58 BOTANICAL GAZETTE [JANUARY 
The genera noted above are supposed to offer about two hundred 
hybrids, and as the observations have been made principally with 
the flora of eastern North America, in a region which probably does 
not furnish more than four thousand species, it is to be seen that 
the questions involved affect about 0.5 per cent. of the flora. Proba- 
bly not more than half of the instances included in the above list could 
be confirmed by actual tests; but on the other hand a closer examina- 
tion of other genera would reveal an equal number of actual occur- 
rences. 
Some of the constituents of the native flora are known to be constant 
unitypic hybrids, and hence have every claim to be regarded as 
species. The type of hybridization most widely different from this 
is illustrated by Bartram’s oak, and between these two diverse modes 
of action may be found. In gaining a more extended and accurate 
knowledge of the manner in which the qualities of separate unified 
strains of plants are alternative, or are interlocked, or fractionized in 
hybridizations, a vantage ground will be gained for the consideration 
of all questions in genetics as to saltations, minute accretions, and 
fluctuations of such characters. 
DEPARTMENT OF BOTANICAL RESEARCH, 
CARNEGIE INSTITUTION OF WASHINGTON. 
