preceding chapters; but as isolated and striking cases of reversion have
here been chiefly insisted on, I wish to guard the reader against supposing
that reversion is due to some rare or accidental combination of
circumstances. When a character, lost during hundreds of generations,
suddenly reappears, no doubt some such combination must occur; but
reversions may be constantly observed, at least to the immediately
preceding generations, in the offspring of most unions. This has been
universally recognised in the case of hybrids and mongrels, but it has been
recognised simply from the difference between the united forms rendering
the resemblance of the offspring to their grandparents or more remote
progenitors of easy detection. Reversion is likewise almost invariably the
rule, as Mr. Sedgwick has shown, with certain diseases. Hence we must
conclude that a tendency to this peculiar form of transmission is an
integral part of the general law of inheritance. {57}
* * * * *
_Monstrosities._--A large number of monstrous growths and of lesser
anomalies are admitted by every one to be due to an arrest of development,
that is to the persistence of an embryonic condition. If every horse or ass
had striped legs whilst young, the stripes which occasionally appear on
these animals when adult would have to be considered as due to the
anomalous retention of an early character, and not as due to reversion.
Now, the leg-stripes in the horse-genus, and some other characters in
analogous cases, are apt to occur during early youth and then to disappear;
thus the persistence of early characters and reversion are brought into
close connexion.
But many monstrosities can hardly be considered as the result of an arrest
of development; for parts of which no trace can be detected in the embryo,
but which occur in other members of the same class of animals or plants,
occasionally appear, and these may probably with truth be attributed to
reversion. For instance: supernumerary mammae, capable of secreting milk,
are not extremely rare in women; and as many as five have been observed.
When four are developed, they are generally arranged symmetrically on each
side of the chest; and in one instance a woman (the daughter of another
with supernumerary mammae) had one mamma, which yielded milk, developed in
the inguinal region. This latter case, when we remember the position of the
mammae in some of the lower animals on both the chest and inguinal region,
is highly remarkable, and leads to the belief that in all cases the
additional mammae in woman are due to reversion. The facts given in the
last chapter on the tendency in supernumerary digits to regrowth after
amputation, indicate their relation to the digits of the lower vertebrate
animals, and lead to the suspicion that their appearance may in some manner
be connected with reversion. But I shall have to recur, in the chapter on
pangenesis, to the abnormal multiplication of organs, and likewise to their
occasional transposition. The occasional development in man of the
coccygeal vertebrae into a short and free tail, though it thus becomes in
one sense more perfectly developed, may at the same time be considered as
an arrest of development, and as a case of reversion. The greater frequency
of a monstrous kind of proboscis in the pig than in any other mammal,
considering the position of the pig {58} in the mammalian series, has
likewise been attributed, perhaps truly, to reversion.[130]
When flowers which are properly irregular in structure become regular
or peloric, the change is generally looked at by botanists as a return
to the primitive state. But Dr. Maxwell Masters,[131] who has ably
discussed this subject, remarks that when, for instance, all the sepals
of a Tropaeolum become green and of the same shape, instead of being
coloured with one alone prolonged into a spur, or when all the petals
of a Linaria become simple and regular, such cases may be due merely to
an arrest of development; for in these flowers all the organs during
their earliest condition are symmetrical, and, if arrested at this
stage of growth, they would not become irregular. If, moreover, the
arrest were to take place at a still earlier period of development, the
result would be a simple tuft of green leaves; and no one probably
would call this a case of reversion. Dr. Masters designates the cases
first alluded to as regular peloria; and others, in which all the
corresponding parts assume a similar form of irregularity, as when all
the petals in a Linaria become spurred, as irregular peloria. We have
no right to attribute these latter cases to reversion, until it can be
shown to be probable that the parent-form, for instance, of the genus
Linaria had had all its petals spurred; for a change of this nature
might result from the spreading of an anomalous structure, in
accordance with the law, to be discussed in a future chapter, of
homologous parts tending to vary in the same manner. But as both forms
of peloria frequently occur on the same individual plant of the
Linaria,[132] they probably stand in some close relation to each other.
On the doctrine that peloria is simply the result of an arrest of
development, it is difficult to understand how an organ arrested at a
very early period of growth should acquire its full functional
perfection;--how a petal, supposed to be thus arrested, should acquire
its brilliant colours, and serve as an envelope to the flower, or a
stamen produce efficient pollen; yet this occurs with many peloric
flowers. That pelorism is not due to mere chance variability, but
either to an arrest of development or to reversion, we may infer from
an observation made by Ch. Morren,[133] namely, that families which
have irregular flowers often "return by these monstrous growths to
their regular form; whilst we never see a regular flower realise the
structure of an irregular one."
Some flowers have almost certainly become more or less completely
peloric through reversion. _Corydalis tuberosa_ properly has one of its
two nectaries colourless, destitute of nectar, only half the size of
the other, and {59} therefore, to a certain extent, in a rudimentary
state; the pistil is curved towards the perfect nectary, and the hood,
formed of the inner petals, slips off the pistil and stamens in one
direction alone, so that, when a bee sucks the perfect nectary, the
stigma and stamens are exposed and rubbed against the insect's body. In
several closely allied genera, as in Dielytra, &c., there are two
perfect nectaries, the pistil is straight, and the hood slips off on
either side, according as the bee sucks either nectary. Now, I have
examined several flowers of _Corydalis tuberosa_, in which both
nectaries were equally developed and contained nectar; in this we see
only the redevelopment of a partially aborted organ; but with this
redevelopment the pistil becomes straight, and the hood slips off in
either direction; so that these flowers have acquired the perfect
structure, so well adapted for insect agency, of Dielytra and its
allies. We cannot attribute these coadapted modifications to chance, or
to correlated variability; we must attribute them to reversion to a
primordial condition of the species.
The peloric flowers of Pelargonium have their five petals in all
respects alike, and there is no nectary; so that they resemble the
symmetrical flowers of the closely allied Geranium-genus; but the
alternate stamens are also sometimes destitute of anthers, the
shortened filaments being left as rudiments, and in this respect they
resemble the symmetrical flowers of the closely allied genus, Erodium.
Hence we are led to look at the peloric flowers of Pelargonium as
having probably reverted to the state of some primordial form, the
progenitor of the three closely related genera of Pelargonium,
Geranium, and Erodium.
In the peloric form of _Antirrhinum majus_, appropriately called the
"_Wonder_," the tubular and elongated flowers differ wonderfully from
those of the common snapdragon; the calyx and the mouth of the corolla
consist of six equal lobes, and include six equal instead of four
unequal stamens. One of the two additional stamens is manifestly formed
by the development of a microscopically minute papilla, which may be
found at the base of the upper lip of the flower in all common
snapdragons, at least in nineteen plants examined by me. That this
papilla is a rudiment of a stamen was well shown by its various degrees
of development in crossed plants between the common and peloric
Antirrhinum. Again, a peloric _Galeobdolon luteum_, growing in my
garden, had five equal petals, all striped like the ordinary lower lip,
and included five equal instead of four unequal stamens; but Mr. R.
Keeley, who sent me this plant, informs me that the flowers vary
greatly, having from four to six lobes to the corolla, and from three
to six stamens.[134] Now, as the members of the two great families to
which the Antirrhinum and Galeobdolon belong are properly pentamerous,
with some of the parts confluent and others suppressed, we ought not to
look at the sixth stamen and the sixth lobe to the corolla in either
case as due to reversion, any more than the additional petals in double
flowers in these same two families. But the case is different with the
fifth stamen in the peloric Antirrhinum, which {60} is produced by the
redevelopment of a rudiment always present, and which probably reveals
to us the state of the flower, as far as the stamens are concerned, at
some ancient epoch. It is also difficult to believe that the other four
stamens and the petals, after an arrest of development at a very early
embryonic age, would have come to full perfection in colour, structure,
and function, unless these organs had at some former period normally
passed through a similar course of growth. Hence it appears to me
probable that the progenitor of the genus Antirrhinum must at some
remote epoch have included five stamens and borne flowers in some
degree resembling those now produced by the peloric form.
Lastly, I may add that many instances have been recorded of flowers,
not generally ranked as peloric, in which certain organs, normally few
in number, have been abnormally augmented. As such an increase of parts
cannot be looked at as an arrest of development, nor as due to the
redevelopment of rudiments, for no rudiments are present, and as these
additional parts bring the plant into closer relationship with its
natural allies, they ought probably to be viewed as reversions to a
primordial condition.
These several facts show us in an interesting manner how intimately certain
abnormal states are connected together; namely, arrests of development
causing parts to become rudimentary or to be wholly suppressed,--the
redevelopment of parts at present in a more or less rudimentary
condition,--the reappearance of organs of which not a vestige can now be
detected,--and to these may be added, in the case of animals, the presence
during youth, and subsequent disappearance, of certain characters which
occasionally are retained throughout life. Some naturalists look at all
such abnormal structures as a return to the ideal state of the group to
which the affected being belongs; but it is difficult to conceive what is
meant to be conveyed by this expression. Other naturalists maintain, with
greater probability and distinctness of view, that the common bond of
connection between the several foregoing cases is an actual, though
partial, return to the structure of the ancient progenitor of the group. If
this view be correct, we must believe that a vast number of characters,
capable of evolution, lie hidden in every organic being. But it would be a
mistake to suppose that the number is equally great in all beings. We know,
for instance, that plants of many orders occasionally become peloric; but
many more cases have been observed in the Labiatae and Scrophulariaceae
than in any other order; and in one genus of the Scrophulariaceae, namely
Linaria, no less {61} than thirteen species have been described in a
peloric condition.[135] On this view of the nature of peloric flowers, and
bearing in mind what has been said with respect to certain monstrosities in
the animal kingdom, we must conclude that the progenitors of most plants
and animals, though widely different in structure, have left an impression
capable of redevelopment on the germs of their descendants.
The fertilised germ of one of the higher animals, subjected as it is to so
vast a series of changes from the germinal cell to old age,--incessantly
agitated by what Quatrefages well calls the _tourbillon vital_,--is perhaps
the most wonderful object in nature. It is probable that hardly a change of
any kind affects either parent, without some mark being left on the germ.
But on the doctrine of reversion, as given in this chapter, the germ
becomes a far more marvellous object, for, besides the visible changes to
which it is subjected, we must believe that it is crowded with invisible
characters, proper to both sexes, to both the right and left side of the
body, and to a long line of male and female ancestors separated by hundreds
or even thousands of generations from the present time; and these
characters, like those written on paper with invisible ink, all lie ready
to be evolved under certain known or unknown conditions.
* * * * *
{62}
CHAPTER XIV.
INHERITANCE _continued_--FIXEDNESS OF CHARACTER--PREPOTENCY--SEXUAL
LIMITATION--CORRESPONDENCE OF AGE.
FIXEDNESS OF CHARACTER APPARENTLY NOT DUE TO ANTIQUITY OF
INHERITANCE--PREPOTENCY OF TRANSMISSION IN INDIVIDUALS OF THE SAME
FAMILY, IN CROSSED BREEDS AND SPECIES; OFTEN STRONGER IN ONE SEX THAN
THE OTHER; SOMETIMES DUE TO THE SAME CHARACTER BEING PRESENT AND
VISIBLE IN ONE BREED AND LATENT IN THE OTHER--INHERITANCE AS LIMITED BY
SEX--NEWLY-ACQUIRED CHARACTERS IN OUR DOMESTICATED ANIMALS OFTEN
TRANSMITTED BY ONE SEX ALONE, SOMETIMES LOST BY ONE SEX
ALONE--INHERITANCE AT CORRESPONDING PERIODS OF LIFE--THE IMPORTANCE OF
THE PRINCIPLE WITH RESPECT TO EMBRYOLOGY; AS EXHIBITED IN DOMESTICATED
ANIMALS; AS EXHIBITED IN THE APPEARANCE AND DISAPPEARANCE OF INHERITED
DISEASES; SOMETIMES SUPERVENING EARLIER IN THE CHILD THAN IN THE
PARENT--SUMMARY OF THE THREE PRECEDING CHAPTERS.
In the two last chapters the nature and force of Inheritance, the
circumstances which interfere with its power, and the tendency to
Reversion, with its many remarkable contingencies, were discussed. In the
present chapter some other related phenomena will be treated of, as fully
as my materials permit.
_Fixedness of Character._
It is a general belief amongst breeders that the longer any character has
been transmitted by a breed, the more firmly it will continue to be
transmitted. I do not wish to dispute the truth of the proposition, that
inheritance gains strength simply through long continuance, but I doubt
whether it can be proved. In one sense the proposition is little better
than a truism; if any character has remained constant during many
generations, it will obviously be little likely, the conditions of life
remaining the same, to vary during the next generation. So, again, in
improving a breed, if care be taken for a length of time to exclude all
inferior individuals, the breed will obviously tend to become truer, as it
will not have been crossed during many generations by an inferior animal.
