PKEEACE. ix 
Third, to tabulate all important observations recorded by 
unknown observers, with the view of ascertaining whether 
they have ever been corroborated by similar or analogous 
observations made by other and independent observers. 
This principle I have found to be of great use in guiding 
my selection of instances, for where statements of fact 
w T hich present nothing intrinsically improbable are found 
to be unconsciously confirmed by^ different observers, they 
have as good a right to be deemed trustworthy as state- 
ments which stand on the single authority of a known ob- 
server, and I have found the former to be at least as abun- 
dant as the latter. Moreover, by getting into the habit 
of always seeking for corroborative cases, I have frequently 
been able to substantiate the assertions of known observers 
by those of other observers as well or better known. 
So much, then, for the principles by which I have 
been guided in the selection of facts. As to the arrange- 
ment of the facts, I have taken the animal kingdom ii, 
ascending order, and endeavoured to give as full a sketch 
as the selected evidence at my disposal permitted of the 
psychology which is distinctive of each class, or order, 
and, in some cases, family, genus, or even species. The 
reason of my entering into greater detail with some 
natural groups than with others scarcely requires explana- 
tion. For it is almost needless to say that if the animal 
kingdom were classified w T ith reference to Psychology in- 
stead of with reference to Anatomy, we should have a very 
different kind of zoological tree from that which is now 
given in our diagrams. There is, indeed, a general and, 
philosophically considered, most important parallelism 
running through the whole animal kingdom between 
structural affinity and mental development; but this 
parallelism is exceedingly rough, and to be traced only 
in broad outlines, so that although it is convenient for 
