IV PREFACE. 



and speculation variously attested by miscellaneous 

 facts ; but, on the other hand, much is more firmly 

 established than any recorded history handed down 

 to us in manuscript. Some parts of the history of 

 life must rank among our most certain facts of 

 knowledge. In the following outline of this history 

 the endeavor will be made to separate carefully those 

 parts which are speculative from those which are 

 based on a more firm foundation, and to give to each 

 part the value it deserves. 



It is well to note at the outset that the facts as 

 collected prove to us that the past history has been 

 a continuous one, and one in which the facts follow 

 each other with such logical consecutiveness that it 

 may be regarded as a logical whole. The history of 

 life has not been one of isolated facts, but a continu- 

 ous flow, in which each step is foreshadowed by the 

 one immediately preceding, and in its turn foretells 

 the next one. This condition of things has led to a 

 development in the life of the world and to a series of 

 facts which science has termed evolution. Whatever 

 be our philosophical understanding of this term, the 

 facts, so far as life is concerned, are beyond question. 

 The history of life has been one of the development 

 of forms and types from each other, or rather from 

 common centres. This law we call organic evolu- 

 tion. The facts upon which organic evolution is 

 based are beyond controversy, although there may 

 be still some dispute as to the interpretation of the 

 facts. No one whose judgment means anything 

 in the scientific world questions that the history of 

 life has been one of growth and development of 