We have previously seen, {63} but without being able to assign any cause,
that, when a new character appears, it is occasionally from the first well
fixed, or fluctuates much, or wholly fails to be transmitted. So it is with
the aggregate of slight differences which characterise a new variety, for
some propagate their kind from the first much truer than others. Even with
plants multiplied by bulbs, layers, &c., which may in one sense be said to
form parts of the same individual, it is well known that certain varieties
retain and transmit through successive bud-generations their newly-acquired
characters more truly than others. In none of these, nor in the following
cases, does there appear to be any relation between the force with which a
character is transmissible and the length of time during which it has
already been transmitted. Some varieties, such as white and yellow
hyacinths and white sweet-peas, transmit their colours more faithfully than
do the varieties which have retained their natural colour. In the Irish
family, mentioned in the twelfth chapter, the peculiar tortoiseshell-like
colouring of the eyes was transmitted far more faithfully than any ordinary
colour. Ancon and Mauchamp sheep and niata cattle, which are all
comparatively modern breeds, exhibit remarkably strong powers of
inheritance. Many similar cases could be adduced.
As all domesticated animals and cultivated plants have varied, and yet are
descended from aboriginally wild forms, which no doubt had retained the
same character from an immensely remote epoch, we see that scarcely any
degree of antiquity ensures a character being transmitted perfectly true.
In this case, however, it may be said that changed conditions of life
induce certain modifications, and not that the power of inheritance fails;
but in every case of failure, some cause, either internal or external, must
interfere. It will generally be found that the parts in our domesticated
productions which have varied, or which still continue to vary,--that is,
which fail to retain their primordial state,--are the same with the parts
which differ in the natural species of the same genus. As, on the theory of
descent with modification, the species of the same genus have been modified
since they branched off from a common progenitor, it follows that the
characters by which they differ from each other have varied whilst other
parts of the organisation have remained unchanged; and it might be argued
that {64} these same characters now vary under domestication, or fail to be
inherited, owing to their lesser antiquity. But we must believe structures,
which have already varied, would be more liable to go on varying, rather
than structures which during an immense lapse of time have remained
unaltered; and this variation is probably the result of certain relations
between the conditions of life and the organisation, quite independently of
the greater or less antiquity of each particular character.
Fixedness of character, or the strength of inheritance, has often been
judged of by the preponderance of certain characters in the crossed
offspring between distinct races; but prepotency of transmission here comes
into play, and this, as we shall immediately see, is a very different
consideration from the strength or weakness of inheritance. It has often
been observed[136] that breeds of animals inhabiting wild and mountainous
countries cannot be permanently modified by our improved breeds; and as
these latter are of modern origin, it has been thought that the greater
antiquity of the wilder breeds has been the cause of their resistance to
improvement by crossing; but it is more probably due to their structure and
constitution being better adapted to the surrounding conditions. When
plants are first subjected to culture, it has been found that, during
several generations, they transmit their characters truly, that is, do not
vary, and this has been attributed to ancient characters being strongly
inherited; but it may with equal or greater probability be consequent on
changed conditions of life requiring a long time for their accumulative
action. Notwithstanding these considerations, it would perhaps be rash to
deny that characters become more strongly fixed the longer they are
transmitted; but I believe that the proposition resolves itself into
this,--that all characters of all kinds, whether new or old, tend to be
inherited, and that those which have already withstood all counteracting
influences and been truly transmitted, will, as a general rule, continue to
withstand them, and consequently be faithfully inherited.
{65}
_Prepotency in the Transmission of Character._
When individuals distinct enough to be recognised, but of the same family,
or when two well-marked races, or two species, are crossed, the usual
result, as stated in the previous chapter, is, that the offspring in the
first generation are intermediate between their parents, or resemble one
parent in one part and the other parent in another part. But this is by no
means the invariable rule; for in many cases it is found that certain
individuals, races, and species are prepotent in transmitting their
likeness. This subject has been ably discussed by Prosper Lucas,[137] but
is rendered extremely complicated by the prepotency sometimes running
equally in both sexes, and sometimes more strongly in one sex than in the
other; it is likewise complicated by the presence of secondary sexual
characters, which render the comparison of mongrels with their
parent-breeds difficult.
It would appear that in certain families some one ancestor, and after him
others in the same family, must have had great power in transmitting their
likeness through the male line; for we cannot otherwise understand how the
same features should so often be transmitted after marriages with various
females, as has been the case with the Austrian Emperors, and as, according
to Niebuhr, formerly occurred in certain Roman families with their mental
qualities.[138] The famous bull Favourite is believed[139] to have had a
prepotent influence on the shorthorn race. It has also been observed[140]
with English race-horses that certain mares have generally transmitted
their own character, whilst other mares of equally pure blood have allowed
the character of the sire to prevail.
The truth of the principle of prepotency comes out more clearly when
certain races are crossed. The improved Shorthorns, notwithstanding
that the breed is comparatively modern, are generally acknowledged to
possess great power in impressing their likeness on all other breeds;
and it is chiefly in consequence of this power that they are so highly
valued {66} for exportation.[141] Godine has given a curious case of a
ram of a goat-like breed of sheep from the Cape of Good Hope, which
produced offspring hardly to be distinguished from himself, when
crossed with ewes of twelve other breeds. But two of these half-bred
ewes, when put to a merino ram, produced lambs closely resembling the
merino breed. Girou de Buzareingues[142] found that of two races of
French sheep the ewes of one, when crossed during successive
generations with merino rams, yielded up their character far sooner
than the ewes of the other race. Sturm and Girou have given analogous
cases with other breeds of sheep and with cattle, the prepotency
running in these cases through the male side; but I was assured on good
authority in South America, that when niata cattle are crossed with
common cattle, though the niata breed is prepotent whether males or
females are used, yet that the prepotency is strongest through the
female line. The Manx cat is tailless and has long hind legs; Dr.
Wilson crossed a male Manx with common cats, and, out of twenty-three
kittens, seventeen were destitute of tails; but when the female Manx
was crossed by common male cats all the kittens had tails, though they
were generally short and imperfect.[143]
In making reciprocal crosses between pouter and fantail pigeons, the
pouter-race seemed to be prepotent through both sexes over the fantail.
But this is probably due to weak power in the fantail rather than to
any unusually strong power in the pouter, for I have observed that
barbs also preponderated over fantails. This weakness of transmission
in the fantail, though the breed is an ancient one, is said[144] to be
general; but I have observed one exception to the rule, namely, in a
cross between a fantail and laugher. The most curious instance known to
me of weak power in both sexes is in the trumpeter pigeon. This breed
has been well known for at least 130 years: it breeds perfectly true,
as I have been assured by those who have long kept many birds: it is
characterised by a peculiar tuft of feathers over the beak, by a crest
on the head, by a most peculiar coo quite unlike that of any other
breed, and by much-feathered feet. I have crossed both sexes with
turbits of two sub-breeds, with almond tumblers, spots, and runts, and
reared many mongrels and recrossed them; and though the crest on the
head and feathered feet were inherited (as is generally the case with
most breeds), I have never seen a vestige of the tuft over the beak or
heard the peculiar coo. Boitard and Corbie[145] assert that this is the
invariable result of crossing trumpeters with any other breed:
Neumeister,[146] however, states that in Germany mongrels have been
obtained, though very rarely, which were furnished with the tuft and
would trumpet: but a pair of these mongrels with a tuft, which I
imported, never trumpeted. Mr. Brent states[147] that the crossed
offspring of a trumpeter were crossed {67} with trumpeters for three
generations, by which time the mongrels had 7-8ths of this blood in
their veins, yet the tuft over the beak did not appear. At the fourth
generation the tuft appeared, but the birds, though now having 15-16ths
trumpeter's blood, still did not trumpet. This case well shows the wide
difference between inheritance and prepotency; for here we have a
well-established old race which transmits it characters faithfully, but
which, when crossed with any other race, has the feeblest power of
transmitting its two chief characteristic qualities.
I will give one other instance with fowls and pigeons of weakness and
strength in the transmission of the same character to their crossed
offspring. The Silk-fowl breeds true, and there is reason to believe is
a very ancient race; but when I reared a large number of mongrels from
a Silk-hen by a Spanish cock, not one exhibited even a trace of the
so-called silkiness. Mr. Hewitt also asserts that in no instance are
the silky feathers transmitted by this breed when crossed with any
other variety. But three birds out of many raised by Mr. Orton from a
cross between a silk-cock and a bantam-hen, had silky feathers.[148] So
that it is certain that this breed very seldom has the power of
transmitting its peculiar plumage to its crossed progeny. On the other
hand, there is a silk sub-variety of the fantail pigeon, which has its
feathers in nearly the same state as in the Silk-fowl: now we have
already seen that fantails, when crossed, possess singularly weak power
in transmitting their general qualities; but the silk sub-variety when
crossed with any other small-sized race invariably transmits its silky
feathers![149]
The law of prepotency comes into action when species are crossed, as
with races and individuals. Gaertner has unequivocally shown[150] that
this is the case with plants. To give one instance: when _Nicotiana
paniculata_ and _vincaeflora_ are crossed, the character of _N.
paniculata_ is almost completely lost in the hybrid; but if _N.
quadrivalvis_ be crossed with _N. vincaeflora_, this later species,
which was before so prepotent, now in its turn almost disappears under
the power of _N. quadrivalvis_. It is remarkable that the prepotency of
one species over another in transmission is quite independent, as shown
by Gaertner, of the greater or less facility with which the one
fertilises the other.
With animals, the jackal is prepotent over the dog, as is stated by
Flourens who made many crosses between these animals; and this was
likewise the case with a hybrid which I once saw between a jackal and
terrier. I cannot doubt, from the observations of Colin and others,
that the ass is prepotent over the horse; the prepotency in this
instance running more strongly through the male than through the female
ass; so that the mule resembles the ass more closely than does the
hinny.[151] The {68} male pheasant, judging from Mr. Hewitt's
descriptions,[152] and from the hybrids which I have seen,
preponderates over the domestic fowl; but the latter, as far as colour
is concerned, has considerable power of transmission, for hybrids
raised from five differently coloured hens differed greatly in plumage.
I formerly examined some curious hybrids in the Zoological Gardens,
between the Penguin variety of the common duck and the Egyptian goose
(_Tadorna Aegyptiaca_); and although I will not assert that the
domesticated variety preponderated over the natural species, yet it had
strongly impressed its unnatural upright figure on these hybrids.
I am aware that such cases as the foregoing have been ascribed by
various authors, not to one species, race, or individual being
prepotent over the other in impressing it character on its crossed
offspring, but to such rules as that the father influences the external
characters and the mother the internal or vital organs. But the great
diversity of the rules given by various authors almost proves their
falseness. Dr. Prosper Lucas has fully discussed this point, and has
shown[153] that none of the rules (and I could add others to those
quoted by him) apply to all animals. Similar rules have been enounced
for plants, and have been proved by Gaertner[154] to be all erroneous.
If we confine our view to the domesticated races of a single species,
or perhaps even to the species of the same genus, some such rules may
hold good; for instance, it seems that in reciprocally crossing various
breeds of fowls the male generally gives colour;[155] but conspicuous
exceptions have passed under my own eyes. In sheep it seems that the
ram usually gives its peculiar horns and fleece to its crossed
offspring, and the bull the presence or absence of horns.
In the following chapter on Crossing I shall have occasion to show that
certain characters are rarely or never blended by crossing, but are
{69} transmitted in an unmodified state from either parent-form; I
refer to this fact here because it is sometimes accompanied on the one
side by prepotency, which thus acquires the false appearance of unusual
strength. In the same chapter I shall show that the rate at which a
species or breed absorbs and obliterates another by repeated crosses,
depends in chief part on prepotency in transmission.
In conclusion, some of the cases above given,--for instance, that of the
trumpeter pigeon,--prove that there is a wide difference between mere
inheritance and prepotency. This latter power seems to us, in our
ignorance, to act in most cases quite capriciously. The very same
character, even though it be an abnormal or monstrous one, such as silky
feathers, may be transmitted by different species, when crossed, either
with prepotent force or singular feebleness. It is obvious, that a
purely-bred form of either sex, in all cases in which prepotency does not
run more strongly in one sex than the other, will transmit its character
with prepotent force over a mongrelized and already variable form.[156]
From several of the above-given cases we may conclude that mere antiquity
of character does not by any means necessarily make it prepotent. In some
cases prepotency apparently depends on the same character being present and
visible in one of the two breeds which are crossed, and latent or invisible
in the other breed; and in this case it is natural that the character which
is potentially present in both should be prepotent. Thus, we have reason to
believe that there is a latent tendency in all horses to be dun-coloured
and striped; and when a horse of this kind is crossed with one of any other
colour, it is said that the offspring are almost sure to be striped. Sheep
have a similar latent tendency to become dark-coloured, and we have seen
with what prepotent force a ram with a few black spots, when crossed with
sheep of various breeds, coloured its offspring. All pigeons have a latent
tendency to become slaty-blue, with certain characteristic marks, and it is
known that, when a bird thus coloured is crossed with one of any other
colour, it is most difficult afterwards to eradicate the blue tint. A
nearly parallel case is offered by those black bantams which, as they grow
{70} old, develop a latent tendency to acquire red feathers. But there are
exceptions to the rule: hornless breeds of cattle possess a latent capacity
to reproduce horns, yet when crossed with horned breeds they do not
invariably produce offspring bearing horns.
