INTRODUCTORY. 



xxi 



mention of that immediately preceding this one — the ninth of the 

 series — as it has been so recently issued as to require little of 

 treatment further. Only, it may be desirable to take notice of it 

 here, in so far that more attention has been paid to the subject of 

 climate and its effects, however much or however little may prove of 

 lasting or only of passing importance. 



Of the present, or tenth volume, I leave it to speak for itself as 

 in continuance of the others ; and it will be seen, I think, that I 

 have adhered to the same plan of work throughout. One small ad- 

 ditional feature I may draw attention to, viz. : raising the question 

 as to the importance or otherwise of some newly defined values in 

 describing tlie position of species in the fauna, such as whether any 

 species be " truly gregarious," or merely " flighting " at times of 

 migration ; whether birds exhibit truly gregarious tendencies all 

 through their lives, or only herd together at times and seasons, in- 

 fluenced by circumstances over which they appear to have little or 

 no control. In other words, whether certain birds spend all their 

 lives in the company of others of their species, or separate at a 

 certain season and become independent of one another's society. 



If these newly introduced values should tend towards any further 

 elucidation of the mysteries of migration, and of dispersal and distri- 

 bution, or even should they not, I hope the object of this series will 

 at least receive recognition as an attempt to throw some new light 

 upon these objects of study. 



