PREFACE TO THE REVISED EDITION. v ij 



College, Oxford. And the statements included under Ani- 

 mal Physiology have been checked by Mr. W. B. Hardy, 

 M.A., Fellow of Gonville and Cams College, Cambridge, 

 Demonstrator of Physiology in the University. Where the 

 discoveries made since 1864 have rendered it needful to 

 change the text, either by omissions or qualifications or in 

 some cases by additions, these gentlemen have furnished 

 me with the requisite information. 



Save in the case of the preliminary portion, bristling 

 with the technicalities of Organic Chemistry (including the 

 pages on " Metabolism "), I have not submitted the proofs, 

 either of the new chapters or of the revised chapters, to the 

 gentlemen above named. The abstention has resulted 

 partly from reluctance to trespass on their time to a greater 

 extent than was originally arranged, and partly from the 

 desire to avoid complicating my own work. During the 

 interval occupied in the preparation of this volume the 

 printers have kept pace with me, and I have feared adding 

 to the entailed attention the further attention which corre- 

 spondence and discussion would have absorbed : feeling that 

 it was better to risk minor inaccuracies than to leave the 

 volume unfinished : an event which at one time appeared 

 probable. I make this statement because, in its absence, 

 one or other of these gentlemen might be held responsible 

 for some error which is not his but mine. 



Yet another explanation is called for. Beyond the ex- 

 position of those general truths constituting the Principles 

 of Biology as commonly accepted, the original edition of 

 this work contained sundry views for which biological 

 opinion did not furnish any authority. Some of these have 