We meet with analogous cases with plants. Striped flowers, though they can
be propagated truly by seed, have a latent tendency to become uniformly
coloured, but when once crossed by a uniformly coloured variety, they ever
afterwards fail to produce striped seedlings.[157] Another case is in some
respects more curious: plants bearing peloric or regular flowers have so
strong a latent tendency to reproduce their normally irregular flowers,
that this often occurs by buds when a plant is transplanted into poorer or
richer soil.[158] Now I crossed the peloric snapdragon (_Antirrhinum
majus_), described in the last chapter, with pollen of the common form; and
the latter, reciprocally, with peloric pollen. I thus raised two great beds
of seedlings, and not one was peloric. Naudin[159] obtained the same result
from crossing a peloric Linaria with the common form. I carefully examined
the flowers of ninety plants of the crossed Antirrhinum in the two beds,
and their structure had not been in the least affected by the cross, except
that in a few instances the minute rudiment of the fifth stamen, which is
always present, was more fully or even completely developed. It must not be
supposed that this entire obliteration of the peloric structure in the
crossed plants can be accounted for by any incapacity of transmission; for
I raised a large bed of plants from the peloric Antirrhinum, artificially
fertilised by its own pollen, and sixteen plants, which alone survived the
winter, were all as perfectly peloric as the parent-plant. Here we have a
good instance of the wide difference between the inheritance of a character
and the power of transmitting it to crossed offspring. The crossed plants,
which perfectly resembled the common snapdragon, were allowed to sow
themselves, and, out of a hundred and twenty-seven seedlings, eighty-eight
proved to be common snapdragons, two were in an intermediate condition
between the peloric and normal state, {71} and thirty-seven were perfectly
peloric, having reverted to the structure of their one grandparent. This
case seems at first sight to offer an exception to the rule formerly given,
namely, that a character which is present in one form and latent in the
other is generally transmitted with prepotent force when the two forms are
crossed. For in all the Scrophulariaceae, and especially in the genera
Antirrhinum and Linaria, there is, as was shown in the last chapter, a
strong latent tendency to become peloric; and there is also, as we have
just seen, a still stronger tendency in all peloric plants to reacquire
their normal irregular structure. So that we have two opposed latent
tendencies in the same plants. Now, with the crossed Antirrhinums the
tendency to produce normal or irregular flowers, like those of the common
Snapdragon, prevailed in the first generation; whilst the tendency to
pelorism, appearing to gain strength by the intermission of a generation,
prevailed to a large extent in the second set of seedlings. How it is
possible for a character to gain strength by the intermission of a
generation, will be considered in the chapter on pangenesis.
On the whole, the subject of prepotency is extremely intricate,--from its
varying so much in strength, even in regard to the same character, in
different animals,--from its running either equally in both sexes, or, as
frequently is the case with animals, but not with plants, much stronger in
the one sex than the other,--from the existence of secondary sexual
characters,--from the transmission of certain characters being limited, as
we shall immediately see, by sex,--from certain characters not blending
together,--and, perhaps, occasionally from the effects of a previous
fertilisation on the mother. It is therefore not surprising that every one
hitherto has been baffled in drawing up general rules on the subject of
prepotency.
_Inheritance as limited by Sex._
New characters often appear in one sex, and are afterwards transmitted to
the same sex, either exclusively or in a much greater degree than to the
other. This subject is important, because with animals of many kinds in a
state of nature, both high and low in the scale, secondary sexual
characters, not in {72} any way directly connected with the organs of
reproduction, are often conspicuously present. With our domesticated
animals, also, these same secondary characters are often found to differ
greatly from the state in which they exist in the parent-species. And the
principle of inheritance as limited by sex shows how such characters might
have been first acquired and subsequently modified.
Dr. P. Lucas, who has collected many facts on this subject, shows[160]
that when a peculiarity, in no manner connected with the reproductive
organs, appears in either parent, it is often transmitted exclusively
to the offspring of the same sex, or to a much greater number of them
than of the opposite sex. Thus, in the family of Lambert, the horn-like
projections on the skin were transmitted from the father to his sons
and grandsons alone; so it has been with other cases of ichthyosis,
with supernumerary digits, with a deficiency of digits and phalanges,
and in a lesser degree with various diseases, especially with
colour-blindness, and a haemorrhagic diathesis, that is, an extreme
liability to profuse and uncontrollable bleeding from trifling wounds.
On the other hand, mothers have transmitted, during several
generations, to their daughters alone, supernumerary and deficient
digits, colour-blindness, and other peculiarities. So that we see that
the very same peculiarity may become attached to either sex, and be
long inherited by that sex alone; but the attachment in certain cases
is much more frequent to one than the other sex. The same peculiarities
also may be promiscuously transmitted to either sex. Dr. Lucas gives
other cases, showing that the male occasionally transmits his
peculiarities to his daughters alone, and the mother to her sons alone;
but even in this case we see that inheritance is to a certain extent,
though inversely, regulated by sex. Dr. Lucas, after weighing the whole
evidence, comes to the conclusion that every peculiarity, according to
the sex in which it first appears, tends to be transmitted in a greater
or lesser degree to that sex.
A few details from the many cases collected by Mr. Sedgwick,[161] may
be here given. Colour-blindness, from some unknown cause, shows itself
much oftener in males than in females; in upwards of two hundred cases
collected by Mr. Sedgwick, nine-tenths related to men; but it is
eminently liable to be transmitted through women. In the case given by
Dr. Earle, members of eight related families were affected during five
generations: these families consisted of sixty-one individuals, namely,
of thirty-two males, of whom nine-sixteenths were incapable of
distinguishing colour, and of twenty-nine females, of whom only
one-fifteenth were thus affected. {73} Although colour-blindness thus
generally clings to the male sex, nevertheless, in one instance in
which it first appeared in a female, it was transmitted during five
generations to thirteen individuals, all of whom were females. A
haemorrhagic diathesis, often accompanied by rheumatism, has been known
to affect the males alone during five generations, being transmitted,
however, through the females. It is said that deficient phalanges in
the fingers have been inherited by the females alone during ten
generations. In another case, a man thus deficient in both hands and
feet, transmitted the peculiarity to his two sons and one daughter; but
in the third generation, out of nineteen grandchildren, twelve sons had
the family defect, whilst the seven daughters were free. In ordinary
cases of sexual limitation, the sons or daughters inherit the
peculiarity, whatever it may be, from their father or mother, and
transmit it to their children of the same sex; but generally with the
haemorrhagic diathesis, and often with colour-blindness, and in some
other cases, the sons never inherit the peculiarity directly from their
fathers, but the daughters, and the daughters alone, transmit the
latent tendency, so that the sons of the daughters alone exhibit it.
Thus, the father, grandson, and great-great-grandson will exhibit a
peculiarity,--the grandmother, daughter, and great-granddaughter having
transmitted it in a latent state. Hence we have, as Mr. Sedgwick
remarks, a double kind of atavism or reversion; each grandson
apparently receiving and developing the peculiarity from his
grandfather, and each daughter apparently receiving the latent tendency
from her grandmother.
From the various facts recorded by Dr. Prosper Lucas, Mr. Sedgwick, and
others, there can be no doubt that peculiarities first appearing in
either sex, though not in any way necessarily or invariably connected
with that sex, strongly tend to be inherited by the offspring of the
same sex, but are often transmitted in a latent state through the
opposite sex.
Turning now to domesticated animals, we find that certain characters
not proper to the parent-species are often confined to, and inherited
by, one sex alone; but we do not know the history of the first
appearance of such characters. In the chapter on Sheep, we have seen
that the males of certain races differ greatly from the females in the
shape of their horns, these being absent in the ewes of some breeds, in
the development of fat in the tail in certain fat-tailed breeds, and in
the outline of the forehead. These differences, judging from the
character of the allied wild species, cannot be accounted for by
supposing that they have been derived from distinct parent-forms. There
is, also, a great difference between the horns of the two sexes in one
Indian breed of goats. The bull zebu is said to have a larger hump than
the cow. In the Scotch deer-hound the two sexes differ in size more
than in any other variety of the dog,[162] and, judging from analogy,
more than in the aboriginal parent-species. The peculiar colour called
tortoise-shell is very rarely seen in a male cat; the males of this
variety being of a rusty tint. A tendency to baldness in man before the
advent of old age is certainly inherited; and in the European, or at
least in the {74} Englishman, is an attribute of the male sex, and may
almost be ranked as an incipient secondary sexual character.
In various breeds of the fowl the males and females often differ
greatly; and these differences are far from being the same with those
which distinguish the two sexes in the parent-species, the _Gallus
bankiva_; and consequently have originated under domestication. In
certain sub-varieties of the Game race we have the unusual case of the
hens differing from each other more than the cocks. In an Indian breed
of a white colour stained with soot, the hens invariably have black
skins, and their bones are covered by a black periosteum, whilst the
cocks are never or most rarely thus characterised. Pigeons offer a more
interesting case; for the two sexes rarely differ throughout the whole
great family, and the males and females of the parent-form, the _C.
livia_, are undistinguishable; yet we have seen that with Pouters the
male has the characteristic quality of pouting more strongly developed
than the female; and in certain sub-varieties[163] the males alone are
spotted or striated with black. When male and female English
carrier-pigeons are exhibited in separate pens, the difference in the
development of the wattle over the beak and round the eyes is
conspicuous. So that here we have instances of the appearance of
secondary sexual characters in the domesticated races of a species in
which such differences are naturally quite absent.
On the other hand, secondary sexual characters which properly belong to the
species are sometimes quite lost, or greatly diminished, under
domestication. We see this in the small size of the tusks in our improved
breeds of the pig, in comparison with those of the wild boar. There are
sub-breeds of fowls in which the males have lost the fine flowing
tail-feathers and hackles; and others in which there is no difference in
colour between the two sexes. In some cases the barred plumage, which in
gallinaceous birds is commonly the attribute of the hen, has been
transferred to the cock, as in the cuckoo sub-breeds. In other cases
masculine characters have been partly transferred to the female, as with
the splendid plumage of the golden-spangled Hamburgh hen, the enlarged comb
of the Spanish hen, the pugnacious disposition of the Game hen, and as in
the well-developed spurs which occasionally appear in the hens of various
breeds. In Polish fowls both sexes are ornamented with a topknot, that of
the male being formed of hackle-like feathers, and this is a new male
character in the genus Gallus. On the whole, as far as I can judge, new
characters are more apt {75} to appear in the males of our domesticated
animals than in the females, and afterwards to be either exclusively or
more strongly inherited by the males. Finally, in accordance with the
principle of inheritance as limited by sex, the appearance of secondary
sexual characters in natural species offers no especial difficulty, and
their subsequent increase and modification, if of any service to the
species, would follow through that form of selection which in my 'Origin of
Species' I have called sexual selection.
_Inheritance at corresponding periods of Life._
This is an important subject. Since the publication of my 'Origin of
Species,' I have seen no reason to doubt the truth of the explanation there
given of perhaps the most remarkable of all the facts in biology, namely,
the difference between the embryo and the adult animal. The explanation is,
that variations do not necessarily or generally occur at a very early
period of embryonic growth, and that such variations are inherited at a
corresponding age. As a consequence of this the embryo, even when the
parent-form undergoes a great amount of modification, is left only slightly
modified; and the embryos of widely-different animals which are descended
from a common progenitor remain in many important respects like each other
and their common progenitor. We can thus understand why embryology should
throw a flood of light on the natural system of classification, for this
ought to be as far as possible genealogical. When the embryo leads an
independent life, that is, becomes a larva, it has to be adapted to the
surrounding conditions in its structure and instincts, independently of
those of its parents; and the principle of inheritance at corresponding
periods of life renders this possible.
This principle is, indeed, in one way so obvious that it escapes attention.
