1920] Forbes: General Discussion 195 



corresponding study of visible variations of structure to a 

 knowledge of the phylogeny of the insect organism. 



One of the environmental conditions to which an insect 

 may adjust itself by reason of this flexibility of its life history is 

 its relation to its food-plant and to its competitors for food. 

 Remembering that the prosperity of a plant-feeding insect is 

 dependent on the abundance and continued growth of its food- 

 plant, and that this plant, for its own best prosperity, must 

 produce for its insect guests timely supplies in quantities which 

 can be spared without actual injury to the plant itself, we see 

 a mutual advantage to insect and plant alike if the draft on 

 the growing plant shall be distributed over as long an interval 

 as possible, in order that the product of continuous growth may 

 go as far as possible to supply the demand. Obviously, the 

 demand of a thousand insects delivered in one day might 

 effectually bankrupt a plant which could honor the draft without 

 embarrassment if it were distributed over a fortnight or a 

 month; and this advantage to the food-plant would react, of 

 course, to the advantage of the feeding insect also. There 

 is thus a standing reward offered to every insect dependent 

 for food on a living and growing organism, for establishing 

 and maintaining an individual variability in its sensitiveness 

 to stimuli such as shall lengthen the period of its depredation. 



Of course, individual differences in the rate and the period 

 of development of the insects of the same generation, and even 

 of the same parentage, are not all due to variations traceable 

 to the egg, but many are consequences of different individual 

 exposures to stimulating or retarding factors; as a consequence 

 of them all, (some original and some incidental) , the effect of an 

 infestation is diluted and diminished by an extension of its 

 period, to the common advantage of the infesting guest and the 

 infested host. 



Furthermore, it seems possible that this depredation period 

 miay be shifted as a whole, so as to come earlier or later in the 

 season, if competition with another species of kindred habit 

 may be thereby evaded. If two species infest a food-plant at 

 the same period, their joint number must be so limited, as a 

 general rule, that their attack will not destroy the food-pro- 

 ducing plant; but if they can come to succeed each other so 

 that each shall have the plant for a time to itself, both may 

 maintain a higher rate of multiplication without permanent 



