26 THE THEORY OF EVOLUTION 



might almost be termed rhythmic order. These 

 peculiarities are the first appearance of new and higher 

 types which were previously absent (larger groups, 

 e.g. classes or families), vigorous development of some 

 of the already- existing types, and the decay and dis- 

 appearance of other and previously very varied ones. 



The Cambrian or pre-Cambrian formations show 

 in a general way organisms for the first time ; they 

 are sharply defined according to depth. None of 

 the classes of Invertebrates which existed were as 

 yet freely divided into genera and species, they show 

 no specialized adaptation to the various environ- 

 ments (i.e. but few families, genera, and species) ; the 

 Trilobites alone form an exception. 



The individuals are still small and simply con- 

 structed ; the Cephalopods, for instance, have simple 

 straight shells in contrast to the highly complicated, 

 curled, and ornamented ones of the Cephalopods of later 

 formations. 1 



All at once there appear, in a definite series of strata, 

 the first fishes, i.e. the first representatives of the 

 family of Vertebrates ; the first land plants also appear. 2 

 The Corals, Starfish, and Graptolites (related to the 

 Bryozoa, but long extinct), which were very rare in the 

 Cambrian formation, become more numerous. 



The Trilobites develop more abundantly, the more 



1 Good tabulated illustrations (one short and one in detail) of the 

 chief groups of animals and plants from these three points of view are 

 given in H. Credner's Elemente der Geologie (1902), pp. 363, 365. 



2 We remark once and for all that ' first appearance ' is intended only 

 to mean ' fresh found.' Perhaps the two are synonymous ; perhaps not. 



