No. 636] GUIXKA PIGS AXD TUBERCULOSIS 



the differences in these respects coukl not he ini( i i)i i i.mI 

 merely as differences in general vigor. A igor f the 

 average in one respect was as likely as not to be found 

 associated with a subnormal record in another respect, 

 the correlation between the records of the families in two 

 respects coming out in most cases substantially zero. 

 Examples of these family differences will be brought out 

 later in connection with the effects of inoculation w^ith 

 tuberculosis. 



Explanation of the Eesults of Inbkeeding and Crossing 

 These results haraionize well, on the whole, with those 

 found by other investigators. It is believed that they can 

 be explained as consequences of the current theory of 

 heredity without recourse to the rather mystical ideals 

 wdiich once prevailed in regard to inbreeding. There 

 appear to be independently inherited factors which af- 

 fect frequency and size of litter, ability to bear the young 

 successfully, vitality and growth as well as for color, 

 pattern and the other characters in which the families 

 differ. There seem to be surprisingly few factors which 

 act on all of these characteristics. The concept, hered- 

 itary vigor, thus becomes merely an expression for the 

 sum of a number of independently inherited qualities 

 and not an entity. 



The factors w^hich cause reduced vigor in any respect 

 appear to be in general recessive. The primary effect 

 of inbreeding is to render homozygous a random group 

 of the factors present in the original stock. Some combi- 

 nation of factors, good, bad and indifferent, thus be- 

 comes fixed in each inbred line. As the recessive factors, 

 tending toward lack of vigor, are as likely to become 

 fixed as the dominant ones there is on the average a de- 

 cline in vigor in each respect. Moreover, owing to the 

 likelihood that many factors for vigor wall be linked 

 genetically with factors causing weakness, it is to be ex- 

 pected that vigor in all respects will be found in very few 

 lines, even where there is careful selection. In the pres- 



