228 GENETICS IN RELATION TO AGRICULTURE 
pletely fertile. East has reported investigations of a cross between 
N. alata and N. langsdorffii. N. alata has flowers the corolla length 
of which averages about 82 mm., whereas the corolla length of flowers 
of N. langsdorffit 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 7:1. A few other species hybrids in this 
group of Nicotiana give highly fertile F; hybrids, for example N. alata x 
N. sandere and N. langsdorffii X N. sandere. 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 Hquus 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 Rérig 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 Kélreuter 
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 
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