We possess a number of races of animals and plants, which, when compared
with each other and with their parent-forms, present conspicuous
differences, both in the immature and mature states. Look at the seeds of
the several kinds of peas, beans, maize, which can be propagated truly, and
see how they differ in size, colour, and shape, whilst the {76} full-grown
plants differ but little. Cabbages on the other hand differ greatly in
foliage and manner of growth, but hardly at all in their seeds; and
generally it will be found that the differences between cultivated plants
at different periods of growth are not necessarily closely connected
together, for plants may differ much in their seeds and little when
full-grown, and conversely may yield seeds hardly distinguishable, yet
differ much when full-grown. In the several breeds of poultry, descended
from a single species, differences in the eggs and chickens, in the plumage
at the first and subsequent moults, in the comb and wattles during
maturity, are all inherited. With man peculiarities in the milk and second
teeth, of which I have received the details, are inheritable, and with man
longevity is often transmitted. So again with our improved breeds of cattle
and sheep, early maturity, including the early development of the teeth,
and with certain breeds of fowl the early appearance of secondary sexual
characters, all come under the same head of inheritance at corresponding
periods.
Numerous analogous facts could be given. The silk-moth, perhaps, offers the
best instance; for in the breeds which transmit their characters truly, the
eggs differ in size, colour, and shape;--the caterpillars differ, in
moulting three or four times, in colour, even in having a dark-coloured
mark like an eyebrow, and in the loss of certain instincts;--the cocoons
differ in size, shape, and in the colour and quality of the silk; these
several differences being followed by slight or barely distinguishable
differences in the mature moth.
But it may be said that, if in the above cases a new peculiarity is
inherited, it must be at the corresponding stage of development; for an egg
or seed can resemble only an egg or seed, and the horn in a full-grown ox
can resemble only a horn. The following cases show inheritance at
corresponding periods more plainly, because they refer to peculiarities
which might have supervened, as far as we can see, earlier or later in
life, yet are inherited at the same period at which they first appeared.
In the Lambert family the porcupine-like excrescences appeared in the
father and sons at the same age, namely, about nine weeks after {77}
birth.[164] In the extraordinary hairy family described by Mr.
Crawfurd,[165] children were produced during three generations with
hairy ears; in the father the hair began to grow over his body at six
years old; in his daughter somewhat earlier, namely, at one year; and
in both generations the milk teeth appeared late in life, the permanent
teeth being afterwards singularly deficient. Greyness of hair at an
unusually early age has been transmitted in some families. These cases
border on diseases inherited at corresponding periods of life, to which
I shall immediately refer.
It is a well-known peculiarity with almond-tumbler pigeons, that the
full beauty and peculiar character of the plumage does not appear until
the bird has moulted two or three times. Neumeister describes and
figures a breed of pigeons in which the whole body is white except the
breast, neck, and head; but before the first moult all the white
feathers acquire coloured edges. Another breed is more remarkable: its
first plumage is black, with rusty-red wing-bars and a crescent-shaped
mark on the breast; these marks then became white, and remain so during
three or four moults; but after this period the white spreads over the
body, and the bird loses its beauty.[166] Prize canary-birds have their
wings and tail black: "this colour, however, is only retained until the
first moult, so that they must be exhibited ere the change takes place.
Once moulted, the peculiarity has ceased. Of course all the birds
emanating from this stock have black wings and tails the first
year."[167] A curious and somewhat analogous account has been
given[168] of a family of wild pied rooks which were first observed in
1798, near Chalfont, and which every year from that date up to the
period of the published notice, viz. 1837, "have several of their brood
particoloured, black and white. This variegation of the plumage,
however, disappears with the first moult; but among the next young
families there are always a few pied ones." These changes of plumage,
which appear and are inherited at various corresponding periods of life
in the pigeon, canary-bird, and rook, are remarkable, because the
parent-species undergo no such change.
Inherited diseases afford evidence in some respects of less value than
the foregoing cases, because diseases are not necessarily connected
with any change in structure; but in other respects of more value,
because the periods have been more carefully observed. Certain diseases
are communicated to the child apparently by a process like inoculation,
and the child is from the first affected; such cases may be here passed
over. Large classes of diseases usually appear at certain ages, such as
St. Vitus's dance in youth, consumption in early mid-life, gout later,
and apoplexy still later; and these are naturally inherited at the same
period. But even in diseases of this class, instances have been
recorded, as with St. Vitus's {78} dance, showing that an unusually
early or late tendency to the disease is inheritable.[169] In most
cases the appearance of any inherited disease is largely determined by
certain critical periods in each person's life, as well as by
unfavourable conditions. There are many other diseases, which are not
attached to any particular period, but which certainly tend to appear
in the child at about the same age at which the parent was first
attacked. An array of high authorities, ancient and modern, could be
given in support of this proposition. The illustrious Hunter believed
in it; and Piorry[170] cautions the physician to look closely to the
child at the period when any grave inheritable disease attacked the
parent. Dr. Prosper Lucas,[171] after collecting facts from every
source, asserts that affections of all kinds, though not related to any
particular period of life, tend to reappear in the offspring at
whatever period of life they first appeared in the progenitor.
As the subject is important, it may be well to give a few instances,
simply as illustrations, not as proof; for proof, recourse must be had
to the authorities above quoted. Some of the following cases have been
selected for the sake of showing that, when a slight departure from the
rule occurs, the child is affected somewhat earlier in life than the
parent. In the family of Le Compte blindness was inherited during three
generations, and no less than thirty-seven children and grandchildren
were all affected at about the same age, namely seventeen or
eighteen.[172] In another case a father and his four children all
became blind at twenty-one years old; in another, a grandmother grew
blind at thirty-five, her daughter at nineteen, and three grandchildren
at the ages of thirteen and eleven.[173] So with deafness, two
brothers, their father and paternal grandfather, all became deaf at the
age of forty.[174]
Esquirol gives several striking instances of insanity coming on at the
same age, as that of a grandfather, father, and son, who all committed
suicide near their fiftieth year. Many other cases could be given, as
of a whole family who became insane at the age of forty.[175] Other
cerebral affections sometimes follow the same rule,--for instance,
epilepsy and apoplexy. A woman died of the latter disease when
sixty-three years old; one of her daughters at forty-three, and the
other at sixty-seven: the latter had twelve children, who all died from
tubercular meningitis.[176] I mention this latter case because it
illustrates a frequent occurrence, namely, a change in the precise
nature of an inherited disease, though still affecting the same organ.
{79}
Asthma has attacked several members of the same family when forty years
old, and other families during infancy. The most different diseases, as
angina pectoris, stone in the bladder, and various affections of the
skin, have appeared in successive generations at nearly the same age.
The little finger of a man began from some unknown cause to grow
inwards, and the same finger in his two sons began at the same age to
bend inwards in a similar manner. Strange and inexplicable neuralgic
affections have caused parents and children to suffer agonies at about
the same period of life.[177]
I will give only two other cases, which are interesting as illustrating
the disappearance as well as the appearance of disease at the same age.
Two brothers, their father, their paternal uncles, seven cousins, and
their paternal grandfather, were all similarly affected by a
skin-disease, called pityriasis versicolor; "the disease, strictly
limited to the males of the family (though transmitted through the
females), usually appeared at puberty, and disappeared at about the age
of forty or forty-five years." The second case is that of four
brothers, who when about twelve years old suffered almost every week
from severe headaches, which were relieved only by a recumbent position
in a dark room. Their father, paternal uncles, paternal grandfather,
and paternal granduncles all suffered in the same way from headaches,
which ceased at the age of fifty-four or fifty-five in all those who
lived so long. None of the females of the family were affected.[178]
It is impossible to read the foregoing accounts, and the many others which
have been recorded, of diseases coming on during three or even more
generations, at the same age in several members of the same family,
especially in the case of rare affections in which the coincidence cannot
be attributed to chance, and doubt that there is a strong tendency to
inheritance in disease at corresponding periods of life. When the rule
fails, the disease is apt to come on earlier in the child than in the
parent; the exceptions in the other direction being vey much rarer. Dr.
Lucas[179] alludes to several cases of inherited diseases coming on at an
earlier period. I have already given one striking instance with blindness
during three generations; and Mr. Bowman remarks that this frequently
occurs with cataract. With cancer there seems to be a peculiar liability to
earlier inheritance: Mr. Paget, who has particularly {80} attended to this
subject, and tabulated a large number of cases, informs me that he believes
that in nine cases out of ten the later generation suffers from the disease
at an earlier period than the previous generation. He adds, "In the
instances in which the opposite relation holds, and the members of later
generations have cancer at a later age than their predecessors, I think it
will be found that the non-cancerous parents have lived to extreme old
ages." So that the longevity of a non-affected parent seems to have the
power of determining in the offspring the fatal period; and we thus
apparently get another element of complexity in inheritance.
The facts, showing that with certain diseases the period of inheritance
occasionally or even frequently advances, are important with respect to the
general descent-theory, for they render it in some degree probable that the
same thing would occur with ordinary modifications of structure. The final
result of a long series of such advances would be the gradual obliteration
of characters proper to the embryo and larva, which would thus come to
resemble more and more closely the mature parent-form. But any structure
which was of service to the embryo or larva would be preserved by the
destruction at this stage of growth of each individual which manifested any
tendency to lose at too early an age its own proper character.
Finally, from the numerous races of cultivated plants and domestic animals,
in which the seed or eggs, the young or old, differ from each other and
from their parent-species;--from the cases in which new characters have
appeared at a particular period, and afterwards have been inherited at the
same period;--and from what we know with respect to disease, we must
believe in the truth of the great principle of inheritance at corresponding
periods of life.
* * * * *
_Summary of the three preceding Chapters._--Strong as is the force of
inheritance, it allows the incessant appearance of new characters. These,
whether beneficial or injurious, of the most trifling importance, such as a
shade of colour in a flower, a coloured lock of hair, or a mere gesture; or
of the highest importance, as when affecting the brain or an organ so
perfect {81} and complex as the eye; or of so grave a nature as to deserve
to be called a monstrosity, or so peculiar as not to occur normally in any
member of the same natural class, are all sometimes strongly inherited by
man, the lower animals, and plants. In numberless cases it suffices for the
inheritance of a peculiarity that one parent alone should be thus
characterised. Inequalities in the two sides of the body, though opposed to
the law of symmetry, may be transmitted. There is a considerable body of
evidence showing that even mutilations, and the effects of accidents,
especially or perhaps exclusively when followed by disease, are
occasionally inherited. There can be no doubt that the evil effects of
long-continued exposure in the parent to injurious conditions are sometimes
transmitted to the offspring. So it is, as we shall see in a future
chapter, with the effects of the use and disuse of parts, and of mental
habits. Periodical habits are likewise transmitted, but generally, as it
would appear, with little force.
Hence we are led to look at inheritance as the rule, and non-inheritance as
the anomaly. But this power often appears to us in our ignorance to act
capriciously, transmitting a character with inexplicable strength or
feebleness. The very same peculiarity, as the weeping habit of trees,
silky-feathers, &c., may be inherited either firmly or not at all by
different members of the same group, and even by different individuals of
the same species, though treated in the same manner. In this latter case we
see that the power of transmission is a quality which is merely individual
in its attachment. As with single characters, so it is with the several
concurrent slight differences which distinguish sub-varieties or races; for
of these, some can be propagated almost as truly as species, whilst others
cannot be relied on. The same rule holds good with plants, when propagated
by bulbs, offsets, &c., which in one sense still form parts of the same
individual, for some varieties retain or inherit through successive
bud-generations their character far more truly than others.
Some characters not proper to the parent-species have certainly been
inherited from an extremely remote epoch, and may therefore be considered
as firmly fixed. But it is doubtful whether length of inheritance in itself
gives fixedness of character; {82} though the chances are obviously in
favour of any character which has long been transmitted true or unaltered,
still being transmitted true as long as the conditions of life remain the
same. We know that many species, after having retained the same character
for countless ages, whilst living under their natural conditions, when
domesticated have varied in the most diversified manner, that is, have
failed to transmit their original form; so that no character appears to be
absolutely fixed. We can sometimes account for the failure of inheritance
by the conditions of life being opposed to the development of certain
characters; and still oftener, as with plants cultivated by grafts and
buds, by the conditions causing new and slight modifications incessantly to
appear. In this latter case it is not that inheritance wholly fails, but
that new characters are continually superadded. In some few cases, in which
both parents are similarly characterised, inheritance seems to gain so much
force by the combined action of the two parents, that it counteracts its
own power, and a new modification is the result.
In many cases the failure of the parents to transmit their likeness is due
to the breed having been at some former period crossed; and the child takes
after his grandparent or more remote ancestor of foreign blood. In other
cases, in which the breed has not been crossed, but some ancient character
has been lost through variation, it occasionally reappears through
reversion, so that the parents apparently fail to transmit their own
likeness. In all cases, however, we may safely conclude that the child
inherits all its characters from its parents, in whom certain characters
are latent, like the secondary sexual characters of one sex in the other.
