namely Azara,[813] states that in Paraguay horses of various colours are
often born with their hair curled and twisted like that on the head of a
negro. This peculiarity is strongly inherited. But what is remarkable is
that the hoofs of these horses "are absolutely like those of a mule." The
hair also of the mane and tail is invariably much shorter than usual, being
only from four {326} to twelve inches in length; so that curliness and
shortness of the hair are here, as with the negro, apparently correlated.
With respect to the horns of sheep, Youatt[814] remarks that "multiplicity
of horns is not found in any breed of much value: it is generally
accompanied by great length and coarseness of the fleece." Several tropical
breeds of sheep, which are clothed with hair instead of wool, have horns
almost like those of a goat. Sturm[815] expressly declares that in
different races the more the wool is curled the more the horns are spirally
twisted. We have seen in the third chapter, where other analogous facts
have been given, that the parent of the Mauchamp breed, so famous for its
fleece, had peculiarly shaped horns. The inhabitants of Angora assert[816]
that "only the white goats which have horns wear the fleece in the long
curly locks that are so much admired; those which are not horned having a
comparatively close coat." From these cases we may conclude that the hair
or wool and the horns vary in a correlated manner. Those who have tried
hydropathy are aware that the frequent application of cold water stimulates
the skin; and whatever stimulates the skin tends to increase the growth of
the hair, as is well shown in the abnormal growth of hair near old inflamed
surfaces. Now, Professor Low[817] is convinced that with the different
races of British cattle thick skin and long hair depend on the humidity of
the climate which they inhabit. We can thus see how a humid climate might
act on the horns--in the first place directly on the skin and hair, and
secondly by correlation on the horns. The presence or absence of horns,
moreover, both in the case of sheep and cattle, acts, as will presently be
shown, by some sort of correlation on the skull.
With respect to hair and teeth, Mr. Yarrell[818] found many of the teeth
deficient in three hairless "_Aegyptian_" dogs, and in a hairless terrier.
The incisors, canines, and premolars suffered most, but in one case all the
teeth, except the large tubercular molar on each side, were deficient. With
man several striking cases have been recorded[819] of inherited baldness
with {327} inherited deficiency, either complete or partial, of the teeth.
We see the same connexion in those rare cases in which the hair has been
renewed in old age, for this has "usually been accompanied by a renewal of
the teeth." I have remarked in a former part of this volume that the great
reduction in the size of the tusks in domestic boars probably stands in
close relation with their diminished bristles, due to a certain amount of
protection; and that the reappearance of the tusks in boars, which have
become feral and are fully exposed to the weather, probably depends on the
reappearance of the bristles. I may add, though not strictly connected with
our present point, that an agriculturist[820] asserts that "pigs with
little hair on their bodies are most liable to lose their tails, showing a
weakness of the tegumental structure. It may be prevented by crossing with
a more hairy breed."
In the previous cases deficient hair, and teeth deficient in number or
size, are apparently connected. In the following cases abnormally redundant
hair, and teeth either deficient or redundant, are likewise connected. Mr.
Crawfurd[821] saw at the Burmese Court a man, thirty years old, with his
whole body, except the hands and feet, covered with straight silky hair,
which on the shoulders and spine was five inches in length. At birth the
ears alone were covered. He did not arrive at puberty, or shed his milk
teeth, until twenty years old; and at this period he acquired five teeth in
the upper jaw, namely four incisors and one canine, and four incisor teeth
in the lower jaw; all the teeth were small. This man had a daughter, who
was born with hair within her ears; and the hair soon extended over her
body. When Captain Yule[822] visited the Court, he found this girl grown
up; and she presented a strange appearance with even her nose densely
covered with soft hair. Like her father, she was furnished with incisor
teeth alone. The King had with difficulty bribed a man to marry her, and of
her two children, one, a boy fourteen months old, had hair growing out of
his ears, with a beard and moustache. This strange peculiarity had,
therefore, been inherited for three generations, with the molar teeth
deficient in the grandfather and mother; whether {328} these teeth would
likewise fail in the infant could not be told. Here is another case
communicated to me by Mr. Wallace on the authority of Dr. Purland, a
dentist: Julia Pastrana, a Spanish dancer, was a remarkably fine woman, but
she had a thick masculine beard and a hairy forehead; she was photographed,
and her stuffed skin was exhibited as a show; but what concerns us is, that
she had in both the upper and lower jaw an irregular double set of teeth,
one row being placed within the other, of which Dr. Purland took a cast.
From the redundancy of the teeth her mouth projected, and her face had a
gorilla-like appearance. These cases and those of the hairless dogs
forcibly call to mind the fact, that the two orders of mammals--namely, the
Edentata and Cetacea--which are the most abnormal in their dermal covering,
are likewise the most abnormal either by deficiency or redundancy of teeth.
The organs of sight and hearing are generally admitted to be homologous,
both with each other and with the various dermal appendages; hence these
parts are liable to be abnormally affected in conjunction. Mr. White Cowper
says "that in all cases of double microphthalmia brought under his notice
he has at the same time met with defective development of the dental
system." Certain forms of blindness seem to be associated with the colour
of the hair; a man with black hair and a woman with light-coloured hair,
both of sound constitution, married and had nine children, all of whom were
born blind; of these children, five "with dark hair and brown iris were
afflicted with amaurosis; the four others, with light-coloured hair and
blue iris, had amaurosis and cataract conjoined." Several cases could be
given, showing that some relation exists between various affections of the
eyes and ears; thus Liebreich states that out of 241 deaf-mutes in Berlin,
no less than fourteen suffered from the rare disease called pigmentary
retinitis. Mr. White Cowper and Dr. Earle have remarked that inability to
distinguish different colours, or colour-blindness, "is often associated
with a corresponding inability to distinguish musical sounds."[823]
{329}
Here is a more curious case: white cats, if they have blue eyes, are almost
always deaf. I formerly thought that the rule was invariable, but I have
heard of a few authentic exceptions. The first two notices were published
in 1829, and relate to English and Persian cats: of the latter, the Rev.
W. T. Bree possessed a female, and he states "that of the offspring
produced at one and the same birth, such as, like the mother, were entirely
white (with blue eyes) were, like her, invariably deaf; while those that
had the least speck of colour on their fur, as invariably possessed the
usual faculty of hearing."[824] The Rev. W. Darwin Fox informs me that he
has seen more than a dozen instances of this correlation in English,
Persian, and Danish cats; but he adds "that, if one eye, as I have several
times observed, be not blue, the cat hears. On the other hand, I have never
seen a white cat with eyes of the common colour that was deaf." In France
Dr. Sichel[825] has observed during twenty years similar facts; he adds the
remarkable case of the iris beginning, at the end of four months, to grow
dark-coloured, and then the cat first began to hear.
This case of correlation in cats has struck many persons as marvellous.
There is nothing unusual in the relation between blue eyes and white fur;
and we have already seen that the organs of sight and hearing are often
simultaneously affected. In the present instance the cause probably lies in
a slight arrest of development in the nervous system in connection with the
sense-organs. Kittens during the first nine days, whilst their eyes are
closed, appear to be completely deaf; I have made a great clanging noise
with a poker and shovel close to their heads, both when they were asleep
and awake, without producing any effect. The trial must not be made by
shouting close to their ears, for they are, even when asleep, extremely
sensitive to a breath of air. Now, as long as the eyes continue closed, the
iris is no doubt blue, for in all the kittens which I have seen this colour
remains for some time after the eyelids open. Hence, if we suppose the
development of the organs of sight and hearing to be arrested at the stage
of the closed eyelids, the eyes would {330} remain permanently blue and the
ears would be incapable of perceiving sound; and we should thus understand
this curious case. As, however, the colour of the fur is determined long
before birth, and as the blueness of the eyes and the whiteness of the fur
are obviously connected, we must believe that some primary cause acts at an
early period.
The instances of correlated variability hitherto given have been chiefly
drawn from the animal kingdom, and we will now turn to plants. Leaves,
sepals, petals, stamens, and pistils are all homologous. In double flowers
we see that the stamens and pistils vary in the same manner, and assume the
form and colour of the petals. In the double columbine (_Aquilegia
vulgaris_), the successive whorls of stamens are converted into
cornucopias, which are enclosed within each other and resemble the petals.
In hose-and-hose flowers the sepals mock the petals. In some cases the
flowers and leaves vary together in tint: in all the varieties of the
common pea, which have purple flowers, a purple mark may be seen on the
stipules. In other cases the leaves and fruit and seeds vary together in
colour, as in a curious pale-leaved variety of the sycamore, which has
recently been described in France,[826] and as in the purple-leaved hazel,
in which the leaves, the husk of the nut, and the pellicle round the kernel
are all coloured purple.[827] Pomologists can predict to a certain extent,
from the size and appearance of the leaves of their seedlings, the probable
nature of the fruit; for, as Van Mons remarks,[828] variations in the
leaves are generally accompanied by some modification in the flower, and
consequently in the fruit. In the Serpent melon, which has a narrow
tortuous fruit above a yard in length, the stem of the plant, the peduncle
of the female flower, and the middle lobe of the leaf, are all elongated in
a remarkable manner. On the other hand, several varieties of Cucurbita,
which have dwarfed stems, all produce, as Naudin remarks with surprise,
leaves of the same peculiar shape. Mr. G. Maw informs me that all the
varieties of the scarlet Pelargoniums which have contracted or imperfect
leaves have contracted flowers: the difference between {331} "Brilliant"
and its parent "Tom Thumb" is a good instance of this. It may be suspected
that the curious case described by Risso,[829] of a variety of the Orange
which produces on the young shoots rounded leaves with winged petioles, and
afterwards elongated leaves on long but wingless petioles, is connected
with the remarkable change in form and nature which the fruit undergoes
during its development.
In the following instance we have the colour and form of the petals
apparently correlated, and both dependent on the nature of the season. An
observer, skilled in the subject, writes,[830] "I noticed, during the year
1842, that every Dahlia, of which the colour had any tendency to scarlet,
was deeply notched--indeed to so great an extent as to give the petals the
appearance of a saw; the indentures were, in some instances, more than a
quarter of an inch deep." Again, Dahlias which have their petals tipped
with a different colour from the rest are very inconstant, and during
certain years some, or even all the flowers, become uniformly coloured; and
it has been observed with several varieties,[831] that when this happens
the petals grow much elongated and lose their proper shape. This, however,
may be due to reversion, both in colour and form, to the aboriginal
species.
* * * * *
In this discussion on correlation, we have hitherto treated of cases in
which we can partly understand the bond of connexion; but I will now give
cases in which we cannot even conjecture, or can only very obscurely see,
what is the nature of the bond. Isidore Geoffroy St. Hilaire, in his work
on Monstrosities, insists,[832] "que certaines anomalies coexistent
rarement entr'elles, d'autres frequemment, d'autres enfin presque
constamment, malgre la difference tres-grande de leur nature, et
quoiqu'elles puissent paraitre _completement independantes_ les unes des
autres." We see something analogous in certain diseases: thus I hear from
Mr. Paget that in a rare affection of the {332} renal capsules (of which
the functions are unknown), the skin becomes bronzed; and in hereditary
syphilis, both the milk and the second teeth assume a peculiar and
characteristic form. Professor Rolleston, also, informs me that the incisor
teeth are sometimes furnished with a vascular rim in correlation with
intra-pulmonary deposition of tubercles. In other cases of phthisis and of
cyanosis the nails and finger-ends become clubbed like acorns. I believe
that no explanation has been offered of these and of many other cases of
correlated disease.
What can be more curious and less intelligible than the fact previously
given, on the authority of Mr. Tegetmeier, that young pigeons of all
breeds, which when mature have white, yellow, silver-blue, or dun-coloured
plumage, come out of the egg almost naked; whereas pigeons of other colours
when first born are clothed with plenty of down? White Pea-fowls, as has
been observed both in England and France,[833] and as I have myself seen,
are inferior in size to the common coloured kind; and this cannot be
accounted for by the belief that albinism is always accompanied by
constitutional weakness; for white or albino moles are generally larger
than the common kind.
To turn to more important characters: the niata cattle of the Pampas are
remarkable from their short foreheads, upturned muzzles, and curved lower
jaws. In the skull the nasal and premaxillary bones are much shortened, the
maxillaries are excluded from any junction with the nasals, and all the
bones are slightly modified, even to the plane of the occiput. From the
analogical case of the dog, hereafter to be given, it is probable that the
shortening of the nasal and adjoining bones is the proximate cause of the
other modifications in the skull, including the upward curvature of the
lower jaw, though we cannot follow out the steps by which these changes
have been effected.
