556 



GENETICS IN RELATION TO AGRICULTURE 



which is given in Fig. 217. But the family was tainted from the be- 

 ginning with the curse of barrenness, which such a system of breeding 

 must inevitably preserve. Shortsighted breeders at the time considered 

 it a fortunate circumstance that Duchess cows were so often barren, 

 for it kept down the number of individuals of this favorite strain and 

 resulted in prices correspondingly high. But as a result of barrenness, 

 the strain eventually ran out completely. In Fig. 218, an attempt 

 has been made to show diagrammatically how barrenness was inherited 

 in this family. The diagram is not complete, for it includes only the 

 females in. the family. Nevertheless it brings out very forcibly how 



Duchess 55th 



4th Duke of Northumber- 

 land (3649) 



Duchess 38th 



Short Tail (2621) 



Duchess 34th 



Norfolk (2377) 



Duchess 33rd 



f Belvedere (1706) 



[ Duchess 32nd 



[ Belvedere (1706) 



[ Duchess 29th 



| 2nd Hubback (1423) 



[ Nonpareil 



[ Belvedere (1706) 



[ Duchess 19th 



FIG. 217. Pedigree of one of the latest Duchess cows, illustrating system of close-breeding 

 followed in maintaining the family. Duchess 55th produced two calves. 



barrenness occurred very early in the family history, and how it re- 

 appeared in about the same proportion of the total population through- 

 out its history. Far from showing an intensification of the defect as a 

 result of inbreeding, this diagram merely illustrates the heredity of a 

 defective family trait. 



Sterility of Hybrids. There is a definite type of sterility which is 

 referable to the effects of species hybridity. We have already had 

 occasion to comment upon this type of sterility in connection with other 

 matters, here we shall however refer to it again with particular emphasis 

 upon certain of its aspects. 



For the higher animals we do not possess much in the way of definite 

 data respective to hybrid sterility. The mule, a familiar and oft-cited 

 example, appears from all accounts to be very nearly completely sterile. 

 The accounts of fertility in mare mules are for the most part shadowed 

 in doubt, but the possibility of a slight fertility should not be denied. 

 The hinny, the homolog of the mule, exhibits as high a degree of ster- 



