( oxvi ) 



anything material to existing knowledge, still less to ofPer, 

 except tentatively, any explanatory hypothesis, I could usefully 

 avail myself of the opportunity you have given me — and 

 which I feel as implying an obligation— by endeavouring to 

 indicate some points to which the extensive opportunities for 

 observation and research presented to this Society of more 

 than 500 Fellows may with advantage be directed. I do so 

 with unfeigned diffidence, for I know that I am addressing 

 some w^ho have devoted to the subject a capacity and an 

 amount of thouglit far in excess of anything I can bring to 

 bear upon it. 



The questions which have been mentioned stand out as of 

 interest and importance and such as we must often be asking 

 ourselves. They may perhaps be followed out in this more 

 detailed form : Why are the individuals of a well-established 

 species fairly constant in number 1 Why is this number so 

 much less in some species than in others ? and especially, Why 

 are some species so rare, and yet so persistent in the numbers 

 of their individuals 1 



In inviting researches on these and cognate questions my 

 data must mostly be supplied by observations made in the 

 British Islands. This is so, partly because among countries 

 of considerable and varied area and possessing a large number 

 of organisms, belonging to widely different types and placed 

 amidst diversified surroundings, there is none which has been 

 more thoroughly searched, I may almost say ransacked, by 

 entomologists, and none whose insects have been more studied 

 by highly-qualified investigators, and therefore in reference to 

 many particulars of which so mvich information of a fairly 

 complete character is available ; and partly because this in- 

 formation is that which has been most accessible to me. For 

 somewhat similar reasons my illustrations must be drawn 

 mainly from the Lepidoptera, with the additional reason that 

 this is the only Ox-der to w^hich I have given any study. These 

 are serious deficiencies, but I would plead that they are not 

 an entire disqualification for the task set before me. 



Productive mul destructice forces — Productive. 

 It is by the conflict between the forces of production and 