Polish fowls have a large tuft of feathers on their heads; and their skulls
are perforated by numerous holes, so that a pin can be driven into the
brain without touching any bone. That this deficiency of bone is in some
way connected with the tuft of feathers is clear from tufted ducks and
geese likewise having {333} perforated skulls. The case would probably be
considered by some authors as one of balancement or compensation. In the
chapter on Fowls, I have shown that with Polish fowls the tuft of feathers
was probably at first small; by continued selection it became larger, and
then rested on a fleshy or fibrous mass; and finally, as it became still
larger, the skull itself became more and more protuberant until it acquired
its present extraordinary structure. Through correlation with the
protuberance of the skull, the shape and even the relative connexion of the
premaxillary and nasal bones, the shape of the orifice of the nostrils, the
breadth of the frontal bone, the shape of the post-lateral processes of the
frontal and squamosal bones, and the direction of the bony cavity of the
ear, have all been modified. The internal configuration of the skull and
the whole shape of the brain have likewise been altered in a truly
marvellous manner.
After this case of the Polish fowl it would be superfluous to do more than
refer to the details previously given on the manner in which the changed
form of the comb, in various breeds of the fowl, has affected the skull,
causing by correlation crests, protuberances, and depressions on its
surface.
With our cattle and sheep the horns stand in close connexion with the size
of the skull, and with the shape of the frontal bones; thus Cline[834]
found that the skull of a horned ram weighed five times as much as that of
a hornless ram of the same age. When cattle become hornless, the frontal
bones are "materially diminished in breadth towards the poll;" and the
cavities between the bony plates "are not so deep, nor do they extend
beyond the frontals."[835]
* * * * *
It may be well here to pause and observe how the effects of correlated
variability, of the increased use of parts, and of the accumulation through
natural selection of so-called spontaneous variations, are in many cases
inextricably commingled. We may borrow an illustration from Mr. Herbert
Spencer, who remarks that, when the Irish elk acquired its gigantic horns,
weighing above one hundred pounds, numerous co-ordinated {334} changes of
structure would have been indispensable,--namely, a thickened skull to
carry the horns; strengthened cervical vertebrae, with strengthened
ligaments; enlarged dorsal vertebrae to support the neck, with powerful
fore-legs and feet; all these parts being supplied with proper muscles,
blood-vessels, and nerves. How then could these admirably co-ordinated
modifications of structure have been acquired? According to the doctrine
which I maintain, the horns of the male elk were slowly gained through
sexual selection,--that is, by the best-armed males conquering the
worse-armed, and leaving a greater number of descendants. But it is not at
all necessary that the several parts of the body should have simultaneously
varied. Each stag presents individual differences, and in the same district
those which had slightly heavier horns, or stronger necks, or stronger
bodies, or were the most courageous, would secure the greater number of
does, and consequently leave a greater number of offspring. The offspring
would inherit, in a greater or less degree, these same qualities, would
occasionally intercross with each other, or with other individuals varying
in some favourable manner; and of their offspring, those which were the
best endowed in any respect would continue multiplying; and so onwards,
always progressing, sometimes in one direction, and sometimes in another,
towards the present excellently co-ordinated structure of the male elk. To
make this clear, let us reflect on the probable steps, as shown in the
twentieth chapter, by which our race and dray-horses have arrived at their
present state of excellence; if we could view the whole series of
intermediate forms between one of these animals and an early unimproved
progenitor, we should behold a vast number of animals, not equally improved
in each generation throughout their entire structure, but sometimes a
little more in one point, and sometimes in another, yet on the whole
gradually approaching in character to our present race or dray-horses,
which are so admirably fitted in the one case for fleetness and in the
other for draught.
Although natural selection would thus[836] tend to give to the {335} male
elk its present structure, yet it is probable that the inherited influence
of use has played an equal or more important part. As the horns gradually
increased in weight, the muscles of the neck, with the bones to which they
are attached, would increase in size and strength; and these parts would
react on the body and legs. Nor must we overlook the fact that certain
parts of the skull and the extremities would, judging by analogy, tend from
the first to vary in a correlated manner. The increased weight of the horns
would also act directly on the skull, in the same manner as, when one bone
is removed in the leg of a dog, the other bone, which has to carry the
whole weight of the body, increases in thickness. But from the facts given
with respect to horned and hornless cattle, it is probable that the horns
and skull would immediately act on each other through the principle of
correlation. Lastly, the growth and subsequent wear and tear of the
augmented muscles and bones would require an increased supply of blood, and
consequently an increased supply of food; and this again would require
increased powers of mastication, digestion, respiration, and excretion.
_Colour as Correlated with Constitutional Peculiarities._
It is an old belief that with man there is a connexion between complexion
and constitution; and I find that some of the best authorities believe in
this to the present day.[837] Thus Dr. Beddoe by his tables shows[838] that
a relation exists between liability to consumption and the colour of the
hair, eyes, and skin. It has been affirmed[839] that, in the French army
which invaded Russia, soldiers having a dark complexion, from the {336}
southern parts of Europe, withstood the intense cold better than those with
lighter complexions from the north; but no doubt such statements are liable
to error.
In the second chapter on Selection I have given several cases proving that
with animals and plants differences in colour are correlated with
constitutional differences, as shown by greater or less immunity from
certain diseases, from the attacks of parasitic plants and animals, from
burning by the sun, and from the action of certain poisons. When all the
individuals of any one variety possess an immunity of this nature, we
cannot feel sure that it stands in any sort of correlation with their
colour; but when several varieties of the same species, which are similarly
coloured, are thus characterised, whilst other coloured varieties are not
thus favoured, we must believe in the existence of a correlation of this
kind. Thus in the United States purple-fruited plums of many kinds are far
more affected by a certain disease than green or yellow-fruited varieties.
On the other hand, yellow-fleshed peaches of various kinds suffer from
another disease much more than the white-fleshed varieties. In the
Mauritius red sugar-canes are much less affected by a particular disease
than the white canes. White onions and verbenas are the most liable to
mildew; and in Spain the green-fruited grapes suffered from the
vine-disease more than other coloured varieties. Dark-coloured pelargoniums
and verbenas are more scorched by the sun than varieties of other colours.
Red wheats are believed to be hardier than white; whilst red-flowered
hyacinths were more injured during one particular winter in Holland than
other coloured varieties. With animals, white terriers suffer most from the
distemper, white chickens from a parasitic worm in their tracheae, white
pigs from scorching by the sun, and white cattle from flies; but the
caterpillars of the silk-moth which yield white cocoons suffered in France
less from the deadly parasitic fungus than those producing yellow silk.
The cases of immunity from the action of certain vegetable poisons, in
connexion with colour, are more interesting, and are at present wholly
inexplicable. I have already given a remarkable instance, on the authority
of Professor Wyman, of all the hogs, excepting those of a black colour,
suffering severely in Virginia from eating the root of the _Lachnanthes
tinctoria_. {337} According to Spinola and others,[840] buckwheat
(_Polygonum fagopyrum_), when in flower, is highly injurious to white or
white-spotted pigs, if they are exposed to the heat of the sun, but is
quite innocuous to black pigs. By two accounts, the _Hypericum crispum_ in
Sicily is poisonous to white sheep alone; their heads swell, their wool
falls off, and they often die; but this plant, according to Lecce, is
poisonous only when it grows in swamps; nor is this improbable, as we know
how readily the poisonous principle in plants is influenced by the
conditions under which they grow.
Three accounts have been published in Eastern Prussia, of white and
white-spotted horses being greatly injured by eating mildewed and
honeydewed vetches; every spot of skin bearing white hairs becoming
inflamed and gangrenous. The Rev. J. Rodwell informs me that his father
turned out about fifteen cart-horses into a field of tares which in parts
swarmed with black aphides, and which no doubt were honeydewed, and
probably mildewed; the horses, with two exceptions, were chesnuts and bays
with white marks on their faces and pasterns, and the white parts alone
swelled and became angry scabs. The two bay horses with no white marks
entirely escaped all injury. In Guernsey, when horses eat fools' parsley
(_Aethusa cynapium_) they are sometimes violently purged; and this plant
"has a peculiar effect on the nose and lips, causing deep cracks and
ulcers, particularly on horses with white muzzles."[841] With cattle,
independently of the action of any poison, cases have been published by
Youatt and Erdt of cutaneous diseases with much constitutional disturbance
(in one instance after exposure to a hot sun) affecting every single point
which bore a white hair, but completely passing over other parts of the
body. Similar cases have been observed with horses.[842]
We thus see that not only do those parts of the skin which bear white hair
differ in a remarkable manner from those bearing {338} hair of any other
colour, but that in addition some great, constitutional difference must
stand in correlation with the colour of the hair; for in the
above-mentioned cases, vegetable poisons caused fever, swelling of the
head, as well as other symptoms, and even death, to all the white or
white-spotted animals.
* * * * *
{339}
CHAPTER XXVI.
LAWS OF VARIATION, _continued_--SUMMARY.
ON THE AFFINITY AND COHESION OF HOMOLOGOUS PARTS--ON THE VARIABILITY OF
MULTIPLE AND HOMOLOGOUS PARTS--COMPENSATION OF GROWTH--MECHANICAL
PRESSURE--RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS OF THE
PLANT, AND OF SEEDS IN THE CAPSULE, AS INDUCING VARIATION--ANALOGOUS OR
PARALLEL VARIETIES--SUMMARY OF THE THREE LAST CHAPTERS.
_On the Affinity of Homologous Parts._--This law was first generalised by
Geoffroy Saint Hilaire, under the expression of _La loi de l'affinite de
soi pour soi_. It has been fully discussed and illustrated by his son,
Isidore Geoffroy, with respect to monsters in the animal kingdom,[843] and
by Moquin-Tandon, with respect to monstrous plants. When similar or
homologous parts, whether belonging to the same embryo or to two distinct
embryos, are brought during an early stage of development into contact,
they often blend into a single part or organ; and this complete fusion
indicates some mutual affinity between the parts, otherwise they would
simply cohere. Whether any power exists which tends to bring homologous
parts into contact seems more doubtful. The tendency to complete fusion is
not a rare or exceptional fact. It is exhibited in the most striking manner
by double monsters. Nothing can be more extraordinary than the manner, as
shown in various published plates, in which the corresponding parts of two
embryos become intimately fused together. This is perhaps best seen in
monsters with two heads, which are united, summit to summit, or face to
face, or, Janus-like, back to back, or obliquely side to side. In one
instance of two heads united almost face to face, but a little obliquely,
four ears were developed, and on one side a perfect face, which was
manifestly formed by the union of two {340} half-faces. Whenever two bodies
or two heads are united, each bone, muscle, vessel, and nerve on the line
of junction seems to seek out its fellow, and becomes completely fused with
it. Lereboullet,[844] who carefully studied the development of double
monsters in fishes, observed in fifteen instances the steps by which two
heads gradually became fused into one. In this and other such cases, no
one, I presume, supposes that the two already formed heads actually blend
together, but that the corresponding parts of each head grow into one
during the further progress of development, accompanied as it always is
with incessant absorption and renovation. Double monsters were formerly
thought to be formed by the union of two originally distinct embryos
developed upon distinct vitelli; but now it is admitted that "their
production is due to the spontaneous divarication of the embryonic mass
into two halves;"[845] this, however, is effected by different methods. But
the belief that double monsters originate from the division of one germ,
does not necessarily affect the question of subsequent fusion, or render
less true the law of the affinity of homologous parts.
The cautious and sagacious J. Mueller,[846] when speaking of Janus-like
monsters, says, that "without the supposition that some kind of affinity or
attraction is exerted between corresponding parts, unions of this kind are
inexplicable." On the other hand, Vrolik, and he is followed by others,
disputes this conclusion, and argues from the existence of a whole series
of monstrosities, graduating from a perfectly double monster to a mere
rudiment of an additional digit, that "an excess of formative power" is the
cause and origin of every monstrous duplicity. That there are two distinct
classes of cases, and that parts may be doubled independently of the
existence of two embryos, is certain; for a single embryo, or even a single
adult animal, may produce doubled organs. Thus Valentin, as quoted by
Vrolik, injured the caudal extremity of an embryo, and three days
afterwards it produced rudiments of a double pelvis and of double hind
limbs. {341} Hunter and others have observed lizards with their tails
reproduced and doubled. When Bonnet divided longitudinally the foot of the
salamander, several additional digits were occasionally formed. But neither
these cases, nor the perfect series from a double monster to an additional
digit, seem to me opposed to the belief that corresponding parts have a
mutual affinity, and consequently tend to fuse together. A part may be
doubled and remain in this state, or the two parts thus formed may
afterwards through the law of affinity become blended; or two homologous
parts in two separate embryos may, through the same principle, unite and
form a single part.
