106 THE BREEDING OF ANIMALS 



104. Unusual fertility. Each species and most varie- 

 ties or breeds of animals have a fairly uniform and normal 

 rate of increase. Thus the normal number of young at a 

 birth in cattle and horses is one. It is also probably 

 true that among sheep one is the normal number of young 

 at each birth. But many breeds of sheep have been 

 so changed by domestication that twins are frequent 

 and triplets are not rare. The quality of fertility is 

 undoubtedly transmitted by heredity. It is therefore 

 possible to increase the normal fertility of the domestic 

 sheep by selection. Among cattle the production of 

 twins has not been regarded as a particularly desirable 

 quality, and hence no attempt has been made to increase 

 the normal birth number of the bovine species. It is 

 not difficult, however, to conceive that it would be com- 

 paratively easy to develop a breed or variety of cattle 

 which would produce twins. Cases of unusual fertility 

 among all classes of the domestic animals are frequent. 

 These are of enough importance from a practical point 

 of view and of sufficient biological significance to be given 

 a place in a discussion on fertility. 



105. Unusual fertility among horses. Exceptional 

 fertility among horses is generally to be found in connec- 

 tion with longevity and active and regular functioning 

 of the breeding powers rather than in unusual numbers 

 of young at a birth. A mare twenty-five years old, owned 

 by R. O'Heren of Illinois, in 1904 was suckling her twen- 

 tieth colt. She was one-half Thoroughbred and one-half 

 Clydesdale and was still strong and active. 1 The Thor- 

 oughbred mare, Fanny Cook, dam of Daniel Lambert, 

 produced fifteen foals and dropped twins at twenty-two 



1 Reported in a letter to the author by R. H. Dunn, Illiopolis, 

 111. 



