I have now enumerated the chief facts which every one would desire to see
connected by some intelligible bond. This can be done, if we make the
following assumptions, and much may be advanced in favour of the chief one.
The secondary assumptions can likewise be supported by various physiological
considerations. It is universally admitted that the cells or units of the body
increase by self-division or proliferation, retaining the same nature, and
that they ultimately become converted into the various tissues and substances
of the body. But besides this means of increase I assume that the units throw
off minute granules which are dispersed throughout the whole system; that
these, when supplied with proper nutriment, multiply by self-division, and are
ultimately developed into units like those from which they were originally
derived. These granules may be called gemmules. They are collected from all
parts of the system to constitute the sexual elements, and their development
in the next generation forms a new being; but they are likewise capable of
transmission in a dormant state to future generations and may then be
developed. Their development depends on their union with other partially
developed or nascent cells 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 unit, not only during the adult state, but during each stage of
development of every organism; but not necessarily during the continued
existence of the same unit. Lastly, I assume that the gemmules in their
dormant state have a mutual affinity for each other, leading to their
aggregation into buds or into the sexual elements. Hence, it is not the
reproductive organs or buds which generate new organisms, but the units of
which each individual is composed. These assumptions constitute the
provisional hypothesis which I have called Pangenesis. Views in many respects
similar have been propounded by various authors. (27/42. Mr. G.H. Lewes
('Fortnightly Review' November 1, 1868 page 506) remarks on the number of
writers who have advanced nearly similar views. More than two thousand years
ago Aristotle combated a view of this kind, which, as I hear from Dr. W. Ogle,
was held by Hippocrates and others. Ray, in his 'Wisdom of God' (2nd edition
1692 page 68), says that "every part of the body seems to club and contribute
to the seed." The "organic molecules" of Buffon ('Hist. Nat. Gen.' edition of
1749 tome 2 pages 54, 62, 329, 333, 420, 425) appear at first sight to be the
same as the gemmules of my hypothesis, but they are essentially different.
Bonnet ('Oeuvres d'Hist. Nat.' tome 5 part 1 1781 4to edition page 334) speaks
of the limbs having germs adapted for the reparation of all possible losses;
but whether these germs are supposed to be the same with those within buds and
the sexual organs is not clear. Prof. Owen says ('Anatomy of Vertebrates'
volume 3 1868 page 813) that he fails to see any fundamental difference
between the views which he propounded in his 'Parthenogenesis' (1849 pages 5-
8), and which he now considers as erroneous, and my hypothesis of pangenesis:
but a reviewer ('Journal of Anat. and Phys.' May 1869 page 441) shows how
different they really are. I formerly thought that the "physiological units"
of Herbert Spencer ('Principles of Biology' volume 1 chapters 4 and 8 1863-64)
were the same as my gemmules, but I now know that this is not the case.
Lastly, it appears from a review of the present work by Prof. Mantegazza
('Nuova Antologia, Maggio' 1868), that he (in his 'Elementi di Igiene' Ediz. 3
page 540) clearly foresaw the doctrine of pangenesis.)
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, as simple as possible, of the hypothesis. If one of the
Protozoa be formed, as it appears under the microscope, of a small mass of
homogeneous gelatinous matter, a minute particle or gemmule thrown off from
any part and nourished under favourable circumstances would reproduce the
whole; but if the upper and lower surfaces were to differ in texture from each
other and from the central portion, then all three parts would have to throw
off gemmules, which when aggregated by mutual affinity would form either buds
or the sexual elements, and would ultimately be developed into a similar
organism. 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 at each stage of development; these gemmules
being developed in union with pre-existing nascent cells in due order of
succession.
Physiologists maintain, as we have seen, that each unit of the body, though to
a large extent dependent on others, is likewise to a certain extent
independent or autonomous, and has the power of increasing by self-division. I
go one step further, and assume that each unit casts off free gemmules which
are dispersed throughout the system, and are capable under proper conditions
of being developed into similar units. Nor can this assumption be considered
as gratuitous and improbable. It is manifest that the sexual elements and buds
include formative matter of some kind, capable of development; and we now know
from the production of graft-hybrids that similar matter is dispersed
throughout the tissues of plants, and can combine with that of another and
distinct plant, giving rise to a new being, intermediate in character. We know
also that the male element can act directly on the partially developed tissues
of the mother-plant, and on the future progeny of female animals. The
formative matter which is thus dispersed throughout the tissues of plants, and
which is capable of being developed into each unit or part, must be generated
there by some means; and my chief assumption is that this matter consists of
minute particles or gemmules cast off from each unit or cell. (27/43. Mr.
Lowne has observed ('Journal of Queckett Microscopical Club' September 23,
1870) certain remarkable changes in the tissues of the larva of a fly, which
makes him believe "it possible that organs and organisms are sometimes
developed by the aggregation of excessively minute gemmules, such as those
which Mr. Darwin's hypothesis demands.")