The law of the affinity and fusion of similar parts applies to the
homologous organs of the same individual animal, as well as to double
monsters. Isidore Geoffroy gives a number of instances of two or more
digits, of two whole legs, of two kidneys, and of several teeth becoming
symmetrically fused together in a more or less perfect manner. Even the two
eyes have been known to unite into a single eye, forming a cyclopean
monster, as have the two ears, though naturally standing so far apart. As
Geoffroy remarks, these facts illustrate in an admirable manner the normal
fusion of various organs which during an early embryonic period are double,
but which afterwards always unite into a single median organ. Organs of
this nature are generally found in a permanently double condition in other
members of the same class. These cases of normal fusion appear to me to
afford the strongest support in favour of the present law. Adjoining parts
which are not homologous sometimes cohere; but this cohesion appears to
result from mere juxtaposition, and not from mutual affinity.
In the vegetable kingdom Moquin-Tandon[847] gives a long list of cases,
showing how frequently homologous parts, such as leaves, petals, stamens,
and pistils, as well as aggregates of homologous parts, such as buds,
flowers, and fruit, become blended into each other with perfect symmetry.
It is interesting to examine a compound flower of this nature, formed of
exactly double the proper number of sepals, petals, stamens, and pistils,
with each whorl of organs circular, and with no trace left of the {342}
process of fusion. The tendency in homologous parts to unite during their
early development, Moquin-Tandon considers as one of the most striking laws
governing the production of monsters. It apparently explains a multitude of
cases, both in the animal and vegetable kingdoms; it throws a clear light
on many normal structures which have evidently been formed by the union of
originally distinct parts, and it possesses, as we shall see in a future
chapter, much theoretical interest.
* * * * *
_On the Variability of Multiple and Homologous Parts._--Isidore
Geoffroy[848] insists that, when any part or organ is repeated many times
in the same animal, it is particularly liable to vary both in number and
structure. With respect to number, the proposition may, I think, be
considered as fully established; but the evidence is chiefly derived from
organic beings living under their natural conditions, with which we are not
here concerned. When the vertebrae, or teeth, or rays in the fins of
fishes, or feathers in the tails of birds, or petals, stamens, pistils, and
seeds in plants, are very numerous, the number is generally variable. The
explanation of this simple fact is by no means obvious. With respect to the
variability in structure of multiple parts, the evidence is not so
decisive; but the fact, as far as it may be trusted, probably depends on
multiple parts being of less physiological importance than single parts;
consequently their perfect standard of structure has been less rigorously
enforced by natural selection.
* * * * *
_Compensation of Growth, or Balancement._--This law, as applied to natural
species, was propounded by Goethe and Geoffroy St. Hilaire at nearly the
same time. It implies that, when much organised matter is used in building
up some one part, other parts are starved and become reduced. Several
authors, especially botanists, believe in this law; others reject it. As
far as I can judge, it occasionally holds good; but its importance has
probably been exaggerated. It is scarcely possible to distinguish between
the supposed effects of such compensation of growth, and the effects of
long-continued selection, which {343} may at the same time lead to the
augmentation of one part and the diminution of another. There can be no
doubt that an organ may be greatly increased without any corresponding
diminution in the adjoining parts. To recur to our former illustration of
the Irish elk, it may be asked what part has suffered in consequence of the
immense development of the horns?
It has already been observed that the struggle for existence does not bear
hard on our domesticated productions; consequently the principle of economy
of growth will seldom affect them, and we ought not to expect to find
frequent evidence of compensation. We have, however, some such cases.
Moquin-Tandon describes a monstrous bean,[849] in which the stipules were
enormously developed, and the leaflets apparently in consequence completely
aborted; this case is interesting, as it represents the natural condition
of _Lathyrus aphaca_, with its stipules of great size, and its leaves
reduced to mere threads, which act as tendrils. De Candolle[850] has
remarked that the varieties of _Raphanus sativus_ which have small roots
yield numerous seed, valuable from containing oil, whilst those with large
roots are not productive in this latter respect; and so it is with
_Brassica asperifolia_. The varieties of the potato which produce tubers
very early in the season rarely bear flowers; but Andrew Knight,[851] by
checking the growth of the tubers, forced the plants to flower. The
varieties of _Cucurbita pepo_ which produce large fruit yield, according to
Naudin, few in number; whilst those producing small fruit yield a vast
number. Lastly, I have endeavoured to show in the eighteenth chapter that
with many cultivated plants unnatural treatment checks the full and proper
action of the reproductive organs, and they are thus rendered more or less
sterile; consequently, in the way of compensation, the fruit becomes
greatly enlarged, and, in double flowers, the petals are greatly increased
in number.
With animals, it has been found difficult to produce cows which should
first yield much milk, and afterwards be capable of {344} fattening well.
With fowls which have large topknots and beards the comb and wattles are
generally much reduced in size. Perhaps the entire absence of the oil-gland
in fantail pigeons may be connected with the great development of their
tails.
* * * * *
_Mechanical Pressure as a Cause of Modifications._--In some few cases there
is reason to believe that mere mechanical pressure has affected certain
structures. Every one knows that savages alter the shape of their infants'
skulls by pressure at an early age; but there is no reason to believe that
the result is ever inherited. Nevertheless Vrolik and Weber[852] maintain
that the shape of the human head is influenced by the shape of the mother's
pelvis. The kidneys in different birds differ much in form, and St.
Ange[853] believes that this is determined by the form of the pelvis, which
again, no doubt, stands in close relation with their various habits of
locomotion. In snakes, the viscera are curiously displaced, in comparison
with their position in other vertebrates; and this has been attributed by
some authors to the elongation of their bodies; but here, as in so many
previous cases, it is impossible to disentangle any direct result of this
kind from that consequent on natural selection. Godron has argued[854] that
the normal abortion of the spur on the inner side of the flower in
Corydalis, is caused by the buds being closely pressed at a very early
period of growth, whilst under ground, against each other and against the
stem. Some botanists believe that the singular difference in the shape both
of the seed and corolla, in the interior and exterior florets in certain
compositous and umbelliferous plants, is due to the pressure to which the
inner florets are subjected; but this conclusion is doubtful.
The facts just given do not relate to domesticated productions, and
therefore do not strictly concern us. But here is a more appropriate case:
H. Mueller[855] has shown that in {345} short-faced races of the dog some
of the molar teeth are placed in a slightly different position from that
which they occupy in other dogs, especially in those having elongated
muzzles; and as he remarks, any inherited change in the arrangement of the
teeth deserves notice, considering their classificatory importance. This
difference in position is due to the shortening of certain facial bones,
and the consequent want of space; and the shortening results from a
peculiar and abnormal state of the basal cartilages of the bones.
_Relative Position of Flowers with respect to the Axis, and of Seeds in the
Capsule, as inducing Variation._
In the thirteenth chapter various peloric flowers were described, and
their production was shown to be due either to arrested development, or
to reversion to a primordial condition. Moquin-Tandon has remarked that
the flowers which stand on the summit of the main stem or of a lateral
branch are more liable to become peloric than those on the sides;[856]
and he adduces, amongst other instances, that of _Teucrium
campanulatum_. In another Labiate plant grown by me, viz. the
_Galeobdolon luteum_, the peloric flowers were always produced on the
summit of the stem, where flowers are not usually borne. In
Pelargonium, a _single_ flower in the truss is frequently peloric, and
when this occurs I have during several years invariably observed it to
be the central flower. This is of such frequent occurrence that one
observer[857] gives the names of ten varieties flowering at the same
time, in every one of which the central flower was peloric.
Occasionally more than one flower in the truss is peloric, and then of
course the additional ones must be lateral. These flowers are
interesting as showing how the whole structure is correlated. In the
common Pelargonium the upper sepal is produced into a nectary which
coheres with the flower-peduncle; the two upper petals differ a little
in shape from the three lower ones, and are marked with dark shades of
colour; the stamens are graduated in length and upturned. In the
peloric flowers, the nectary aborts; all the petals become alike both
in shape and colour; the stamens are generally reduced in number and
become straight, so that the whole flower resembles that of the allied
genus Erodium. The correlation between these changes is well shown when
one of the two upper petals alone loses its dark mark, for in this case
the nectary does not entirely abort, but is usually much reduced in
length.[858]
{346}
Morren has described[859] a marvellous flask-shaped flower of the
Calceolaria, nearly four inches in length, which was almost completely
peloric; it grew on the summit of the plant, with a normal flower on
each side; Prof. Westwood also has described[860] three similar peloric
flowers, which all occupied a central position on the flower-branches.
In the Orchideous genus, Phalaenopsis, the terminal flower has been
seen to become peloric.
In a Laburnum-tree I observed that about a fourth part of the racemes
produced terminal flowers which had lost their papilionaceous
structure. These were produced after almost all the other flowers on
the same racemes had withered. The most perfectly pelorised examples
had six petals, each marked with black striae like those on the
standard-petal. The keel seemed to resist the change more than the
other petals. Dutrochet has described[861] an exactly similar case in
France, and I believe these are the only two instances of pelorism in
the laburnum which have been recorded. Dutrochet remarks that the
racemes on this tree do not properly produce a terminal flower, so
that, as in the case of the Galeobdolon, their position as well as
their structure are both anomalies, which no doubt are in some manner
related. Dr. Masters has briefly described another leguminous
plant,[862] namely, a species of clover, in which the uppermost and
central flowers were regular or had lost their papilionaceous
structure. In some of these plants the flower-heads were also
proliferous.
Lastly, Linaria produces two kinds of peloric flowers, one having
simple petals, and the other having them all spurred. The two forms, as
Naudin remarks,[863] not rarely occur on the same plant, but in this
case the spurred form almost invariably stands on the summit of the
spike.
The tendency in the terminal or central flower to become peloric more
frequently than other flowers, probably results from "the bud which
stands on the end of a shoot receiving the most sap; it grows out into
a stronger shoot than those situated lower down."[864] I have discussed
the connection between pelorism and a central position, partly because
some few plants are known normally to produce a terminal flower
different in structure from the lateral ones; but chiefly on account of
the following case, in which we see a tendency to variability or to
reversion connected with the same position. A great judge of
Auriculas[865] states that when an Auricula throws up a side bloom it
is pretty sure to keep its character; but that if it grows from the
centre or heart of the plant, whatever the colour of the edging ought
to be, "it is just as likely to come in any other class as in the one
to which it properly belongs." This is so notorious a {347} fact, that
some florists regularly pinch off the central trusses of flowers.
Whether in the highly improved varieties the departure of the central
trusses from their proper type is due to reversion, I do not know. Mr.
Dombrain insists that, whatever may be the commonest kind of
imperfection in each variety, this is generally exaggerated in the
central truss. Thus one variety "sometimes has the fault of producing a
little green floret in the centre of the flower," and in central blooms
these become excessive in size. In some central blooms, sent to me by
Mr. Dombrain, all the organs of the flower were rudimentary in
structure, of minute size, and of a green colour, so that by a little
further change all would have been converted into small leaves. In this
case we clearly see a tendency to prolification--a term which, I may
explain to those who have never attended to botany, means the
production of a branch or flower, or head of flowers, out of another
flower. Now Dr. Masters[866] states that the central or uppermost
flower on a plant is generally the most liable to prolification. Thus,
in the varieties of the Auricula, the loss of their proper character
and a tendency to prolification, and in other plants a tendency to
prolification and pelorism, are all connected together, and are due
either to arrested development, or to reversion to a former condition.
The following is a more interesting case; Metzger[867] cultivated in
Germany several kinds of maize brought from the hotter parts of
America, and he found, as has been previously described, that in two or
three generations the grains became greatly changed in form, size, and
colour; and with respect to two races he expressly states that in the
first generation, whilst the lower grains on each head retained their
proper character, the uppermost grains already began to assume that
character which in the third generation all the grains acquired. As we
do not know the aboriginal parent of the maize, we cannot tell whether
these changes are in any way connected with reversion.
