vi Preface. 
grown much beyond the bulk of the original lectures, 
I have thought it desirable to publish the whole in 
the form of three separate works. Of these the first 
—or that which deals with the purely historical side 
of biological science—may be allowed to stand over 
for an indefinite time. The second is the one which 
is now brought out. and which, as its sub-title signifies, 
is devoted to the general theory of organic evolution 
as this was left by the stupendous labours of Darwin. 
As soon as the translations shall have been completed, 
the third portion will follow (probably in the Autumn 
season), under the sub-title, “ Post-Darwinian Ques- 
tions,” 
As the present volume is thus intended to be merely 
a systematic exposition of what may be termed the 
Darwinism of Darwin, and as on this account it is 
likely to prove of more service to general readers than 
to professed naturalists, I have been everywhere care- 
ful to avoid assuming even the most elementary know- 
ledge of natural science on the part of those to whom 
the exposition is addressed. The case, however, will 
be different as regards the next volume, where I shall 
have to deal with the important questions touching 
Heredity, Utility, Isolation, &c., which have been 
raised since the death of Mr. Darwin, and which are 
now being debated with such salutary vehemence by 
the best naturalists of our time. 
My obligations to the Senatus of the University 
of Edinburgh, and to the Board of Management of 
the Royal Institution, have already been virtually 
