82 LIFE ON THE EARTH. 



the arrangement is purposely made to shew by the 

 blue tint those classes which suffer diminution with 

 time, and by the red tint those which from small 

 beginnings grow to great preponderance, while the 

 yellow tint is assigned to classes which scarcely ap- 

 pear in the early period, but swell out in the middle 

 of the scale so as to equal or overmatch either of 

 the other classes Thus appears in a striking light 

 the great difference between the systems of oceanic 

 life in earlier and later periods, the nature of this 

 difference, and something of the method of varia- 

 tion which binds the whole into one plan, and con- 

 nects the dawn of created life with this our breathing 

 world. 



In all the great periods the numerically prevalent 

 life among the Invertebrata of the Sea appears in the 

 Molluscous division 1 . 



In the three grand periods the order of preva- 

 lence is thus found. 



Caenozoic Period . . . Gasteropoda, Dimyaria, Monomyaria, 

 Echinodermata, Zoophyta, Crustacea, 

 Cephalopoda, Brachiopoda. 



Mesozoic Period ... Dimyaria, Cephalopoda, Gasteropoda, 

 Monomyaria, Echinodermata, Brachio- 

 poda, Zoophyta, Crustacea. 



1 Only in the very earliest zone, Crustacea appear to predomi- 

 nate over Brachiopoda. 