In the two following cases, reversion, as influenced by the position of
the seed in the capsule, evidently acts. The Blue Imperial pea is the
offspring of the Blue Prussian, and has larger seed and broader pods
than its parent. Now Mr. Masters, of Canterbury, a careful observer and
a raiser of new varieties of the pea, states[868] that the Blue
Imperial always has a strong tendency to revert to its parent-stock,
and the reversion "occurs in this manner: the last (or uppermost) pea
in the pod is frequently much smaller than the rest; and if these small
peas are carefully collected and sown separately, very many more, in
proportion, will revert to their origin, than those taken from the
other parts of the pod." Again M. Chate[869] says that in raising
seedling stocks he succeeds in getting eighty per cent. to bear double
flowers, by leaving only a few of the secondary branches to seed; but
in addition to this, "at the time of extracting the seeds, the upper
portion of the pod is separated and {348} placed aside, because it has
been ascertained that the plants coming from the seeds situated in this
portion of the pod, give eighty per cent. of single flowers." Now the
production of single-flowering plants from the seed of double-flowering
plants is clearly a case of reversion. These latter facts, as well as
the connection between a central position and pelorism and
prolification, show in an interesting manner how small a
difference--namely a little greater freedom in the flow of sap towards
one part of the same plant--determines important changes of structure.
* * * * *
_Analogous or Parallel Variation._--By this term I wish to express that
similar characters occasionally make their appearance in the several
varieties or races descended from the same species, and more rarely in the
offspring of widely distinct species. We are here concerned, not as
hitherto with the causes of variation, but with the results; but this
discussion could not have been more conveniently introduced elsewhere. The
cases of analogous variation, as far as their origin is concerned, may be
grouped, disregarding minor subdivisions, under two main heads; firstly,
those due to unknown causes having acted on organic beings with nearly the
same constitution, and which consequently vary in an analogous manner; and
secondly, those due to the reappearance of characters which were possessed
by a more or less remote progenitor. But these two main divisions can often
be only conjecturally separated, and graduate, as we shall presently see,
into each other.
Under the first head of analogous variations, not due to reversion, we
have the many cases of trees belonging to quite different orders which
have produced pendulous and fastigate varieties. The beech, hazel, and
barberry have given rise to purple-leaved varieties; and as Bernhardi
has remarked,[870] a multitude of plants, as distinct as possible, have
yielded varieties with deeply-cut or laciniated leaves. Varieties
descended from three distinct species of Brassica have their stems, or
so-called roots, enlarged into globular masses. The nectarine is the
offspring of the peach; and the varieties of both these trees offer a
remarkable parallelism in the fruit being white, red, or yellow
fleshed--in being clingstones or freestones--in the flowers being large
or small--in the leaves being serrated or crenated, furnished with
globose or reniform glands, or quite destitute of glands. It should be
remarked that each variety of the nectarine has not derived its
character from a corresponding variety of the peach. The several
varieties also of a closely allied genus, namely the apricot, differ
from each other in nearly the same parallel manner. There is no reason
{349} to believe that in any of these cases long-lost characters have
reappeared, and in most of them this certainly has not occurred.
Three species of Cucurbita have yielded a multitude of races, which
correspond so closely in character that, as Naudin insists, they may be
arranged in an almost strictly parallel series. Several varieties of
the melon are interesting from resembling in important characters other
species, either of the same genus or of allied genera; thus, one
variety has fruit so like, both externally and internally, the fruit of
a perfectly distinct species, namely, the cucumber, as hardly to be
distinguished from it; another has long cylindrical fruit twisting
about like a serpent; in another the seeds adhere to portions of the
pulp; in another the fruit, when ripe, suddenly cracks and falls into
pieces; and all these highly remarkable peculiarities are
characteristic of species belonging to allied genera. We can hardly
account for the appearance of so many unusual characters by reversion
to a single ancient form; but we must believe that all the members of
the family have inherited a nearly similar constitution from an early
progenitor. Our cereal and many other plants offer similar cases.
With animals we have fewer cases of analogous variation, independently
of direct reversion. We see something of the kind in the resemblance
between the short-muzzled races of the dog, such as the pug and
bulldog; in feather-footed races of the fowl, pigeon, and canary-bird;
in horses of the most different races presenting the same range of
colour; in all black-and-tan dogs having tan-coloured eye-spots and
feet, but in this latter case reversion may possibly have played a
part. Low has remarked[871] that several breeds of cattle are
"sheeted,"--that is, have a broad band of white passing round their
bodies like a sheet; this character is strongly inherited and sometimes
originates from a cross; it may be the first step in reversion to an
original or early type, for, as was shown in the third chapter, white
cattle with dark ears, feet, and tip of tail formerly existed, and now
exist in a feral or semi-feral condition in several quarters of the
world.
Under our second main division, namely, of analogous variations due to
reversion, the best cases are afforded by animals, and by none better
than by pigeons. In all the most distinct breeds sub-varieties
occasionally appear coloured exactly like the parent rock-pigeon, with
black wing-bars, white loins, banded tail, &c.; and no one can doubt
that these characters are simply due to reversion. So with minor
details; turbits properly have white tails, but occasionally a bird is
born with a dark-coloured and banded tail; pouters properly have white
primary wing-feathers, but not rarely a "sword-flighted" bird, that is,
one with the few first primaries dark-coloured, appears; and in these
cases we have characters proper to the rock-pigeon, but new to the
breed, evidently appearing from reversion. In some domestic varieties
the wing-bars, instead of being simply black, as in the rock-pigeon,
are beautifully edged with different zones of colour, and they then
present a striking analogy with the wing-bars in certain natural
species of the same family, such as _Phaps chalcoptera_; and this may
probably be accounted for by {350} all the forms descended from the
same remote progenitor having a tendency to vary in the same manner.
Thus also we can perhaps understand the fact of some Laugher-pigeons
cooing almost like turtle-doves, and of several races having
peculiarities in their flight, for certain natural species (viz. _C.
torquatrix_ and _palumbus_) display singular vagaries in this respect.
In other cases a race, instead of imitating in character a distinct
species, resembles some other race; thus certain runts tremble and
slightly elevate their tails, like fantails; and turbits inflate the
upper part of their oesophagus, like pouter-pigeons.
It is a common circumstance to find certain coloured marks persistently
characterising all the species of a genus, but differing much in tint;
and the same thing occurs with the varieties of the pigeon: thus,
instead of the general plumage being blue with the wing-bars black,
there are snow-white varieties with red bars, and black varieties with
white bars; in other varieties the wing-bars, as we have seen, are
elegantly zoned with different tints. The Spot pigeon is characterised
by the whole plumage being white, excepting the tail and a spot on the
forehead; but these parts may be red, yellow, or black. In the
rock-pigeon and in many varieties the tail is blue, with the outer
edges of the outer feathers white; but in one sub-variety of the
monk-pigeon we have a reversed variation, for the tail is white, except
the outer edges of the outer feathers, which are black.[872]
With some species of birds, for instance with gulls, certain coloured
parts appear as if almost washed out, and I have observed exactly the
same appearance in the terminal dark tail-bar in certain pigeons, and
in the whole plumage of certain varieties of the duck. Analogous facts
in the vegetable kingdom could be given.
Many sub-varieties of the pigeon have reversed and somewhat lengthened
feathers on the back part of their heads, and this is certainly not due
to reversion to the parent-species, which shows no trace of such
structure; but when we remember that sub-varieties of the fowl, turkey,
canary-bird, duck, and goose, all have topknots or reversed feathers on
their heads; and when we remember that scarcely a single large natural
group of birds can be named, in which some members have not a tuft of
feathers on their heads, we may suspect that reversion to some
extremely remote form has come into action.
Several breeds of the fowl have either spangled or pencilled feathers;
and these cannot be derived from the parent-species, the _Gallus
bankiva_; though of course it is possible that an early progenitor of
this species may have been spangled, and a still earlier or a later
progenitor may have been pencilled. But as many gallinaceous birds are
spangled or pencilled, it is a more probable view that the several
domestic breeds of the fowl have acquired this kind of plumage from all
the members of the family inheriting a tendency to vary in a like
manner. The same principle may account for the ewes in certain breeds
of sheep being hornless, like the females of some other hollow-horned
ruminants; it may account for certain domestic cats having
slightly-tufted ears, like those of the lynx; and for the skulls of
domestic rabbits often differing from each {351} other in the same
characters by which the skulls of the various species of the genus
Lepus differ.
I will only allude to one other case, already discussed. Now that we
know that the wild parent of the ass has striped legs, we may feel
confident that the occasional appearance of stripes on the legs of the
domestic ass is due to direct reversion; but this will not account for
the lower end of the shoulder-stripe being sometimes angularly bent or
slightly forked. So, again, when we see dun and other coloured horses
with stripes on the spine, shoulders, and legs, we are led, from
reasons formerly given, to believe that they reappear from direct
reversion to the wild parent-horse. But when horses have two or three
shoulder-stripes with one of them occasionally forked at the lower end,
or when they have stripes on their faces, or as foals are faintly
striped over nearly their whole bodies, with the stripes angularly bent
one under the other on the forehead, or irregularly branched in other
parts, it would be rash to attribute such diversified characters to the
reappearance of those proper to the aboriginal wild horse. As three
African species of the genus are much striped, and as we have seen that
the crossing of the unstriped species often leads to the hybrid
offspring being conspicuously striped--bearing also in mind that the
act of crossing certainly causes the reappearance of long-lost
characters--it is a more probable view that the above-specified stripes
are due to reversion, not to the immediate wild parent-horse, but to
the striped progenitor of the whole genus.
I have discussed this subject of analogous variation at considerable
length, because, in a future work on natural species, it will be shown that
the varieties of one species frequently mock distinct species--a fact in
perfect harmony with the foregoing cases, and explicable only on the theory
of descent. Secondly, because these facts are important from showing, as
remarked in a former chapter, that each trifling variation is governed by
law, and is determined in a much higher degree by the nature of the
organisation, than by the nature of the conditions to which the varying
being has been exposed. Thirdly, because these facts are to a certain
extent related to a more general law, namely, that which Mr. B. D.
Walsh[873] has called the "Law of _Equable Variability_," or, as he
explains it, "if any given character is very variable in one species of a
group, it will tend to be variable in allied species; and if any given
character is perfectly constant in one species of a group, it will tend to
be constant in allied species."
This leads me to recall a discussion in the chapter on Selection, in which
it was shown that with domestic races, which are {352} now undergoing rapid
improvement, those parts or characters which are the most valued vary the
most. This naturally follows from recently selected characters continually
tending to revert to their former less improved standard, and from their
being still acted on by the same agencies, whatever these may be, which
first caused the characters in question to vary. The same principle is
applicable to natural species, for, as stated in my 'Origin of Species,'
generic characters are less variable than specific characters; and the
latter are those which have been modified by variation and natural
selection, since the period when all the species belonging to the same
genus branched off from a common progenitor, whilst generic characters are
those which have remained unaltered from a much more remote epoch, and
accordingly are now less variable. This statement makes a near approach to
Mr. Walsh's law of Equable Variability. Secondary sexual characters, it may
be added, rarely serve to characterise distinct genera, for they usually
differ much in the species of the same genus, and are highly variable in
the individuals of the same species; we have also seen in the earlier
chapters of this work how variable secondary sexual characters become under
domestication.
_Summary of the three previous Chapters, on the Laws of Variation._
In the twenty-third chapter we have seen that changed conditions
occasionally act in a definite manner on the organisation, so that all, or
nearly all, the individuals thus exposed become modified in the same
manner. But a far more frequent result of changed conditions, whether
acting directly on the organisation or indirectly through the reproductive
system being affected is indefinite and fluctuating variability. In the
three latter chapters we have endeavoured to trace some of the laws by
which such variability is regulated.
Increased use adds the size of a muscle, together with the blood-vessels,
nerves, ligaments, the crests of bone to which these are attached, the
whole bone and other connected bones. So it is with various glands.
Increased functional activity strengthens the sense-organs. Increased and
intermittent pressure thickens the epidermis; and a change in the nature of
the food sometimes modifies the coats of the stomach, and increases or
{353} decreases the length of the intestines. Continued disuse, on the
other hand, weakens and diminishes all parts of the organisation. Animals
which during many generations have taken but little exercise, have their
lungs reduced in size, and as a consequence the bony fabric of the chest,
and the whole form of the body, become modified. With our anciently
domesticated birds, the wings have been little used, and they are slightly
reduced; with their decrease, the crest of the sternum, the scapulae,
coracoids, and furcula, have all been reduced.
With domesticated animals, the reduction of a part from disuse is never
carried so far that a mere rudiment is left, but we have good reason to
believe that this has often occurred under nature. The cause of this
difference probably is that with domestic animals not only sufficient time
has not been granted for so profound a change, but that, from not being
exposed to a severe struggle for life, the principle of the economy of
organisation does not come into action. On the contrary, we sometimes see
that structures which are rudimentary in the parent-species become
partially redeveloped in their domesticated progeny. When rudiments are
formed or left under domestication, they are the result of a sudden arrest
of development, and not of long-continued disuse with the absorption of all
superfluous parts; nevertheless they are of interest, as showing that
rudiments are the relics of organs once perfectly developed.
