364 ANIMAL BIOLOGY 



to the genus Equus which has continued from the Pleistocene 

 epoch to the present. This genus includes the modern Horse, 

 Equus caballus, with one functional digit on each foot and vestiges 

 of two more (digits 2 and 4) as the splint bones. (Fig. 234.) 



In this outline of what must be interpreted as the fossil ancestors 

 of the Horse of to-day, we have merely selected several representa- 

 tive forms to emphasize changes in foot structure. But the reader 

 will realize that many other equally significant changes were in- 

 volved in the transformation — during perhaps ten million gen- 

 erations — of an Eohippus type into that of Equus. This much 

 appears certain to the biologist: 'In early Eocene times there 

 lived small five-toed hoofed quadrupeds of generalized type, that 

 the descendants of these were gradually specialized throughout 

 long ages along similar but by and by divergent lines, that they 

 lost toe after toe till only the third remained, that they became 

 taller and swifter, that they gained longer necks, more complex 

 teeth, and larger brains. So from the short-legged splay-footed 

 plodders of the Eocene marshes there were evolved light-footed 

 horses running on tiptoe on the dry plains." 



Truly, the stupendous and ever increasing record of ancient 

 forms of life is not that of a disordered multitude. Newly dis- 

 covered fossil remains, one after another, fall into the scheme 

 of a common tree of descent — descent with change. 



4. Embryology 



If evolution is a fact, one would expect to find evidences of the 

 genetic relationships of organisms in their embryological develop- 

 ment from egg to adult. Under former headings we have inciden- 

 tally mentioned embryological data which point toward evolution, 

 so that now attention may be confined to an attempt to make clear 

 a fact of first importance — the history of the individual frequently 

 corresponds in broad outlines to the history of the race as indicated 

 by evidence from comparative anatomy, etc. If we have in mind 

 the earlier discussion of Vertebrate anatomy, just a few examples 

 will suffice to suggest the type of evidence which supports this 

 so-called recapitulation theory, or biogenetic law. (Page 96.) 



Lower Vertebrates, such as the Fishes, have a heart composed of 

 two chief chambers: an auricle which receives blood from the 

 body as a whole and a ventricle which pumps it to the gills on its 

 way to supply all parts of the body. Among the members of the 