But I have further to assume that the gemmules in their undeveloped state are
capable of largely multiplying themselves by self-division, like independent
organisms. Delpino insists that to "admit of multiplication by fissiparity in
corpuscles, analogous to seeds or buds...is repugnant to all analogy." But
this seems a strange objection, as Thuret (27/44. 'Annales des Sc. Nat.' 3rd
series Bot. tome 14 1850 page 244.) has seen the zoospore of an alga divide
itself, and each half germinated. Haeckel divided the segmented ovum of a
siphonophora into many pieces, and these were developed. Nor does the extreme
minuteness of the gemmules, which can hardly differ much in nature from the
lowest and simplest organisms, render it improbable that they should grow and
multiply. A great authority, Dr. Beale (27/45. 'Disease Germs' page 20.), says
"that minute yeast cells are capable of throwing off buds or gemmules, much
less than the 1/100000 of an inch in diameter;" and these he thinks are
"capable of subdivision practically ad infinitum."
A particle of small-pox matter, so minute as to be borne by the wind, must
multiply itself many thousandfold in a person thus inoculated; and so with the
contagious matter of scarlet fever. (27/46. See some very interesting papers
on this subject by Dr. Beale in 'Medical Times and Gazette' September 9, 1865
pages 273, 330.) It has recently been ascertained (27/47. Third Report of the
R. Comm. on the Cattle Plague as quoted in 'Gardener's Chronicle' 1866 page
446.) 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 will 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 will appear still more improbable; but here
again we should remember that many rudimentary and useless organs 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 ovule, and within each
spermatozoon or pollen-grain, and as some animals and plants produce an
astonishing number of pollen-grains and ovules (27/48. Mr. F. Buckland found
6,867,840 eggs in a cod-fish ('Land and Water' 1868 page 62). An Ascaris
produces about 64,000,000 eggs (Carpenter's 'Comp. Phys.' 1854 page 590). Mr.
J. Scott, of the Royal Botanic Garden of Edinburgh, calculated, in the same
manner as I have done for some British Orchids ('Fertilisation of Orchids'
page 344), the number of seeds in a capsule of an Acropera and found the
number to be 371,250. Now this plant produces several flowers on a raceme, and
many racemes during a season. In an allied genus, Gongora, Mr. Scott has seen
twenty capsules produced on a single raceme; ten such racemes on the Acropera
would yield above seventy-four millions of seed.), the number and minuteness
of the gemmules must be something inconceivable. But considering how minute
the molecules are, and how many go to the formation of the smallest granule of
any ordinary substance, this difficulty with respect to the gemmules is not
insuperable. From the data arrived at by Sir W. Thomson, my son George finds
that a cube of 1/10000 of an inch of glass or water must consist of between 16
million millions, and 131 thousand million million molecules. No doubt the
molecules of which an organism is formed are larger, from being more complex,
than those of an inorganic substance, and probably many molecules go to the
formation of a gemmule; but when we bear in mind that a cube of 1/10000 of an
inch is much smaller than any pollen-grain, ovule or bud, we can see what a
vast number of gemmules one of these bodies might contain.
The gemmules derived from each part or organ must be thoroughly dispersed
throughout the whole system. We know, for instance, that even a minute
fragment of a leaf of a Begonia will reproduce the whole plant; and that if a
fresh-water worm is chopped into small pieces, each will reproduce the whole
animal. Considering also the minuteness of the gemmules and the permeability
of all organic tissues, the thorough dispersion of the gemmules is not
surprising. That matter may be readily transferred without the aid of vessels
from part to part of the body, we have a good instance in a case recorded by
Sir J. Paget of a lady, whose hair lost its colour at each successive attack
of neuralgia and recovered it again in the course of a few days. With plants,
however, and probably with compound animals, such as corals, the gemmules do
not ordinarily spread from bud to bud, but are confined to the parts developed
from each separate bud; and of this fact no explanation can be given.
The assumed elective affinity of each gemmule for that particular cell which
precedes it in due order of development is supported by many analogies. In all
ordinary cases of sexual reproduction, the male and female elements certainly
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 formative matter of each part contained within the ovules
and the male element act on each other by some law of special affinity, so
that corresponding parts affect one another; thus, a calf produced from a
short-horned cow by a long-horned bull has its horns affected by the union of
the two forms, and the offspring from two birds with differently coloured
tails have their tails affected.
