PREFACE. 



IN course of the preparation of critical summaries, such as the 

 articles " Reproduction " or " Sex," contributed by one of 

 us to the " Encyclopaedia Britannica," or the account of recent 

 progress annually prepared for the Zoological Record by the 

 other, we have not only naturally accumulated considerable 

 material towards a general theory of the subject, but have come 

 to take up an altered and unconventional view upon the general 

 questions of biology, particularly upon that of the factors of 

 organic evolution. Hence this little book has the difficult task 

 of inviting the criticism of the biological student, although 

 primarily addressing itself to the general reader or beginner. 

 The specialist therefore must not expect exhaustiveness, despite 

 a good deal of small type and bibliography, over which other 

 readers (for whose sakes technicalities have also been kept 

 down as much as possible) may lightly skim. 



Our central thesis has been, in the first place, to present an 

 outline of the main processes for the continuance of organic 

 life with such unity as our present knowledge renders possible ; 

 and in the second, to point the way towards the interpretation 

 of these processes in those ultimate biological terms which 

 physiologists are already reaching as regards the functions of 

 individual life, — those of the constructive and destructive 

 changes (anabolism and katabolism) of living matter or proto- 

 plasm. 



