CHAPTER ill. 



GREGA3IOUPNESS ^ND SOCIABILITY OF SPECIES- 



The terms "gregarious" and "sociable" have been defined on 

 page 5. From f he considerations developed in the Chapter just 

 concluded, it will be evident that a gregarious species is one which, 

 by its superior tenacity or exclusiveness, or both combined, over- 

 comes all its neighbours and survives all other contrary conditions 

 in the struggle for existence, while a sociable species is one which, 

 being neither exclusive nor so tenacious as to yield at few or no 

 points to its neighbours, nor the sole species capable of growing in 

 the given soil or locality, is yet tenacious enough to resist complete 

 banishment. It hence follows that a species that is generally gre- 

 garious may nevertheless under certain circumstances become 

 sporadic, and one that is generally sporadic may similarly become 

 gregarious. As a general rule, the more extreme the nature of the 

 soil or climate in any case is, the more likely is the species for 

 which it is best adapted to become gregarious; while average cha- 

 racters of soil and climate favour sociability. 



What we have said above will at once become apparent from 

 a study of the few examples of gregarious and sociable trees which 

 follow and in which the principal characters that combine to render 

 them gregarious or sociable, as the case may be, are simply enume- 

 rated. To have attempted to explain and discuss them in a full 

 manner would have carried us beyond the limits laid down for the 

 present Part of the Manual. 



Sal 



This tree disappears the moment it reaches a too binding soil 

 resting on a more or less impermeable sub-soil. Although in a 

 comparatively dry climate it can grow on fairly stiff soils, the sub- 

 soil must always be freely permeable. Hot or excessively cold winds, 

 or drought in April-May also restrict its extension. It is thus, to 

 a very remarkable extent, wanting in tenacity as against certain 

 widely prevailing unfavourable influences of soil and climate, so 

 that its distribution is limited by sharp boundary lines ; but within 



