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THE AMERICAN NATURALIST [Vol. LV 



relation by assuming that a certain average quantity is 

 inherited in the individuals of different strains which 

 determines the intensity in the tendency towards the de- 

 velopment of tumors. This intensity may depend on the 

 average number and character of multiple factors favor- 

 ing tumor growth which is characteristic of a strain. A 

 special kind of factors or a larger number of factors 

 causes both a higher incidence and an earlier appearance 

 of cancer in a certain strain. 



(h) In addition to this intensity which is a character- 

 istic of the strains in general, there is a peculiar tumor 

 age in certain strains which is independent of the tumor 

 rate. Strains with a similar tumor rate may differ in 

 their tumor age, and in hylirids tumor rate and tumor age 

 may be inherited separately in the offspring. Thus two 

 of our high tumor rate strains formed by the crossing 

 of the same male (European 151) with two sisters (first 

 and second daughter of Xo. 10, respectively) with tumor 

 rates of 72 per cent, and ptM' cent., respectively, have 

 relatively late tumors, in lioth onlx IT) ])er cent, of the 

 Iniiiors a])i)e;irii]g in the lii'^t :iue p(MMo,l. Tn the Tream- 

 Kr,-IIsli liyl)ri(U thr tiiinu,- r;itrs in two strains xwiv sini- 



the tumor age approaclied tlu' hite ..n.' of thr rivnin par- 

 ent; in tlie other it was nearer the eai ly i>uv of the l-:uli>h 

 parent. 



We may therefore assume that in addition to tlu' sum 

 total of rnnlti|)le factors which determine at the same 

 tinif auv and tumor i-ate, there are special factors which 



7). In general tlie cancer rate in mice is not a sex-linked 

 character. Either the cancer rate ami auc ot' the father 

 or mother strain may predominate in the cancer rate of 

 the offspring. This fact does not. howcvfi-. exclude the 

 possil^ility that in certain cases a sc\ linkcl factor may 

 .■ntrr a^ nnv of the multiple factors which in all |)robabil- 

 ity (h'terniine the inheritance of cancer, ('ertain of our 

 observations suggest such a possibility. We found, for 



