HYBRIDS
The subject treated in this and the next chapter is one of the most
interesting to mankind, and is surrounded by extraordinary prejudice,
sentiment, and ignorance. It is one upon which really trustworthy
information is to a very large extent absent–and difficult to obtain.
I cannot profess to supply this deficiency, but I can put the matter
before the reader.
It is a well-established fact that the various "kinds" of animals and
of plants do not breed promiscuously with one another. The individuals
of a "species" only breed with other individuals of that "species."
They do not even, as a habit, breed with the individuals of an allied
species. So nearly universal is this rule that it was for a long time
held by naturalists to be an absolute definition of "a species," that
it is a group of individuals capable of producing fertile young by
breeding with one another and incapable of producing fertile young by
mating with individuals of another such group, which were, therefore,
held to constitute a distinct species. The practical importance of this
definition was that it could, in a large number of instances among
animals, and still more amongst plants, be made use of as a test and
decided by experiment.
It is a curious fact that popular belief amongst country-folk and
those who have opportunities of coming to a conclusion on so simple
and direct a question has never accepted this law of the limitation
of species in breeding as more than a general rule to which it has
always been supposed that frequent exceptions occur. I mention this
not in order to add that "there is always some basis of truth in
these popular beliefs," but on the contrary to point out that popular
beliefs on such matters are very frequently altogether erroneous, and
though their origin can sometimes be explained, it is rare to find
that they are due, in however small a degree, to true observation and
inference. Where the subject under consideration has the obscurity
and strong fascination for the natural man which all that relates to
the processes of life, growth, and reproduction possess, we find that
traditional fancies of the most unwarrantable kind are current, and
hold their ground with tenacity even at the present day. Some 250 years
ago, and earlier–in fact, before the commencement of that definite
epoch of "the New Philosophy" marked by the foundation of the Royal
Society of London–any queer-looking animal brought from remote lands,
and any misshapen monstrosity born of cattle, sheep, dogs, or men, was
"explained," and confidently regarded as a "hybrid," the result of a
"cross" or irregular coupling of two distinct species of animals to
which the "monster" presented some fanciful resemblance. Whole books
were devoted to the description and picturing of such supposed examples
of mis-begotten progeny.
The belief in the existence of such extraordinary hybrids is still
common among so-called "well-educated" people. I have with difficulty
avoided causing annoyance and offence to a friend, a celebrated
painter, by refusing to admit that a deformed cat, of which he gave me
an account, was a hybrid between a cat and a rabbit. A very eminent
person whom I was conducting some years ago round the galleries of the
Natural History Museum, declared, as we stood in front of the specimen
of the Okapi of the Congo Forest, that it was clearly a hybrid between
the giraffe and the zebra. He insisted that it was obvious that such
was its explanation, and pointed to its striped haunches and legs, and
its cloven hoofs and giraffe-like head. I failed to change his opinion.
It is the fact–ascertained by careful observation of natural
occurrences and by experiment–that, in spite of the almost absolute
law or general truth to the effect that the members of a species
(whether of plant or animal) only produce fertile offspring by mating
with members of that same species, yet there are rare instances
known in which individuals of two distinct but allied species have
mated and produced fertile offspring. The cases in which such unions
have resulted in the production of offspring, but in which the
offspring so produced prove to be infertile–that is, incapable of
producing offspring in their turn–are much more numerous. An important
distinction has also to be made between cases of either fertile or
infertile hybrid-production which occur spontaneously in nature, and
those in which man by separating the parent animals or plants from
their natural conditions of life, or by bringing about impregnation (as
in "pollinating" one flower with the pollen-dust of another) succeeds
in obtaining a "cross" or "hybrid," whether fertile or infertile, not
known to occur in "wild" (that is, not humanly controlled) nature.
The rarest case would be that of the production of fertile hybrids
in uncontrolled natural conditions. Such possibly occur in the case
of some fishes in which the fertilization of the eggs takes place in
water, the fertilizing microscopic sperms passing from the males like
dust into the water and thus reaching the eggs laid by the females.
Occasionally hybrids are thus produced between some common fresh-water
fishes–species of the same genus–and between species of flat-fish,
such as the turbot and the brill, though it is difficult to be sure
that the rare hybrids so produced are fertile even if they attain
to maturity. The same is true as to certain small flowering plants
having distinct regions of natural distribution and occurrence.
At the confines of the regions proper to two such allied species,
insects passing from one to the other do sometimes effect a reciprocal
fertilization of the two species, and a natural hybrid is the result.
Here, again, it is difficult to follow the subsequent history of the
hybrids, but it is believed that in some instances they are fertile,
and that the hybrid race is only gradually merged by subsequent
crossing into one or other of the parent species. Not a single instance
is on record of the production of a "natural" hybrid (that is to say,
one produced in natural conditions without man's interference), whether
fertile or infertile, between two species of the larger animals (such
as between horse and ass or zebra and ass, or between lion and tiger
or any of the species of cats, or between species of bears) or birds
(such as pheasants of various species, including the jungle cock, the
wild original of our domestic fowl, or between various species of
ducks, various species of geese, or between various species of the
grouse-birds).
Nevertheless, in conditions brought about by man–that is to say,
confinement in cages or paddocks, or at any rate removal from their
native climate and home–all the groups of species just cited commonly
and frequently produce hybrids _inter se_, that is, one or more species
of the horse group thus inter-breed with one another, so will certain
species of cats, certain species of bears, many species of pheasants,
also of ducks, of geese, and of grouse. In nearly every case the
hybrids so produced are infertile; they will not mate with a similar
hybrid, and even when mated with one of the parent species rarely
produce offspring, though they sometimes do so. The best cases of the
production of fertile hybrids are between species of flowering plants
brought to this country from widely separated regions. The surprising
and instructive result has been obtained that a cross between two
allied species (that is, of one and the same "genus") which will fail
altogether or "come to nothing" as infertile hybrids–if the two species
crossed are from the same or contiguous regions–yet will yield readily
vigorous fertile hybrid offspring when the two species (always, of
course, of one and the same genus) have their native homes in widely
separate parts of the world–as, for instance, the Indian Himalaya range
and the South American Andean range.
