PREFACE 
TO 
THE THIRD EDITION. 
Foil all that distinguishes the present from the second 
edition the reader has to thank Professor Bonney. 
He has added occasional footnotes (distinguished by 
square brackets), and he has given, in the form of an 
appendix, a careful summary of the more important 
memoirs published since 1874. 
My own contribution is merely the fulfilment of a 
pleasant duty — the expression of my sincere gratitude 
to Professor Bonney for the ready kindness with which 
he undertook a difficult task, and for the care and 
skill with which he has completed it. 
I must also he allowed the satisfaction of expressing 
my obligations to Captain Wharton, R.N., Hydro- 
grapher to the Admiralty, for an interesting series of 
notes, which are embodied by Professor Bonney in the 
present edition. 
Francis Darwin. 
Cambridge : February 28, 1889. 
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