THE LAWS OF EVOLUTION EMPHASIZED. 369 



the results attained by evolution in specific, generic, and the 

 higher orders of differentiation. 



A Statement of the Laws of Evolution Emphasized by Fossils. — 

 The analysis of the facts regarding the order of succession and 

 modification of organisms derived from this critical study of 

 fossils suggests the following to have been some of the chief 

 laws of the evolution by which the present conditions of the 

 organic world have arisen : 



(ist) An orderly succession in the geological history of 

 organisms, which in - the main has resulted in an increasing 

 differentiation of structure and specialization of function with 

 the progress of geological time. The general name for this 

 process is evolution. 



(2d) While the whole organism is concerned in this evo- 

 tion, certain parts of an organism (or certain of the morpho- 

 logical characters) exhibit the evohition more rapidly than do 

 other parts or characters. 



(3d) When these characters are arranged in the order of 

 relative rank of importance in the economy of the organism, 

 the characters of least importance (the varietal and specific 

 characters) exhibit the evolution most constantly and persist- 

 ently, but at a very sloiv rate, chronologically considered. 



(4th) The characters of higher rank (the branch, class, 

 ordinal, and family characters) ivere relatively more rapid in 

 the expression of their initial evolution and thereafter zvere 

 very constant in each successive race. 



(5th) These two tendencies are expressive of the two 

 fundamental laws of evolution — variability and heredity. 

 Variability is recognized as a common law of organism, ac- 

 cording to which, in the ordinary process of generation, 

 slight changes are continually taking place in the morphologi- 

 cal features of the offspring as compared with the parent form. 



Heredity is a common law of the organism, according to 

 which a character once acquired in the parent tends, in the 

 process of ordinary generation, to be repeated with increasing 

 precision, and to result in the transmission of characters with- 

 out change from generation to generation. The process of 

 evolution is the combined result of the interaction of these two 

 antagonistic laws of the organism. 



