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 invariably 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
(16/1. 'Journal de Physiolog.' tome 2 1859 page 385.) 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 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 Kolreuter, and more especially
by Gartner, 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, etc.' (16/2. December
1863 page 484.) 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,
Hamburghs, etc. 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 (16/3. On 'The Varieties of Wheat' page 66.) remarks, "the
Talavera wheat, from flowering much earlier than any other kind, is sure to
continue pure." In different parts of 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 Sir J. Sulivan,
three months earlier than those on the lowland; 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 in this 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 (16/4. Rengger 'Saugethiere von Paraguay'
s. 336.) 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 have received distinct
names; and a native proprietor of rank (16/5. See a memoir by MM. Lherbette
and De Quatrefages in 'Bull. Soc. d'Acclimat.' tome 8 July 1861 page 312.)
asserts that horses of three of these races, whilst living a free life, almost
always refuse to mingle and cross, and will even attack one another.
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 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." (16/6. For the Norfolk sheep see Marshall 'Rural Economy of Norfolk'
volume 2 page 136. See Rev. L. Landt 'Description of Faroe' page 66. For the
ancon sheep see 'Phil. Transact.' 1813 page 90.) With respect to fallow-deer,
which live in a semi-domesticated condition, Mr. Bennett (16/7. White 'Nat.
Hist. of Selbourne' edited by Bennett page 39. With respect to the origin of
the dark-coloured deer see 'Some Account of English Deer Parks' by E.P.
Shirley, Esq.) 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 the instinct, or simply to their
extreme wildness, or whether confinement had rendered them sterile, as often
occurs, cannot be determined.
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 dovecote pigeon seems to have an actual
aversion towards the several fancy breeds (16/8. 'The Dovecote' by the Rev.
E.S. Dixon page 155; Bechstein 'Naturgesch. Deutschlands' b. 4 1795 page 17.)
yet all have 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, etc., 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 is not a safe
guide for predicting 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 (16/9. 'Cattle' page 202.) 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 (16/10. Mr. J. Wilkinson in 'Remarks
addressed to Sir J. Sebright' 1820 page 38.), 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.
In the case of plants Gartner states that he fertilised thirteen heads (and
subsequently nine others) on a dwarf maize bearing yellow seed (16/11.
'Bastarderzeugung' s. 87, 169. See also the Table at the end of volume.) with
pollen of a tall maize having red seed; and one head alone produced good seed,
but 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 Gartner 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 seeds were intermediate in structure,
extremely variable, and perfectly fertile. (16/12. 'Bastarderzeugung' s. 87,
577.) In like manner Prof. Hildebrand (16/13. 'Bot. Zeitung' 1868 page 327.)
could not succeed in fertilising the female flowers of a plant bearing brown
grains with pollen from a certain kind bearing yellow grains; although other
flowers on the same plant, which were fertilised with their own pollen,
yielded good seed. No one, I believe, even suspects that these varieties of
maize are distinct species; but had the hybrids been in the least sterile, no
doubt Gartner 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, namely, of a first cross made with difficulty, but yielding perfectly
fertile hybrids. (16/14. Mr. Shirreff formerly thought ('Gardener's Chronicle'
1858 page 771) that the offspring from a cross between certain varieties of
wheat became sterile in the fourth generation; but he now admits ('Improvement
of the Cereals' 1873) that this was an error.)
The following case is much more remarkable, and evidently perplexed Gartner,
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 Gartner actually raised in the
case of both species one variety from the seed of the other. Now in two of his
works (16/15. 'Kenntniss der Befruchtung' s. 137; 'Bastarderzeugung' s. 92,
181. On raising the two varieties from seed see s. 307.) 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 end of his volume. In one
instance he gives (16/16. 'Bastarderzeugung' s. 216.) the following details;
but I must premise that Gartner, 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 seeds; so that we have the proportion of
1000 to 908, according to Gartner'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 Gartner 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
Gartner 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. (16/17. The results have
since been published in 'Journ. Asiatic Soc. of Bengal' 1867 page 145.) He
repeated some of Gartner's experiments on distinct species, but obtained only
fluctuating results, some confirmatory, the greater number contradictory;
nevertheless these seem hardly sufficient to overthrow the conclusion arrived
at by Gartner from experiments tried on a larger scale. Mr. Scott also
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 comparison, the average number of
good seed in each of their capsules 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 similarly-coloured varieties of the same species were more
fertile than the unions of dissimilarly-coloured varieties; when all the cases
are grouped together, the difference of fertility is as 100 to 86. Some
additional trials were made, and altogether thirty-six similarly-coloured
unions yielded thirty-five good 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.
