April, '12] HESSIAN FLY DISCUSSION 107 



grasses. In Kansas I have found it developing as abundantly on 

 Agroiyyron sniithii as upon wheat. 



T. J. Headlee: I realize that the amount of data, on which the 

 new point that I have to ])rcsent is based, is so limited that the point 

 must largely l)e taken as in the nature of a suggestion. I would, 

 however, say that I have some negative data that I am not submitting 

 in this paper. 



A. D. Hopkins: Mr. Chairman, it is naturally very gratifying to 

 have this verification of my theory on this subject. I might state 

 that in Bulletin No. 67 of the West Virginia Agricultural Experiment 

 Station, August 1900, entitled, "The Hessian Fly in West Virginia 

 and How to Prevent Losses from its Ravages," I discussed the theory 

 on pages 242 to 246, under the head of ''A Law of Definite Normal 

 Rate of Difference in the Periodical Phenomena of Plants and Ani- 

 mals." I will read the three paragraphs in which the essential features 

 of the theory are specified. 



"First, that, under similar conditions of land surfaces other than 

 altitude, there is a definite normal rate of difference in time in periodi- 

 cal phenomena of plants and animals for all differences in latitude 

 and altitude. 



Second, that, under normal conditions, the rate of the average 

 difference in the dates of the beginning or ending of such phenomena 

 is not far from one day for each fifteen minutes of latitude, and one 

 day for each 100 feet of altitude. 



Third, that the dates of commencing or ending of a given period 

 vary with the season, — t-he weather and local physical conditions, 

 such as exposure and character of soil, — but that the rate of difference 

 under each condition is the same." 



This is founded on evidence furnished by natural phenomena which 

 can be observed by any one and which reflect the combined influences 

 that numerous and complex elements exert on plants and animals, such 

 as temperature, humidity, character of soil, exposure, and a number of 

 other factors. The relative difference in any given phenomenon is 

 demonstrated by the relative activity of plants and animals in the 

 spring. When activity begins in any given locality, as for example 

 the opening of the buds, it will do so under the influence of the com- 

 bination of factors prevailing there. 



Therefore, if we note, in any given season, the appearance of a 

 phenomenon at any given place, we can prophesj' within a few days 

 of when the same phenomenon will occur at another place, north or 

 south or east or west at varying altitudes, if we know the latitude and 

 altitude. I have verified this over and over again. I have much 

 data, from Maine to Florida and from Washington to the high moun- 



