86 LIFE ON THE EARTH. 



genera of the Palaeozoic systems belong to this divi- 

 sion; the same is true for the Mesozoic series; and 

 even in the Caenozoie strata Alcyonoid Zoophyta are 

 rare. Here, then, is one great order of Zoophyta 

 continuous through the whole series. The genera 

 change with the successive deposits, but there is one 

 remarkable law of structure which is characteristic 

 of period. The radiating plates of the Coral are in 

 a young state pretty regular in number and in the 

 mode of division. In all the Palaeozoic Strata the 

 primary or principal plates are four, or some multiple 

 of four; in all the Mesozoic and Caenozoic Strata 

 they are six, or some multiple of six. This striking 

 generalization, due to Milne Edwards and the late 

 Jules Haime, is thought to be subject to no more 

 than solitary exceptions. It suggests the reflec- 

 tion that the persistence of characters which we 

 observe in modern living nature was quite as re- 

 markable in ancient organizations, and throws a 

 heavy weight into the scale against the doctrine of 

 the later forms of life being derived from earlier 

 types, through natural variations integrated by time. 



Echinodermata. Six fossil groups represent this 

 beautiful class of animals, viz. Crinoidea, Blastoidea, 

 Cystoidea, Ophiuroidea, Asteroidea, Echinoidea. Of 

 these, two are only known fossil (Blastoidea and 

 Cystoidea), and they belong to the Palaeozoic Strata. 



