PREFACE 



TO THE REVISED AND E^^LARGED EDITION. 



Rapid in all directions, scientific progress has during 

 the last generation been more rapid in the direction of 

 Biology than in any other; and had this work been one 

 dealing with Biology at large, the hope of bringing it up to 

 date could not have been rationally entertained. But it is a 

 work on the Principles of Biology ; and to bring an expo- 

 sition of these up to date, seemed not impossible with such 

 small remnant of energy as is left me. Slowly, and often 

 interrupted by ill-health, I have in the course of the last 

 two years, completed this first volume of the final edition. 



Numerous additions have proved needful. What was 

 originally said about vital changes of matter has been sup- 

 plemented by a chapter on '' Metabolism," Under the title 

 " The Dynamic Element in Life," I have added a chapter 

 which renders less inadequate the conception of Life pre- 

 viously expressed. A gap in preceding editions, which 

 should have been occupied by some pages on '' Structure," 

 is now filled up. Those astonishing actions in cell-nuclei 

 which the microscope has of late revealed, will be found 

 briefly set forth under the head of " Cell-Life and Cell- 

 Multiplication." Further f^vidence and further thought 

 have resulted in a supplementary chapter on " Genesis, 

 Heredity, and Variation " ; in which certain views enun- 



