296 HOW CROPS GROW. 
and vegetable kingdoms the limits within which hybridiza 
tion is possible appear to be very narrow. It is only be 
tween closely allied species that fecundation can take place. 
Wheat, oats, and barley, show no tendency to “ mix”; the 
pollen of one of these similar plants being incapable of 
fertilizing the ovules of the others. 
In flower and fruit-culture, hybridization is practised or 
attempted, as a means of producing new kinds. Thus the 
celebrated Rogers’ Seedling Grapes are believed to be hy- 
brids between the European grape, Vitis vinifera, and 
the allied but distinct Vitis labrusea, of North America. 
Hybridization between plants is effected, if at all, by 
removing from the flower of one kind, the stamens before 
they shed their pollen, and dusting the summit of the pistil 
with pollen from another kind. 
The mixing of different varieties, as commonly happens 
among maize, melons, etc., is not properly hybridization, 
this word being used in the long-established sense. _We 
are thus led to brief notice of the meaning of the terms 
species and variety, and of the distinctions employed in 
botanical classification. 
Species.—The idea of species as distinct from variety 
which has been held by most scientific authorities hither- 
to, is based primarily on the faculty of continued repro- 
duction. The horse is a species comprising many vari- 
eties. Any two of these varieties by sexual union may 
propagate the species. The same is true of the ass. The 
horse and the ass by sexual union produce a hybrid—the 
mule,—but the sexual union of mules is without result. 
They cannot continue the mule as a distinct kind of ani- 
mal—as 1 species. Among animals a species therefore com- 
prises all those individuals which are related by common 
origin or fraternity, and which are capable of sexual fer 
tility. This conception involves original and permaneot _ 
differences between different species. 
