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[1] 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 Gärtner, 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.’[2] 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[3] 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[4] 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[5] 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.”[6] With respect
to fallow-deer, which live in a semi-domesticated condition, Mr.
Bennett[7] 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[8] 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[9] 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,[10] 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 Gärtner states that he fertilised thirteen heads
(and subsequently nine others) on a dwarf maize bearing yellow seed[11]
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
monœcious, and therefore do not require castration, yet I should have
suspected some accident in the manipulation, had not Gärtner 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.[12] In like
manner Prof. Hildebrand[13] 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
Gärtner 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.[14]
The following case is much more remarkable, and evidently perplexed
Gärtner, 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 Gärtner actually raised in the case
of both species one variety from the seed of the other. Now in two of
his works[15] 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] the following
details; but I must premise that Gärtner, 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 Gärtner’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 Gärtner 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
Gärtner 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.[17] He
repeated some of Gärtner’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 Gärtner 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. phœniceum_
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. phœniceum_ 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.[18]
This remarkable fact of the sexual affinity of similarly-coloured
varieties, as observed by Gärtner 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[19] 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;[20] 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,[21] 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,[22] 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.[23] Sageret[24] 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[25] 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.[26] 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._[27]
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,[28] 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[29] 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,[30] namely, that
plants, known from their intermediate character and sterility to be
hybrids between Ægilops 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,[31] 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.[32]
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.[33]
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.
REFERENCES
[1] ‘Journal de Physiolog.,’ tom. ii., 1859, p. 385.
[2] Dec. 1863, p. 484.
[3] On ‘The Varieties of Wheat,’ p. 66.
[4] Rengger, ‘Säugethiere von Paraguay,’ s. 336.
[5] _See_ a memoir by MM. Lherbette and De Quatrefages, in ‘Bull. Soc.
d’Acclimat.,’ tom. viii., July, 1861, p. 312.
[6] For the Norfolk sheep, _see_ Marshall’s ‘Rural Economy of
Norfolk,’ vol. ii. p. 136. _See_ Rev. L. Landt’s ‘Description of
Faroe,’ p. 66. For the ancon sheep, _see_ ‘Phil. Transact.,’ 1813, p.
90.
[7] White’s ‘Nat. Hist. of Selbourne,’ edited by Bennett, p. 39. With
respect to the origin of the dark-coloured deer, _see_ ‘Some Account
of English Deer Parks,’ by E. P. Shirley, Esq.
[8] ‘The Dovecote,’ by the Rev. E. S. Dixon, p. 155; Bechstein,
‘Naturgesch. Deutschlands,’ b. iv., 1795, s. 17.
[9] ‘Cattle,’ p. 202.
[10] Mr. J. Wilkinson, in ‘Remarks addressed to Sir J. Sebright,’
1820, p. 38.
[11] ‘Bastarderzeugung,’ s. 87, 169. _See also_ the Table at the end
of volume.
[12] ‘Bastarderzeugung,’ s. 87, 577.
[13] ‘Bot. Zeitung,’ 1868, p. 327.
[14] Mr. Shirreff formerly thought (‘Gard. Chron.,’ 1858, p. 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.
[15] ‘Kenntniss der Befruchtung,’ s. 137; ‘Bastarderzeugung,’ s. 92,
181. On raising the two varieties from seed, _see_ s. 307.
[16] ‘Bastarderzeugung,’ s. 216.
[17] The results have since been published in ‘Journ. Asiatic Soc. of
Bengal,’ 1867, p. 145.
[18] The following facts, given by Kölreuter in his ‘Dritte
Fortsetzung,’ ss. 34, 39, appear at first sight strongly to confirm
Mr. Scott’s and Gärtner’s statements; and to a certain limited extent
they do so. Kölreuter 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 (Kölreuter,
‘Dritte Forts.’ s. 39, and Gärtner, ‘Bastarderz.,’ _passim_) being a
perfectly well-ascertained power. But the force of the foregoing facts
is much lessened by Gärtner’s numerous experiments, for, differently
from Kölreuter, 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.
[19] ‘Amaryllidaceæ,’ 1837, p. 366. Gärtner has made a similar
observation.
[20] Kölreuter first observed this fact, ‘Mém. de l’Acad. de St.
Petersburg,’ vol. iii. p. 127. _See also_ C. K. Sprengel, ‘Das
Entdeckte Geheimniss,’ s. 345.
[21] Namely, Barbarines, Pastissons, Giraumous: ‘Annal. des Sc. Nat.’
tom. xxx., 1833, pp. 398 and 405.
[22] ‘Mémoire sur les Cucurbitaceæ,’ 1826, pp. 46, 55.
[23] ‘Annales des Sc. Nat.,’ 4th series, tom. vi. M. Naudin considers
these forms as undoubtedly varieties of _Cucurbita pepo._
[24] ‘Mém. Cucurb.,’ p. 8.
[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._
[26] Kölreuter 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 Gärtner
(‘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.
[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.,’
vol. viii., 1864, p. 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. p. 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.
[28] ‘Act. Acad. St. Petersburg,’ 1780, part ii. pp. 84, 100.
[29] ‘Annales des Sc. Nat.’ tom. xxi. (1st series), p. 61.
[30] ‘Bull. Bot. Soc. de France,’ Dec. 27th, 1861, tom. viii. p. 612.
[31] Quoted by Isid. Geoffroy St. Hilaire ‘Hist. Naturelle Générale,’
tom. iii. p. 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,’ vol. ii., 1867, p. 457 _et seq._
[32] For cats and dogs, etc., _see_ Bellingeri in ‘Annal. des Sc.
Nat.,’ 2nd series, Zoolog. tom. xii. p. 155. For ferrets, Bechstein,
‘Naturgeschichte Deutschlands,’ b. i. 1801, s. 786, 795. For rabbits,
ditto, s. 1123, 1131; and Bronn’s ‘Geschichte der Natur.,’ b. ii. s.
99. For mountain sheep, ditto, s. 102. For the fertility of the wild
sow, _see_ Bechstein ‘Naturgesch. Deutschlands,’ b. i., 1801, s. 534;
for the domestic pig, Sidney’s edit. of Youatt on the Pig, 1860, p.
62. With respect to Lapland, _see_ Acerbi’s ‘Travels to the North
Cape,’ Eng. translat., vol. ii. p. 222. About the Highland cows, _see_
Hogg on Sheep, p. 263.
[33] For the eggs of _Gallus bankiva, see_ Blyth, in ‘Annals and Mag.
of Nat. Hist.,’ 2nd series, vol. i., 1848, p. 456. For wild and tame
ducks, Macgillivray, ‘British Birds,’ vol. v. p. 37; and ‘Die Enten,’
s. 87. For wild geese, L. Lloyd, ‘Scandinavian Adventures,’ vol. ii.
1854, p. 413; and for tame geese, ‘Ornamental Poultry,’ by Rev. E. S.
Dixon, p. 139. On the breeding of Pigeons, Pistor, ‘Das Ganze der
Taubenzucht,’ 1831, s. 46; and Boitard and Corbié ‘Les Pigeons,’ p.
158. With respect to peacocks, according to Temminck (‘Hist. Nat. Gén.
des Pigeons,’ etc., 1813, tom. ii. p. 41), the hen lays in India even
as many as twenty eggs; but according to Jerdon and another writer
(quoted in Tegetmeier’s ‘Poultry Book,’ 1866, pp. 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.