Palaeontology. 173 



are rather discontinuous series, of which all the members 

 change in a definite direction, and obviously form steps 

 in a line of development which culminates in the last- 

 extinct or still-existing- representatives." Zittel refers 

 to such instances as the succession from Hyracotherium, 

 or it may be from Phenacodus, through Palaplotherium, 

 Anchilopus, Anchitherium, and Hipparion, to the single- 

 toed horse. To this best- known instance might be 

 added that of the camels, the pigs, the crocodiles, the 

 amioid fishes, the ammonoids, &c. At the same time, 

 it must in fairness be noted that the palaeontologists 

 remain in darkness in regard to many of the most 

 momentous origins in the history of life. For what is 

 really known as to the ancestry of Mammals, Birds, 

 Reptiles, Amphibians, or Fishes, not to mention many 

 an Invertebrate stock? 



One of the most interesting and important of modern 

 palaeontological problems is whether there are chrono- 

 logical series of fossil embryonic types corresponding 

 to the different stages in the development of a modern 

 form. Is there palaeontological evidence of that gen- 

 eralization which appealed so strongly to Agassiz 

 though he was unable to see its evolutionary import 

 Haeckel's "Biogenetic Law". If this law be crudely and 

 carelessly interpreted as implying an exact correspond- 

 ence between individual and racial history, the answer 

 must be an emphatic negative. As we have seen, care- 

 ful embryological work points to the fact that the em- 

 bryo, say of a fowl or duck, pig or rabbit, exhibits 

 from a very early stage individual characters peculiar 

 to fowl or duck, pig or rabbit characters which date 

 from the respective origins of these species. There is 

 certainly no detailed or exact recapitulation, but this 

 does not exclude the possibility that there may be fossil 

 forms which bear a general resemblance to the youthful 

 stages of modern forms. 



"In spite of these drawbacks," Von Zittel says, "fossil em- 

 bryonic types are not entirely awanting, even among Inverte- 

 brates. The palaeozoic Belinuridae are bewilderingly like the 

 larvae of the living Limulus ; the Pentacrinoid larva of Antedon 

 is nearer many fossil crinoids than is, the full-grown animal; 



