558 



GENETICS IN RELATION TO AGRICULTURE 



female hybrids with the zebu, Bos indicus. With the yak, Bibos grun- 

 iens; the gayal, Bibos frontalis, the gaur, Bibos gaurus, and the bison, 

 Bison americanus, the female hybrids with the domestic cow are fertile, 

 but the males are sterile. The banteng, Bibos sondaicus, and the zebu 

 behave like this latter series in giving fertile female and sterile male 

 offspring. In this respect they resemble Detlefsen's and Castle and 

 Wright's results with species crosses among guinea-pigs, the female 

 hybrids of which were fertile, the males sterile. 



Among domesticated birds in particular the reproductive powers are 

 strongly disturbed by hybridization. Not only are such hybrids often 



FIG. 219. Abnormal reduction divisions in spermatogenesis of the mule. (After 



Wodsedalek.) 



sterile, but very frequently the sexual organs develop in an abnormal 

 fashion strongly suggestive of intersexualism of the kind exhibited by 

 Goldschmidt's Lymantria hybrids. Smith and Thomas have examined 

 sterile hybrids between species of pheasants. They found that very 

 often ovarian degeneration or imperfect development occurs in the 

 females, as a consequence of which a marked tendency exists to assume 

 plumage patterns and characters peculiar to the male. 



Here we are dealing rather definitely with a type of sterility different 

 from that which characterizes different families within a species or 

 breed or different mutant types of Drosophila, the sterility here appears 

 to be more deep-seated and strangely enough, far from being associated 

 with a general diminution in vigor, the vigor and size of the hybrids are 

 often very augmented. We are not surprised, therefore, to find that 

 profound disturbances in the hereditary mechanism occur in such hybrids. 

 Wodsedalek has shown that irregular reduction divisions occur in the 

 mule (Fig. 219). Smith and Thomas have shown specifically that in 

 sterile hybrid pheasants of both sexes the abnormal behavior and de- 



