Vlll 



PREFACE 



number of illustrative cases has been kept within a moderate 

 compass. A good many of the examples have been chosen from 

 American natural history, as being appropriate to a book in- 

 tended primarily for American readers. The facts are largely 

 given on the authority of others, and I wish to express my 

 gratitude for the abundant assistance received from American 

 colleagues, especially from the staffs of the American Museum 

 in New York, and of the Boston Museum of Natural History. 

 In connexion with the particular subjects personal acknowledg- 

 ments are made. 



Dr. F. M. Chapman was so good as to supervise the prepara- 

 tion of the coloured Plate of Colaptes, and to authorize the loan 

 of the Plate representing the various forms of Helminthophila, 

 which is taken from his North American Warblers. i 



I am under obligation to Messrs. Macmillan & Co., for per- 

 mission to reproduce several figures from Materials for the Study 

 of Variation, illustrating subjects which I wished to treat in 

 new associations, and to M. Leduc for leave to use Fig. 9. 



In conclusion I thank my friends in Yale for the high honour 

 they did me by their invitation to contribute to the series of 

 Silliman Lectures, and for much kindness received during a 

 delightful sojourn in that genial home of learning. 



