14 PREFACE. 



matters, but this is the inevitable result of the particular 

 point of view of the author. 



Nearly all of the subjects treated are of controversial 

 nature, but that is the present state of biological science. 

 Attempts to crystallize incomplete information into laws 

 lead to theory rather than to fact, and the subject 

 passes from theory to theory in search of the fact. 

 This explains why the consideration of certain subjects 

 may lead the reader to a final interrogation point or 

 may end without a personal expression by the author in 

 favor of one or the other side of the question. 



It is hoped that the problems of Blood-relationship, 

 Infection, Immunity, Parasitism, Inheritance, Mutila- 

 tion, Regeneration, Grafting, and Senescence, which have 

 been presented at greater length than in other writings 

 upon Biology, may be useful to the reader. 



It is hoped that the writing will not be found too 

 technical to be beyond the comprehension of any intelli- 

 gent reader, though it must be admitted that some ac- 

 quaintance with the sciences will be of decided advan- 

 tage to him. 



The author expresses his sincere thanks to his friend 

 and colleague, Professor Charles H. Shaw, A.M., Ph.D., 

 for many valuable suggestions and criticisms. 



PHILADELPHIA, PA. JOSEPH McFARLAND. 



