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 completely independent of their systematic
affinity, 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 jalappa 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. jalappa, 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 reciprocal 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.
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