168 The Mechanism of Evolution in Leptinotaesa 



already present to make a considerable array of possible pure-breeding Fj 

 extractive races. The F^ in this combination shows nothing of interest with 

 these lines ; decemlineata is dominant, and the fraternity is as in the other com- 

 binations of these species, except that the elytral color is variable from yellow 

 to red, with no observed regularity of proportions. Fj is complicated, and I 

 have never tried to analyze out the entire conditions present, as it required 

 space and labor that I could not afford to devote to this relatively unimportant 

 portion of the general project. 



These experiments establish first, the point that decemlineata is the dominant 

 form in crossings of these two species, and this has an important relation to 

 some of the experiments to be presented later. It suggests also that in the test- 

 ing of species in initial crosses it is in all respects best to reduce them to a 

 modal biotypic condition, from which races all contaminating agents or char- 

 acters have been removed, or the lines otherwise reduced to basal conditions in 

 which the agents productive of diversity were eliminated. When this is done it 

 is possible to test fully the true method of reaction between the two species and 

 the extent to which dissociation occurs in crossing. From this as a point of 

 departure it is possible in any instance to test by experiment the action of added 

 gametic agents or the role of external conditions in the production of hetero- 

 geneity of action and product in the crossing of the two in experimental con- 

 ditions or in nature. 



As far as the crosses of these two species are concerned, further description 

 at this point would add only uninteresting details and no points of general 

 interest. Added data of the crossing of them appears from time to time in other 

 portions of this report as it has a bearing upon the operations under discussion. 

 At all points, however, it should be kept in mind that these two species when in 

 combination are not in crosses dissociated to any considerable extent, and that 

 the basal form, comprising those agents that produce the form as measured by 

 the form-index, with which are associated many characteristic and non-dissoci- 

 able characters, remains intact in the intercrossings, and only some of the more 

 easily displaced agents are shifted. The condition in this cross is therefore 

 quite different from that of decemlineata and ohlongata, in which the reaction 

 between the two reminds one of an extensive disruption of the two systems 

 when they meet in the F^ heterozygotes. The main point in presenting these 

 details in this report is to make clear the fact that the reactions of these species 

 in crossing are in no respect different in principle from the reactions found in 

 many other organisms studied, as measured by the neo-Mendelian reaction in 

 the last ten years. 



LEPTINOTARSA MULTIT/ENIATA X LEPTINOTARSA OBLONGATA. 



The crossing of these two species presents essentially the same set of problems 

 that are present in muUitceniata and decemlineata. It is necessary that the 

 materials be reduced to homogeneous conditions, to modal biotypic lines, before 

 the crossing is made. When this is done and the cross is made between muUi- 

 tceniata modal biotype and oblongata modal biotype, the reaction is a perfectly 

 simple one, monohybrid in all respects, without any dissociation of either 

 gametic system. This cross is not complicated by the juvenile body-colors, 

 which are the same, yellow, in both, and the only juvenile difference is the 



