370 GEOLOGICAL BIOLOGY. 



(6th) The mode of the evolution consists in the acquire- 

 ment of new characters by variation, and in the acceleration 

 or the retardation of the development of characters already- 

 acquired. 



(7th) The cause of the evolution is of a twofold nature — 

 extrinsic and intri^isic. 



In the first case, extrinsic evolution, the direction and 

 specific character of the modifications appear to be determined 

 by the conditions of environment — using that term in its broad- 

 est sense for all the outward conditions of life in which the 

 individual organism finds itself after birth. Adjustment of 

 the organism to the environment, struggle for existence, and 

 natural selection are the terms under which extrinsic evolu- 

 tion is commonly defined. 



The intrinsic cause of evolution acts previous to the indi- 

 vidual birth, and it seems to be at the foundation of varia- 

 bility. The mode and manner of expression of this kind of 

 evolution are more difficult to define than in the case of ex- 

 trinsic evolution ; but the facts of Paleontology clearly indicate 

 that such a cause exists, prior to the morphological appear- 

 ance of each individual and species. 



(8th) In this discussion classification is recognized as an 

 orderly and epitomized formulation of the facts already known 

 regarding the extent and kind of differentiation actually at- 

 tained in the evolution of the characters of organisms. The 

 statistics of classification are therefore available for expressing, 

 numerically, the relations existing between organic characters 

 and time and place; and it is observed that the numerical re- 

 lations of the different kinds of organisms to the time and the 

 place of their appearance point with overwhelming force to 

 the conclusion, that acquirement of morphological difference 

 is co-ordinate with both the passage of geological time and the 

 divergence of the conditions of external environment in which 

 the organisms have lived. 



