PREFACE. 
Yll 
might not meet with a sale, proportioned to the expense of the 
undertaking ; he, therefore, considered it to be necessary to adapt it, 
as much as possible, to readers in general. Another consideration 
also influenced him to this — ^he thereby hoped to lead those to 
the study of this part of Nature’s works, to whom, as yet, they 
might be unknown. The epistolary form, from the familiarity of 
style which it admits, was therefore adopted; especially since it 
yielded the opportunity of introducing such a portion of intro- 
ductory matter, as might secure the comprehending of the discussion 
by those, to whom it might be, in a great measure, new. In 
doing this he was, however, aware that he was trespassing on those 
who were already informed on these subjects ; but, reckoning on 
the complacency and liberality with which men of true learning 
promote the diffusion of knowledge, he considered their excuse as 
already obtained, and no longer viewed this as an objection. 
Gratitude directs, that he should declare how much he is himself 
indebted to the prevalence of this generous disposition. With 
grateful remembrance of the kindnesses he received from the late 
Dr. Gray, Secretary of the Royal Society, he acknowledges his ready 
assistance on many occasions, as well as the kind facilities he in- 
dulged him with, during his examinations of the numerous valuable 
specimens, contained in that department of the British Museum 
which he so ably superintended. To Dr. Shaw he is also obliged 
for similar attentions. To Charles Hawkins, Esq., William Long, 
Esq., George Chandler, Esq., Sir Charles Blick, John Heaviside, 
Esq., Sir William Blizard, and Henry Cline, Esq. the Curators of 
the Hunterian Museum, he is under great obligations for like 