When, after a long succession of bud-generations, a flower or fruit becomes
separated into distinct segments, having the colours or other attributes of
both parent-forms, we cannot doubt that these characters were latent in the
earlier buds, though they could not then be detected, or could be detected
only in an intimately commingled state. So it is with animals of crossed
parentage, which with advancing years occasionally exhibit characters
derived from one of their two parents, of which not a trace could at first
be perceived. Certain monstrosities, which resemble what naturalists call
the typical form of the group in question, {83} apparently come under the
same law of reversion. It is assuredly an astonishing fact that the male
and female sexual elements, that buds, and even full-grown animals, should
retain characters, during several generations in the case of crossed
breeds, and during thousands of generations in the case of pure breeds,
written as it were in invisible ink, yet ready at any time to be evolved
under the requisite conditions.
What these conditions are, we do not in many cases at all know. But the act
of crossing in itself, apparently from causing some disturbance in the
organisation, certainly gives a strong tendency to the reappearance of
long-lost characters, both corporeal and mental, independently of those
derived from the cross. A return of any species to its natural conditions
of life, as with feral animals and plants, favours reversion; though it is
certain that this tendency exists, we do not know how far it prevails, and
it has been much exaggerated. On the other hand, the crossed offspring of
plants which have had their organisation disturbed by cultivation, are more
liable to reversion than the crossed offspring of species which have always
lived under their natural conditions.
When distinguishable individuals of the same family, or races, or species
are crossed, we see that the one is often prepotent over the other in
transmitting its own character. A race may possess a strong power of
inheritance, and yet when crossed, as we have seen with trumpeter-pigeons,
yield to the prepotency of every other race. Prepotentcy of transmission
may be equal in the two sexes of the same species, but often runs more
strongly in one sex. It plays an important part in determining the rate at
which one race can be modified or wholly absorbed by repeated crosses with
another. We can seldom tell what makes one race or species prepotent over
another; but it sometimes depends on the same character being present and
visible in one parent, and latent or potentially present in the other.
Characters may first appear in either sex, but oftener in the male than in
the female, and afterwards be transmitted to the offspring of the same sex.
In this case we may feel confident that the peculiarity in question is
really present though latent in the opposite sex; hence the father may
transmit through his daughter any character to his grandson; and the mother
{84} conversely to her granddaughter. We thus learn, and the fact is an
important one, that transmission and development are distinct powers.
Occasionally these two powers seem to be antagonistic, or incapable of
combination in the same individual; for several cases have been recorded in
which the son has not directly inherited a character from his father, or
directly transmitted it to his son, but has received it by transmission
through his non-affected mother, and transmitted it through his
non-affected daughter. Owing to inheritance being limited by sex, we can
see how secondary sexual characters may first have arisen under nature;
their preservation and accumulation being dependent on their service to
either sex.
At whatever period of life a new character first appears, it generally
remains latent in the offspring until a corresponding age is attained, and
then it is developed. When this rule fails, the child generally exhibits
the character at an earlier period than the parent. On this principle of
inheritance at corresponding periods, we can understand how it is that most
animals display from the germ to maturity such a marvellous succession of
characters.
Finally, though much remains obscure with respect to Inheritance, we may
look at the following laws as fairly well established. Firstly, a tendency
in every character, new and old, to be transmitted by seminal and bud
generation, though often counteracted by various known and unknown causes.
Secondly, reversion or atavism, which depends on transmission and
development being distinct powers: it acts in various degrees and manners
through both seminal and bud generation. Thirdly, prepotency of
transmission, which may be confined to one sex, or be common to both sexes
of the prepotent form. Fourthly, transmission, limited by sex, generally to
the same sex in which the inherited character first appeared. Fifthly,
inheritance at corresponding periods of life, with some tendency to the
earlier development of the inherited character. In these laws of
Inheritance, as displayed under domestication, we see an ample provision
for the production, through variability and natural selection, of new
specific forms.
* * * * *
{85}
CHAPTER XV.
ON CROSSING.
FREE INTERCROSSING OBLITERATES THE DIFFERENCES BETWEEN ALLIED
BREEDS--WHEN THE NUMBERS OF TWO COMMINGLING BREEDS ARE UNEQUAL, ONE
ABSORBS THE OTHER--THE RATE OF ABSORPTION DETERMINED BY PREPOTENCY OF
TRANSMISSION, BY THE CONDITIONS OF LIFE, AND BY NATURAL SELECTION--ALL
ORGANIC BEINGS OCCASIONALLY INTERCROSS; APPARENT EXCEPTIONS--ON CERTAIN
CHARACTERS INCAPABLE OF FUSION; CHIEFLY OR EXCLUSIVELY THOSE WHICH HAVE
SUDDENLY APPEARED IN THE INDIVIDUAL--ON THE MODIFICATION OF OLD RACES,
AND THE FORMATION OF NEW RACES, BY CROSSING--SOME CROSSED RACES HAVE
BRED TRUE FROM THEIR FIRST PRODUCTION--ON THE CROSSING OF DISTINCT
SPECIES IN RELATION TO THE FORMATION OF DOMESTIC RACES.
In the two previous chapters, when discussing reversion and prepotency, I
was necessarily led to give many facts on crossing. In the present chapter
I shall consider the part which crossing plays in two opposed
directions,--firstly, in obliterating characters, and consequently in
preventing the formation of new races; and secondly, in the modification of
old races, or in the formation of new and intermediate races, by a
combination of characters. I shall also show that certain characters are
incapable of fusion.
The effects of free or uncontrolled breeding between the members of the
same variety or of closely allied varieties are important; but are so
obvious that they need not be discussed at much length. It is free
intercrossing which chiefly gives uniformity, both under nature and under
domestication, to the individuals of the same species or variety, when they
live mingled together and are not exposed to any cause inducing excessive
variability. The prevention of free crossing, and the intentional matching
of individual animals, are the corner-stones of the breeder's art. No man
in his senses would expect to improve or modify a breed in any particular
manner, or keep an old breed true and distinct, unless he separated his
animals. The killing of inferior animals in each generation comes to the
{86} same thing as their separation. In savage and semi-civilised
countries, where the inhabitants have not the means of separating their
animals, more than a single breed of the same species rarely or never
exists. In former times, even in a country so civilised as North America,
there were no distinct races of sheep, for all had been mingled
together.[180] The celebrated agriculturist Marshall[181] remarks that
"sheep that are kept within fences, as well as shepherded flocks in open
countries, have generally a similarity, if not a uniformity, of character
in the individuals of each flock;" for they breed freely together, and are
prevented from crossing with other kinds; whereas in the unenclosed parts
of England the unshepherded sheep, even of the same flock, are far from
true or uniform, owing to various breeds having mingled and crossed. We
have seen that the half-wild cattle in the several British parks are
uniform in character in each; but in the different parks, from not having
mingled and crossed during many generations, they differ in a slight
degree.
We cannot doubt that the extraordinary number of varieties and
sub-varieties of the pigeon, amounting to at least one hundred and fifty,
is partly due to their remaining, differently from other domesticated
birds, paired for life when once matched. On the other hand, breeds of cats
imported into this country soon disappear, for their nocturnal and rambling
habits render it hardly possible to prevent free crossing. Rengger[182]
gives an interesting case with respect to the cat in Paraguay: in all the
distant parts of the kingdom it has assumed, apparently from the effects of
the climate, a peculiar character, but near the capital this change has
been prevented, owing, as he asserts, to the native animal frequently
crossing with cats imported from Europe. In all cases like the foregoing,
the effects of an occasional cross will be augmented by the increased
vigour and fertility of the crossed offspring, of which fact evidence will
hereafter be given; for this will lead to the mongrels increasing more
rapidly than the pure parent-breeds.
{87}
When distinct breeds are allowed to cross freely, the result will be a
heterogenous body; for instance, the dogs in Paraguay are far from uniform,
and can no longer be affiliated to their parent-races.[183] The character
which a crossed body of animals will ultimately assume must depend on
several contingencies,--namely, on the relative numbers of the individuals
belonging to the two or more races which are allowed to mingle; on the
prepotency of one race over the other in the transmission of character; and
on the conditions of life to which they are exposed. When two commingled
breeds exist at first in nearly equal numbers, the whole will sooner or
later become intimately blended, but not so soon, both breeds being equally
favoured in all respects, as might have been expected. The following
calculation[184] shows that this is the case: if a colony with an equal
number of black and white men were founded, and we assume that they marry
indiscriminately, are equally prolific, and that one in thirty annually
dies and is born; then "in 65 years the number of blacks, whites, and
mulattoes would be equal. In 91 years the whites would be 1-10th, the
blacks 1-10th, and the mulattoes, or people of intermediate degrees of
colour, 8-10ths of the whole number. In three centuries not 1-100th part of
the whites would exist."
When one of two mingled races exceeds the other greatly in number, the
latter will soon be wholly, or almost wholly, absorbed and lost.[185] Thus
European pigs and dogs have been largely introduced into the islands of the
Pacific Ocean, and the native races have been absorbed and lost in the
course of about fifty or sixty years;[186] but the imported races no doubt
were favoured. Rats may be considered as semi-domesticated animals. Some
snake-rats (_Mus alexandrinus_) escaped in the Zoological Gardens of
London, "and for a long time afterwards the keepers frequently caught
cross-bred rats, at first half-breds, afterwards with less and less of the
character of the snake-rat, till at length all traces of it
disappeared."[187] On the other hand, {88} in some parts of London,
especially near the docks, where fresh rats are frequently imported, an
endless variety of intermediate forms may be found between the brown,
black, and snake rat, which are all three usually ranked as distinct
species.
How many generations are necessary for one species or race to absorb
another by repeated crosses has often been discussed;[188] and the
requisite number has probably been much exaggerated. Some writers have
maintained that a dozen, or score, or even more generations, are necessary;
but this in itself is improbable, for in the tenth generation there will be
only 1-1024th part of foreign blood in the offspring. Gaertner found,[189]
that with plants one species could be made to absorb another in from three
to five generations, and he believes that this could always be effected in
from six to seventh generations. In one instance, however, Koelreuter[190]
speaks of the offspring of _Mirabilis vulgaris_, crossed during eight
successive generations by _M. longiflora_, as resembling this latter
species so closely, that the most scrupulous observer could detect "vix
aliquam notabilem differentiam;"--he succeeded, as he says, "ad plenariam
fere transmutationem." But this expression shows that the act of absorption
was not even then absolutely complete, though these crossed plants
contained only the 1-256th part of _M. vulgaris_. The conclusions of such
accurate observers as Gaertner and Koelreuter are of far higher worth than
those made without scientific aim by breeders. The most remarkable
statement which I have met with of the persistent endurance of the effects
of a single cross is given by Fleischmann,[191] who, in reference to German
sheep, says "that the original coarse sheep have 5500 fibres of wool on a
square inch; grades of the third or fourth Merino cross produced about
8000, the twentieth cross 27,000, the perfect pure Merino blood 40,000 to
48,000." So that in this case common German sheep crossed twenty times
successively with Merinos have not by any means acquired wool as fine as
that of the pure breed. In all cases, the rate of absorption will {89}
depend largely on the conditions of life being favourable to any particular
character; and we may suspect that there would be under the climate of
Germany a constant tendency to degeneration in the wool of Merinos, unless
prevented by careful selection; and thus perhaps the foregoing remarkable
case may be explained. The rate of absorption must also depend on the
amount of distinguishable difference between the two forms which are
crossed, and especially, as Gaertner insists, on prepotency of transmission
in the one form over the other. We have seen in the last chapter that one
of two French breeds of sheep yielded up its character, when crossed with
Merinos, very much slower than the other; and the common German sheep
referred to by Fleischmann may present an analogous case. But in all cases
there will be during many subsequent generations more or less liability to
reversion, and it is this fact which has probably led authors to maintain
that a score or more of generations are requisite for one race to absorb
another. In considering the final result of the commingling of two or more
breeds, we must not forget that the act of crossing in itself tends to
bring back long-lost characters not proper to the immediate parent-forms.
With respect to the influence of the conditions of life on any two breeds
which are allowed to cross freely, unless both are indigenous and have long
been accustomed to the country where they live, they will, in all
probability, be unequally affected by the conditions, and this will modify
the result. Even with indigenous breeds, it will rarely or never occur that
both are equally well adapted to the surrounding circumstances; more
especially when permitted to roam freely, and not carefully tended, as will
generally be the case with breeds allowed to cross. As a consequence of
this, natural selection will to a certain extent come into action, and the
best fitted will survive, and this will aid in determining the ultimate
character of the commingled body.
How long a time it would require before such a crossed body of animals
would assume within a limited area a uniform character no one can say; that
they would ultimately become uniform from free intercrossing, and from the
survival of the fittest, we may feel assured; but the character thus
acquired would rarely or never, as we may infer from the several previous
{90} considerations, be exactly intermediate between that of the two
parent-breeds. With respect to the very slight differences by which the
individuals of the same sub-variety, or even of allied varieties, are
characterised, it is obvious that free crossing would soon obliterate such
small distinctions. The formation of new varieties, independently of
selection, would also thus be prevented; except when the same variation
continually recurred from the action of some strongly predisposing cause.
