290 Illinois State Lahornfory of Natural liistorij. 



the matter to a decisive test on that species. The occurrence 

 of a spontaneous outbreak of the common cabbage worm 

 fiacherie among the lot under experiment, would of course 

 arrest the progress of the experiment, and might even so mask 

 the result as to mislead. 



This accident, in fact, occurred to my first two experiments, 

 begun August 9 and 10. Not only did the cabbage worm 

 affection appear in both the experimental breeding cages and 

 the checks, but the latter lot as well as the former gave evi- 

 dence of infection from our silkworm material. The latter fact 

 convinced me that my arrangements were inadequate for the 

 protection of my check lots against accidental infection with 

 the experimental material. These lots were placed at a distance 

 from those purposely exposed to disease, but in another part of 

 the same large hall, and were attended by the same assistant. 

 In previous experiments, not yet detailed, with other larvse, I 

 had already had evidence of slight unintentional infection of 

 the check lots by this too close association with those under 

 treatment, and now arranged another experiment on a wholly 

 different plan. 



Careful examination was made of all the cabbage fields near 

 Normal, and one was selected which showed no trace of the 

 proper disease of the cabbage worm. From this field two lots 

 of caterpillars were selected, twenty-five in each, those for 

 experiment by the assistant whose duty it was to make the infec- 

 tions, and the check lot, by an intelligent student of the Normal 

 school, who did not visit my zoological laboratory at all. The 

 first lot was brought to the office and placed in a clean and dis- 

 infected cage in the usual place, but the second or check lot was 

 taken by the student mentioned directly to his own home and 

 confined in a new breeding cage. Care was taken that both lots 

 should be fed and treated alike, except for the infection, but no 

 opportunity was given for any communication between them. 

 The results in this case were more satisfactory, and confirmed 

 my suspicion that our check lots had not before been sufficiently 

 isolated. 



History of the Infected Lot. — The food of the twenty-five 

 cabbage worms selected especially for experiment, was sprayed on 

 the 6th September with beef broth infected nearly a month pre- 



