilar animals share any similarity of embryonic development at 

 all? The most reasonable explanation seems to be inheritance from com- 

 mon ancestry. In accordance with this view we may picture two species 

 descended from a common ancestor. That ancestor had a certain pattern 



