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 FI 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 Kolreuter, 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- 



