PREFACE. 



TN 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 toward 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 toward 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 katabolismj of living matter or 

 protoplasm. 



But while Books I. and II. are thus the more important, and such 

 chapters as "Hermaphroditism," "Parthenogenesis," "Alternation 

 of Generations," have only a subordinate and comparatively technical 

 interest, it will be seen that our theme raises nearly all the burning 

 questions of biology. Hence, for instance, a running discussion and 

 criticism of the speculative views of Professor Weismann, to which 

 their very recent introduction to English readers has awakened so wide 

 an interest. At once of less technical difficulty, and in some respects 

 even wider issues, is the discussion of Mr. Darwin's theory of sexual 

 selection, reopened by the other leading contribution to the year's 

 biological literature which we owe to Mr. Alfred Russel Wallace.* 

 Besides entering this controversy at the outset of the volume, we have 

 in the sequel attempted to show that the view taken of the processes 



*" Darwinism." No. 115 and No. 116 of the Humboldt Library. 



