364 FOUNDATIONS OF BIOLOGY 



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. " (Thomson.) 



4. Embryology 



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

 of the genetic relationships of organisms in their individual 

 development from egg to adult, that is in ontogeny. Under 

 former headings we have incidentally mentioned embryo- 

 logical 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 (ONTOGENY) 

 frequently corresponds in broad outlines to the history of the 

 race (PHYLOGENY) as indicated by evidence from comparative 

 anatomy, etc. If we have in mind the earlier discussion of 

 Vertebrate anatomy, one or two examples will suffice to 

 suggest the type of evidence which supports this so-called 



RECAPITULATION THEORY, Or BIOGENETIC LAW. 



Lower Vertebrates, such as the Fishes, have a heart com- 

 posed 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 next higher group, the Amphibia (Frogs, 

 Toads, etc.), the auricle is divided into two parts, while the 

 ventricle remains as before. Thus these forms have a three- 

 chambered heart. Passing to the Reptiles, we find that 



