160 



The Mechanism of Evolution in Leptinotaesa 



have been found to interchange in the crossing of these two species. The com- 

 moner are shown in table 21. 



Tabu: 21. 



These six pairs of agents, which are recognizable as being shifted in crossing, 

 give, if they act independently, a possible hypothetical Fg array as follows : 



(1) Number of individual combinations (4°) or 4' = 4096. 



(2) Number of terms in the series (3°) or 3* = 729. 



(3) Number of stable combinations (2") or 2' = 64. 



To this series could be added the combinations of some of these into associ- 

 ations that act singly, and also the dissociation of some of the groups into com- 

 ponent simpler ones. Thus, in color of the body there has been obtained no 

 hypodermal color, white, pale yellow, yellow, chrome yellow, orange, red, and 

 Vermillion in pure-breeding strains out of this cross, and localized sometimes in 

 the pronotum, sometimes in the elytra, or at others uniform over the body as a 

 whole; these, of course, in homozygous-acting races that behave when crossed 

 with other races as pure Mendelian pairs. I have not the least doubt that if 

 time, space, and labor Avere available one could easily obtain from this cross 

 several hundred of pure-breeding lines, each distinctive in one or more char- 

 acters, alternative to one another throughout. 



Two points of general interest come out in this complicated reaction : (1) the 

 dissociation of each of the gametic systems and the numerous interchanges that 

 are shown as the result of this cross; (2) the action of agents and groups of 

 agents as units, irrespective of the number of contained agents present. This 

 second point is of especial importance as showing that the type of reaction is not 

 one that involves the ultimate nature of the interchanging agents. The entire 

 series of reactions, the numerous stable end-products, presents a condition that is 

 strikingly in contrast with that shown in the crosses of species described in the 

 previous chapter. In plate 18 I have shown some of the homozygous biotypic 

 lines that have been derived as the products of the intercrossing of these two 

 species. Those shown are all derived from the middle group of P^, as far as 

 the complications presented in the adult condition are concerned, and consist 

 only of a few combinations of form and color. When these are added characters 

 in the juvenile stages, very many lines, each with its own sequence of characters, 

 are possible of derivation from this cross. The details of the production are not 

 worth presenting, involving only routine operations, with interplay of external 

 agents for the attainment of certain end-results. Each one of the 20 conditions 

 shown in plate 18 is biotypic for its adult characters and with ease can be made 

 biotypic for its entire constitution. The 20 shown can be made 40 by change 

 in larval color alone, yellow and red larval races being possible of each one of 

 these, and so on the biotypic lines could be multiplied. 



