SPECIES HYBRIDIZATION 227 
of the wild agouti pattern, therefore, lends additional support to the 
conclusion that these two agouti factors, although occupying homologous 
loci in the hereditary systems of the two species are different from each 
other. 
The Forms of Species Hybrids.—Thus far we have dealt with two 
species crosses which have given satisfactory indications of behavior 
essentially in accord with generally accepted Mendelian principles. 
The remainder of the chapter will be devoted to general considerations 
respecting species hybrids and to particular cases which do not give 
entirely satisfactory evidence of Mendelian behavior. In common 
with most variety hybrids, species hybrids display marked uniformity 
in the first generation and equality of reciprocal crosses. Exceptions, 
however, occur to both these conditions and these we shall take up 
later in the discussion. 
With respect to the characters which they display species hybrids 
usually represent an intermediate condition as compared with the parents. 
We may refer this condition to a mixture of dominant factors derived 
from both parents and in some cases to actual intermediate expression 
of contrasted allelomorphs, as is not uncommon in variety hybrids. 
The intermediacy of F; in species crosses is a well-known phenomenon 
and is so common that it may be regarded as the rule. This condition 
was well known to the older hybridists, such as Kélreuter, Gartner, 
Naudin, and Focke, all of whom investigated extensive series of species 
hybrids with respect to the characters both of the immediate hybrid and 
of its progeny. The intermediate condition however, is not universal, 
for examples are known of all conditions from that of strict intermediacy 
to a condition so nearly resembling one parent in certain cases that only 
slight character differences or sterility establish the existence of an 
actual cross. 
Intermediate species hybrids are so common that it seems super- 
fluous to call especial attention to them, nevertheless this will be done 
in order to point out the relation of the intermediate condition to other 
characteristic features of species hybridization. In the first place the 
intermediate condition is not associated with any particular degree of 
fertility in the hybrids. Partial sterility is a common characteristic 
of wide crosses, and in fact this sterility in some cases appears to be 
complete. The Antirrhinum species hybrids are intermediate in practi- 
cally all characters, but they are apparently completely fertile. Such 
cases are, however, uncommon in species hybridization, but neverthe- 
less a few others have been studied. Baur and Lotsy have reported 
other species hybrids in Antirrhinum which give fertile intermediate 
hybrids. 
Some species hybrids in Nicotiana are known to be very nearly com- 
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