DISCUSSION OF MIGRATIONS. 11 



that lie had not read Mr. Affleck's statement of the theory. Dr. Gor- 

 ham had still another reason for his conclusions which was in itself 

 sufficient to prove the theory. He collected pupa3 late in the season y 

 and finding that all of them were parasitized he concluded that the 

 entire fall brood had been destroyed by parasites. From the descrip- 

 tion which Dr. Gorham published of the parasite, it is evident that it 

 was Pimpla conquisitor. His account of its operations is very interest- 

 ing, being, we believe, the first published notice of any parasite infest- 

 ing the cotton- worm. And the fact that this Ichneumon infests the last 

 brood of A. argillacea to a very great extent has been confirmed by 

 the experience of the past year. Still many pupa3 of this brood do es- 

 cape; and we therefore infer that Dr. Gorham made his collections 

 late in the season after the unparasitized individuals had emerged 

 from the pupa state, and before the parasites themselves had completed 

 their development. 



The data upon which Dr. Burnett founded his theory were very in- 

 sufficient. It is surprising that a man of his scientific attainments 

 should have proposed a theory simply upon the statements of an un- 

 trained observer. For there is no evidence that Dr. Burnett ever studied 

 the subject in the field ; although, as we learn from the notice of his life, 

 written by the late Professor Wyman,* he passed several winters in the 

 South. Each of the three reasons which Dr. Burnett brings forward as 

 proof of the truth of his theory has already been shown to be a mistaken 

 idea. The insect, in the adult state at least, is not " so sensitive to the 

 cold as to quickly die iu an atmosphere even approaching the freezing- 

 point." It does not appear only "at intervals of every three years." 

 Neither does the pest appear first upon the seaboard and progress 

 gradually inland. 



In the case of Mr. Grote we find the first instance among those who 

 have proposed the theory of migrations, of a writer who is both a trained 

 entomologist of high standing, and one who based his conclusions on 

 extended personal observations in the field. It is also worthy of note 

 that Mr. Grote's researches were made in one of the localities which has 

 suffered most from the ravages of the cotton- worm, and one in which, 

 as we have already indicated, we believe the species to hibernate. For 

 these reasons Mr. Grote's essay in particular should be carefully studied 

 by one treating of the theory of migrations. Such study taken in con- 

 nection with what is now known respecting the first appearance of the 

 worms in the spring will, we believe, reveal the fact that Mr. Grote's 

 observations were not made, as he supposed, upon the first brood, but 

 upon the brood to which we have referred as the first crop, i. e. the 

 second brood, or in some cases the third brood. The statement that 

 "the earliest period at which I have noticed the young worm was the 

 last week in June, and its usual appearance was in July, sometimes as 

 late as the latter part of the mouth" is sufficient to prove this. For we 



' Proc. Bost, Soc. Nat. Hist. V. 65. 