Corporeal, periodical, and mental habits, though the latter have been
almost passed over in this work, become changed under domestication, and
the changes are often inherited. Such changed habits in any organic being,
especially when living a free life, would often lead to the augmented or
diminished use of various organs, and consequently to their modification.
From long-continued habit, and more especially from the occasional birth of
individuals with a slightly different constitution, domestic animals and
cultivated plants become to a certain extent acclimatised, or adapted to a
climate different from that proper to the parent-species.
Through the principle of correlated variability, when one part varies other
parts vary,--either simultaneously, or one after the other. Thus an organ
modified during an early embryonic period affects other parts subsequently
developed. When an {354} organ, such as the beak, increases or decreases in
length, adjoining or correlated parts, as the tongue and the orifice of the
nostrils, tend to vary in the same manner. When the whole body increases or
decreases in size, various parts become modified; thus with pigeons the
ribs increase or decrease in number and breadth. Homologous parts, which
are identical during their early development and are exposed to similar
conditions, tend to vary in the same or in some connected manner,--as in
the case of the right and left sides of the body, of the front and hind
limbs, and even of the head and limbs. So it is with the organs of sight
and hearing; for instance, white cats with blue eyes are almost always
deaf. There is a manifest relation throughout the body between the skin and
its various appendages of hair, feathers, hoofs, horns, and teeth. In
Paraguay, horses with curly hair have hoofs like those of a mule; the wool
and the horns of sheep vary together; hairless dogs are deficient in their
teeth; men with redundant hair have abnormal teeth, either deficient or in
excess. Birds with long wing-feathers usually have long tail-feathers. When
long feathers grow from the outside of the legs and toes of pigeons, the
two outer toes are connected by membrane; for the whole leg tends to assume
the structure of the wing. There is a manifest relation between a crest of
feathers on the head and a marvellous amount of change in the skull of
various fowls; and in a lesser degree, between the greatly elongated,
lopping ears of rabbits and the structure of their skulls. With plants, the
leaves, various parts of the flower, and the fruit, often vary together in
a correlated manner.
In some cases we find correlation without being able even to conjecture
what is the nature of the connexion, as with various correlated
monstrosities and diseases. This is likewise the case with the colour of
the adult pigeon, in connexion with the presence of down on the young bird.
Numerous curious instances have been given of peculiarities of
constitution, in correlation with colour, as shown by the immunity of
individuals of some one colour from certain diseases, from the attacks of
parasites, and from the action of certain vegetable poisons.
Correlation is an important subject; for with species, and in a lesser
degree with domestic races, we continually find that {355} certain parts
have been greatly modified to serve some useful purpose; but we almost
invariably find that other parts have likewise been more or less modified,
without our being able to discover any advantage in the change. No doubt
great caution is necessary in coming to this conclusion, for it is
difficult to overrate our ignorance on the use of various parts of the
organisation; but from what we have now seen, we may believe that many
modifications are of no direct service, having arisen in correlation with
other and useful changes.
Homologous parts during their early development evince an affinity for each
other,--that is, they tend to cohere and fuse together much more readily
than other parts. This tendency to fusion explains a multitude of normal
structures. Multiple and homologous organs are especially liable to vary in
number and probably in form. As the supply of organised matter is not
unlimited, the principle of compensation sometimes comes into action; so
that, when one part is greatly developed, adjoining parts or functions are
apt to be reduced; but this principle is probably of much less importance
than the more general one of the economy of growth. Through mere mechanical
pressure hard parts occasionally affect soft adjoining parts. With plants
the position of the flowers on the axis, and of the seeds in the capsule,
sometimes leads, through a freer flow of sap, to changes of structure; but
these changes are often due to reversion. Modifications, in whatever manner
caused, will be to a certain extent regulated by that co-ordinating power
or _nisus formativus_, which is in fact a remnant of one of the forms of
reproduction, displayed by many lowly organised beings in their power of
fissiparous generation and budding. Finally, the effects of the laws, which
directly or indirectly govern variability, may be largely influenced by
man's selection, and will so far be determined by natural selection that
changes advantageous to any race will be favoured and disadvantageous
changes checked.
Domestic races descended from the same species, or from two or more allied
species, are liable to revert to characters derived from their common
progenitor, and, as they have much in common in their constitutions, they
are also liable under changed conditions to vary in the same manner; from
these {356} two causes analogous varieties often arise. When we reflect on
the several foregoing laws, imperfectly as we understand them, and when we
bear in mind how much remains to be discovered, we need not be surprised at
the extremely intricate manner in which our domestic productions have
varied, and still go on varying.
* * * * *
{357}
CHAPTER XXVII.
PROVISIONAL HYPOTHESIS OF PANGENESIS.
PRELIMINARY REMARKS.--FIRST PART:--THE FACTS TO BE CONNECTED UNDER A
SINGLE POINT OF VIEW, NAMELY, THE VARIOUS KINDS OF REPRODUCTION--THE
DIRECT ACTION OF THE MALE ELEMENT ON THE FEMALE--DEVELOPMENT--THE
FUNCTIONAL INDEPENDENCE OF THE ELEMENTS OR UNITS OF THE
BODY--VARIABILITY--INHERITANCE--REVERSION.
SECOND PART:--STATEMENT OF THE HYPOTHESIS--HOW FAR THE NECESSARY
ASSUMPTIONS ARE IMPROBABLE--EXPLANATION BY AID OF THE HYPOTHESIS OF THE
SEVERAL CLASSES OF FACTS SPECIFIED IN THE FIRST PART--CONCLUSION.
In the previous chapters large classes of facts, such as those bearing on
bud-variation, the various forms of inheritance, the causes and laws of
variation, have been discussed; and it is obvious that these subjects, as
well as the several modes of reproduction, stand in some sort of relation
to each other. I have been led, or rather forced, to form a view which to a
certain extent connects these facts by a tangible method. Every one would
wish to explain to himself, even in an imperfect manner, how it is possible
for a character possessed by some remote ancestor suddenly to reappear in
the offspring; how the effects of increased or decreased use of a limb can
be transmitted to the child; how the male sexual element can act not solely
on the ovule, but occasionally on the mother-form; how a limb can be
reproduced on the exact line of amputation, with neither too much nor too
little added; how the various modes of reproduction are connected, and so
forth. I am aware that my view is merely a provisional hypothesis or
speculation; but until a better one be advanced, it may be serviceable by
bringing together a multitude of facts which are at present left
disconnected by any efficient cause. As Whewell, the historian of the
inductive sciences, remarks:--"Hypotheses may often be of service to
science, when they involve a certain portion of incompleteness, and even of
error." Under this point of view I venture to advance the hypothesis of
Pangenesis, which {358} implies that the whole organisation, in the sense
of every separate atom or unit, reproduces itself. Hence ovules and
pollen-grains,--the fertilised seed or egg, as well as buds,--include and
consist of a multitude of germs thrown off from each separate atom of the
organism.
In the First Part I will enumerate as briefly as I can the groups of facts
which seem to demand connection; but certain subjects, not hitherto
discussed, must be treated at disproportionate length. In the Second Part
the hypothesis will be given; and we shall see, after considering how far
the necessary assumptions are in themselves improbable, whether it serves
to bring under a single point of view the various facts.
PART I.
Reproduction may be divided into two main classes, namely, sexual and
asexual. The latter is effected in many ways--by gemmation, that is by the
formation of buds of various kinds, and by fissiparous generation, that is
by spontaneous or artificial division. It is notorious that some of the
lower animals, when cut into many pieces, reproduce so many perfect
individuals: Lyonnet cut a Nais or freshwater worm into nearly forty
pieces, and these all reproduced perfect animals.[874] It is probable that
segmentation could be carried much further in some of the protozoa, and
with some of the lowest plants each cell will reproduce the parent-form.
Johannes Mueller thought that there was an important distinction between
gemmation and fission; for in the latter case the divided portion, however
small, is more perfectly organised; but most physiologists are now
convinced that the two processes are essentially alike.[875] Prof. Huxley
remarks, "fission is little more than a peculiar {359} mode of budding,"
and Prof. H. J. Clark, who has especially attended to this subject, shows
in detail that there is sometimes "a compromise between self-division and
budding." When a limb is amputated, or when the whole body is bisected, the
cut extremities are said to bud forth; and as the papilla, which is first
formed, consists of undeveloped cellular tissue like that forming an
ordinary bud, the expression is apparently correct. We see the connection
of the two processes in another way; for Trembley observed that with the
hydra the reproduction of the head after amputation was checked as soon as
the animal began to bud.[876]
Between the production, by fissiparous generation, of two or more complete
individuals, and the repair of even a very slight injury, we have, as
remarked in a former chapter, so perfect and insensible a gradation, that
it is impossible to doubt that they are connected processes. Between the
power which repairs a trifling injury in any part, and the power which
previously "was occupied in its maintenance by the continued mutation of
its particles," there cannot be any great difference; and we may follow Mr.
Paget in believing them to be the selfsame power. As at each stage of
growth an amputated part is replaced by one in the same state of
development, we must likewise follow Mr. Paget in admitting "that the
powers of development from the embryo are identical with those exercised
for the restoration from injuries: in other words, that the powers are the
same by which perfection is first achieved, and by which, when lost, it is
recovered."[877] Finally, we may conclude that the several forms of
gemmation, and of fissiparous generation, the repair of injuries, the
maintenance of each part in its proper state, and the growth or progressive
development of the whole structure of the embryo, are all essentially the
results of one and the same great power.
_Sexual Generation._--The union of the two sexual elements seems to make a
broad distinction between sexual and asexual reproduction. But the
well-ascertained cases of Parthenogenesis prove that the distinction is not
really so great as it at first appears; for ovules occasionally, and even
in some cases {360} frequently, become developed into perfect beings,
without the concourse of the male element. J. Mueller and others admit that
ovules and buds have the same essential nature. Certain bodies, which
during their early development cannot be distinguished by any external
character from true ovules, nevertheless must be classed as buds, for
though formed within the ovarium they are incapable of fertilisation. This
is the case with the germ-balls of the Cecidomyide larvae, as described by
Leuckart.[878] Ovules and the male element, before they become united,
have, like buds, an independent existence.[879] Both have the power of
transmitting every single character possessed by the parent-form. We see
this clearly when hybrids are paired _inter se_, for the characters of
either grandparent often reappear, either perfectly or by segments, in the
progeny. It is an error to suppose that the male transmits certain
characters and the female other characters; though no doubt, from unknown
causes, one sex sometimes has a stronger power of transmission than the
other.
It has been maintained by some authors that a bud differs essentially from
a fertilised germ, by always reproducing the perfect character of the
parent-stock; whilst fertilised germs become developed into beings which
differ, in a greater or less degree, from each other and from their
parents. But there is no such broad distinction as this. In the eleventh
chapter, numerous cases were given showing that buds occasionally grow into
plants having new and strongly marked characters; and varieties thus
produced can be propagated for a length of time by buds, and occasionally
by seed. Nevertheless, it must be admitted that beings produced sexually
are much more liable to vary than those produced asexually; and of this
fact a partial explanation will hereafter be attempted. The variability in
both cases is determined by the same general causes, and is governed by the
same laws. Hence new varieties arising from buds cannot be distinguished
from those arising from seed. Although bud-varieties usually retain their
character during {361} successive bud-generations, yet they occasionally
revert, even after a long series of bud-generations, to their former
character. This tendency to reversion in buds is one of the most remarkable
of the several points of agreement between the offspring from bud and
seminal reproduction.
There is, however, one difference between beings produced sexually and
asexually, which is very general. The former usually pass in the course of
their development from a lower to a higher grade, as we see in the
metamorphoses of insects and in the concealed metamorphoses of the
vertebrata; but this passage from a lower to a higher grade cannot be
considered as a necessary accompaniment of sexual reproduction, for hardly
anything of the kind occurs in the development of Aphis amongst insects, or
with certain crustaceans, cephalopods, or with any of the higher vascular
plants. Animals propagated asexually by buds or fission are on the other
hand never known to undergo a retrogressive metamorphosis; that is, they do
not first sink to a lower, before passing on to their higher and final
stage of development. But during the act of asexual production or
subsequently to it, they often advance in organisation, as we see in the
many cases of "alternate generation." In thus speaking of alternate
generation, I follow those naturalists who look at the process as
essentially one of internal budding or of fissiparous generation. Some of
the lower plants, however, such as mosses and certain algae, according to
Dr. L. Radlkofer,[880] when propagated asexually, do undergo a
retrogressive metamorphosis. We can to a certain extent understand, as far
as the final cause is concerned, why beings propagated by buds should so
rarely retrogress during development; for with each organism the structure
acquired at each stage of development must be adapted to its peculiar
habits. Now, with beings produced by gemmation,--and this, differently from
sexual reproduction, may occur at any period of growth,--if there were
places for the support of many individuals at some one stage of
development, the simplest plan would be that they should be multiplied by
gemmation at that stage, and not that they should first retrograde in their
development to an earlier or simpler structure, which might not be fitted
for the surrounding conditions.
