THE ANIMALS OF THE PAST 



433 



small, and the hoof of the middle toe rounder. In spite of 

 the great difference between the one-toed foot of the living 

 horse and the dog's five-toed foot, there was once a kind of 

 horse which had a five-toed 

 foot, and there is after all a 

 close relationship between 

 the foot of the horse and 

 the foot of the dog. 



342. Parallelism of em- 

 bryonic stages with fossil 

 series, One of the most im- 

 portant truths of paleon- 

 tology is that the ancient 

 groups in any type agree 

 more or less closely in 

 structure with the embryos 

 or with the larval stages of 

 the living representatives of 

 the same group. Embryos 

 are generalized organisms 

 simple in structure as com- 

 pared with the adult ani- 

 mal. The earlier represen- 

 tatives of any class or type 



of animal are likewise simple and devoid of specializa- 

 tion. And there is a curious parallelism, which is not 

 accidental, in the resemblance in the successive stages of 

 animal life in a series of fossils to the successive stages in 

 the embryo of recent forms. The persistence of heredity 

 is undoubtedly the cause of this parallelism. By its influ- 

 ence ancestral traits are repeated in the embryo, even 

 though the characters thus produced give way in later 

 development to further specialization or growth along 

 other lines. This great truth has been stated in these 

 words : " The life-history of the individual is an epitome 

 of the life-history of the group to which it belongs." This 



FIG. 254. Feet in fossil pedigree of horse. 

 After MARSH, a, Equus, Quaternary 

 (recent) ; b, Pliohippus, Pliocene ; c, 

 Protohippus, Lower Pliocene ; d, Mo- 

 Mppus, Miocene; e, Mesohippus, Lower 

 Miocene ; /, Orohippus, Eocene. 



