180 The Mechanism of Evolution in Leptinotaesa 



thereof, the presence of which is dominant to its absence in typical Mendelian 

 reactions. In this species cross the typically Mendelian reaction, trihybrid, is 

 entirely a product of the number of the dissociated groups, and independent of 

 the character or size thereof. 



In the crosses between the species decemlineata, oblongata, and muUitceniaia, 

 another aspect of the same problem appears, namely, the complexity or simplicity 

 of the reaction depending upon the complexity of the gametic constitution, as in 

 the cross of decemlinealaxmultitceniata, where factors added to the complex 

 also added complexity to the type of reaction resulting. The cross of decem- 

 lineata and oblongata, however, showed still other conditions in the extensive 

 dissociation that followed the combination of these two in a hybrid reaction. In 

 decemlineata xmuUitceniata the modal biotypic conditions in homozygous lines 

 reacted in strict monohybrid reactions ; added factors to either disrupted this and 

 also induced dissociation in the gametic complexes; and so through the entire 

 series it is uniformly the result that the reaction which follows the combination 

 of these species gametic systems is the direct product of their differences, and the 

 dissociation resulting depends upon the number of equivalent factorial groups 

 capable of metathesis. This sequence of events following the cross we may call 

 the Mendelian reaction and not Mendelian heredity. 



The chief point that I desire to make in this connection is that in the crossing 

 of species direct from nature there is but one fundamental type of action, 

 which depends entirely on the factorial constitution of the gametes combined, 

 multiplied by the conditions of the medium at time of combination, and that 

 the resultant reaction is the direct product of their differences and the capacity 

 for metatheses between equivalent factorial groups. If the reaction proceeds 

 under conditions in the medium that are common to and act in like manner upon 

 the two systems, the reaction and products will appear in proportions and com- 

 binations that are the product of chance, but if the conditions of the medium 

 act to accelerate action in one gametic system or to retard it more or less than in 

 the other, then the dissociation and combinations are disturbed and the result- 

 ant array of products is entirely dependent upon the nature and action of this 

 active agent in the medium. This Mendelian reaction is our first and at present 

 most important means of investigating the gametic constitution and action of 

 organisms. It is not a law of heredity ; it tells us nothing of the way characters 

 are maintained through countless generations of similar organisms nor why 

 characters exist at all ; but it sliows a first method of investigating the factorial 

 agents in the gametes that are, in ways at present unknown, concerned in the 

 production of the characters in endless repetitions. 



But once for all let us discard the notion of characters and of kindred concep- 

 tions, and recognize that the presence of a character in an organism is tliere as in 

 the inorganic, the end-result of the interactions of the component non-living sub- 

 stances, and so the physical basis of these characters that are used here as mere 

 indicators of the presence and type of reaction present may be only HO groups 

 in the material substance that bridges the gap between generations. We do not 

 know the exact agents that produce these end-results, but their precision of 

 action, and the certainty of their duplication, times without number, in the same 

 line of living beings can admit of no other viewpoint than that the production of 

 a character is, in organisms, the same resultant of the physical interactions of 

 the materials of composition, and due to the same precision of process that pro- 



