118 



The Mechanism of Evolution" in Leptinotaesa 



In figure 8 I have put in condensed form a schematic presentation of the 

 general results that have been obtained in the study of the role of external con- 

 ditions in modifying the reactions in crossing. In these experiments there are 

 two important agents — the value ot Ac and the composition of the climatic 

 conditions in the medium; but since the value of 4c is the direct product of the 

 conditions and duration of the race m the medium, the external complex plays 

 the major role in the production of changes in the Mendelian reaction. I have 

 used two sets of conditions, as follows : 



Table 8b. 



With stocks of known standard reactions, when crossed, under the conditions 

 of climatic complex Z, with the Ac values 40 and 60 respectively, the uniform 

 result is the production in Fi of all signaticoUis type, and these in Fj breed true ; 

 but the polygon of all fraternities that have been tested are trimodal, whereas 

 the line continues to appear to breed true as a sig'naitcoZHs-appearing type, as 

 shown in case 1, figure 8. Under these conditions the desiccation is intense and 

 the result is to accelerate ontogeny, so that the reaction in Pj is to alter the 

 arrangement in all of the gametes, and even in the P^ heterozygotes, so that they 

 are in aspect signaticoUis. This is a striking result, and although the result 

 breeds true in ordinary terms, it is truly in all instances thus far seen hetero- 

 zygous, as may be demonstrated by biometric analysis of the individual fra- 

 ternities, or by application of the test reaction. Both tests are equally good, but 

 the biometric test must not be applied to more than the members of a single 

 fraternity if the results are to be clear and useful. In table 9 I have given the 



