228 GENETICS IN RELATION TO AGRICULTURE 



pletely fertile. East has reported investigations of a cross between 

 N. (data and AT. langsdorffii. N. alata has flowers the corolla length 

 of which averages about 82 mm., whereas the corolla length of flowers 

 of N. langsdorffii averages not over 22 mm., so that N. alata flowers are 

 nearly four times as large as those of N. langsdorffii. In addition to 

 these differences there are other distinct differences between the two 

 species. Nevertheless examination indicated that there was little, if 

 any, diminution in fertility in F\. A few other species hybrids in this 

 group of Nicotiana give highly fertile F\ hybrids, for example N. alata X 

 N. sanderce and N. langsdorffii X N. sanderce. There appears to be 

 little reason for not regarding these as species hybrids, although it should 

 be stated that some investigators feel inclined to restrict the species 

 concept to forms which display a certain degree of partial sterility in F\. 

 Such a line of separation must, however, be purely arbitrary since it 

 can be shown that fertile species hybrids merely represent one of the 

 extremes in a continuous series extending from complete fertility to 

 complete sterility. 



Since partial sterility is such a characteristic feature of species hybridi- 

 zation, it is not surprising to find that diminution in fertility is not 

 associated with any particular kind of character expression in the hybrids. 

 Intermediate hybrids as well as those which more or less resemble one 

 of the parents usually, therefore, display a considerably diminished 

 fertility. Not much has been done with such hybrids for aside from 

 exceptional instances sterility presents at once a bar to their further 

 analysis and to their use for economical purposes. A familiar example, 

 the mule, a cross between Equus caballus and E. asinus, has given no 

 authentic case of the production of offspring, although produced for many 

 centuries under domestication and in vast numbers. Among plants so 

 many examples occur that it is of no advantage whatever to attempt 

 an enumeration of them here. The student who is particularly inter- 

 ested in such matters will find that excellent compilations of species 

 hybrids in plants have been made by Gartner and Focke; and Ackermann, 

 Przibram, and Rorig have performed a similar service for the animal 

 kingdom. 



In tobacco a large number of species hybrids occur which give partially 

 sterile intermediate hybrids. The genus Nicotiana had been much 

 employed in hybridization investigations providing as it did the first 

 instance of hybridization in the plant kingdom when in 1760 Kolreuter 

 crossed N. rustica and N. paniculata. The hybrid thus obtained was 

 intermediate in its characters and was only slightly fertile. Varying 

 comments have been made as to the exact expression of the characters 

 of this hybrid as compared with those of its parents, but apparently 

 careful scrutiny reveals the influence of both parents in practically 