{362}
From the several foregoing considerations we may conclude that the
difference between sexual and asexual generation is not nearly so great as
it at first appears; and we have already seen that there is the closest
agreement between gemmation, fissiparous generation, the repair of
injuries, and ordinary growth or development. The capacity of fertilisation
by the male element seems to be the chief distinction between an ovule and
a bud; and this capacity is not invariably brought into action, as in the
cases of parthenogenetic reproduction. We are here naturally led to inquire
what the final cause can be of the necessity in ordinary generation for the
concourse of the two sexual elements.
Seeds and ova are often highly serviceable as the means of disseminating
plants and animals, and of preserving them during one or more seasons in a
dormant state; but unimpregnated seeds or ova, and detached buds, would be
equally serviceable for both purposes. We can, however, indicate two
important advantages gained by the concourse of the two sexes, or rather of
two individuals belonging to opposite sexes; for, as I have shown in a
former chapter, the structure of every organism appears to be especially
adapted for the concurrence, at least occasionally, of two individuals. In
nearly the same manner as it is admitted by naturalists that hybridism,
from inducing sterility, is of service in keeping the forms of life
distinct and fitted for their proper places; so, when species are rendered
highly variable by changed conditions of life, the free intercrossing of
the varying individuals will tend to keep each form fitted for its proper
place in nature; and crossing can be effected only by sexual generation,
but whether the end thus gained is of sufficient importance to account for
the first origin of sexual intercourse is very doubtful. Secondly, I have
shown, from the consideration of a large body of facts, that, as a slight
change in the conditions of life is beneficial to each creature, so, in an
analogous manner, is the change effected in the germ by sexual union with a
distinct individual; and I have been led, from observing the many
widely-extended provisions throughout nature for this purpose, and from the
greater vigour of crossed organisms of all kinds, as proved by direct
experiments, as well as from the evil effects of close interbreeding when
long {363} continued, to believe that the advantage thus gained is very
great. Besides these two important ends, there may, of course, be others,
as yet unknown to us, gained by the concourse of the two sexes.
Why the germ, which before impregnation undergoes a certain amount of
development, ceases to progress and perishes, unless it be acted on by the
male element; and why conversely the male element, which is enabled to keep
alive for even four or five years within the spermatheca of a female
insect, likewise perishes, unless it acts on or unites with the germ, are
questions which cannot be answered with any certainty. It is, however,
possible that both sexual elements perish, unless brought into union,
simply from including too little formative matter for independent existence
and development; for certainly they do not in ordinary cases differ in
their power of giving character to the embryo. This view of the importance
of the quantity of formative matter seems probable from the following
considerations. There is no reason to suspect that the spermatozoa or
pollen-grains of the same individual animal or plant differ from each
other; yet Quatrefages has shown in the case of the Teredo,[881] as did
formerly Prevost and Dumas with other animals, that more than one
spermatozoon is requisite to fertilise an ovule. This has likewise been
clearly proved by Newport,[882] who adds the important fact, established by
numerous experiments, that, when a very small number of spermatozoa are
applied to the ova of Batrachians, they are only partially impregnated and
the embryo is never fully developed: the first step, however, towards
development, namely, the partial segmentation of the yelk, does occur to a
greater or less extent, but is never completed up to granulation. The rate
of the segmentation is likewise determined by the number of the
spermatozoa. With respect to plants, nearly the same results were obtained
by Koelreuter and Gaertner. This last careful observer found,[883] after
making successive trials on a Malva with more and more pollen-grains, that
even thirty grains did not fertilise a single seed; but when forty grains
were applied to the {364} stigma, a few seeds of small size were formed.
The pollen-grains of Mirabilis are extraordinarily large, and the ovarium
contains only a single ovule; and these circumstances led Naudin[884] to
make the following interesting experiments: a flower was fertilised by
three grains and succeeded perfectly; twelve flowers were fertilised by two
grains, and seventeen flowers by a single grain, and of these one flower
alone in each lot perfected its seed; and it deserves especial notice that
the plants produced by these two seeds never attained their proper
dimensions, and bore flowers of remarkably small size. From these facts we
clearly see that the quantity of the peculiar formative matter which is
contained within the spermatozoa and pollen-grains is an all-important
element in the act of fertilisation, not only in the full development of
the seed, but in the vigour of the plant produced from such seed. We see
something of the same kind in certain cases of parthenogenesis, that is,
when the male element is wholly excluded; for M. Jourdan[885] found that,
out of about 58,000 eggs laid by unimpregnated silk-moths, many passed
through their early embryonic stages, showing that they were capable of
self-development, but only twenty-nine out of the whole number produced
caterpillars. Therefore it is not an improbable view that deficient bulk or
quantity in the formative matter, contained within the sexual elements, is
the main cause of their not having the capacity of prolonged separate
existence and development. The belief that it is the function of the
spermatozoa to communicate life to the ovule seems a strange one, seeing
that the unimpregnated ovule is already alive and continues for a
considerable time alive. We shall hereafter see that it is probable that
the sexual elements, or possibly only the female element, include certain
primordial cells, that is, such as have undergone no differentiation, and
which are not present in an active state in buds.
_Graft-hybrids._--When discussing in the eleventh chapter the curious case
of the _Cytisus adami_, facts were given which render it to a certain
degree probable, in accordance with the belief of some distinguished
botanists, that, when the tissues of two plants {365} belonging to distinct
species or varieties are intimately united, buds are afterwards
occasionally produced which, like hybrids, combine the characters of the
two united forms. It is certain that when trees with variegated leaves are
grafted or budded on a common stock, the latter sometimes produces buds
bearing variegated leaves; but this may perhaps be looked at as a case of
inoculated disease. Should it ever be proved that hybridised buds can be
formed by the union of two distinct vegetative tissues, the essential
identity of sexual and asexual reproduction would be shown in the most
interesting manner; for the power of combining in the offspring the
characters of both parents, is the most striking of all the functions of
sexual generation.
_Direct Action of the Male Element on the Female._--In the chapter just
referred to, I have given abundant proofs that foreign pollen occasionally
affects the mother-plant in a direct manner. Thus, when Gallesio fertilised
an orange-flower with pollen from the lemon, the fruit bore stripes of
perfectly characterised lemon-peel: with peas, several observers have seen
the colour of the seed-coats and even of the pod directly affected by the
pollen of a distinct variety; so it has been with the fruit of the apple,
which consists of the modified calyx and upper part of the flower-stalk.
These parts in ordinary cases are wholly formed by the mother-plant. We
here see the male element affecting and hybridising not that part which it
is properly adapted to affect, namely the ovule, but the partially
developed tissues of a distinct individual. We are thus brought half-way
towards a graft-hybrid, in which the cellular tissue of one form, instead
of its pollen, is believed to hybridise the tissues of a distinct form. I
formerly assigned reasons for rejecting the belief that the mother-plant is
affected through the intervention of the hybridised embryo; but even if
this view were admitted, the case would become one of graft-hybridism, for
the fertilised embryo and the mother-plant must be looked at as distinct
individuals.
With animals which do not breed until nearly mature, and of which all the
parts are then fully developed, it is hardly possible that the male element
should directly affect the female. But we have the analogous and perfectly
well-ascertained case of the male element of a distinct form, as with the
{366} quagga and Lord Morton's mare, affecting the ovarium of the female,
so that the ovules and offspring subsequently produced by her when
impregnated by other males are plainly affected and hybridised by the first
male.
_Development._--The fertilised germ reaches maturity by a vast number of
changes: these are either slight and slowly effected, as when the child
grows into the man, or are great and sudden, as with the metamorphoses of
most insects. Between these extremes we have, even within the same class,
every gradation: thus, as Sir J. Lubbock has shown,[886] there is an
Ephemerous insect which moults above twenty times, undergoing each time a
slight but decided change of structure; and these changes, as he further
remarks, probably reveal to us the normal stages of development which are
concealed and hurried through, or suppressed, in most other insects. In
ordinary metamorphoses, the parts and organs appear to become changed into
the corresponding parts in the next stage of development; but there is
another form of development, which has been called by Professor Owen
metagenesis. In this case "the new parts are not moulded upon the inner
surface of the old ones. The plastic force has changed its course of
operation. The outer case, and all that gave form and character to the
precedent individual, perish and are cast off; they are not changed into
the corresponding parts of the new individual. These are due to a new and
distinct developmental process," &c.[887] Metamorphosis, however, graduates
so insensibly into metagenesis, that the two processes cannot be distinctly
separated. For instance, in the last change which Cirripedes undergo, the
alimentary canal and some other organs are moulded on pre-existing parts;
but the eyes of the old and the young animal are developed in entirely
different parts of the body; the tips of the mature limbs are formed within
the larval limbs, and may be said to be metamorphosed from them; but their
basal portions and the whole thorax are developed in a plane actually at
right angles to the limbs and thorax of the larva; and this {367} may be
called metagenesis. The metagenetic process is carried to an extreme degree
in the development of some Echinoderms, for the animal in the second stage
of development is formed almost like a bud within the animal of the first
stage, the latter being then cast off like an old vestment, yet sometimes
still maintaining for a short period an independent vitality.[888]
If, instead of a single individual, several were to be thus developed
metagenetically within a pre-existing form, the process would be called one
of alternate generation. The young thus developed may either closely
resemble the encasing parent-form, as with the larvae of Cecidomyia, or may
differ to an astonishing degree, as with many parasitic worms and with
jelly-fishes; but this does not make any essential difference in the
process, any more than the greatness or abruptness of the change in the
metamorphoses of insects.
The whole question of development is of great importance for our present
subject. When an organ, the eye for instance, is metagenetically formed in
a part of the body where during the previous stage of development no eye
existed, we must look at it as a new and independent growth. The absolute
independence of new and old structures, which correspond in structure and
function, is still more obvious when several individuals are formed within
a previous encasing form, as in the cases of alternate generation. The same
important principle probably comes largely into play even in the case of
continuous growth, as we shall see when we consider the inheritance of
modifications at corresponding ages.
We are led to the same conclusion, namely, the independence of parts
successively developed, by another and quite distinct group of facts. It is
well known that many animals belonging to the same class, and therefore not
differing widely from each other, pass through an extremely different
course of development. Thus certain beetles, not in any way remarkably
different from others of the same order, undergo what has been called a
hyper-metamorphosis--that is, they pass through an early stage wholly
different from the ordinary grub-like larva. In the same sub-order of
crabs, namely, the Macroura, as Fritz {368} Mueller remarks, the river
cray-fish is hatched under the same form which it ever afterwards retains;
the young lobster has divided legs, like a Mysis; the Palaemon appears
under the form of a Zoea, and Peneus under the Nauplius-form; and how
wonderfully these larval forms differ from each other, is known to every
naturalist.[889] Some other crustaceans, as the same author observes, start
from the same point and arrive at nearly the same end, but in the middle of
their development are widely different from each other. Still more striking
cases could be given with respect to the Echinodermata. With the Medusae or
jelly-fishes Professor Allman observes, "the classification of the Hydroida
would be a comparatively simple task if, as has been erroneously asserted,
generically-identical medusoids always arose from generically-identical
polypoids; and on the other hand, that generically-identical polypoids
always gave origin to generically-identical medusoids." So, again, Dr.
Strethill Wright remarks, "in the life-history of the Hydroidae any phase,
planuloid, polypoid, or medusoid, may be absent."[890]
According to the belief now generally accepted by our best naturalists, all
the members of the same order or class, the Macrourous crustaceans for
instance, are descended from a common progenitor. During their descent they
have diverged much in structure, but have retained much in common; and this
divergence and retention of character has been effected, though they have
passed and still pass through marvellously different metamorphoses. This
fact well illustrates how independent each structure must be from that
which precedes and follows it in the course of development.
_The Functional Independence of the Elements or Units of the
Body._--Physiologists agree that the whole organism consists of a multitude
of elemental parts, which are to a great extent independent of each other.