(16/18. The following facts, given by Kolreuter in his 'Dritte Fortsetzung'
ss. 34, 39, appear at first sight strongly to confirm Mr. Scott's and
Gartner's statements; and to a certain limited extent they do so. Kolreuter
asserts, from innumerable observations, that insects incessantly carry pollen
from one species and variety of Verbascum to another; and I can confirm this
assertion; yet he found that the white and yellow varieties of Verbascum
lychnitis often grew wild mingled together: moreover, he cultivated these two
varieties in considerable numbers during four years in his garden, and they
kept true by seed; but when he crossed them, they produced flowers of an
intermediate tint. Hence it might have been thought that both varieties must
have a stronger elective affinity for the pollen of their own variety than for
that of the other; this elective affinity, I may add of each species for its
own pollen (Kolreuter 'Dritte Forts.' s. 39 and Gartner 'Bastarderz.' passim)
being a perfectly well-ascertained power. But the force of the foregoing facts
is much lessened by Gartner's numerous experiments, for, differently from
Kolreuter, he never once got ('Bastarderz.' s. 307) an intermediate tint when
he crossed the yellow and white flowered varieties of Verbascum. So that the
fact of the white and yellow varieties keeping true to their colour by seed
does not prove that they were not mutually fertilised by the pollen carried by
insects from one to the other.)
This remarkable fact of the sexual affinity of similarly-coloured varieties,
as observed by Gartner 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 (16/19.
'Amaryllidaceae' 1837 page 366. Gartner has made a similar observation.) has
remarked that variously-coloured double varieties of the Hollyhock (Althea
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 (16/20. Kolreuter first observed this fact,
'Mem. de l'Acad. de St. Petersburg' volume 3 page 127. See also C.K. Sprengel
'Das Entdeckte Geheimniss' s. 345.); 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 of each variety was prepotent on its own stigma
over that of all other varieties, but I have no evidence on this point. 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 cannot have been injured by,
castration. Girou de Buzareingues crossed what he designates three varieties
of gourd (16/21. Namely Barbarines, Pastissons, Giraumous: 'Annal. des Sc.
Nat.' tome 30 1833 pages 398 and 405.), 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 (16/22. 'Memoire sur les Cucurbitaceae' 1826 pages 46, 55.), 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. (16/23. 'Annales des Sc. Nat.' 4th series tome 6. M. Naudin considers
these forms as undoubtedly varieties of Cucurbita pepo.) Sageret (16/24. 'Mem.
Cucurb.' page 8.) 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. Kolreuter minutely
describes five varieties of the common tobacco (16/25. 'Zweite Forts.' s. 53
namely Nicotiana major vulgaris; (2) perennis; (3) transylvanica; (4) a sub-
var. of the last; (5) major latifol. fl. alb.) which were reciprocally
crossed, and the offspring were intermediate in character and as fertile as
their parents: from this fact Kolreuter 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. (16/26. Kolreuter was so much
struck with this fact that he suspected that a little pollen of N. glutinosa
in one of his experiments might have accidentally got mingled with that of
var. perennis, and thus aided its fertilising power. But we now know
conclusively from Gartner ('Bastarderz.' s. 34, 43) that the pollen of two
species never acts CONJOINTLY on a third species; still less will the pollen
of a distinct species, mingled with a plant's own pollen, if the latter be
present in sufficient quantity, have any effect. The sole effect of mingling
two kinds of pollen is to produce in the same capsule seeds which yield
plants, some taking after the one and some after the other parent.) So that
the sexual capacity of this one variety has certainly been in some degree
modified, so as to approach in nature that of N. glutinosa. (16/27. Mr. Scott
has made some observations on the absolute sterility of a purple and white
primrose (Primula vulgaris) when fertilised by pollen from the common primrose
('Journal of Proc. of Linn. Soc.' volume 8 1864 page 98); but these
observations require confirmation. I raised a number of purple-flowered long-
styled seedlings from seed kindly sent me by Mr. Scott, and, though they were
all in some degree sterile, they were much more fertile with pollen taken from
the common primrose than with their own pollen. Mr. Scott has likewise
described a red equal-styled cowslip (P. veris ibid page 106), which was found
by him to be highly sterile when crossed with the common cowslip; but this was
not the case with several equal-styled red seedlings raised by me from his
plant. This variety of the cowslip presents the remarkable peculiarity of
combining male organs in every respect like those of the short-styled form,
with female organs resembling in function and partly in structure those of the
long-styled form; so that we have the singular anomaly of the two forms
combined in the same flower. Hence it is not surprising that these flowers
should be spontaneously self-fertile in a high degree.)
