SO THE DOCTRINE OF DESCENT. 



to the present day. The formations mostly have their 

 characteristic organisms, but almost everywhere the con- 

 necting links have been exhibited. Everything conduces 

 to show that it is a question of evolution, not revolution. 

 Wherever there seems to be a sudden break, the case is 

 the same as in the revolutions of human history, in which 

 likewise only reforms long-prepared, and practically 

 necessary, come to a rapid issue. 



If we sum up the result of the comparison of fossil with 

 living animal life, we are first of all struck by the accord- 

 ance between the grades succeeding one another in the 

 order of time, and the members now 7 ranged side by side 

 in the system. Secondly, when this is confirmed, the 

 parallelism between the geological succession of animals 

 and the grades of the individual development of.present 

 animals follows as a matter of course. Agassiz, in his 

 great work on fossil fishes, pointed out this fact with 

 irresistible force, and confirmed it by fresh, valuable, and 

 convincing observations in his later works, down to his 

 investigations as to the development and growth of 

 corals. The same examples which served in the preced- 

 ing chapter to illustrate the parallelism of individual de- 

 velopment with the systematic stages, may be repeated 

 here ; though many newer and very striking instances 

 have been brought to light by the special researches 

 of the last ten years. To express this relation, Agassiz 

 introduced the term " embryonic types,'' or " embryonic 

 representatives." Thus the stalked stone lilies are the 

 embryonic types of the present genus Comatula ; the 

 most ancient Echinidse are the embryonic representatives 

 of the higher families of the Clypeastrae and Spatangidae ; 

 the Mastodon, on account of its persistent molar teeth, 



