':258 
HYBRIDISM. 
Chap. VIIL 
ences in every part of the flower, even in the pollen, in 
the fruit, and in the cotyledons, can be crossed* Annual 
and perennial plants, deciduous and evergreen trees, 
plants inhabiting different stations and fitted for ex¬ 
tremely different climates, can often be crossed with ease. 
By a reciprocal cross between two species, I mean 
the case, for instance, of a stallion-horse being first 
crossed with a female-ass, and then a male-ass with a 
mare: these two species may then be said to have been 
reciprocally crossed. There is often the widest possible 
difference in the facility of making reciprocal crosses. 
Such cases are highly important, for they prove that 
the capacity in any two species to cross is often com¬ 
pletely independent of their systematic afSnity, or of 
any recognisable difference in their whole organisation. 
On the other hand, these cases clearly show that the 
capacity for crossing is connected with constitutional 
differences imperceptible by us, and confined to the 
reproductive system. This difference in the result of 
reciprocal crosses between the same two species was 
long ago observed by Kolreuter. To give an instance : 
Mirabilis jalapa can easily be fertilised by the pollen 
of M. longiflora, and the hybrids thus produced are 
sufficiently fertile; but Kolreuter tried more than two 
hundred times, during eight following years, to fertilise 
reciprocally M. longiflora with the pollen of M. jalapa, 
and utterly failed. Several other equally striking cases 
could be given. Thuret has observed the same fact 
with certain sea-weeds or Fuci. Gartner, moreover, 
found that this difference of facility in making reci¬ 
procal crosses is extremely common in a lesser degree. 
He has observed it even between forms so closely related 
(as Matthiola annua and glabra) that many botanists 
rank them only as varieties. It is also a remarkable 
fact, that hybrids raised from reciprocal crosses, though 
