PREFACE TO THE THIRD REVISED 

 EDITION 



It is a feature of scientific knowledge to be always improv- 

 ing, and, owing to advances since the publication of the 

 earlier editions, many of the matters dealt with in this book 

 appear in a new and clearer light. But since the book aims 

 primarily to point out the epochs of advancement as well as 

 to depict the conditions under which, and the spirit in which 

 advances have been consummated, the subject matter of the 

 text does not quickly become obsolete. 



While retaining substantially its original form, some altera- 

 tions have been made: several pages have been rewritten to 

 convey more clearly the meaning, as in reference to Mendel's 

 discovery, and some additions have been introduced, as com- 

 ments on isolation and orthogenesis as factors of organic 

 evolution. The important contributions of Fritz Schaudinn 

 have been noted and the discussion of the antiquity of man 

 has been considerably extended. Several new portraits have 

 been substituted for those of the earlier editions and the por- 

 trait of Schaudinn has been added. 



W. A. L. 



March, 191 5. 