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 fact, and others, 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 (16/28. 'Act. Acad. St.
Petersburg' 1780 part 2 pages 84, 100.), 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
experiment would in all cases be surrounded by many difficulties. Dureau de la
Malle, who has so closely studied classical literature, states (16/29.
'Annales des Sc. Nat.' tome 21 1st series page 61.) 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 (16/30.
'Bull. Bot. Soc. de France' December 27, 1861 tome 8 page 612.), 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.
The indirect evidence in favour of the Pallasian doctrine appears to me to be
extremely strong. In the earlier chapters I have shown that our various breeds
of the dog are descended from several wild species; and this probably is the
case with sheep. There can be no doubt that the Zebu or humped Indian ox
belongs to a distinct species from European cattle: the latter, moreover, are
descended from two forms, which may be called either species or races. We have
good evidence that our domesticated pigs belong to at least two specific
types, S. scrofa and indicus. Now a widely extended analogy leads to the
belief that if these several allied species, when first reclaimed, had been
crossed, they would have exhibited, both in their first unions and in their
hybrid offspring, some degree of sterility. Nevertheless, the several
domesticated races descended from them are now all, as far as can be
ascertained, perfectly fertile together. If this reasoning be trustworthy, and
it is apparently sound, we must admit the Pallasian doctrine that long-
continued domestication tends to eliminate that sterility which is natural to
species when crossed in their aboriginal state.
ON INCREASED FERTILITY FROM DOMESTICATION AND CULTIVATION.
Increased fertility from domestication, without any reference to crossing, may
be here briefly considered. This subject bears indirectly on two or three
points connected with the modification of organic beings. As Buffon long ago
remarked (16/31. Quoted by Isid. Geoffroy St. Hilaire 'Hist. Naturelle
Generale' tome 3 page 476. Since this MS. has been sent to press a full
discussion on the present subject has appeared in Mr. Herbert Spencer's
'Principles of Biology' volume 2 1867 page 457 et seq.), domestic animals
breed oftener in the year and produce more young at a birth than wild animals
of the same species; they, also, sometimes breed at an earlier age. The case
would hardly have deserved further notice, had not some authors lately
attempted to show that fertility increases and decreases in an inverse ratio
with the amount of food. This strange doctrine has apparently arisen from
individual animals when supplied with an inordinate quantity of food, and from
plants of many kinds when grown on excessively rich soil, as on a dunghill,
becoming sterile: but to this latter point I shall have occasion presently to
return. With hardly an exception, our domesticated animals, which have been
long habituated to a regular and copious supply of food, without the labour of
searching for it, are more fertile than the corresponding wild animals. It is
notorious how frequently cats and dogs breed, and how many young they produce
at a birth. The wild rabbit is said generally to breed four times yearly, and
to produce each time at most six young; the tame rabbit breeds six or seven
times yearly, producing each time from four to eleven young; and Mr. Harrison
Weir tells me of a case of eighteen young having been produced at a birth, all
of which survived. The ferret, though generally so closely confined, is more
prolific than its supposed wild prototype. The wild sow is remarkably
prolific; she often breeds twice in the year, and bears from four to eight and
sometimes even twelve young; but the domestic sow regularly breeds twice a
year, and would breed oftener if permitted; and a sow that produces less than
eight at a birth "is worth little, and the sooner she is fattened for the
butcher the better." The amount of food affects the fertility of the same
individual: thus sheep, which on mountains never produce more than one lamb at
a birth, when brought down to lowland pastures frequently bear twins. This
difference apparently is not due to the cold of the higher land, for sheep and
other domestic animals are said to be extremely prolific in Lapland. Hard
living, also, retards the period at which animals conceive; for it has been
found disadvantageous in the northern islands of Scotland to allow cows to
bear calves before they are four years old. (16/32. For cats and dogs etc. see
Bellingeri in 'Annal. des Sc. Nat.' 2nd series, Zoolog. tome 12 page 155. For
ferrets Bechstein 'Naturgeschichte Deutschlands' b. 1 1801 s. 786, 795. For
rabbits ditto s. 1123, 1131; and Bronn 'Geschichte der Natur.' b. 2 s. 99. For
mountain sheep ditto s. 102. For the fertility of the wild sow, see Bechstein
'Naturgesch. Deutschlands' b. 1 1801 s. 534; for the domestic pig Sidney's
edition of 'Youatt on the Pig' 1860 page 62. With respect to Lapland see
Acerbi 'Travels to the North Cape' English translation volume 2 page 222.
