( 476 ) 



CHAPTER XXV. 



THE SUCCESSION OF LIFE IN CLASSES AND ORDERS OF ANIMALS. 



The Constituents of Faunas. When we turn from the theoretical 

 or inductive aspect of paleontology to its stratigraphical facts, the 

 duration in time of species and genera is found to be different in the 

 several groups of organisms. This may be a defect in our classification 

 by which characters of different chronological value have to be made 

 use of in distinguishing animal types in the several orders and classes. 

 But it may also be a consequence of the struggle for existence by 

 which the varying collocation of representatives of different groups of 

 animals favours the extinction of some, the unchanged survival of 

 others, and the varied development of special types of life in definite 

 periods of time. Absence of enemies, such as indicates to us undis- 

 turbed nutrition, is calculated to result in large physical growth and 

 abundant increase of individuals. And then the incoming of multi- 

 tudinous enemies upon races which have not been inured to the con- 

 flict is likely to exterminate such groups. Professor Owen has specu- 

 lated on the way in which the advent of a higher type of life 

 modifies the physical structure of a lower type, and is disposed to 

 attribute the changes which converted the ancient Teleosaurian type of 

 crocodile into the modern type of crocodile, as exemplified in larger 

 forelimbs and other distinctive characters of the skeleton, to the ad- 

 vent of mammalia, which furnished a new kind of prey. 1 When the 

 higher and lower types existed under similar conditions and as com- 

 petitors, the advent of the higher type, however it might stimulate 

 specialisation in the skeleton, would probably tend to arrest organic 

 development in the less specialised groups of lower organic grade. We 

 are disposed to look to this principle for an interpretation of the 

 circumstance that fishes and amphibians of the Primary rocks, and the 

 reptiles of the Secondary rocks, when compared with their nearest 

 living allies, approximate to higher types, and present a more varied 

 development than the surviving groups of the same class. Similarly, 

 the Tetrabranchiata have given place to the Dibranchiata ; and the 

 Entomostraca have not the dominance in later times which they had 

 in the Primary period. We are therefore disposed to urge that time 

 will not be misspent which is used in analysing a fauna into its 

 elements, in tracing the history of the representatives of the several 



1 Q. J. G. S., vol. xxxiv. p. 426. 