Hence we may conclude that free crossing has in all cases played an
important part in giving to all the members of the same domestic race, and
of the same natural species, uniformity of character, though largely
modified by natural selection and by the direct action of the surrounding
conditions.
_On the possibility of all organic beings occasionally intercrossing._--But
it may be asked, can free crossing occur with hermaphrodite animals and
plants? All the higher animals, and the few insects which have been
domesticated, have separated sexes, and must inevitably unite for each
birth. With respect to the crossing of hermaphrodites, the subject is too
large for the present volume, and will be more properly treated in a
succeeding work. In my 'Origin of Species,' however, I have given a short
abstract of the reasons which induce me to believe that all organic beings
occasionally cross, though perhaps in some cases only at long intervals of
time.[192] I will here just recall the fact that many plants, though
hermaphrodite in structure, are unisexual in function;--such as those
called by C. K. Sprengel _dichogamous_, in which the pollen and stigma of
the same flower are matured at different periods; or those called by me
_reciprocally dimorphic_, in which the flower's own pollen is not fitted to
fertilise its own stigma; or again, the many kinds in which curious
mechanical contrivances exist, effectually preventing self-fertilisation.
There are, however, many hermaphrodite plants which are not in any way
specially constructed to favour intercrossing, but which nevertheless
commingle almost as freely as animals with separated sexes. This is the
case with cabbages, radishes, and onions, as I know from {91} having
experimented on them: even the peasants of Liguria say that cabbages must
be prevented "from falling in love" with each other. In the orange tribe,
Gallesio[193] remarks that the amelioration of the various kinds is checked
by their continual and almost regular crossing. So it is with numerous
other plants.
Nevertheless some cultivated plants can be named which rarely intercross,
as the common pea, or which never intercross, as I have reason to believe
is the case with the sweet-pea (_Lathyrus odoratus_); yet the structure of
these flowers certainly favours an occasional cross. The varieties of the
tomato and aubergine (_Solanum_) and pimenta (_Pimenta vulgaris?_) are
said[194] never to cross, even when growing alongside each other. But it
should be observed that these are all exotic plants, and we do not know how
they would behave in their native country when visited by the proper
insects.
It must also be admitted that some few natural species appear under our
present state of knowledge to be perpetually self-fertilised, as in the
case of the Bee Ophrys (_O. apifera_), though adapted in its structure to
be occasionally crossed. The _Leersia oryzoides_ produces minute enclosed
flowers which cannot possibly be crossed, and these alone, to the exclusion
of the ordinary flowers, have as yet been known to yield seed.[195] A few
additional and analogous cases could be advanced. But these facts do not
make me doubt that it is a general law of nature that the individuals of
the same species occasionally intercross, and that some great advantage is
derived from this act. It is well known (and I shall hereafter have to give
instances) that some plants, both indigenous and naturalised, rarely or
never produce flowers; or, if they flower, never produce seeds. But no one
is thus led to doubt that it is a general law of nature that phanerogamic
plants should produce flowers, and that these flowers should produce seed.
When they fail, we believe that such plants would perform their proper
functions under different conditions, or that they formerly did so and will
do so again. On analogous grounds, I believe that the few flowers {92}
which do not now intercross, either would do so under different conditions,
or that they formerly fertilised each other at intervals--the means for
effecting this being generally still retained--and they will do so again at
some future period, unless indeed they become extinct. On this view alone,
many points in the structure and action of the reproductive organs in
hermaphrodite plants and animals are intelligible,--for instance, the male
and female organs never being so completely enclosed as to render access
from without impossible. Hence we may conclude that the most important of
all the means for giving uniformity to the individuals of the same species,
namely, the capacity of occasionally intercrossing, is present, or has been
formerly present, with all organic beings.
_On certain Characters not blending._--When two breeds are crossed
their characters usually become intimately fused together; but some
characters refuse to blend, and are transmitted in an unmodified state
either from both parents or from one. When grey and white mice are
paired, the young are not piebald nor of an intermediate tint, but are
pure white or of the ordinary grey colour: so it is when white and
common collared turtle-doves are paired. In breeding Game fowls, a
great authority, Mr. J. Douglas, remarks, "I may here state a strange
fact: if you cross a black with a white game, you get birds of both
breeds of the clearest colour." Sir R. Heron crossed during many years
white, black, brown, and fawn-coloured Angora rabbits, and never once
got these colours mingled in the same animal, but often all four
colours in the same litter.[196] Additional cases could be given, but
this form of inheritance is very far from universal even with respect
to the most distinct colours. When turnspit dogs and ancon sheep, both
of which have dwarfed limbs, are crossed with common breeds, the
offspring are not intermediate in structure, but take after either
parent. When tailless or hornless animals are crossed with perfect
animals, it frequently, but by no means invariably, happens that the
offspring are {93} either perfectly furnished with these organs or are
quite destitute of them. According to Rengger, the hairless condition
of the Paraguay dog is either perfectly or not at all transmitted to
its mongrel offspring; but I have seen one partial exception in a dog
of this parentage which had part of its skin hairy, and part naked; the
parts being distinctly separated as in a piebald animal. When Dorking
fowls with five toes are crossed with other breeds, the chickens often
have five toes on one foot and four on the other. Some crossed pigs
raised by Sir R. Heron between the solid-hoofed and common pig had not
all four feet in an intermediate condition, but two feet were furnished
with properly divided, and two with united hoofs.
Analogous facts have been observed with plants: Major Trevor Clarke
crossed the little, glabrous-leaved, annual stock (_Matthiola_), with
pollen of a large, red-flowered, rough-leaved, biennial stock, called
_cocardeau_ by the French, and the result was that half the seedlings
had glabrous and the other half rough leaves, but none had leaves in an
intermediate state. That the glabrous seedlings were the product of the
rough-leaved variety, and not accidentally of the mother-plant's own
pollen, was shown by their tall and strong habit of growth.[197] In the
succeeding generations raised from the rough-leaved crossed seedlings,
some glabrous plants appeared, showing that the glabrous character,
though incapable of blending with and modifying the rough leaves, was
all the time latent in this family of plants. The numerous plants
formerly referred to, which I raised from reciprocal crosses between
the peloric and common Antirrhinum, offer a nearly parallel case; for
in the first generation all the plants resembled the common form, and
in the next generation, out of one hundred and thirty-seven plants, two
alone were in an intermediate condition, the others perfectly
resembling either the peloric or common form. Major Trevor Clarke also
fertilised the above-mentioned red-flowered stock with pollen from the
purple Queen stock, and about half the seedlings scarcely differed in
habit, and not at all in the red colour of the flower, from the
mother-plant, the other half bearing blossoms of a rich purple, closely
like those of the paternal plant. Gaertner crossed many white and
yellow-flowered species and varieties of Verbascum; and these colours
were never blended, but the offspring bore either pure white or pure
yellow blossoms; the former in the larger proportion.[198] Dr. Herbert
raised many seedlings, as he informed me, from Swedish turnips crossed
by two other varieties, and these never produced flowers of an
intermediate tint, but always like one of their parents. I fertilised
the purple sweet-pea (_Lathyrus odoratus_), which has a dark
reddish-purple standard-petal and violet-coloured wings and keel, with
pollen of the painted-lady sweet-pea, which has a pale cherry-coloured
standard, and almost white wings and keel; and from the same pod I
twice raised plants perfectly resembling both sorts; the greater number
resembling the father. So perfect was the resemblance, that I should
have thought there had {94} been some mistake, if the plants which were
at first identical with the paternal variety, namely, the painted-lady,
had not later in the season produced, as mentioned in a former chapter,
flowers blotched and streaked with dark purple. I raised grandchildren
and great-grandchildren from these crossed plants, and they continued
to resemble the painted-lady, but during the later generations became
rather more blotched with purple, yet none reverted completely to the
original mother-plant, the purple sweet-pea. The following case is
slightly different, but still shows the same principle: Naudin[199]
raised numerous hybrids between the yellow _Linaria vulgaris_ and the
purple _L. purpurea_, and during three successive generations the
colours kept distinct in different parts of the same flower.
From such cases as the foregoing, in which the offspring of the first
generation perfectly resemble either parent, we come by a small step to
those cases in which differently coloured flowers borne on the same
root resemble both parents, and by another step to those in which the
same flower or fruit is striped or blotched with the two parental
colours, or bears a single stripe of the colour or other characteristic
quality of one of the parent-forms. With hybrids and mongrels it
frequently or even generally happens that one part of the body
resembles more or less closely one parent and another part the other
parent; and here again some resistance to fusion, or, what comes to the
same thing, some mutual affinity between the organic atoms of the same
nature, apparently comes into play, for otherwise all parts of the body
would be equally intermediate in character. So again, when the
offspring of hybrids or mongrels, which are themselves nearly
intermediate in character, revert either wholly or by segments to their
ancestors, the principle of the affinity of similar, or the repulsion
of dissimilar atoms, must come into action. To this principle, which
seems to be extremely general, we shall recur in the chapter on
pangenesis.
It is remarkable, as has been strongly insisted upon by Isidore
Geoffroy St. Hilaire in regard to animals, that the transmission of
characters without fusion occurs most rarely when species are crossed;
I know of one exception alone, namely, with the hybrids naturally
produced between the common and hooded crow (_Corvus corone_ and
_cornix_), which, however, are closely allied species, differing in
nothing except colour. Nor have I met with any well-ascertained cases
of transmission of this kind, even when one form is strongly prepotent
over another, when two races are crossed which have been slowly formed
by man's selection, and therefore resemble to a certain extent natural
species. Such cases as puppies in the same litter closely resembling
two distinct breeds, are probably due to super-foetation,--that is, to
the influence of two fathers. All the characters above enumerated,
which are transmitted in a perfect state to some of the offspring and
not to others,--such as distinct colours, nakedness of skin, smoothness
of leaves, absence of horns or tail, additional toes, pelorism, dwarfed
structure, &c.,--have all been known to appear suddenly in individual
animals and plants. From this fact, and from the several slight,
aggregated differences which distinguish domestic races and species
from {95} each other, not being liable to this peculiar form of
transmission, we may conclude that it is in some way connected with the
sudden appearance of the characters in question.
_On the Modification of old Races and the Formation of new Races by
Crossing._--We have hitherto chiefly considered the effects of crossing in
giving uniformity of character; we must now look to an opposite result.
There can be no doubt that crossing, with the aid of rigorous selection
during several generations, has been a potent means in modifying old races,
and in forming new ones. Lord Orford crossed his famous stud of greyhounds
once with the bulldog, which breed was chosen from being deficient in
scenting powers, and from having what was wanted, courage and perseverance.
In the course of six or seven generations all traces of the external form
of the bulldog were eliminated, but courage and perseverance remained.
Certain pointers have been crossed, as I hear from the Rev. W. D. Fox, with
the foxhound, to give them dash and speed. Certain strains of Dorking fowls
have had a slight infusion of Game blood; and I have known a great fancier
who on a single occasion crossed his turbit-pigeons with barbs, for the
sake of gaining greater breadth of beak.
