120 



The Mechanism of Evolution in Leptinotaesa 



data that has been derived from experiments of this character extending from 

 1904 to 1910 and carried in all far enough to make certain of the result. In 

 the 11 tests of this sort that are given, the uniform result is the entire failure 

 of these heterozygous types to split up in any of the 8313 individuals in the 

 record. All of the lines have been tested at one time or another, either bio- 

 metrically or by the test reaction, and all of them showed unmistakable hetero- 

 zygous conditions in the fraternities, either in the trimodal polygon, with the 

 modes on the modes of the parent species, or the recovery of the diversa form 

 from the signaticoUis strain in the manner described in the preceding pages. 



When the same materials are crossed under the conditions of the climatic 

 complex Y, the stereotyped result is the production in F^ of the signaticoUis type 

 and mid-type in about equal numbers, as is shown in plate 10, and in diagram 

 in figure 8, case 2, In table 10 is given the data from 5 experiments, showing 

 the uniformity of behavior in case 2, and aside from the change in the reactions 

 of the masked heterozygotes, the remainder of the population reacts in perfectly 

 regular manner. 



The behavior and modifications of reactions shown in figure 8, cases 1 and 2, 

 only follow when the values of Ac are at about 40 in signaticoUis and 60 in 

 diversa. As the two values approach, the reactions change, and many transi- 

 tion stages have been found between the two types represented, so that in the 

 end the reaction is that shown in case 3, in figure 8, in which the reactions are 

 normal monohybrid in all respects. This type of reaction, shown in case 3, 

 is of interest in that it is not disturbed by the conditions of the medium, even 

 when more extreme than those in complexes X and Y. For example, stocks of 

 these two species have been sent from the laboratory at Chicago to the plant 

 at the Desert Laboratory at Tucson, where the conditions given them were 

 desert surroundings; nevertheless, there has not been any change in the pro- 

 duction of the Fj of the usual mid-type array, with F^ showing the three 

 usual types. In the same manner, in the experimental conditions in the 

 laboratory, the conditions of the medium do not, within limits, change the re- 

 action of the crossing of the two species in any respect when the Ac values are 

 the same. In table 11 are given some arrays of the crossing of these two when 

 the Ac values are the same. 



Table 11. 