About the Highland cows see 'Hogg on Sheep' page 263.)
[Birds offer still better evidence of increased fertility from domestication:
the hen of the wild Gallus bankiva lays from six to ten eggs, a number which
would be thought nothing of with the domestic hen. The wild duck lays from
five to ten eggs; the tame one in the course of the year from eighty to one
hundred. The wild grey-lag goose lays from five to eight eggs; the tame from
thirteen to eighteen, and she lays a second time; as Mr. Dixon has remarked,
"high-feeding, care, and moderate warmth induce a habit of prolificacy which
becomes in some measure hereditary." Whether the semi-domesticated dovecote
pigeon is more fertile than the wild rock-pigeon, C. livia, I know not; but
the more thoroughly domesticated breeds are nearly twice as fertile as
dovecotes: the latter, however, when caged and highly fed, become equally
fertile with house pigeons. I hear from Judge Caton that the wild turkey in
the United States does not breed when a year old, as the domesticated turkeys
there invariably do. The peahen alone of domesticated birds is rather more
fertile, according to some accounts, when wild in its native Indian home, than
in Europe when exposed to our much colder climate. (16/33. For the eggs of
Gallus bankiva see Blyth in 'Annals and Mag. of Nat. Hist.' 2nd series volume
1 1848 page 456. For wild and tame ducks Macgillivray 'British Birds' volume 5
page 37; and 'Die Enten' s. 87. For wild geese L. Lloyd 'Scandinavian
Adventures' volume 2 1854 page 413; and for tame geese 'Ornamental Poultry' by
Rev. E.S. Dixon page 139. On the breeding of Pigeons Pistor 'Das Ganze der
Taubenzucht' 1831 s. 48; and Boitard and Corbie 'Les Pigeons' page 158. With
respect to peacocks, according to Temminck 'Hist. Nat. Gen. des Pigeons' etc.
1813 tome 2 page 41, the hen lays in India even as many as twenty eggs; but
according to Jerdon and another writer quoted in Tegetmeier 'Poultry Book'
1866 pages 280, 282, she there lays only from four to nine or ten eggs: in
England she is said, in the 'Poultry Book' to lay five or six, but another
writer says from eight to twelve eggs.)
With respect to plants, no one would expect wheat to tiller more, and each ear
to produce more grain, in poor than in rich soil; or to get in poor soil a
heavy crop of peas or beans. Seeds vary so much in number that it is difficult
to estimate them; but on comparing beds of carrots in a nursery garden with
wild plants, the former seemed to produce about twice as much seed. Cultivated
cabbages yielded thrice as many pods by measure as wild cabbages from the
rocks of South Wales. The excess of berries produced by the cultivated
asparagus in comparison with the wild plant is enormous. No doubt many highly
cultivated plants, such as pears, pineapples, bananas, sugar-cane, etc., are
nearly or quite sterile; and I am inclined to attribute this sterility to
excess of food and to other unnatural conditions; but to this subject I shall
recur.]
In some cases, as with the pig, rabbit, etc., and with those plants which are
valued for their seed, the direct selection of the more fertile individuals
has probably much increased their fertility; and in all cases this may have
occurred indirectly, from the better chance of some of the numerous offspring
from the more fertile individuals having been preserved. But with cats,
ferrets, and dogs, and with plants like carrots, cabbages, and asparagus,
which are not valued for their prolificacy, selection can have played only a
subordinate part; and their increased fertility must be attributed to the more
favourable conditions of life under which they have long existed.