118 



Prof. Packard's recent amended Report on Forest Insects is an example 

 of this kind of work, the vahie of which must constantly have been felt 

 by every member of this Association. Even in Jamaica, where we have 

 to deal with a very difterent fauna, it is continually consulted on all 

 sorts of points and has to be placed among the two dozen or so books 

 which are always in the workroom at one's elbow. 



I have thought that this Association might suitably consider the 

 desirability of contiTiuing and greatly extending this kind of research, 

 and the following notes are put together as a slight contribution to the 

 discussion of the subject. 



In the first place, we are very familiar with the fact that some insects 

 are strictly confined to one genus or even one s^jecies of jjlant, while 

 others seem almost omnivorous. In 1879 I found the larvte of DeilepMla 

 eupJwrhi(v upon Sea-spurge in Madeira and brought some half-grown 

 and young larvtie to England. In the neighborhood of Chiselhurst, 

 where I was stopping, no Sea-spurge was to be obtained, and, as I could 

 by no means get the larv;e to eat any of the Euphorbijie that grew there, 

 they all perished. Yet a nearly related siiecies, DeilepJiila Uneata, is 

 one of the most promiscuous feeders among the Lepidoptera. From this 

 and many other instances which might be given, it appears that the 

 habits of exclusive or promiscuous feeding are not generic in their 

 range, but vary greatly among members of a single genus. 



Considering this from a Darwinian point of view, we may perhaps 

 trace out a sort of cycle of events, comj)rising the rise, multii^lication, 

 decrease, and extinction of a species. This is not a suitable time for 

 going into great detail in a matter of this sort, but briefly, I suppose 

 the course of events may often have been as follows : 



Supi^ose a common and widely distributed sjiecies, which lives on 

 several plants, to be attacked by many enemies, so that it is in danger 

 of not being able to maintain itself. The individuals living on every 

 kind of plant will vary somewhat, and there will be a tendency for dif- 

 ferent variations to survive on different plants, owing to the fact that 

 each kind of plant constitutes a somewhat difterent environment. For 

 example, if the insect lives on the bark of trees the tendency will be for 

 a flat variety to preponderate or survive on a tree with smooth bark 

 and a narrow variety on a tree with crevices in the bark, etc. Now 

 the enemies of this insect, or at least the most serious of them, will 

 be in the habit of examining the several plants it infests, and in this 

 examination will naturally look for most and see most readily the 

 typical or ordinary form, not that which has begun to diverge from the 

 normal. Consequently, the diverging races will be specially favored by 

 immunity from attack, whatever the character of their divergence, even 

 though not obviously protective, and the tendency will be to accentu- 

 ate the differences, and ultimately to lead to the formation of a number 

 of new species, each confined to a single species or x)erhaps genus of 

 plants. 



