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 
Belvedere (1706) 
Short Tail (2621) | 
Duchess 32nd 
4th Duke of Northumber- 
land (3649) | Belvedere (1706) 
| Duchess 34th 
Duchess 29th 
Duchess 55th 
ee nd Hubback (1423) 
Norfolk (2377) 4 
| Nonpareil 
Duchess 38th 
Belvedere (1706) 
Duchess 33rd 
Duchess 19th 
Fie. 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- 
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