PREFACE 
THE present volume consists mainly of a reprint of two 
volumes of essays — Contributions to the Theory of Natural 
Selection, which appeared in 1870, with a second edition in 
1871, and has now been many years out of print; and, 
Tropical Nature and Other Essays, which appeared in 1878. 
In preparing a new edition of these works to appear as 
a single volume I have thought it advisable to omit two 
essays—that on “The Malayan Papilionide” as being too 
technical for general readers, and that on “The Distribution 
of Animals as indicating Geographical Changes,” which con- 
tains nothing that is not more fully treated in my other 
works. Another essay — ‘“By-Paths in the Domain of 
Biology ”—has also been partly omitted, one portion of it 
forming a short chapter on “The Antiquity and Origin of 
Man,” while another portion has been incorporated in the 
chapter on “The Colours of Animals and Sexual Selection.” 
More than compensating for these omissions are two new 
chapters—“ The Antiquity of Man in North America” and 
“The Debt of Science to Darwin.” 
Many corrections and some important additions have been 
made to the text, the chief of which are indicated in the 
table given below ; and to facilitate reference the two original 
works have separate headings, and form Parts I. and II. of 
the present volume. 
