170 



The Journal of Heredity 



as a whole. This table allows us to 

 make comparisons and determine any 

 significant difference between the aver- 

 age age of the superior cows and the 

 (comparatively) inferior cows, 

 i It will be noted in table I that 

 the greatest difference in age is four 

 months and that this occurs in the 

 case of the sires. This difference 

 might seem to be somewhat significant 

 in the light of the probable error as a 

 measure of its probable significance. 

 The difference is more than three 

 times the probable error but the prob- 

 able error in this case is small. We 

 must remember, however, that four 

 months is not a very marked difference 

 and this difference loses a large part of 

 its significance when we study the 

 group averages. There is more differ- 

 ence between the groups within the 

 classes even here than there is between 

 the classes themselves. This is still 

 more noticeable in other places in the 

 table where the class difference is not 

 so great. Take for example, the case 

 of the dams, the great granddams, or 

 the maternal grand sire. In each of 

 these cases, the difference is probably 

 insignificant in the light of the prob- 

 able error, and in each case, the differ- 

 ence between the groups within the 

 class is greater than the difference 

 between the classes. 



The real difference in age between 

 the various parents and generations is 

 more clearly shown by means of a 

 pedigree. Such a pedigree is given in 

 Figure 12. 



It is evident that there can be no 

 increased hereditary value due to the 

 age of the dams, for in every case ex- 

 cept one, the dams of the Inferior Class 

 average older, though not significantly 

 so, than the dams of the Superior Class. 

 In the one case, that of the paternal 

 granddam, they average exactly alike. 



From a study of Table I and the 

 pedigree, Figure 12, it is apparent 

 that there is no significant difference 

 between the ages of parents of the 

 Inferior and of the Superior Classes. 

 In only one case is the difference out- 

 side the limits defined by three times 

 the probable error. In this case, the 



small difference between the class 

 averages can hardly be considered sig- 

 nificant when there is so great a differ- 

 ence between the group averages within 

 the class. 



BULLS ARE NO MORE VALUABLE 

 PARENTS AS THEY GROW OLD 



Turning our attention again to the 

 pedigree, Figure 12, we note that in 

 every case the Superior sires on the top 

 line of the pedigree are older than the 

 corresponding inferior sires. The dif- 

 ference is probably not significant 

 even in the case of the sires, but the 

 tendency persists. 



This led to a study to determine 

 whether a bull produced superior 

 offspring as he grew older. From a 

 study of four bulls having a total of 

 five hundred and six daughters, it 

 seems that bulls may or may not have 

 better daughters as they grow older, 

 depending on the quality of animals to 

 which they are bred. As a bull proves 

 his superior worth, he is very likely to 

 be bred to better cows and produce 

 better offspring. This probably ac- 

 counts for the difference shown in the 

 pedigree. This whole matter will be 

 fully discussed in a later paper. Thus 

 the difference in age of sires can be 

 very readily assigned to the use of the 

 sires and there is no indication that 

 the sires are any better when they are 

 old than when young and immature. 

 In fact the frequency distribution, 

 Figures 13 and 14, tends to show that 

 the young parent of either sex is just as 

 valuable as a parent when immature 

 as it is in later life. The frequency 

 curves include only the sires and dams 

 of the first generation. 



FREQUENCY DISTRIBUTION ACCORD- 

 ING TO AGE OF SIRE 



In the frequency curves, Figures 

 13 and 14, it will be observed that 

 the curves run very nearly parallel 

 throughout. The curves start high, 

 indicating that a large percentage 

 of cows in both the Superior and 

 Inferior Classes are sired by very 

 young bulls. In this connection, it 

 will be remembered from Table I that 



