214 



The Mechanism of Evolution in Leptinotarsa 



are produced in the larvae. In the Fg extracted dgnaticollis types there is a 

 regular proportion of the fraternity in which the normal type of spots has been 

 replaced by that from undecimlineata, giving a yellow larva with the usual num- 

 ber of black tergal markings, but without the characteristic fusions. An 

 extracted race of this sort does not have the tendency to produce the direc- 

 tions of fusions that the normal species does; the typical spiracular spot of 



Fig. 29. — Diagram showing centers of coloration 

 on the abdominal segments in the larval stages of 

 L. diversa and L. signattcollia, and directions in 

 which fusion between these elements may take 

 place. 



signaticollis has been replaced by an equally specific condition from another 

 species by the reaction in crossing. In figure 30 I have shown these two types 

 of larvae, which are different enough in pattern so that anyone can at once 

 distinguish the two. When two such biotypic races of signaticollis are crossed 

 typical Mendelian arrays are always produced in Fg, showing the remarkably 

 exact and specific nature of the productive factors and determiners of these 



Fig. 30. — Showing pattern In third larval stage of L. undecimlineata and in 

 L. signaticolUs and effects produced In the larvae by the crossing of these two, which 

 results in the production of extracted races of signaticolUs in F2 having larvae 

 of two distinct types In the third stage; those like the original parent stock (C), 

 and those Intermediate between the two parental stock (B), In that in the latter 

 extracted race the different elements never fuse, hut remain separated as a stable 

 characteristic, so that the amount of pigment presenting this stable race (B) Is 

 Intermediate between the conditions represented In the two parents. 



trivial, nonutile characters. There are clearly present and operative in this 

 instance the general color-factor for black pigment (Cm), the black pigment 

 producer (Bm), the general body-pattern factor (PIII), and a specific 

 spiracular pattern-determiner (Sp) that are intimately concerned in the pro- 

 duction of these spots, which are absent if one of these essential elements in 

 its array of producers is absent. If either Cm or Bm is absent, no color is pro- 



