PREFACE. is 



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 1 have found to be of great use in guiding 

 my selection of instances, for where statements of fact 

 which 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 in 

 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 with 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 



