PALEONTOLOGY 



103 



we believe, had, like the jellyfishes, but a single opening into 

 the alimentary canal. The lower vertebrates repeat this con- 

 dition in the course of their 

 embryonic development. The 

 higher vertebrates no longer 

 use the blastopore even while 

 embryos, but they retain it as a 

 transient rudiment. Of facts 

 like these we have no satis- 

 factory explanation except the 

 theory of evolution, with its 

 corollary that the development 

 of the individual tends to be 

 a recapitulation of the race 

 history. 



The relation of the phenom- 

 ena of paleontology to the theory 

 of evolution. 



In the phenomena of com- 

 parative anatomy and compara- 

 tive embryology we see much 

 that is intelligible only with 



FIG. 24. Longitudinal sections of gas- 

 the aid Of the theory Of eVO- trulae of various animals. [After HAECKEL.] 



Intirm Tn the nhpnnmprm of A - Ofaworm.&^/^a. B. Of a star fish, 



1UUOn - Uraster. C. Of a crustacean. D. Of a 



T-o1^/f/-0/~rrw Anf^ VIOTT^ fV>^ o/- snai1 ' LymntBus. E. Of Amphioxus, a lowly 



paleontology we have the ac- relative of the vertebrates> A Digestive 

 tual record of this evolution in cavity - ' Blast P re - 

 the remains of the animals and plants which have lived in 

 the past. The record is very imperfect, to be sure, but so 

 far as it goes it is an actual record. Only very unusual 



