vi PREFACE. 
of lectures delivered at the Lowell Institute in 
Boston, without any thought of their subsequent 
publication. Notes were, however, taken of 
them at the time, and I very willingly assented 
to the suggestion of some of my listeners, that 
they should be recorded in the form of articles 
for the Atlantic Monthly. They still retain some- 
thing of the repetition which is needed in a 
public course of scientific lectures in order to 
keep the connection of the subjects clearly before 
the mind of a popular audience. An attempt 
to change this character would have amounted 
to writing the whole course anew, —a task for 
which I had neither time nor inclination. I have 
endeavored to avoid technicalities as far as pos- 
sible in dealing with subjects many of which 
are quite unfamiliar to the general mind; and the 
closing chapter of the book, which has been in- 
corporated in the volume, but did not appear in 
the Atlantic Monthly, is the only one especially 
addressed to the professional naturalist. 
L. AGASSIZ. 
Nawant, August 22, 1868. 
