AUTHOR’S PREFACE. 
Ir is one of the essential advantages of the present age, 
that the bond of union connecting the different branches of 
natural science is daily becoming more intimate, and it is to 
the contributions which they reciprocally afford each other that 
we are indebted for a great portion of the progress which the 
physical sciences have lately made. This circumstance there- 
fore renders it so much the more remarkable, that, notwith- 
standing the many efforts of distinguished men, the anatomy 
and physiology of animals and plants should remain almost 
isolated, though advancing side by side, and that the conclu- 
sions deducible from the one department should admit only of - 
a remote and extremely cautious application to the other. Of 
late, the two sciences have for the first time begun to be more 
and more intimately allied. The object of the present treatise 
is to prove the most intimate connexion of the two kingdoms 
of organic nature, from the similarity in the laws of develop- 
ment of the elementary parts of animals and plants. 
The principal result of this investigation is, that one com- 
mon principle of development forms the basis for every sepa- 
rate elementary particle of all organised bodies, just as all 
crystals, notwithstanding the diversity of their figures, are 
formed according to similar laws. I have endeavoured to 
explain the design of such a comparison more fully in the 
commencement of the third section of this treatise, and will 
now lay before the reader those data which are of most im- 
portance in an historical point of view im reference to the 
development of this idea. 
