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 



the aid Of the theory Of eVO- 



lution. In the phenomena of 



paleontology we have the ac- 



tual record of this evolution in 



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 



FIG. 24. Longitudinal sections of gas- 

 trulae of various animals. [After HAECKEL.] 



cavity - ' Blastopore - 



