HYBKIDISM. 
Chap, VIII. 
248 
It is certain, on the one hand, that the sterility of 
various species when crossed is so different in degree 
and graduates away so insensibly, and, on the other 
hand, that the fertility of pure species is so easily 
affected by various circumstances, that for all practical 
purposes it is most ^difficult to say where perfect fer¬ 
tility ends and sterility begins. I think no better evi¬ 
dence of this can be required than that the two most 
experienced observers who have ever lived, namely, 
Kolreuter and Gartner, should have arrived at dia¬ 
metrically opposite conclusions in regard to the very 
same species. It is also most instructive to compare— 
but I have not space here to enter on details—the evi¬ 
dence advanced by our best botanists on the question 
whether certain doubtful forms should be ranked as 
species or varieties, with the evidence from fertility ad¬ 
duced by different hybridisers, or by the same author, 
from experiments made during different years. It can 
thus be shown that neither sterility nor fertility affords 
any clear distinction between species and varieties; but 
that the evidence from this source graduates away, and 
is doubtful in the same degree as is the evidence derived 
from other constitutional and structural differences. 
In regard to the sterility of hybrids in successive 
generations; though Gartner was enabled to rear some 
hybrids, carefully guarding them from a cross with 
either pure parent, for six or seven, and in one case for 
ten generations, yet he asserts positively that their fer¬ 
tility never increased, but generally greatly decreased. 
I do not doubt that this is usually the case, and that 
the fertility often suddenly decreases in the first few 
generations. Nevertheless I believe that in all these 
experiments the fertility has been diminished by an 
independent cause, namely, from close interbreeding. 
I have collected so large a body of facts, showing 