This has been found in crossing species of rhododendrons, of orchids,
and of many other plants with which horticulturists occupy themselves
for commercial purposes. It is in some ways the reverse of what one
might expect. It would be reasonable to suppose that allied species
from the same climate and geographical region would have more affinity
and be more readily hybridized than species from widely remote and
physically differing regions. But the reverse is the case, many
thriving hybrid stocks which duly fertilize and set their seed are now
in cultivation, having been produced by the union of parent species
from "the opposite ends of the earth."
The consideration of this case throws some light on the significance
of the non-occurrence of natural hybrids and of the very remarkable
and curious fact that hybrids are so usually sterile. When we come
to think of it, the natural preliminary assumption should be (as is
that of unsophisticated humanity) that any animal or plant might, so
far as possibilities go, breed with any other; and the questions to
be answered are: (1) What advantage to a species is it not to be able
to hybridize with other species, and (2) how–that is to say, by what
structure or by what subtle chemical differences or other features in
their make-up and habit–are they prevented from so hybridizing? Then we
come on further to the question, Why should a hybrid, once produced,
fail to bear healthy eggs or sperms according to its sex, although it
grows up to full size and is to all appearances mature? And why should
hybrids between parents of origin locally remote from one another not
show this failure, but behave like ordinary healthy organisms?
In the full solution of these inquiries we should get very near to some
of the most important secrets of the living body which have still to
be searched out. But a reply to these questions which is probably in
large measure true, and serves to help us in the further collection
and examination of facts, is as follows: First, the production and
maintenance of "species" of plants and of animals by survival of
favourable variations in the struggle for existence (Darwin and
Wallace's theory of the origin of species) requires the maintenance of
the purity of the favourable stock which survives in the struggle. If
it were continually liable to hybridization by other species it would
never establish its own distinctive features. It would deteriorate
by departing from those characteristics which have been "naturally
selected" and have rendered it a successful "species." Thus the
breeder, when he has selected a stock for propagation which approaches
the standard at which he is aiming, keeps it apart, and does not allow
it to be "crossed" by other stock. One of the qualities "naturally
selected" in "the wild" is the power of resistance to fertilization by
neighbouring species.
This power of resistance or immunity to fertilization by other species
may be attained by several different methods. Amongst these are (1)
a difference in the season of breeding or sexual ripening; (2) the
production of secretions (whether by plant or by animal) which poison
or paralyse the fertilizing sperms of allied and locally associated
species, but are harmless to those of the secreting species; (3) the
mechanical differences of size, etc., which prevent the fertilizing
material of a strange species from gaining access to the egg-cells;
(4) psychical activities (antipathies) in the case of animals or mere
attraction and repulsion by odoriferous substances, which serve to
repel a strange species, but are attractive to individuals of the same
species; (5) finally, a chemical and physiological incompatibility
between the sperms of one species and the germs of another (as distinct
from the attraction or repulsion of the entire living individual),
which, even when all other difficulties are absent or have been
overcome, may be, and frequently is, present, so that the spermatozoon
cannot penetrate the egg-cell even when resting upon it, but may be
paralysed or repelled, and in any case is not guided and drawn into the
aperture of the egg-covering, called the micropyle, or "little entry,"
so as to fuse with and fertilize the egg.
The operation of these hindrances to hybrid fertilization and breeding
have been ascertained in several different instances. It is not always
possible, and certainly not easy to ascertain, which is at work in
any and every case. But we can well conceive that one or other of
these agencies have been developed and accentuated by survival of
the fittest, so as to protect a species against fertilization by a
neighbouring species, and thus to enable it to maintain its own "bundle
of characteristics" free from the swamping effects of "mixture" (that
is, "hybridization") with another species. It is also thus intelligible
that an allied species from a distant land against which our native
species and its closer ancestry–struggling for purity of race–have had
no occasion or opportunity to develop a repelling protection–will have
no such difficulty in effecting the fertilization of the native species
as have those adjacent species against whose intrusions the latter is
specifically moulded and selected by long generations of severe natural
selection.
The failure of hybrids generally to ripen their ova and sperm so as to
reproduce themselves is a subject upon which, considering its enormous
importance and significance, singularly little has been done in the
way of investigation. Fifty years ago it was usually taught that the
mule, between the horse and the ass, so largely produced under human
superintendence for transport service, was unable to breed owing to
some deformity in the reproductive passages. Even now no adequate study
of the subject has been made, but it appears that whilst a female mule
can be, and sometimes is, successfully mated to a horse or an ass,
giving birth to a foal, the male mule does not produce fully-formed
spermatozoa. What precisely is the nature of this failure, what the
ultimate microscopic condition of the sperm cells in infertile male
mules, or in any other infertile male hybrids, has not yet been
properly worked out by modern cytological methods. It would be a matter
of vast interest to determine what is the difference in the structure
of the sperm-cells of a fertile and of an infertile male hybrid. At
present, so far as I know, this has not been done.
So far what I have written applies to hybridization–the inter-breeding
of distinct species. A similar but by no means identical subject is
that of the inter-breeding of distinct races or varieties of one
species, and the production of "mongrels." "Mongrels" are to races what
"hybrids" are to species. To this branch of the subject belongs the
study of the effects of intermarriage between distinct races of men.