60 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



of the guinea-pig or of other classes of hybrids. The depression in 

 the composite curve of the | wild females from the 120th day to the 

 340th day was due largely to our eager haste to breed these unusual 

 hybrids as soon and as frequently as possible. Furthermore, I should 

 not consider the composite curve as trustworthy as the skeletal dimen- 

 sions; because adult weights are more variable than adult skeletal 

 dimensions; and because possible errors in judgment arise, especially 

 when one subconsciously tries to avoid a bias in favor of ''too much 

 heterozygosis" in smoothing the individual growth curves from which 

 the composite curves were calculated. 



The species cross between the horse and ass gives the well-known 

 vigor for which the mule is so highly valued. Darwin (1876) pointed 

 out that cross-bred plants were often more vigorous than the inbred 

 parents. East and Hayes (1912) have concluded that the vigor is in 

 a measure proportional to the number of factors in a heterozygous 

 condition. Our | wild hybrids were undoubtedly heterozygous in 

 many factors, but we can not be sure that the more vigorous were 

 heterozygous in a greater number of factors. What part sterility may 

 play is also unknown. 



(3) The I wild of both sexes clearly lacked the vigor which charac- 

 terized the I wild. The composite curves of the males and females 

 lie entirely below those of the ^ wild. The greater part of both also 

 lies below the guinea-pig and the | wild. Although these I wild were 

 produced by mating the vigorous ^ wild back to the larger of the 

 original two parent species, it is obvious that both the males and 

 females were smaller at all ages than the ^ wild, and also smaller than 

 the guinea-pig during the larger part of their growth curve. 



If we regard the sexes separately, it will be seen that the I wild 

 males averaged less than the guinea-pig throughout the greater part 

 of their growth curve, for they lie distinctly below these up to the age 

 of 360 days. Their curves take an unexpected rise at the age of 340 

 days, but from personal experience with these animals I am led to 

 believe that this was due to the obesity of a number of older males which 

 were kept alone to prevent fighting. The difference between the I wild 

 males and the | wild is quite apparent, for they are separated by an 

 average of about 150 grams during a large part of their growth. It 

 is difficult to ascertain how much significance to attach to the aver- 

 age difference between the I wild males and their smaller parent, the 

 guinea-pig. They were consistently lower at all ages than the smaller 

 race of guinea-pig males until 360 days, although the difference was 

 not great. 



The I wild females resembled their brothers in many respects. They 

 likewise lie below the guinea-pig during the greater part of the growth 

 curve, for they were smaller up to the age of 260 days. Their growth 



