528 CAROLINE BURLING THOMPSON 



Thompson and Snyder ('19), attempting to answer the question 

 of the mode of origin of the termite castes, suggested that the 

 castes might be interpreted either as a series of fluctuating varia- 

 tions or as mutations ''comparable to the series of mutations 

 found in Drosophila." To-day, the writer, influenced by the 

 recent work of Morgan and his school, especially by their interpre- 

 tation of the genetic behavior of Oenothera lamarckiana, be- 

 heves that termite castes should be interpreted as comparable to 

 the offspring of Oenothera, as arising by segregation from a 

 heterozygous parent form. In modern terminology, therefore, 

 the termite castes are not mutants, in the sense of the progeny of 

 Drosophila, arising once for all from a mutating parent, and then 

 breeding true, but are rather segregants, in the sense of the off- 

 spring of Oenothera lamarckiana, arising generation after genera- 

 tion by the splitting and recombination of the genes of a hetero- 

 zygous parent form. Aly views on this point therefore, are in 

 general agreement with those of Imms, except in the use of the 

 term mutant, which cannot to-day be apphed with exactness to 

 the recurrent termite castes. 



With another theoretical point advanced by Imms I am unable 

 to agree. Imms ('19, p. 146) says of the wingless third form of 

 Archotermopsis, which he terms the 'worker-hke' form: ''I con- 

 sider that they exhibit the first step in the evolution of the worker 

 caste." . . . . ''At the same time they afford a clue to the 

 possible origin of the worker, which appears to have arisen as a 

 mutation of the nymphal stage and not of the winged adult.^' 

 (ItaUcs mine.) The view that the wingless sterile worker is 

 merely a physiological phase of the wingless fertile third form, 

 and one a step to the other, has also tempted the writer (Thomp- 

 son and Snyder, '20), but a careful study of any termite genus 

 mth both castes gives strong e\ddence that the two castes are 

 morphologically distinct. The fertile third form of higher ter- 

 mites, like the first form, is probably heterozygous, and produces 

 among its offspring sterile workers, but we lack as yet actual 

 proof of this. We do know, however, that the first form gives 

 rise to both third forms and workers. Imms' statement, that 

 workers may have arisen as mutations of the njonphal stage, and 



