ROBERTSON: MULE AND HORSE TWINS. 297 



REFERENCES. 



Castle, W. E. 1916. Genetics and Eugenics. Cambridge, Harvard 



Univ. Press. 

 LiLLiE, F. R. 1917. The Freemartin; A Study of the Sex Hormones in 



the Foetal Life of Cattle. Journal Exp. Zool., vol. 23, No. 2. 

 MUMFORD, F. B. 1917. The Breeding of Animals. New York, The Mac- 



millan Co. 

 Newman, H. H. 1917. The Biology of Twins (Mammals). University 



of Chicago Press. 



EXPLANATION OF PLATES. 

 PLATE I. 



Ii. Black percheron stallion. 



12. Black mare. 



13. Full blood Arabian stallion; spotted over hips. 



14. Wild pony mare from herd ; color was black with large white spots. 



111. McDuff, a black percheron stallion. Service charges, $25. 



112. Black mare. 



113. Arabian stallion; white, with some black spots. 



114. Oregon mare, branded ; color, sorrel ; had white points from knees 



and hocks down, and a blaze in face. 

 nil. Horse stallion; dark bay. 

 IIIo. Ass stallion. 



1113. Ass stallion; black. 



1114. Ass stallion. 



1115. Ass stallion. 



Hie. Horse stallion; black. Not related to III9. Used to tease before 



breeding to ass stallion. Was two years old, coming three, at 



the time. 

 III7. Ass stallion, "Mammoth" breed; gray. Owned by farmer W. T. 



Watson of Barnard, Kan. Used ten minutes after Ille- 

 Ills- Ass stallion. 

 III9. Sorrel mare with white face and white points. (See figures 1, 2 



and 3.) Face spot slightly to right. Foaled 1892. Mother of the 



mule twins, IVe, IV7; the horse-mule twins, IVio, IVn; and 



IV12, IV13. Died winter of 1915-'16. 

 IIIio. Half sister to III9, dam of twins IV14 and IV15. Was owned by Mr. 



Loy's mother. 

 IIIii. Horse stallion. 

 IVi. Horse (gelding), sorrel. Foaled 1899. Markings and color (see fig. 



4) same as mother III9. 

 IV2. Horse stallion; sorrel. 



IV3. Mare; solid bay. Foaled 1901. Mother of twins Vi and V2. 

 IV4. Horse (gelding) ; solid bay. Foaled 1902. 

 IV5. Mule (male). 

 IVe, IV7. Pair of male mule twins; one black, one brown. Black one died 



48 hours after birth. "Its bowels did n't work. Veterinary gave 



it oil ; then it scoured to death." 



20— Sci. Bull. X. 



