150 The Mechanism of Evolution in Leptinotaesa 



yielded to test and have been capable of some dissociation, showing the presence 

 of the lost characters in the erratic type. In this no indication of dissociation or 

 disintegration of any sort has been produced, and the type is as stable and fixed 

 in all tests thus far made as are any of the types direct from nature which we 

 call species. I can only suspend judgment as to its production. It would be of 

 interest to know the reason for the limitation of the production of these lines to 

 one side of the cross and its realization in less than 1 per cent of the total 

 crosses made. These are problems for the future, and as time and opportunity 

 permit I hope to be able to penetrate farther into the mechanism and nature of 

 this method of the production of stable, physiologically isolated groups. This 

 represents perhaps the most complex type of interspecific cross, with com- 

 plete permanent combination and production of new groups, that of diversax 

 signaticollis the simplest, with no dissociation and the non-contamination of the 

 gametic system by their entering and passing through the F^ hybrid combina- 

 tion. Between these two extremes are all sorts of intermediate conditions, some 

 of which have been given in the preceding chapter, and others are to be presented 

 in the remainder of this. 



The point of most interest shown in the series is the reaction of the nutritive 

 activities as a unit group being retained in one totality in the combination in 

 dominant positions, so far as the feeding and foods that are capable of use are 

 concerned. Interesting new relations must, however, have been established in 

 the new combination, so that the products of this new nutritive arrangement 

 and limitation may be used by the agents productive of the characteristics of the 

 female line. The adjustments that have been made in the type to bring this 

 about would be of much interest if known, and might give important informa- 

 tion regarding the inheritance of metabolic relations and capa,cities, as well as 

 of the gametic agents present that are responsible therefor. It is certain that 

 the agents in the gamete, productive of restricted nutritive relations, are as 

 specific and capable of dissociation and metathesis and of entering into new 

 associations as are any of the other gametic agents experimented with. Further, 

 they have the ability to produce in the new position some of the same end- 

 results that were shown in the original association, and this in many respects is 

 suggestive of the transfer only of determining agents, rather than the transfer 

 of the total nutritive complex. At any rate, it is certain that agents of the most 

 fundamental nature are able to act as units in the interchange, and recombina- 

 tions have taken place in the production of this race. Other experiments in 

 which the food-relations are subject to analysis have shown the presence in the 

 gamete of a general basic growth and metabolism factor, with which are asso- 

 ciated several determining agents or determiners that may be dissociated and 

 replaced in the combination. In this series the probability is that the food 

 determining agent, determiner F„ from diversa, has combined with the chro- 

 matic receptor of the female parent, CMR^, the combination being productive 

 of the limitation as to food. What became of the determiner F„ from the female 

 line remains to be discovered. 



The importance of reactions of the kind here described in evolution in nature, 

 I believe, merits attention. By this method of reaction there arises at once a 

 precise, isolated group with capacities and characters derived from its pro- 

 genitors fitting it to be successful from the start as an element in the fauna of 



