THE LOCUST PROBLEM .299 



appears to be explainable as the result of a chemical influence, 

 derived from the food-plant, acting upon each successive 

 generation. Specific host-preference and its repetition in each 

 generation seems to afford an analogy with what Pavlov has 

 termed a conditioned reflex. The conditioning takes place during 

 the developmental cycle of the insect and influences the chemo- 

 tropic responses of the female during oviposition. It determines 

 the preference for selecting the same host-species upon which 

 her own developmental stages were passed. The faculty for 

 conditioning, which appears to be present in all insects to a 

 greater or lesser degree, is almost certainly inherited. The 

 chemical influence derived from the food-plant, on the other 

 hand, seems to determine the direction along which the con- 

 ditioning shall take place. It has already been shown that there 

 is some evidence that specific host-selection may become intensified 

 in successive generations, or, in other words, intensified specific 

 conditioning may occur. The question naturally arises as to 

 whether this is due to the passing on and accumulation in 

 successive generations of some chemical agency derived from the 

 food-plant ? There is, again, experimental evidence showing 

 that if an insect be transferred to a different food-plant, the 

 effects of this conditioning may become lost and a new conditioning 

 substituted. If, on the other hand, a specific conditioning be 

 repeated through a sufficient number of generations as to become 

 unalterable, it would appear to have directly affected the germ 

 cells of the insect. Up to the present, however, we have not 

 adequate experimental data in order to determine whether a 

 permanent effect can be so induced and reflected in a rigid host- 

 selection. The available evidence will hardly be considered by 

 most biologists to be sufficiently well grounded to demonstrate a 

 principle of inheritance so open to controversy. 



Ecological Aspects of the Locust Problem 



The more or less irregular periodicity of locust outbreaks has 

 long attracted attention, and various explanations have been 

 advanced to account for the phenomenon. There seems to be 

 very little doubt that the periodicity is the result of certain 



