VARIATION IN PEDIGREE-CULTURES 207 



DISCONTINUOUS VARIATION IN PEDIGREE-CULTURES 1 



By Dr. D. T. MACDOUGAL 



DIRECTOR, DEPARTMENT OF BOTANICAL RESEARCH, CARNEGIE INSTITUTION OF WASHINGTON 



"I T EREDIT Y may be defined as that appurtenant function 

 -■ — *- of living matter by which qualities, characters and capaci- 

 ties are transmitted through successive generations. The absolute 

 identity, or measurable expression, of the inherited characters may be 

 qualified by partial and individual fluctuations about a norm in a 

 continuous series in the progenies, and these characters, singly or in 

 groups, may be subject to dominations, recessions, integrations and 

 resolutions in hybridizations, or to various forms of combination, 

 actual or apparent. In the case of discontinuous variations or muta- 

 tion, single or unit characters, or constellations of them, may be 

 activated, or converted into a latent or perlatent condition. 



Methods of Investigation 



Owing to the stimulus of recent discovery, attention is focused at 

 the present time upon the ultimate result of fluctuating variability 

 as influenced by various agents, in the origin and fate of hereditary 

 strains, species, races or physiologically unified groups of organisms, 

 and upon the probable part played in the matter by the saltatory 

 movements, which are being brought to notice so plentifully in all 

 quarters. In connection with the last-named feature, the behavior of 

 the individual qualities or unit-characters in hybridizations are being 

 studied with enormous zeal as offering a ready analysis of the action 

 of inheritance. The comparative ease with which hybrid combina- 

 tions of plants are effected, and the simplicity of the subsequent 

 resolutions in the progeny, render this phase of the subject easy of 

 attack, and results are being obtained, which, if one may judge from 

 recent literature, do not seem available to many writers. 



It needs but a moment's consideration to bring the realization that 

 the entire subject offers some of the most abstruse and difficult prob- 

 lems in the whole realm of biological science. Intricate and elusive in 

 their physiological complexity, we may hope to uncover the main 

 factors by perfected methods in research upon the ultimate mechanical 

 basis of heredity coupled with a refinement of technique in dealing 

 with the course of inheritance as we trace it from generation to 



1 Lecture given at Marine Biological Laboratory, Woods Holl, July 20, 1906. 