Each organ, says Claude Bernard,[891] {369} has its proper life, its
autonomy; it can develop and reproduce itself independently of the
adjoining tissues. The great German authority, Virchow,[892] asserts still
more emphatically that each system, as the nervous or osseous system, or
the blood, consists of an "enormous mass of minute centres of action....
Every element has its own special action, and even though it derive its
stimulus to activity from other parts, yet alone effects the actual
performance of its duties.... Every single epithelial and muscular
fibre-cell leads a sort of parasitical existence in relation to the rest of
the body.... Every single bone-corpuscle really possesses conditions of
nutrition peculiar to itself." Each element, as Mr. Paget remarks, lives
its appointed time, and then dies, and, after being cast off or absorbed,
is replaced.[893] I presume that no physiologist doubts that, for instance,
each bone-corpuscle of the finger differs from the corresponding corpuscle
in the corresponding joint of the toe; and there can hardly be a doubt that
even those on the corresponding sides of the body differ, though almost
identical in nature. This near approach to identity is curiously shown in
many diseases in which the same exact points on the right and left sides of
the body are similarly affected; thus Mr. Paget[894] gives a drawing of a
diseased pelvis, in which the bone has grown into a most complicated
pattern, but "there is not one spot or line on one side which is not
represented, as exactly as it would be in a mirror, on the other."
Many facts support this view of the independent life of each minute element
of the body. Virchow insists that a single bone-corpuscle or a single cell
in the skin may become diseased. The spur of a cock, after being inserted
into the eye of an ox, lived for eight years, and acquired a weight of 306
grammes, or nearly fourteen ounces.[895] The tail of a pig has been grafted
into the middle of its back, and reacquired sensibility. Dr. Ollier[896]
inserted a piece of periosteum from the bone of a young dog under the skin
of a rabbit, and true bone was developed. A multitude of similar facts
could be given. The {370} frequent presence of hairs and of perfectly
developed teeth, even teeth of the second dentition, in ovarian
tumours,[897] are facts leading to the same conclusion.
Whether each of the innumerable autonomous elements of the body is a cell
or the modified product of a cell, is a more doubtful question, even if so
wide a definition be given to the term, as to include cell-like bodies
without walls and without nuclei.[898] Professor Lionel Beale uses the term
"germinal matter" for the contents of cells, taken in this wide
acceptation, and he draws a broad distinction between germinal matter and
"formed material" or the various products of cells.[899] But the doctrine
of _omnis cellula e cellula_ is admitted for plants, and is a widely
prevalent belief with respect to animals.[900] Thus Virchow, the great
supporter of the cellular theory, whilst allowing that difficulties exist,
maintains that every atom of tissue is derived from cells, and these from
pre-existing cells, and these primarily from the egg, which he regards as a
great cell. That cells, still retaining the same nature, increase by
self-division or proliferation, is admitted by almost every one. But when
an organism undergoes a great change of structure during development, the
cells, which at each stage are supposed to be directly derived from
previously-existing cells, must likewise be greatly changed in nature; this
change is apparently attributed by the supporters of the cellular doctrine
to some inherent power which the cells possess, and not to any external
agency.
Another school maintains that cells and tissues of all kinds may be formed,
independently of pre-existing cells, from plastic lymph or blastema; and
this it is thought is well exhibited in the repair of wounds. As I have not
especially attended to histology, it would be presumptuous in me to express
an opinion on the two opposed doctrines. But every one appears to admit
that the body consists of a multitude of "organic units,"[901] {371} each
of which possesses its own proper attributes, and is to a certain extent
independent of all others. Hence it will be convenient to use indifferently
the terms cells or organic units or simply units.
_Variability and Inheritance._--We have seen in the twenty-second chapter
that variability is not a principle co-ordinate with life or reproduction,
but results from special causes, generally from changed conditions acting
during successive generations. Part of the fluctuating variability thus
induced is apparently due to the sexual system being easily affected by
changed conditions, so that it is often rendered impotent; and when not so
seriously affected, it often fails in its proper function of transmitting
truly the characters of the parents to the offspring. But variability is
not necessarily connected with the sexual system, as we see from the cases
of bud-variation; and although we may not be able to trace the nature of
the connexion, it is probable that many deviations of structure which
appear in sexual offspring result from changed conditions acting directly
on the organisation, independently of the reproductive organs. In some
instances we may feel sure of this, when all, or nearly all the individuals
which have been similarly exposed are similarly and definitely affected--as
in the dwarfed and otherwise changed maize brought from hot countries when
cultivated in Germany; in the change of the fleece in sheep within the
tropics; to a certain extent in the increased size and early maturity of
our highly-improved domesticated animals; in inherited gout from
intemperance; and in many other such cases. Now, as such changed conditions
do not especially affect the reproductive organs, it seems mysterious on
any ordinary view why their product, the new organic being, should be
similarly affected.
How, again, can we explain to ourselves the inherited effects of the use or
disuse of particular organs? The domesticated duck flies less and walks
more than the wild duck, and its limb-bones have become in a corresponding
manner diminished and increased in comparison with those of the wild duck.
A horse is trained to certain paces, and the colt inherits similar
consensual movements. The domesticated rabbit becomes tame from close
confinement; the dog intelligent from associating with man; the retriever
is taught to fetch and carry: and these {372} mental endowments and bodily
powers are all inherited. Nothing in the whole circuit of physiology is
more wonderful. How can the use or disuse of a particular limb or of the
brain affect a small aggregate of reproductive cells, seated in a distant
part of the body, in such a manner that the being developed from these
cells inherits the characters of either one or both parents? Even an
imperfect answer to this question would be satisfactory.
Sexual reproduction does not essentially differ, as we have seen, from
budding or self-division, and these processes graduate through the repair
of injuries into ordinary development and growth; it might therefore be
expected that every character would be as regularly transmitted by all the
methods of reproduction as by continued growth. In the chapters devoted to
inheritance it was shown that a multitude of newly-acquired characters,
whether injurious or beneficial, whether of the lowest or highest vital
importance, are often faithfully transmitted--frequently even when one
parent alone possesses some new peculiarity. It deserves especial attention
that characters appearing at any age tend to reappear at a corresponding
age. We may on the whole conclude that in all cases inheritance is the
rule, and non-inheritance the anomaly. In some instances a character is not
inherited, from the conditions of life being directly opposed to its
development; in many instances, from the conditions incessantly inducing
fresh variability, as with grafted fruit-trees and highly cultivated
flowers. In the remaining cases the failure may be attributed to reversion,
by which the child resembles its grandparents or more remote progenitors,
instead of its parents.
This principle of Reversion is the most wonderful of all the attributes of
Inheritance. It proves to us that the transmission of a character and its
development, which ordinarily go together and thus escape discrimination,
are distinct powers; and these powers in some cases are even antagonistic,
for each acts alternately in successive generations. Reversion is not a
rare event, depending on some unusual or favourable combination of
circumstances, but occurs so regularly with crossed animals and plants, and
so frequently with uncrossed breeds, that it is evidently an essential part
of the principle of inheritance. We know that {373} changed conditions have
the power of evoking long-lost characters, as in the case of some feral
animals. The act of crossing in itself possesses this power in a high
degree. What can be more wonderful than that characters, which have
disappeared during scores, or hundreds, or even thousands of generations,
should suddenly reappear perfectly developed, as in the case of pigeons and
fowls when purely bred, and especially when crossed; or as with the zebrine
stripes on dun-coloured horses, and other such cases? Many monstrosities
come under this same head, as when rudimentary organs are redeveloped, or
when an organ which we must believe was possessed by an early progenitor,
but of which not even a rudiment is left, suddenly reappears, as with the
fifth stamen in some Scrophulariaceae. We have already seen that reversion
acts in bud-reproduction; and we know that it occasionally acts during the
growth of the same individual animal, especially, but not exclusively, when
of crossed parentage,--as in the rare cases described of individual fowls,
pigeons, cattle, and rabbits, which have reverted as they advanced in years
to the colours of one of their parents or ancestors.
We are led to believe, as formerly explained, that every character which
occasionally reappears is present in a latent form in each generation, in
nearly the same manner as in male and female animals secondary characters
of the opposite sex lie latent, ready to be evolved when the reproductive
organs are injured. This comparison of the secondary sexual characters
which are latent in both sexes, with other latent characters, is the more
appropriate from the case recorded of the Hen, which assumed some of the
masculine characters, not of her own race, but of an early progenitor; she
thus exhibited at the same time the redevelopment of latent characters of
both kinds and connected both classes. In every living creature we may feel
assured that a host of lost characters lie ready to be evolved under proper
conditions. How can we make intelligible, and connect with other facts,
this wonderful and common capacity of reversion,--this power of calling
back to life long-lost characters? {374}
PART II.
I have now enumerated the chief facts which every one would desire to
connect by some intelligible bond. This can be done, as it seems to me, if
we make the following assumptions; if the first and chief one be not
rejected, the others, from being supported by various physiological
considerations, will not appear very improbable. It is almost universally
admitted that cells, or the units of the body, propagate themselves by
self-division or proliferation, retaining the same nature, and ultimately
becoming converted into the various tissues and substances of the body. But
besides this means of increase I assume that cells, before their conversion
into completely passive or "formed material," throw off minute granules or
atoms, which circulate freely throughout the system, and when supplied with
proper nutriment multiply by self-division, subsequently becoming developed
into cells like those from which they were derived. These granules for the
sake of distinctness may be called cell-gemmules, or, as the cellular
theory is not fully established, simply gemmules. They are supposed to be
transmitted from the parents to the offspring, and are generally developed
in the generation which immediately succeeds, but are often transmitted in
a dormant state during many generations and are then developed. Their
development is supposed to depend on their union with other partially
developed cells or gemmules which precede them in the regular course of
growth. Why I use the term union, will be seen when we discuss the direct
action of pollen on the tissues of the mother-plant. Gemmules are supposed
to be thrown off by every cell or unit, not only during the adult state,
but during all the stages of development. Lastly, I assume that the
gemmules in their dormant state have a mutual affinity for each other,
leading to their aggregation either into buds or into the sexual elements.
Hence, speaking strictly, it is not the reproductive elements, nor the
buds, which generate new organisms, but the cells themselves throughout the
body. These assumptions constitute the provisional hypothesis which I have
called Pangenesis. Nearly {375} similar views have been propounded, as I
find, by other authors, more especially by Mr. Herbert Spencer;[902] but
they are here modified and amplified.
{376}
Before proceeding to show, firstly, how far these assumptions are in
themselves probable, and secondly, how far they connect and explain the
various groups of facts with which we are concerned, it may be useful to
give an illustration of the hypothesis. If one of the simplest Protozoa be
formed, as appears under the microscope, of a small mass of homogeneous
gelatinous matter, a minute atom thrown off from any part and nourished
under favourable circumstances would naturally reproduce the whole; but if
the upper and lower surfaces were to differ in texture from the central
portion, then all three parts would have to throw off atoms or gemmules,
which when aggregated by mutual affinity would form either buds or the
sexual elements. Precisely the same view may be extended to one of the
higher animals; although in this case many thousand gemmules must be thrown
off from the various parts of the body. Now, when the leg, for instance, of
a salamander is cut off, a slight crust forms over the wound, and beneath
this crust the uninjured cells or units of bone, muscle, nerves, &c., are
supposed to unite with the diffused gemmules of those cells which in the
perfect leg come next in order; and these as they become slightly developed
unite with others, and so on until a papilla of soft cellular tissue, the
"budding leg," is formed, and in time a perfect leg.[903] Thus, that
portion of the leg which had {377} been cut off, neither more nor less,
would be reproduced. If the tail or leg of a young animal had been cut off,
a young tail or leg would have been reproduced, as actually occurs with the
amputated tail of the tadpole; for gemmules of all the units which compose
the tail are diffused throughout the body at all ages. But during the adult
state the gemmules of the larval tail would remain dormant, for they would
not meet with pre-existing cells in a proper state of development with
which to unite. If from changed conditions or any other cause any part of
the body should become permanently modified, the gemmules, which are merely
minute portions of the contents of the cells forming the part, would
naturally reproduce the same modification. But gemmules previously derived
from the same part before it had undergone any change, would still be
diffused throughout the organisation, and would be transmitted from
generation to generation, so that under favourable circumstances they might
be redeveloped, and then the new modification would be for a time or for
ever lost. The aggregation of gemmules derived from every part of the body,
through their mutual affinity, would form buds, and their aggregation in
some special manner, apparently in small quantity, together probably with
the presence of gemmules of certain primordial cells, would constitute the
sexual elements. By means of these illustrations the hypothesis of
pangenesis has, I hope, been rendered intelligible.
