156 IV' CONFKRKNCK INTI.ItN ATIONAI.K HF. C.KNKTiniJE. 



ON ACCLIMATIZATION BY RECOMBINATION OF MENDELIAN FACTORS 



SIMMAIiV 



Il is cvidiMil lliat in (•onsidcriiit; Uic (Hicsliiiii of Iho nllfiiilidii ni' ;i Ivpfi 

 llio incndelian idoa of llio recomiiinalion of cliaraclers is of greal impoilancc 

 If a quanlitative character, for inslance size, is concerned wilh sevoral iiidc- 

 [KMidant mendelian faclors, which by varions coml)inations givc riso (o dill'crcnl 

 tirades of tliis character, then il bcconies possible loproducca graduai change in 

 Ihe populalion, by means of seleclion of individuals,and repealed hybridizalions. 



The exlenl, and the duralion of such a change, dépend on the nnmber and 

 llienalure of Ihe unit factors. In my opinion \ve are only on Ihe Ihreshold of 

 any real underslanding of Ihe inherilance of quantilalive variation. Il is 

 possible that in the case of certain organisms and characters, quantitative 

 variation niay be more complicated, and dépend on a niuch larger numb(>r ol' 

 factors, Ihan in olher cases. l'^or Ihis rcason, il cannol be denied, even l'roni 

 Ihe mendelian point of view, that the probabililies of Ihe occurence of a change 

 in the type may be very di lièrent, and similarly that il may be slower in some 

 cases than in others, allhough Ihere must ahvays be a fixed limil lo the changes 

 occurring by the recombinalion of faclors. By long continuai research much 

 light should be Ihrown on the question. 



According to my expérimental researches wilh the ccreals, therc can be 

 no doubl Ihal there is a certain relation belween Ihose adaplivc changes of 

 plants callcd heredilary acclimatization, and the regrouping of mendelian 

 factors. I hâve suggesled on the basis of experiments wilh autumn wheal. 

 pnblishcd in 1901, that the increased résistance to cold which certain délicate 

 varielies acquirc, afler being grown in Sweden for a séries of years, dépends 

 on the fact that allhough they may be morphologically uniform, in reality 

 Ihere arc varions types présent, which differin their powers of résistance. The 

 increased résistance to cold of such varielies is direclly due to the increase in 

 the numbers of the more résistant types. At thaï lime I said nothingabonl the 

 nature or origin of thèse forms and did not describe Ihem as mutations which 

 appeared from lime to lime, and of which only the most résistant survi\ éd. 



The facts cannot be explained so simply. On Ihe contrary, ail my 

 researches tend to show thaï the numerous types which can lie dislingnishcd, 

 bolh in the characters of résistance lo cold, precocity, and otber quantilalive 

 characters, are produced by varions comljinalions of certain mendelian factors, 

 Ihe origin of thèse factors being unknown. 



From crosses made belween types which dill'erfrom each olher only quan- 

 litalively, either in respect of precocity or of résistance to cold, there results 

 ahvays definite ségrégation, which however can only be proved by analysis of 

 Iheprogeny of each F, planl. In Ihe course of my experiments in hybridizalion 

 I hâve examined several hundreds of F- familles. W'ith regard to Ihc precocity 

 of wheal and of oals, the most important results (with Ihe exceplion of Ihe 

 proof of the occurrence of ségrégation) were as foUows. 



Isl. — Ségrégation from a cross belween true breeding forms \\ hich dilTcr 

 from each olher in precocity, lias never given the simple mendelian ralio 

 1:2:1 but a much more complicated ralio. 



