No. 641] INHERITANCE OF CANCER IN MICE 



515 



but even in such cases the increase was moderate; there 

 was never observed among our material a sudden transi- 

 tion from a low to a high rate tumor strain. The increase 

 as well as the decrease in the tumor rate was caused by 

 the same factor; whether one or the other should prevail 

 depends more or less on chance, and in different material 

 the number of strains showing the one or the other varia- 

 tion may be expected to differ. It has been maintained 

 that in strains which have been inbred foi- a loni>- ]i('i'iod 

 of time and in which a decrease in fertilil>- occniTpd as 

 the result of the inbreeding, developmonl of (•.•nicrr- \:\]{(^< 

 the place of the lost fertility. In inliici ilinu' cnici r re- 

 places reproduction, as it has been exi)rcss( (l 1)\ Mnud 

 Slye. In our material such a substitution did not take 

 place; in inbreeding mice vanishing fertility was not re- 

 placed by the development of cancer under ordinary 

 conditions. Inbreeding does not lead to an increased 

 cancer rate. 



3. If we cross strains with a similar tumor rate, the 

 offspring inherits the tumor rate common to l)oth par- 

 ents; if both parents differ in tnnior rate, tlic tiinior rate 

 of the offspring is on tln^ wiiolt^ intcrnicdinie l)ct\veen 

 those of the parents, lint all <lc-T(rs of i iitcrnirdiMcy 

 are observed. In our material the nuni])er of strains in 

 which the rate of the parent with the higher tumor in- 

 cidence dominated was on the whole greater than the 

 contrary one. 



We selected for our hybridizations especially strains 

 which differed markedly in their tumor rate and other 

 characteristics and which had been followed over long 

 periods of time and had been found consistent in their 

 behavioi-. TIk' Kiiu'li^li as a representative of a high 



Unv minor rate >irain were e>i,eeia!l\ Miitable. for this 

 purpose. In tlie tiiajoritv- (»f ea-e- we -eh .'ted few in<li\ id- 

 uals for hybridization, eithiM' one ]iiah' and one female or 

 one male and sevei-al feinah'-. \Ve followed the offspring 

 through several generations. The near relatives of the 



