CHAPTER IX - HYBRIDISM
Hybrids and Mongrels compared, independently of their fertility
Independently of the question of fertility, the offspring of species
and of varieties when crossed may be compared in several other
respects. Gartner, whose strong wish it was to draw a distinct line
between species and varieties, could find very few, and, as it seems
to me, quite unimportant differences between the so-called hybrid
offspring of species, and the so-called mongrel offspring of
varieties. And, on the other hand, they agree most closely in many
important respects.
I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gartner admits that hybrids from
species which have long been cultivated are often variable in the
first generation; and I have myself seen striking instances of this
fact. Gartner further admits that hybrids between very closely
allied species are more variable than those from very distinct
species; and this shows that the difference in the degree of
variability graduates away. When mongrels and the more fertile hybrids
are propagated for several generations, an extreme amount of
variability in the offspring in both cases is notorious; but some
few instances of both hybrids and mongrels long retaining a uniform
character could be given. The variability, however, in the
successive generations of mongrels is, perhaps, greater than in
hybrids.
This greater variability in mongrels than in hybrids does not seem
at all surprising. For the parents of mongrels are varieties, and
mostly domestic varieties (very few experiments having been tried on
natural varieties), and this implies that there has been recent
variability, which would often continue and would augment that arising
from the act of crossing. The slight variability of hybrids in the
first generation, in contrast with that in the succeeding generations,
is a curious fact and deserves attention. For it bears on the view
which I have taken of one of the causes of ordinary variability;
namely, that the reproductive system from being eminently sensitive to
changed conditions of life, fails under these circumstances to perform
its proper function of producing offspring closely similar in all
respects to the parent-form. Now hybrids in the first generation are
descended from species (excluding those long-cultivated) which have
not had their reproductive systems in any way affected, and they are
not variable; but hybrids themselves have their reproductive systems
seriously affected, and their descendants are highly variable.
But to return to our comparison of mongrels and hybrids: Gartner
states that mongrels are more liable than hybrids to revert to
either parent-form; but this, if it be true, is certainly only a
difference in degree. Moreover, Gartner expressly states that
hybrids from long cultivated plants are more subject to reversion than
hybrids from species in their natural state; and this probably
explains the singular difference in the results arrived at by
different observers: thus Max Wichura doubts whether hybrids ever
revert to their parent-forms, and he experimented on uncultivated
species of willows; whilst Naudin, on the other hand, insists in the
strongest terms on the almost universal tendency to reversion in
hybrids, and he experimented chiefly on cultivated plants. Gartner
further states that when any two species, although most closely allied
to each other, are crossed with a third species, the hybrids are
widely different from each other; whereas if two very distinct
varieties of one species are crossed with another species, the hybrids
do not differ much. But this conclusion, as far as I can make out,
is founded on a single experiment; and seems directly opposed to the
results of several experiments made by Kolreuter.
Such alone are the unimportant differences which Gartner is able
to point out between hybrid and mongrel plants. On the other hand, the
degrees and kinds of resemblance in mongrels and in hybrids to their
respective parents, more especially in hybrids produced from nearly
related species, follow according to Gartner the same laws. When two
species are crossed, one has sometimes a prepotent power of impressing
its likeness on the hybrid. So I believe it to be with varieties of
plants; and with animals one variety certainly often has this
prepotent power over another variety. Hybrid plants produced from a
reciprocal cross, generally resemble each other closely; and so it
is with mongrel plants from a reciprocal cross. Both hybrids and
mongrels can be reduced to either pure parent-form, by repeated
crosses in successive generations with either parent.
These several remarks are apparently applicable to animals; but
the subject is here much complicated, partly owing to the existence of
secondary sexual characters; but more especially owing to prepotency
in transmitting likeness running more strongly in one sex than in
the other, both when one species is crossed with another, and when one
variety is crossed with another variety. For instance, I think those
authors are right who maintain that the ass has a prepotent power over
the horse, so that both the mule and the hinny resemble more closely
the ass than the horse; but that the prepotency runs more strongly
in the male than in the female ass, so that the mule, which is the
offspring of the male ass and mare, is more like an ass, than is the
hinny, which is the offspring of the female ass and stallion.
Much stress has been laid by some authors on the supposed fact, that
it is only with mongrels that the offspring are not intermediate in
character, but closely resemble one of their parents; but this does
sometimes occur with hybrids, yet I grant much less frequently than
with mongrels. Looking to the cases which I have collected of
cross-bred animals closely resembling one parent, the resemblances
seem chiefly confined to characters almost monstrous in their
nature, and which have suddenly appeared- such as albinism,
melanism, deficiency of tail or horns, or additional fingers and toes;
and do not relate to characters which have been slowly acquired
through selection. A tendency to sudden reversions to the perfect
character of either parent would, also, be much more likely to occur
with mongrels, which are descended from varieties often suddenly
produced and semi-monstrous in character, than with hybrids, which are
descended from species slowly and naturally produced On the whole, I
entirely agree with Dr. Prosper Lucas, who, after arranging an
enormous body of facts with respect to animals, comes to the
conclusion that the laws of resemblance of the child to its parents
are the same, whether the two parents differ little or much from
each other, namely, in the union of individuals of the same variety,
or of different varieties, or of distinct species.
Independently of the question of fertility and sterility, in all
other respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look
at species as having been specially created, and at varieties as
having been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.
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