260 
HYBEIDISM. 
Chap. VIII. 
from this cross. That the fertility of hybrids is not 
related to the degree in which they resemble in exter¬ 
nal appearance either parent. And lastly, that the 
facility of making a first cross between any two species 
is not always governed by their systematic afSnity or 
degree of resemblance to each other. This latter 
statement is clearly proved by reciprocal crosses 
between the same two species, for according as the 
one species or the other is used as the father or 
the mother, there is generally some difference, and 
occasionally the widest possible difference, in the 
facility of effecting an union. The hybrids, more¬ 
over, produced from reciprocal crosses often differ in 
fertility. 
Now do these complex and singular rules indicate 
that species have been endowed with sterility simply 
to prevent their becoming confounded in nature? I 
think not. For why should the sterility be so extremely 
different in degree, when various species are crossed, all 
of which we must suppose it would be equally important 
to keep from blending together? Why should the 
degree of sterility be innately variable in the individuals 
of the same species ? Why should some species cross 
with facility, and yet produce very sterile hybrids; 
and other species cross with extreme difficulty, and yet 
produce fairly fertile hybrids ? Why should there often 
be so great a difference in the result of a reciprocal 
cross between the same two species ? Why, it may 
even be asked, has the production of hybrids been per¬ 
mitted? to grant to species the special power of pro¬ 
ducing hybrids, and then to stop their further propaga¬ 
tion by different degrees of sterility, not strictly related 
to the facility of the first union between their parents, 
seems to be a strange arrangement. 
The foregoing rules and facts, on the other hand, ap^ 
