220 GENETICS IN RELATION TO AGRICULTURE 
although vigorous and normally developed, are totally sterile. The mule 
is a familiar example, many others could be given, but they will be con- 
sidered elsewhere along with the problem of sterility in species hybrids. 
For the present we shall consider one of the simpler cases in which the 
species hybridized, although differing very markedly in morphological 
characters, produce hybrids which appear to be fully fertile. 
Species Hybrids in Antirrhinum.—Baur crossed the wild Antirrhinum 
molle with the common garden snapdragon, Antirrhinum majus, and 
Baur and Lotsy have made extensive studies of the progenies obtained in 
successive generations of this cross and of other species hybrids in 
Antirrhinum. Antirrhinum majus and A. molle differ strikingly in a 
large number of morphological characters. The size proportions and 
general characteristics of the common snapdragon of the garden are fa- 
miliar to everyone. It is a strong growing erect herbaceous plant, about 
three feet high producing spikes of large zygomorphic flowers. Under 
the careful attention of commercial seedsmen it has produced a very large 
number of varieties which differ in the form and color of the flowers, in 
height and in other characteristics. Antirrhinum molle on the other 
hand is a low growing prostrate plant which is profusely branched and 
produces flowers about one-third as large as those of majus, but very like 
them in form and general appearance. The species differs from majus 
also in being apparently totally self-sterile, so that with respect to their 
genetic constitution plants of molle are normally heterozygous to some 
extent. Since molle occurs in nature in a number of slightly different 
forms its self-sterility must not be lost sight of in interpreting the results 
of hybridization between it and majus. F, of the reciprocal crosses 
molle X majus and majus X molle are completely self-fertile, and identical 
in every respect. Minor differences did occur but they were of such a 
nature that they could be accounted for as a result of the slight degree of 
heterozygosis of the particular plant of molle which was used as a parent. 
Baur employed a peloric majus for crossing with molle in order that he 
might follow a known factor difference throughout the investigation. 
The F, plants in this experiment bore zygomorphic flowers, a fact which 
indicated a corresponding behavior as regards dominance for the factor 
for zygomorphic flowers in molle and majus. Six Fy plants differing 
slightly in their characters were selected as parents for the F. generation. 
Lotsy grew the progeny of five of these, obtaining from them 624 Fy» 
plants. The general conclusions which Baur.and Lotsy have drawn 
from a study of these J’, plants is that the extreme range of forms dis- 
played, so great that no two plants resembled each other in all their 
characters, is a result of Mendelian segregation and recombination of 
characters. The diversity, however, was so great as to preclude the 
application of any exact factor analysis to the case. 
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