CROSSING OF RACES AND SPECIES IN THE ANIMAL AND VEGETABLE
KINGDOMS.—MONGRELS AND HYBRIDS.
I. Sexual unions in plants, as in animals, can take place between
individuals of the _same species_ and the _same race_; further,
between _different races_ of the _same species_, and, finally,
between _different species_. In the two latter cases we have what is
called a cross. This crossing itself is differently named according
to whether it takes place between different races or different
species. In the first case it produces a _mongrel_, in the second
a _hybrid_. When the cross-unions are fertile the product of the
union of mongrels is called a _mongrel_, the product of the union of
hybrids a _hybrid_.
If the difference of the relations existing between the _race_ and
the _species_ has been properly understood, we ought to be inclined
to admit that mongrels and hybrids would not present the same
phenomena; experience and observation confirm this presentiment.
We have, therefore, in this crossing a means of judging whether
the human groups are only _races of a single species_, or rather
_distinct species_. For this purpose it will be sufficient to study
the phenomena which, in other organised and living beings, accompany
the production of mongrels and hybrids, and then to compare with
both the phenomena which characterise the crosses effected between
human groups. If, in the latter case, the phenomena are those
which characterise _hybridism_, we must conclude that the groups
are specifically distinct, and admit the _multiplicity_ of human
species. If, however, crosses between human groups, morphologically
different, are accompanied by phenomena peculiar to the production of
_mongrels_, we shall only be justified in considering these groups as
races of one species; we must take our stand upon the doctrine of the
_Specific Unity_ of all mankind.
The question before us becomes then entirely a physiological one,
and depends simply upon observation and experiment. For its solution
we must again turn our attention to plants as well as to animals.
It is in the phenomena of reproduction that the two kingdoms show
the greatest resemblance. This is not a case of mere _analogy_, but
almost of _identity_, and it is not the superior which lowers itself
but the inferior which is raised. We might say that, ennobled by the
importance of the function, the plant, as far as its reproductive
system is concerned, becomes, for the time, animal.
II. In these kingdoms the unions between _races of the same species_,
that is to say, the _production of mongrels_, may be accomplished
without any intervention on the part of man, or it may take place
under his direction. It is consequently either natural or artificial.
Mongrels among plants could only be recognised after the discovery
of the distinction of the sexes in 1744. The honour of this great
discovery belongs to Linnæus. He at once comprehended the importance
of the subject, and even exaggerated it, as we shall presently see.
Linnæus admitted that cross-unions, which had been observed for
centuries between animals, might be repeated between plants. And he
thus explained the appearance of variegated tulips in the midst of
borders originally formed of uniformly coloured flowers. Observation
and experiment have confirmed the views of the founder of the natural
sciences again and again. Moreover, it has been observed that the
crossing may become apparent in all parts of the plant by a mixture
of characters similar to that exhibited by the colouring of the
tulips. M. Naudin, among others, who, during one year, watched the
development of more than 1200 gourds, saw the seeds of a single
fruit reproduce all the races contained in the garden in which his
observations were made. _Superfetation_ had taken place. It is a
fact of great importance, as it demonstrates the equality of action
enjoyed by the pollen of all these races, which, morphologically,
differ so widely from each other. No better example could be given of
the faculty of crossing _between races_.
The natural and spontaneous production of mongrels among animals
presents the same characters. Facilitated by locomotion it is
accomplished every day in our houses, our poultry-yards, and our
farms. The difficulty does not consist in the accomplishment of the
cross but in its prevention, and in the preservation of the purity
of the race. The careful observations made by Isidore Geoffroy at
the Paris Museum, have shown that with sheep, dogs, pigs, and fowls,
mongrels between the most different races were invariably fertile.
Here again the phenomena of superfetation was often proved. Bitches
produced, by males of several races successively, young which showed
three or four distinct sources. Here the case was the same as with
the gourds of M. Naudin.
We see that man has found no difficulty in breeding mongrels, and
that, when he has wished to do so for any purpose whatever, he has
been able to regulate it by merely choosing the animal or plant.
This kind of union has, indeed, been long in daily practice for the
amelioration, modification, and diversification of the living beings
upon which human industry is exercised. It is useless to insist
upon facts which are known to all gardeners and breeders, and I
shall confine myself to one remark, the importance of which will be
understood later.
We have already seen that in the endeavour to _perfect_ a vegetable
or animal race, the physiological equilibrium has sometimes been
destroyed at the expense of the reproductive power. In such cases,
crossing with another race which is less modified, generally revives
the extinguished fertility. For example, the English pigs imported
into the middle of France by M. de Ginestous became sterile after
several generations. Upon crossing them with a leaner and less
perfect local race, their fertility returned.