In the foregoing cases breeds have been crossed once, for the sake of
modifying some particular character; but with most of the improved races of
the pig, which now breed true, there have been repeated crosses,--for
instance, the improved Essex owes its excellence to repeated crosses with
the Neapolitan, together probably with some infusion of Chinese blood.[200]
So with our British sheep: almost all the races, except the Southdown, have
been largely crossed; "this, in fact, has been the history of our principal
breeds."[201] To give an example, the "Oxfordshire Downs" now rank as an
established breed.[202] They were produced about the year 1830 by crossing
"Hampshire and in some instances Southdown ewes with Cotswold rams:" now
the Hampshire ram was itself produced by repeated crosses between the
native {96} Hampshire sheep and Southdowns; and the long-woolled Cotswold
were improved by crosses with the Leicester, which latter again is believed
to have been a cross between several long-woolled sheep. Mr. Spooner, after
considering the various cases which have been carefully recorded, concludes
"that from a judicious pairing of cross-bred animals it is practicable to
establish a new breed." On the Continent the history of several crossed
races of cattle and of other animals has been well ascertained. To give one
instance: the King of Wurtemberg, after twenty-five years' careful
breeding, that is after six or seven generations, made a new breed of
cattle from a cross between a Dutch and Swiss breed, combined with other
breeds.[203] The Sebright bantam, which breeds as true as any other kind of
fowl, was formed about sixty years ago by a complicated cross.[204] Dark
Brahmas, which are believed by some fanciers to constitute a distinct
species, were undoubtedly formed[205] in the United States, within a recent
period, by a cross between Chittagongs and Cochins. With plants I believe
there is little doubt that some kinds of turnips, now extensively
cultivated, are crossed races; and the history of a variety of wheat which
was raised from two very distinct varieties, and which after six years'
culture presented an even sample, has been recorded on good authority.[206]
Until quite lately, cautious and experienced breeders, though not averse to
a single infusion of foreign blood, were almost universally convinced that
the attempt to establish a new race, intermediate between two widely
distinct races, was hopeless: "they clung with superstitious tenacity to
the doctrine of purity of blood, believing it to be the ark in which alone
true safety could be found."[207] Nor was this conviction unreasonable:
when two distinct races are crossed, the offspring of the first generation
are generally nearly uniform in character; but even this sometimes fails to
be the case, especially with crossed dogs and fowls, the young of which
from the first are sometimes much {97} diversified. As cross-bred animals
are generally of large size and vigorous, they have been raised in great
numbers for immediate consumption. But for breeding they are found to be
utterly useless; for though they may be themselves uniform in character,
when paired together they yield during many generations offspring
astonishingly diversified. The breeder is driven to despair, and concludes
that he will never form an intermediate race. But from the cases already
given, and from others which have been recorded, it appears that patience
alone is necessary; as Mr. Spooner remarks, "nature opposes no barrier to
successful admixture; in the course of time, by the aid of selection and
careful weeding, it is practicable to establish a new breed." After six or
seven generations the hoped-for result will in most cases be obtained; but
even then an occasional reversion, or failure to keep true, may be
expected. The attempt, however, will assuredly fail if the conditions of
life be decidedly unfavourable to the characters of either
parent-breed.[208]
Although the grandchildren and succeeding generations of cross-bred animals
are generally variable in an extreme degree, some curious exceptions to the
rule have been observed, both with crossed races and species. Thus Boitard
and Corbie[209] assert that from a Pouter and a Runt "a Cavalier will
appear, which we have classed amongst pigeons of pure race, because it
transmits all its qualities to its posterity." The editor of the 'Poultry
Chronicle'[210] bred some bluish fowls from a black Spanish cock and a
Malay hen; and these remained true to colour "generation after generation."
The Himalayan breed of rabbits was certainly formed by crossing two
sub-varieties of the silver-grey rabbit; although it suddenly assumed its
present character, which differs much from that of either parent-breed, yet
it has ever since been easily and truly propagated. I crossed some Labrador
and Penguin ducks, and recrossed the mongrels with Penguins; afterwards,
most of the ducks reared during three generations were nearly uniform in
character, being brown with a white crescentic mark on the lower part of
the breast, {98} and with some white spots at the base of the beak; so that
by the aid of a little selection a new breed might easily have been formed.
In regard to crossed varieties of plants, Mr. Beaton remarks[211] that
"Melville's extraordinary cross between the Scotch kale and an early
cabbage is as true and genuine as any on record;" but in this case no doubt
selection was practised. Gaertner[212] has given five cases of hybrids, in
which the progeny kept constant; and hybrids between _Dianthus armeria_ and
_deltoides_ remained true and uniform to the tenth generation. Dr. Herbert
likewise showed me a hybrid from two species of Loasa which from its first
production had kept constant during several generations.
We have seen in the earlier chapters, that some of our domesticated
animals, such as dogs, cattle, pigs, &c., are almost certainly descended
from more than one species, or wild race, if any one prefers to apply this
latter term to forms which were enabled to keep distinct in a state of
nature. Hence the crossing of aboriginally distinct species probably came
into play at an early period in the formation of our present races. From
Ruetimeyer's observations there can be little doubt that this occurred with
cattle; but in most cases some one of the forms which were allowed to cross
freely, will, it is probable, have absorbed and obliterated the others. For
it is not likely that semi-civilized men would have taken the necessary
pains to modify by selection their commingled, crossed, and fluctuating
stock. Nevertheless, those animals which were best adapted to their
conditions of life would have survived through natural selection; and by
this means crossing will often have indirectly aided in the formation of
primeval domesticated breeds.
Within recent times, as far as animals are concerned, the crossing of
distinct species has done little or nothing in the formation or
modification of our races. It is not yet known whether the species of
silk-moth which have been recently crossed in France will yield permanent
races. In the fourth chapter I alluded with some hesitation to the
statement that a new breed, between the hare and rabbit, called leporides,
had been formed in France, and was found capable of propagating {99}
itself; but it is now positively affirmed[213] that this is an error. With
plants which can be multiplied by buds and cuttings, hybridisation has done
wonders, as with many kinds of Roses, Rhododendrons, Pelargoniums,
Calceolarias, and Petunias. Nearly all these plants can be propagated by
seed; most of them freely; but extremely few or none come true by seed.
Some authors believe that crossing is the chief cause of variability,--that
is, of the appearance of absolutely new characters. Some have gone so far
as to look at it as the sole cause; but this conclusion is disproved by
some of the facts given in the chapter on Bud-variation. The belief that
characters not present in either parent or in their ancestors frequently
originate from crossing is doubtful; that they occasionally thus arise is
probable; but this subject will be more conveniently discussed in a future
chapter on the causes of Variability.
A condensed summary of this and of the three following chapters, together
with some remarks on Hybridism, will be given in the nineteenth chapter.
* * * * *
{100}
CHAPTER XVI.
CAUSES WHICH INTERFERE WITH THE FREE CROSSING OF VARIETIES--INFLUENCE OF
DOMESTICATION ON FERTILITY.
DIFFICULTIES IN JUDGING OF THE FERTILITY OF VARIETIES WHEN
CROSSED--VARIOUS CAUSES WHICH KEEP VARIETIES DISTINCT, AS THE PERIOD OF
BREEDING AND SEXUAL PREFERENCE--VARIETIES OF WHEAT SAID TO BE STERILE
WHEN CROSSED--VARIETIES OF MAIZE, VERBASCUM, HOLLYHOCK, GOURDS, MELONS,
AND TOBACCO, RENDERED IN SOME DEGREE MUTUALLY STERILE--DOMESTICATION
ELIMINATES THE TENDENCY TO STERILITY NATURAL TO SPECIES WHEN
CROSSED--ON THE INCREASED FERTILITY OF UNCROSSED ANIMALS AND PLANTS
FROM DOMESTICATION AND CULTIVATION.
The domesticated races of both animals and plants, when crossed, are with
extremely few exceptions quite prolific,--in some cases even more so than
the purely bred parent-races. The offspring, also, raised from such crosses
are likewise, as we shall see in the following chapter, generally more
vigorous and fertile than their parents. On the other hand, species when
crossed, and their hybrid offspring, are almost invariability in some
degree sterile; and here there seems to exist a broad and insuperable
distinction between races and species. The importance of this subject as
bearing on the origin of species is obvious; and we shall hereafter recur
to it.
It is unfortunate how few precise observations have been made on the
fertility of mongrel animals and plants during several successive
generations. Dr. Broca[214] has remarked that no one has observed whether,
for instance, mongrel dogs, bred _inter se_, are indefinitely fertile; yet,
if a shade of infertility be detected by careful observation in the
offspring of natural forms when crossed, it is thought that their specific
distinction is proved. But so many breeds of sheep, cattle, pigs, dogs, and
poultry, have been crossed and recrossed in various ways, that any
sterility, if it had existed, would from being injurious {101} almost
certainly have been observed. In investigating the fertility of crossed
varieties many sources of doubt occur. Whenever the least trace of
sterility between two plants, however closely allied, was observed by
Koelreuter, and more especially by Gaertner, who counted the exact number
of seed in each capsule, the two forms were at once ranked as distinct
species; and if this rule be followed, assuredly it will never be proved
that varieties when crossed are in any degree sterile. We have formerly
seen that certain breeds of dogs do not readily pair together; but no
observations have been made whether, when paired, they produce the full
number of young, and whether the latter are perfectly fertile _inter se_;
but, supposing that some degree of sterility were found to exist,
naturalists would simply infer that these breeds were descended from
aboriginally distinct species; and it would be scarcely possible to
ascertain whether or not this explanation was the true one.
The Sebright Bantam is much less prolific than any other breed of fowls,
and is descended from a cross between two very distinct breeds, recrossed
by a third sub-variety. But it would be extremely rash to infer that the
loss of fertility was in any manner connected with its crossed origin, for
it may with more probability be attributed either to long-continued close
interbreeding, or to an innate tendency to sterility correlated with the
absence of hackles and sickle tail-feathers.
Before giving the few recorded cases of forms, which must be ranked as
varieties, being in some degree sterile when crossed, I may remark that
other causes sometimes interfere with varieties freely intercrossing. Thus
they may differ too greatly in size, as with some kinds of dogs and fowls:
for instance, the editor of the 'Journal of Horticulture, &c.,'[215] says
that he can keep Bantams with the larger breeds without much danger of
their crossing, but not with the smaller breeds, such as Games, Hamburgs,
&c. With plants a difference in the period of flowering serves to keep
varieties distinct, as with the various kinds of maize and wheat: thus
Colonel Le Couteur[216] remarks, "the Talavera wheat, from flowering much
earlier than any other kind, is sure to continue pure." In different parts
of {102} the Falkland Islands the cattle are breaking up into herds of
different colours; and those on the higher ground, which are generally
white, usually breed, as I am informed by Admiral Sulivan, three months
earlier than those on the lowlands; and this would manifestly tend to keep
the herds from blending.
Certain domestic races seem to prefer breeding with their own kind; and
this is a fact of some importance, for it is a step towards that
instinctive feeling which helps to keep closely allied species in a state
of nature distinct. We have now abundant evidence that, if it were not for
this feeling, many more hybrids would be naturally produced than is the
case. We have seen in the first chapter that the alco dog of Mexico
dislikes dogs of other breeds; and the hairless dog of Paraguay mixes less
readily with the European races, than the latter do with each other. In
Germany the female Spitz-dog is said to receive the fox more readily than
will other dogs; a female Australian Dingo in England attracted the wild
male foxes. But these differences in the sexual instinct and attractive
power of the various breeds may be wholly due to their descent from
distinct species. In Paraguay the horses have much freedom, and an
excellent observer[217] believes that the native horses of the same colour
and size prefer associating with each other, and that the horses which have
been imported from Entre Rios and Banda Oriental into Paraguay likewise
prefer associating together. In Circassia six sub-races of the horse are
known and have received distinct names; and a native proprietor of
rank[218] asserts that horses of three of these races, whilst living a free
life, almost always refuse to mingle and cross, and will even attack each
other.
It has been observed, in a district stocked with heavy Lincolnshire and
light Norfolk sheep, that both kinds, though bred together, when turned
out, "in a short time separate to a sheep;" the Lincolnshires drawing off
to the rich soil, and the Norfolks to their own dry light soil; and as long
as there is plenty of grass, "the two breeds keep themselves as distinct as
rooks and pigeons." In this case different habits of {103} life tend to
keep the races distinct. On one of the Faroe islands, not more than half a
mile in diameter, the half-wild native black sheep are said not to have
readily mixed with the imported white sheep. It is a more curious fact that
the semi-monstrous ancon sheep of modern origin "have been observed to keep
together, separating themselves from the rest of the flock, when put into
enclosures with other sheep."[219] With respect to fallow deer, which live
in a semi-domesticated condition, Mr. Bennett[220] states that the dark and
pale coloured herds, which have long been kept together in the Forest of
Dean, in High Meadow Woods, and in the New Forest, have never been known to
mingle: the dark-coloured deer, it may be added, are believed to have been
first brought by James I. from Norway, on account of their greater
hardiness. I imported from the island of Porto Santo two of the feral
rabbits, which differ, as described in the fourth chapter, from common
rabbits; both proved to be males, and, though they lived during some years
in the Zoological Gardens, the superintendent, Mr. Bartlett, in vain
endeavoured to make them breed with various tame kinds; but whether this
refusal to breed was due to any change in instinct, or simply to their
extreme wildness; or whether confinement had rendered them sterile, as
often occurs, cannot be told.
Whilst matching for the sake of experiment many of the most distinct breeds
of pigeons, it frequently appeared to me that the birds, though faithful to
their marriage vow, retained some desire after their own kind. Accordingly
I asked Mr. Wicking, who has kept a larger stock of various breeds together
than any man in England, whether he thought that they would prefer pairing
with their own kind, supposing that there were males and females enough of
each; and he without hesitation answered that he was convinced that this
was the case. It has often been noticed that the dovecot pigeon seems to
have an actual aversion towards the several fancy breeds;[221] yet all have
{104} certainly sprung from a common progenitor. The Rev. W. D. Fox informs
me that his flocks of white and common Chinese geese kept distinct.
These facts and statements, though some of them are incapable of proof,
resting only on the opinion of experienced observers, show that some
domestic races are led by different habits of life to keep to a certain
extent separate, and that others prefer coupling with their own kind, in
the same manner as species in a state of nature, though in a much less
degree.
