ce PREFACE, 
investigation on this subject. It is regarded as a 
desideratum by all, and as the consummation of 
botanical science by many, that plants should be 
so arranged, as that their assemblages should 
agree, not only in external forms, but in internal 
qualities and operative powers. Certain general 
agreements of this kind evidently prevail through- 
out nature ; yet they are so varied, and subject to 
so many exceptions, that it is difficult to establish 
them by general scientific descriptions, and when 
they are rendered too minute they seem to lose 
much of their importance. It is perhaps as easy 
to know the properties of plants from their exter- 
nal habit, as to understand the characters of man- 
kind from their physiognomy. Accurate obser- 
vers know more than they can communicate the 
means of knowing to others, yet the most accu- 
rate are liable to be mistaken. Many vegetables 
of the closest affinity and resemblance, even spe- 
cies of the same genus, differ wholly from each 
other in their effects. Witness the species of 
Cucumis, Convolvulus, and Solanum, some of 
which are salutary, and others highly deleteri- 
ous. Nevertheless there are many general truths, 
or at least general probabilities, by which every 
one would be influenced, and which have so much 
importance, that they will never be forgotten. 