All these facts, and their inevitable consequences, have been
admitted by every naturalist who has studied the question. Even
Darwin has recognised the truth of them in his valuable work upon
the _Variation of Animals and Plants_. At that time he confined
himself to the conclusion that the crosses between some races of
plants are less fertile than between others, a proposition which
no one would think of denying. He has gone further in the latest
editions of his work upon the Origin of Species. Without bringing
forward clear facts, the meaning of which would go further than the
wise conclusions he had previously admitted, he invokes our relative
ignorance of what takes place among wild _varieties_, and concludes
that we must admit that the crosses between varieties must always be
perfectly fertile. This is one of those appeals to the unknown, one
of those arguments where even our ignorance is invoked as a proof,
which we too often meet with in Darwin, who is often carried away by
his convictions. I shall have to return to this point, but I here
make the statement as an established fact, on the authority even of
Darwin, that all _known_ facts attest the _perfect fertility_ of
mongrels.
Finally, the formation of _crosses between races_, or the production
of _mongrels_, is spontaneous, and may be promoted by man without the
least difficulty; the results are as certain as those with the union
of individuals of the same race; in certain cases, indeed, fertility
is increased or revived under the influence of this crossing.
Crosses _between species, or hybrids,_ will exhibit facts of an
entirely contrary nature.
III. The formation of hybrids, as of mongrels, may be either natural
or artificial.
The former is so rare that eminent naturalists have doubted its
reality. There are, however, according to M. Decaisne, a score of
well proved examples among plants. What is this number compared with
the thousands of mongrels produced every day under our eyes. And yet
the material conditions of fertility are identically the same with
races as with species, and our botanical gardens, which group numbers
of species side by side, facilitate crossing still more.
Among wild animals living in liberty hybrids are still more rare.
It is unknown, for example, among mammalia, according to Isidore
Geoffroy, whose experience has here a double value. The order of
birds alone presents some facts of this kind, nearly all of which are
in the order of Gallinæ. According to Valenciennes, they are unknown
among fishes. In domestication and captivity spontaneous crossing
between different species is a little less rare.
The intelligent intervention of man has multiplied unions of this
kind in a remarkable manner, especially among plants, but without
being able to extend their limits. Linnæus thought crossing was
possible between species of _different families_. But in 1761
Koelreuter showed that he was mistaken. From these investigations,
which were carried on for twenty-seven years, and from those of
M. Naudin, his worthy rival, it appears that artificial crossing
between species of _different families never_ succeeds, and _very
rarely_ between species of _different genera_; that it is always very
difficult, and demands the most minute precautions to insure success;
that it often fails between species of the same genus closely allied
in appearance, and finally, that there are whole families among which
hybrids are impossible. Amongst the latter figures the family of the
cucurbitaceæ, so thoroughly studied by M. Naudin, where the most
perfect mongrels were produced spontaneously. We could not imagine,
evidently, a more complete contrast.
This contrast is carried into the minutest details. For example, any
flower which has in the least possible degree undergone the action of
pollen of its own species becomes absolutely insensible to the action
of pollen of a different species. How different to the equality of
action displayed by the several pollens of most distant races!
All experimenters agree further in declaring that even in the unions
between species which have been most successful, the fertility is
constantly diminished, and often in immense proportions. The head of
the Papaver somnifera generally contains 2000 seeds or more. In a
hybrid of this species Gœrtner only found six which had been matured;
all the rest were more or less abortive. Here again, what a contrast
between the crossing productive of such fertility in M. De Ginestous’
English pigs.
Hybridism in animals presents exactly the same phenomena as in
plants. Man has been able, by diverting and deceiving animal
instincts, to multiply crosses between species. But he has not been
able to extend the very narrow limits at which these phenomena cease.
Not one fertile union has taken place between different families;
they are very rare between genera, and even between species they are
far from numerous, a fact the more remarkable as animal hybridation
is an ancient institution. The mule was known to the Hebrews before
the time of David, and to the Greeks in the age of Homer. _Titires_
and _musmons_, products of crossings between the he-goat and the
sheep and the ram with the she-goat, received their distinctive names
from the Romans.
The uncertainty of the result is another point of resemblance between
animal and vegetable hybrids. The same experiments executed with
the same care and by equally clever experimenters have sometimes
succeeded and sometimes failed without any apparent cause. Buffon
and Daubenson often tried to reproduce titires and musmons. They
succeeded twice, while Isidore Geoffroy has invariably failed. The
formation of crosses between the hare and the rabbit, which has
frequently been attempted in various parts of the globe, appears only
to have been successful four or five times at the most. The pretended
cross between the camel and the dromedary, admitted by Buffon and
quoted by Nott, is certainly a fable, after the details which M. De
Khanikoff kindly gave me, and which I have published elsewhere. We
may, therefore, draw this conclusion from known facts, that there
are only two species of mammals, the ass and the horse, the crossing
of which is almost universally and invariably fertile.
Finally, crossing _between species_, or _hybridation_, is extremely
exceptional among plants and animals when left to themselves; man can
only produce them with great difficulty in the two kingdoms, and then
only between a very limited number of species; when he has succeeded,
the fertility is almost constantly diminished, and often to a very
considerable extent.