With respect to sterility from the crossing of domestic races, I know
of no well-ascertained case with animals. This fact, seeing the great
difference in structure between some breeds of pigeons, fowls, pigs,
dogs, &c., is extraordinary, in contrast with the sterility of many
closely allied natural species when crossed; but we shall hereafter
attempt to show that it is not so extraordinary as it at first appears.
And it may be well here to recall to mind that the amount of external
difference between two species will not safely guide us in foretelling
whether or not they will breed together,--some closely allied species
when crossed being utterly sterile, and others which are extremely
unlike being moderately fertile. I have said that no case of sterility
in crossed races rests on satisfactory evidence; but here is one which
at first seems trustworthy. Mr. Youatt,[222] and a better authority
cannot be quoted, states, that formerly in Lancashire crosses were
frequently made between longhorn and shorthorn cattle; the first cross
was excellent, but the produce was uncertain; in the third or fourth
generation the cows were bad milkers; "in addition to which, there was
much uncertainty whether the cows would conceive; and full one-third of
the cows among some of these half-breds failed to be in calf." This at
first seems a good case; but Mr. Wilkinson states,[223] that a breed
derived from this same cross was actually established in another part
of England; and if it had failed in fertility, the fact would surely
have been noticed. Moreover, supposing that Mr. Youatt had proved his
case, it might be argued that the sterility was wholly due to the two
parent-breeds being descended from primordially distinct species.
I will give a case with plants, to show how difficult it is to get
sufficient evidence. Mr. Sheriff, who has been so successful in the
formation of new races of wheat, fertilised the Hopetoun with the
Talavera; in the first and second generations the produce was
intermediate in character, but in the fourth generation "it was found
to consist of many varieties; nine-tenths of the florets proved barren,
and many of the seeds seemed shrivelled abortions, void of vitality,
and the whole race was evidently verging to extinction."[224] Now,
considering how little these {105} varieties of wheat differ in any
important character, it seems to me very improbable that the sterility
resulted, as Mr. Sheriff thought, from the cross, but from some quite
distinct cause. Until such experiments are many times repeated, it
would be rash to trust them; but unfortunately they have been rarely
tried even once with sufficient care.
Gaertner has recorded a more remarkable and trustworthy case: he
fertilised thirteen panicles (and subsequently nine others) on a dwarf
maize bearing yellow seed[225] with pollen of a tall maize having red
seed; and one head alone produced good seed, only five in number.
Though these plants are monoecious, and therefore do not require
castration, yet I should have suspected some accident in the
manipulation had not Gaertner expressly stated that he had during many
years grown these two varieties together, and they did not
spontaneously cross; and this, considering that the plants are
monoecious and abound with pollen, and are well known generally to
cross freely, seems explicable only on the belief that these two
varieties are in some degree mutually infertile. The hybrid plants
raised from the above five seed were intermediate in structure,
extremely variable, and perfectly fertile.[226] No one, I believe, has
hitherto suspected that these varieties of maize are distinct species;
but had the hybrids been in the least sterile, no doubt Gaertner would
at once have so classed them. I may here remark, that with undoubted
species there is not necessarily any close relation between the
sterility of a first cross and that of the hybrid offspring. Some
species can be crossed with facility, but produce utterly sterile
hybrids; others can be crossed with extreme difficulty, but the hybrids
when produced are moderately fertile. I am not aware, however, of any
instance quite like this of the maize with natural species, namely, of
a first cross made with difficulty, but yielding perfectly fertile
hybrids.
The following case is much more remarkable, and evidently perplexed
Gaertner, whose strong wish it was to draw a broad line of distinction
between species and varieties. In the genus Verbascum, he made, during
eighteen years, a vast number of experiments, and crossed no less than
1085 flowers and counted their seeds. Many of these experiments
consisted in crossing white and yellow varieties of both _V. lychnitis_
and _V. blattaria_ with nine other species and their hybrids. That the
white and yellow flowered plants of these two species are really
varieties, no one has doubted; and Gaertner actually raised in the case
of both species one variety from the seed of the other. Now in two of
his works[227] he distinctly asserts that crosses between
similarly-coloured flowers yield more seed than between
dissimilarly-coloured; so that the yellow-flowered variety of either
species (and conversely with the white-flowered variety), when crossed
with pollen of its own kind, yields more seed than when crossed with
that of the white variety; and so it is when differently coloured
species are crossed. The general results may be seen in the Table at
the {106} end of his volume. In one instance he gives[228] the
following details; but I must premise that Gaertner, to avoid
exaggerating the degree of sterility in his crosses, always compares
the _maximum_ number obtained from a cross with the _average_ number
naturally given by the pure mother-plant. The white-variety of _V.
lychnitis_, naturally fertilised by its own pollen, gave from an
_average_ of twelve capsules ninety-six good seeds in each; whilst
twenty flowers fertilised with pollen from the yellow variety of this
same species, gave as the _maximum_ only eighty-nine good seed; so that
we have the proportion of 1000 to 908, according to Gaertner's usual
scale. I should have thought it possible that so small a difference in
fertility might have been accounted for by the evil effects of the
necessary castration; but Gaertner shows that the white variety of _V.
lychnitis_, when fertilised first by the white variety of _V.
blattaria_, and then by the yellow variety of this species, yielded
seed in the proportion of 622 to 438; and in both these cases
castration was performed. Now the sterility which results from the
crossing of the differently coloured varieties of the same species, is
fully as great as that which occurs in many cases when distinct species
are crossed. Unfortunately Gaertner compared the results of the first
unions alone, and not the sterility of the two sets of hybrids produced
from the white variety of _V. lychnitis_ when fertilised by the white
and yellow varieties of _V. blattaria_, for it is probable that they
would have differed in this respect.
Mr. J. Scott has given me the results of a series of experiments on
Verbascum, made by him in the Botanic Gardens of Edinburgh. He repeated
some of Gaertner's experiments on distinct species, but obtained only
fluctuating results; some confirmatory, but the greater number
contradictory; nevertheless these seem hardly sufficient to overthrow
the conclusions arrived at by Gaertner from experiments tried on a much
larger scale. In the second place Mr. Scott experimented on the
relative fertility of unions between similarly and
dissimilarly-coloured varieties of the same species. Thus he fertilised
six flowers of the yellow variety of _V. lychnitis_ by its own pollen,
and obtained six capsules, and calling, for the sake of having a
standard of comparison, the average number of good seed in each one
hundred, he found that this same yellow variety, when fertilised by the
white variety, yielded from seven capsules an average of ninety-four
seed. On the same principle, the white variety of _V. lychnitis_ by its
own pollen (from six capsules), and by the pollen of the yellow variety
(eight capsules), yielded seed in the proportion of 100 to 82. The
yellow variety of _V. thapsus_ by its own pollen (eight capsules), and
by that of the white variety (only two capsules), yielded seed in the
proportion of 100 to 94. Lastly, the white variety of _V. blattaria_ by
its own pollen (eight capsules), and by that of the yellow variety
(five capsules), yielded seed in the proportion of 100 to 79. So that
in every case the unions of dissimilarly-coloured varieties of the same
species were less fertile than the unions of similarly-coloured
varieties; when all the cases are grouped together, the difference of
fertility is as 86 to 100. Some additional trials were made, and
altogether thirty-six similarly-coloured unions yielded thirty-five
good {107} capsules; whilst thirty-five dissimilarly-coloured unions
yielded only twenty-six good capsules. Besides the foregoing
experiments, the purple _V. phoeniceum_ was crossed by a rose-coloured
and a white variety of the same species; these two varieties were also
crossed together, and these several unions yielded less seed than _V.
phoeniceum_ by its own pollen. Hence it follows from Mr. Scott's
experiments, that in the genus Verbascum the similarly and
dissimilarly-coloured varieties of the same species behave, when
crossed, like closely allied but distinct species.[229]
This remarkable fact of the sexual affinity of similarly-coloured
varieties, as observed by Gaertner and Mr. Scott, may not be of very
rare occurrence; for the subject has not been attended to by others.
The following case is worth giving, partly to show how difficult it is
to avoid error. Dr. Herbert[230] has remarked that variously-coloured
double varieties of the hollyhock (_Althaea rosea_) may be raised with
certainty by seed from plants growing close together. I have been
informed that nurserymen who raise seed for sale do not separate their
plants; accordingly I procured seed of eighteen named varieties; of
these, eleven varieties produced sixty-two plants all perfectly true to
their kind; and seven produced forty-nine plants, half of which were
true and half false. Mr. Masters of Canterbury has given me a more
striking case; he saved seed from a great bed of twenty-four named
varieties planted in closely adjoining rows, and each variety
reproduced itself truly with only sometimes a shade of difference in
tint. Now in the hollyhock the pollen, which is abundant, is matured
and nearly all shed before the stigma of the same flower is ready to
receive it;[231] and as bees covered with pollen incessantly fly from
plant to plant, it would appear that adjoining varieties could not
escape being crossed. As, however, this does not occur, it appeared to
me probable that the pollen {108} of each variety was prepotent on its
own stigma over that of all other varieties. But Mr. C. Turner of
Slough, well known for his success in the cultivation of this plant,
informs me that it is the doubleness of the flowers which prevents the
bees gaining access to the pollen and stigma; and he finds that it is
difficult even to cross them artificially. Whether this explanation
will fully account for varieties in close proximity propagating
themselves so truly by seed, I do not know.
The following cases are worth giving, as they relate to monoecious
forms, which do not require, and consequently have not been injured by,
castration. Girou de Buzareingues crossed what he designates three
varieties of gourd,[232] and asserts that their mutual fertilisation is
less easy in proportion to the difference which they present. I am
aware how imperfectly the forms in this group were until recently
known; but Sageret,[233] who ranked them according to their mutual
fertility, considers the three forms above alluded to as varieties, as
does a far higher authority, namely, M. Naudin.[234] Sageret[235] has
observed that certain melons have a greater tendency, whatever the
cause may be, to keep true than others; and M. Naudin, who has had such
immense experience in this group, informs me that he believes that
certain varieties intercross more readily than others of the same
species; but he has not proved the truth of this conclusion; the
frequent abortion of the pollen near Paris being one great difficulty.
Nevertheless, he has grown close together, during seven years, certain
forms of Citrullus, which, as they could be artificially crossed with
perfect facility and produced fertile offspring, are ranked as
varieties; but these forms when not artificially crossed kept true.
Many other varieties, on the other hand, in the same group cross with
such facility, as M. Naudin repeatedly insists, that without being
grown far apart they cannot be kept in the least true.
Another case, though somewhat different, may be here given, as it is
highly remarkable, and is established on excellent evidence. Koelreuter
minutely describes five varieties of the common tobacco,[236] which
were reciprocally crossed, and the offspring were intermediate in
character and as fertile as their parents: from this fact Koelreuter
inferred that they are really varieties; and no one, as far as I can
discover, seems to have doubted that such is the case. He also crossed
reciprocally these five varieties with _N. glutinosa_, and they yielded
very sterile hybrids; but those raised from the _var. perennis_,
whether used as the father or mother plant, were not so sterile as the
hybrids from the four other varieties.[237] So that the sexual {109}
capacity of this one variety has certainly been in some degree
modified, so as to approach in nature that of _N. glutinosa_.[238]
These facts with respect to plants show that in some few cases certain
varieties have had their sexual powers so far modified, that they cross
together less readily and yield less seed than other varieties of the same
species. We shall presently see that the sexual functions of most animals
and plants are eminently liable to be affected by the conditions of life to
which they are exposed; and hereafter we shall briefly discuss the conjoint
bearing of this and other facts on the difference in fertility between
crossed varieties and crossed species.
_Domestication eliminates the tendency to Sterility which is general with
Species when crossed._
This hypothesis was first propounded by Pallas,[239] and has been adopted
by several authors. I can find hardly any direct facts in its support; but
unfortunately no one has compared, in the case of either animals or plants,
the fertility of anciently domesticated varieties, when crossed with a
distinct species, with that of the wild parent-species when similarly
crossed. No one has compared, for instance, the fertility of _Gallus
bankiva_ and of the domesticated fowl, when crossed with a distinct species
of Gallus or Phasianus; and the {110} experiment would in all cases be
surrounded by many difficulties. Dureau de la Malle, who has so closely
studied classical literature, states[240] that in the time of the Romans
the common mule was produced with more difficulty than at the present day;
but whether this statement may be trusted I know not. A much more
important, though somewhat different, case is given by M. Groenland,[241]
namely, that plants, known from their intermediate character and sterility
to be hybrids between Aegilops and wheat, have perpetuated themselves under
culture since 1857, _with a rapid but varying increase of fertility in each
generation_. In the fourth generation the plants, still retaining their
intermediate character, had become as fertile as common cultivated wheat.