THE LAWS OF EVOLUTION EMPHASIZED. 365 



to either, and the reader may find many admirable treatises 

 giving account of this aspect of evolution. 



Method and Purpose in the Selection of the Evidence here 

 Set Forth. The facts which have been selected in these chap- 

 ters have been chosen for the purpose of ascertaining what 

 the geological history of organisms has been. 



Examples have been taken and analyzed to ascertain what 

 has been the .particular law of succession in particular cases 

 where the evidence was full enough to be relied upon. If the 

 interpretation of these selected cases has been correct, the 

 principles discovered may be applied to other cases. 



The facts have been examined for the purpose of learning 

 (i) what the fossils indicate has been the order of succession 

 in the initiation of different forms of organisms; (2) what rela- 

 tion this succession bears to the relative importance of the 

 characters in the economy of the individual organism, as 

 shown by the systematic classification of the Animal Kingdom ; 

 and (3) what have been the determining causes by which the 

 multitudinous differences in organic structure have been 

 brought about. The first consideration in their selection 

 was that they should be from among those of which the most 

 perfect record is preserved. The cases already cited in evi- 

 dence are not selected because they are the most important 

 examples, nor because they illustrate only the most impor- 

 tant laws of evolution, but they are selected because they are 

 the best examples to show what the geological records testify 

 regarding the history of organisms. 



Different Kinds of Evidence Borne by Living and Fossil Organ- 

 isms. Living organisms present the best evidence of the laws 

 of ontogenetic development, because they furnish illustration 

 of each stage in the development. A continuous series of the 

 stages of development of a single organism is more satisfac- 

 tory evidence of the essential nature of that development, 

 than would be any number of detached exhibitions of sundry 

 stages of development of different organisms. 



So it is believed that the evidence borne by a series of 

 fossils preserved in each stage of the geological record, of 

 which specimens are well preserved and described from the 

 first to the last, and which show the beginning, dominance, 



