192 INBREEDING AND OUTBREEDING 



ferences in many characters, yet their hybrids' give no 

 indication of any lessened fertility. 



(2) The hybrid may have the same or greater vigor 

 than the parents and at the same time show reduced fer- 

 tility or even total sterility. This is a common result with 

 many species hybrids. The increase in growth is often- 

 times extreme. The cross between the garden radish, 

 Raphanus sativus, and the cabbage, Brassica oleracea, 

 two species belonging to different genera, gives plants of 

 rampant growth which are very nearly, if not completely, 

 sterile, as shown by Sageret 191 nearly a century ago and 

 by Gravatt 85 in recent times (Fig. 38). In most of these 

 cases no seed can be produced by self-fertilization, but 

 back crossing with one or the other parents is sometimes 

 successful. Animal hybrids frequently show sterility in 

 the males and partial or complete fertility in the females. 

 This is the condition in Cavia species hybrids (Detlef- 

 sen 47 ) and in crosses made between the buffalo, Bison 

 americanus; the yak, Bibos gruniens; the gayal, Bibos 

 frontalis; the gaur, Bibos gaurus, and the domestic cow, 

 Bos taurus. 



(3) The species hybrid may exhibit a reduced size and 

 a decline in vigor combined with complete sterility. The 

 phenomenon shows in various degrees. For example, 

 East and Hayes 59 made several Nicotiana crosses in 

 which the seed would not germinate, although both em- 

 bryo and endosperm tissue was formed. Crosses between 

 Nicotiana tabacum and N. paniculata, and between N. 

 rustica and N. alata resulted in seed which germinated, 

 but the plants were weak and died before flowering, appar- 

 ently because of inability to utilize the starch formed. 