* * * * *
Physiologists maintain, as we have seen, that each cell, though to a large
extent dependent on others, is likewise, to a certain extent, independent
or autonomous. I go one small step further, and assume that each cell casts
off a free gemmule, which is capable of reproducing a similar cell. There
is some analogy between this view and what we see in compound animals and
in the flower-buds on the same tree; for these are distinct individuals
capable of true or seminal reproduction, yet have parts in common and are
dependent on each other; thus {378} the tree has its bark and trunk, and
certain corals, as the Virgularia, have not only parts, but movements in
common.
The existence of free gemmules is a gratuitous assumption, yet can hardly
be considered as very improbable, seeing that cells have the power of
multiplication through the self-division of their contents. Gemmules differ
from true ovules or buds inasmuch as they are supposed to be capable of
multiplication in their undeveloped state. No one probably will object to
this capacity as improbable. The blastema within the egg has been known to
divide and give birth to two embryos; and Thuret[904] has seen the zoospore
of an alga divide itself, and both halves germinate. An atom of small-pox
matter, so minute as to be borne by the wind, must multiply itself many
thousand-fold in a person thus inoculated.[905] It has recently been
ascertained[906] that a minute portion of the mucous discharge from an
animal affected with rinderpest, if placed in the blood of a healthy ox,
increases so fast that in a short space of time "the whole mass of blood,
weighing many pounds, is infected, and every small particle of that blood
contains enough poison to give, within less than forty-eight hours, the
disease to another animal."
The retention of free and undeveloped gemmules in the same body from early
youth to old age may appear improbable, but we should remember how long
seeds lie dormant in the earth and buds in the bark of a tree. Their
transmission from generation to generation may appear still more
improbable; but here again we should remember that many rudimentary and
useless organs are transmitted and have been transmitted during an
indefinite number of generations. We shall presently see how well the
long-continued transmission of undeveloped gemmules explains many facts.
As each unit, or group of similar units throughout the body, casts off its
gemmules, and as all are contained within the smallest egg or seed, and
within each spermatozoon or pollen-grain, their number and minuteness must
be something {379} inconceivable. I shall hereafter recur to this
objection, which at first appears so formidable; but it may here be
remarked that a cod-fish has been found to produce 4,872,000 eggs, a single
Ascaris about 64,000,000 eggs, and a single Orchidaceous plant probably as
many million seeds.[907] In these several cases, the spermatozoa and
pollen-grains must exist in considerably larger numbers. Now, when we have
to deal with numbers such as these, which the human intellect cannot grasp,
there is no good reason for rejecting our present hypothesis on account of
the assumed existence of cell-gemmules a few thousand times more numerous.
The gemmules in each organism must be thoroughly diffused; nor does this
seem improbable considering their minuteness, and the steady circulation of
fluids throughout the body. So it must be with the gemmules of plants, for
with certain kinds even a minute fragment of a leaf will reproduce the
whole. But a difficulty here occurs; it would appear that with plants, and
probably with compound animals, such as corals, the gemmules do not spread
from bud to bud, but only through the tissues developed from each separate
bud. We are led to this conclusion from the stock being rarely affected by
the insertion of a bud or graft from a distinct variety. This non-diffusion
of the gemmules is still more plainly shown in the case of ferns; for Mr.
Bridgman[908] has proved that, when spores (which it should be remembered
are of the nature of buds) are taken from a monstrous part of a frond, and
others from an ordinary part, {380} each reproduces the form of the part
whence derived. But this non-diffusion of the gemmules from bud to bud may
be only apparent, depending, as we shall hereafter see, on the nature of
the first-formed cells in the buds.
The assumed elective affinity of each gemmule for that particular cell
which precedes it in the order of development is supported by many
analogies. In all ordinary cases of sexual reproduction the male and female
elements have a mutual affinity for each other: thus, it is believed that
about ten thousand species of Compositae exist, and there can be no doubt
that if the pollen of all these species could be, simultaneously or
successively, placed on the stigma of any one species, this one would elect
with unerring certainty its own pollen. This elective capacity is all the
more wonderful, as it must have been acquired since the many species of
this great group of plants branched off from a common progenitor. On any
view of the nature of sexual reproduction, the protoplasm contained within
the ovules and within the sperm-cells (or the "spermatic force" of the
latter, if so vague a term be preferred) must act on each other by some law
of special affinity, either during or subsequently to impregnation, so that
corresponding parts alone affect each other; thus, a calf produced from a
short-horned cow by a long-horned bull has its horns and not its horny
hoofs affected by the union of the two forms, and the offspring from two
birds with differently coloured tails have their tails and not their whole
plumage affected.
The various tissues of the body plainly show, as many physiologists have
insisted,[909] an affinity for special organic substances, whether natural
or foreign to the body. We see this in the cells of the kidneys attracting
urea from the blood; in the worrara poison affecting the nerves; upas and
digitalis the muscles; the Lytta vesicatoria the kidneys; and in the
poisonous matter of many diseases, as small-pox, scarlet-fever,
hooping-cough, glanders, cancer, and hydrophobia, affecting certain
definite parts of the body or certain tissues or glands.
The affinity of various parts of the body for each other during {381} their
early development was shown in the last chapter, when discussing the
tendency to fusion in homologous parts. This affinity displays itself in
the normal fusion of organs which are separate at an early embryonic age,
and still more plainly in those marvellous cases of double monsters in
which each bone, muscle, vessel, and nerve in the one embryo, blends with
the corresponding part in the other. The affinity between homologous organs
may come into action with single parts, or with the entire individual, as
in the case of flowers or fruits which are symmetrically blended together
with all their parts doubled, but without any other trace of fusion.
It has also been assumed that the development of each gemmule depends on
its union with another cell or unit which has just commenced its
development, and which, from preceding it in order of growth, is of a
somewhat different nature. Nor is it a very improbable assumption that the
development of a gemmule is determined by its union with a cell slightly
different in nature, for abundant evidence was given in the seventeenth
chapter, showing that a slight degree of differentiation in the male and
female sexual elements favours in a marked manner their union and
subsequent development. But what determines the development of the gemmules
of the first-formed or primordial cell in the unimpregnated ovule, is
beyond conjecture.
It must also be admitted that analogy fails to guide us towards any
determination on several other points: for instance, whether cells, derived
from the same parent-cell, may, in the regular course of growth, become
developed into different structures, from absorbing peculiar kinds of
nutriment, independently of their union with distinct gemmules. We shall
appreciate this difficulty if we call to mind, what complex yet symmetrical
growths the cells of plants yield when they are inoculated by the poison of
a gall-insect. With animals various polypoid excrescences and tumours are
now generally admitted[910] to be the direct product, through
proliferation, of normal cells which have become abnormal. In the regular
growth and repair of bones, the tissues undergo, as Virchow remarks,[911] a
whole series of permutations and substitutions. "The cartilage-cells may be
{382} converted by a direct transformation into marrow-cells, and continue
as such; or they may first be converted into osseous and then into
medullary tissue; or lastly, they may first be converted into marrow and
then into bone. So variable are the permutations of these tissues, in
themselves so nearly allied, and yet in their external appearance so
completely distinct." But as these tissues thus change their nature at any
age, without any obvious change in their nutrition, we must suppose in
accordance with our hypothesis that gemmules derived from one kind of
tissue combine with the cells of another kind, and cause the successive
modifications.
It is useless to speculate at what period of development each organic unit
casts off its gemmules; for the whole subject of the development of the
various elemental tissues is as yet involved in much doubt. Some
physiologists, for instance, maintain that muscle or nerve-fibres are
developed from cells, which are afterwards nourished by their own proper
powers of absorption; whilst other physiologists deny their cellular
origin; and Beale maintains that such fibres are renovated exclusively by
the conversion of fresh germinal matter (that is the so-called nuclei) into
"formed material." However this may be, it appears probable that all
external agencies, such as changed nutrition, increased use or disuse, &c.,
which induced any permanent modification in a structure, would at the same
time or previously act on the cells, nuclei, germinal or formative matter,
from which the structures in question were developed, and consequently
would act on the gemmules or cast-off atoms.
There is another point on which it is useless to speculate, namely, whether
all gemmules are free and separate, or whether some are from the first
united into small aggregates. A feather, for instance, is a complex
structure, and, as each separate part is liable to inherited variations, I
conclude that each feather certainly generates a large number of gemmules;
but it is possible that these may be aggregated into a compound gemmule.
The same remark applies to the petals of a flower, which in some cases are
highly complex, with each ridge and hollow contrived for special purposes,
so that each part must have been separately modified, and the modifications
transmitted; consequently, separate gemmules, according to our hypothesis,
{383} must have been thrown off from each cell or part. But, as we
sometimes see half an anther or a small portion of a filament becoming
petaliform, or parts or mere stripes of the calyx assuming the colour and
texture of the corolla, it is probable that with petals the gemmules of
each cell are not aggregated together into a compound gemmule, but are
freely and separately diffused.
* * * * *
Having now endeavoured to show that the several foregoing assumptions are
to a certain extent supported by analogous facts, and having discussed some
of the most doubtful points, we will consider how far the hypothesis brings
under a single point of view the various cases enumerated in the First
Part. All the forms of reproduction graduate into each other and agree in
their product; for it is impossible to distinguish between organisms
produced from buds, from self-division, or from fertilised germs; such
organisms are liable to variations of the same nature and to reversion of
character; and as we now see that all the forms of reproduction depend on
the aggregation of gemmules derived from the whole body, we can understand
this general agreement. It is satisfactory to find that sexual and asexual
generation, by both of which widely different processes the same living
creature is habitually produced, are fundamentally the same.
Parthenogenesis is no longer wonderful; in fact, the wonder is that it
should not oftener occur. We see that the reproductive organs do not
actually create the sexual elements; they merely determine or permit the
aggregation of the gemmules in a special manner. These organs, together
with their accessory parts, have, however, high functions to perform; they
give to both elements a special affinity for each other, independently of
the contents of the male and female cells, as is shown in the case of
plants by the mutual reaction of the stigma and pollen-grains; they adapt
one or both elements for independent temporary existence, and for mutual
union. The contrivances for these purposes are sometimes wonderfully
complex, as with the spermatophores of the Cephalopoda. The male element
sometimes possesses attributes which, if observed in an independent animal,
would be put down to instinct guided by sense-organs, as when the {384}
spermatozoon of an insect finds its way into the minute micropyle of the
egg, or as when the antherozoids of certain algae swim by the aid of their
ciliae to the female plant, and force themselves into a minute orifice. In
these latter cases, however, we must believe that the male element has
acquired its powers, on the same principle with the larvae of animals,
namely by successive modifications developed at corresponding periods of
life: we can hardly avoid in these cases looking at the male element as a
sort of premature larva, which unites, or, like one of the lower algae,
conjugates, with the female element. What determines the aggregation of the
gemmules within the sexual organs we do not in the least know; nor do we
know why buds are formed in certain definite places, leading to the
symmetrical growth of trees and corals, nor why adventitious buds may be
formed almost anywhere, even on a petal, and frequently upon healed
wounds.[912] As soon as the gemmules have aggregated themselves,
development apparently commences, but in the case of buds is often
afterwards suspended, and in the case of the sexual elements soon ceases,
unless the elements of the opposite sexes combine; even after this has
occurred, the fertilised germ, as with seeds buried in the ground, may
remain during a lengthened period in a dormant state.
The antagonism which has long been observed,[913] though exceptions
occur,[914] between active growth and the power of sexual
reproduction--between the repair of injuries and gemmation--and with
plants, between rapid increase by buds, rhizomes, &c., and the production
of seed, is partly explained by the gemmules not existing in sufficient
numbers for both processes. {385} But this explanation hardly applies to
those plants which naturally produce a multitude of seeds, but which,
through a comparatively small increase in the number of the buds on their
rhizomes or offsets, yield few or no seed. As, however, we shall presently
see that buds probably include tissue which has already been to a certain
extent developed or differentiated, some additional organised matter will
thus have been expended.
From one of the forms of Reproduction, namely, spontaneous self-division,
we are led by insensible steps to the repair of the slightest injury; and
the existence of gemmules, derived from every cell or unit throughout the
body and everywhere diffused, explains all such cases,--even the wonderful
fact that, when the limbs of the salamander were cut off many times
successively by Spallanzani and Bonnet, they were exactly and completely
reproduced. I have heard this process compared with the recrystallisation
which occurs when the angles of a broken crystal are repaired; and the two
processes have this much in common, that in the one case the polarity of
the molecules is the efficient cause, and in the other the affinity of the
gemmules for particular nascent cells.