XIV. PREFACE. 
reasons in the economy of Nature for such actions imposed 
on creatures apparently inadequate to the task, the secondary 
causes inducing birds to undertake their appointed flights, 
the means by which the extreme punctuality of these 
migrations is attained to, the nature and amount of the 
instinct by which they are impelled, the variations of that 
instinct or guiding agent according to locality, and so 
forth ;—all, questions of intricacy, of deep interest separately 
considered, and still more so when embodied into, and 
made to tell in the commonwealth of science. No man 
really ,sensible of the manner in which the data and 
maxims of philosophy are arrived at, will for a moment 
doubt the importance of local facts and their concomitant 
theories as assistant in the construction of general or in- 
clusive theories, any more than he will hesitate to admit 
the effect of one part or department of science on another 
in the attainment of truth. Theory moreover may be shewn 
to be useful in another way, namely that by entertaining 
a preconception a man is naturally induced to search for 
corroborative facts in a greater variety of directions and to 
a greater extent than he would do if not prepossessed. 
But still itis abundantly manifest that a theory of too in- 
clusive a nature or what is termed a “ leading theory” is a 
prepossession which acts most prejudicially on future in- 
vestigations and discoveries. 
My plan has been to avoid those departments in which 
I possessed trifling knowledge, namely Mineralogy and 
Entomology, and since the Botany of South Devon has 
fallen into very able hands* I thought it would amount 
* Flora Devoniensis.—Banks’s Flora of Plymouth and Deyon- 
port.—Dr, Jacob’s West Devon and Cornwall Flora. 
