594 Species According to the Theory of Mutation. 
to an enlargement of genera. On the other hand it would 
put a check on the splitting up of genera which has been 
so much in vogue of recent years, and not less to the 
elevation of subgenera to the rank of genera. In practice, 
however, the application of HERBERT'S suggestion has 
proved impossible. At that time there were only a few 
generic hybrids, but their number has undergone a con- 
siderable increase ; partly no doubt through the arbitrary 
splitting up of genera mentioned above, but partly also 
through the rapid accumulation of experimental data. 
Berber is and Mahonia (e. g. B. Neuberti) could well be 
united into one genus. The suggested union of rye and 
wheat, a hybrid between which has been raised by RIM- 
PAU, 1 into a single genus Frumentum, is not likely to 
win much favor; and the fact that BURBANK, in Cali- 
fornia, has raised a hybrid bet ween Nicotiana and Petunia 
which he calls Nicotunia, 2 will hardly be regarded as a 
sufficient ground for a systematic union of these two 
genera. There are now about 150 bi-generic hybrids 
amongst the Orchids, especially between the genera Lae- 
lia, Cattleya, Epidendrum and Sophronitis, as also be- 
tween Zygopctalum, Colax and Batemannia. 3 
The practical difficulties which stand in the way of 
HERBERT'S proposal are, on the one hand, the fact that 
the limits to possible hybrid combinations are by no means 
definite, and on the other hand, the objection, which has 
so often been raised, that crosses are exceptional phe- 
nomena and that it would therefore be impossible to 
1 W. RIMPAU, Kreuzungsprodukte landwirthschaftlicher Cultur- 
pHanzen. Landwirthsch. Jahrb., 1891, p. 20, and PI. VI, Fig. 58. 
2 LUTHER BURBANK, Neiv Creations in Fruits and Floivers (Bur- 
bank's Experiment Grounds, Santa Rosa, California), 1893, with a 
figure of the Nicotunia. 
3 C. C. HURST, Journ. Roy. Hort. Soc., Vol. XXIV, pp. 102, 125. 