The various tissues of the body plainly show, as many physiologists have
insisted (27/49. Paget 'Lectures on Pathology' page 27; Virchow 'Cellular
Pathology' translated by Dr. Chance pages 123, 126, 294. Claude Bernard 'Des
Tissus Vivants' pages 177, 210, 337; Muller 'Physiology' English translation
page 290.), 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 curare affecting certain nerves; Lytta vesicatoria the
kidneys; and the poisonous matter of various diseases, as small-pox, scarlet-
fever, hooping-cough, glanders, and hydrophobia, affecting certain definite
parts of the body. 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 precedes it in due order of growth. That
the formative matter within the pollen of plants, which by our hypothesis
consists of gemmules, can unite with and modify the partially developed cells
of the mother-plant, we have clearly seen in the section devoted to this
subject. As the tissues of plants are formed, as far as is known, only by the
proliferation of pre-existing cells, we must conclude that the gemmules
derived from the foreign pollen do not become developed into new and separate
cells, but penetrate and modify the nascent cells of the mother-plant. This
process may be compared with what takes place in the act of ordinary
fertilisation, during which the contents of the pollen-tubes penetrate the
closed embryonic sac within the ovule, and determine the development of the
embryo. According to this view, the cells of the mother-plant may almost
literally be said to be fertilised by the gemmules derived from the foreign
pollen. In this case and in all others the proper gemmules must combine in due
order with pre-existing nascent cells, owing to their elective affinities. A
slight difference in nature between the gemmules and the nascent cells would
be far from interfering with their mutual union and development, for we well
know in the case of ordinary reproduction that such slight differentiation in
the sexual elements favours in a marked manner their union and subsequent
development, as well as the vigour of the offspring thus produced.
Thus far we have been able by the aid of our hypothesis to throw some obscure
light on the problems which have come before us; but it must be confessed that
many points remain altogether doubtful. Thus it is useless to speculate at
what period of development each unit of the body casts off its gemmules, as
the whole subject of the development of the various tissues is as yet far from
clear. We do not know whether the gemmules are merely collected by some
unknown means at certain seasons within the reproductive organs, or whether
after being thus collected they rapidly multiply there, as the flow of blood
to these organs at each breeding season seems to render probable. Nor do we
know why the gemmules collect to form buds in certain definite places, leading
to the symmetrical growth of trees and corals. We have no means of deciding
whether the ordinary wear and tear of the tissues is made good by means of
gemmules, or merely by the proliferation of pre-existing cells. If the
gemmules are thus consumed, as seems probable from the intimate connection
between the repair of waste, regrowth, and development, and more especially
from the periodical changes which many male animals undergo in colour and
structure, then some light would be thrown on the phenomena of old age, with
its lessened power of reproduction and of the repair of injuries, and on the
obscure subject of longevity. The fact of castrated animals, which do not cast
off innumerable gemmules in the act of reproduction, not being longer-lived
than perfect males, seems opposed to the belief that gemmules are consumed in
the ordinary repair of wasted tissues; unless indeed the gemmules after being
collected in small numbers within the reproductive organs are there largely
multiplied. (27/50. Prof. Ray Lankester has discussed several of the points
here referred to as bearing on pangenesis, in his interesting essay, 'On
Comparative Longevity in Man and the Lower Animals' 1870 pages 33, 77, etc.)
That the same cells or units may live for a long period and continue
multiplying without being modified by their union with free gemmules of any
kind, is probable from such cases as that of the spur of a cock which grew to
an enormous size when grafted into the ear of an ox. How far units are
modified during their normal growth by absorbing peculiar nutriment from the
surrounding tissues, independently of their union with gemmules of a distinct
nature, is another doubtful point. (27/51. Dr. Ross refers to this subject in
his 'Graft Theory of Disease' 1872 page 53.) We shall appreciate this
difficulty by calling to mind what complex yet symmetrical growths the cells
of plants yield when inoculated by the poison of a gall-insect. With animals
various polypoid excrescences and tumours are generally admitted (27/52.
Virchow 'Cellular Pathology' translated by Dr. Chance 1860 pages 60, 162, 245,
441, 454.) 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 (27/53. Ibid pages 412-426.), a whole
series of permutations and substitutions. "The cartilage cells may be
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.
We have good reason to believe that several gemmules are requisite for the
development of one and the same unit or cell; for we cannot otherwise
understand the insufficiency of a single or even of two or three pollen-grains
or spermatozoa. But we are far from knowing whether the gemmules of all the
units are free and separate from one another, 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 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 flowers, which are sometimes highly complex
structures, with each ridge and hollow contrived for a special purpose, so
that each part must have been separately modified, and the modifications
transmitted; consequently, separate gemmules, according to our hypothesis,
must have been thrown off from each cell or unit. But, as we sometimes see
half an anther or a small portion of a filament becoming petali-form, 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 free and separate. Even in so simple
a case as that of a perfect cell, with its protoplasmic contents, nucleus,
nucleolus, and walls, we do not know whether or not its development depends on
a compound gemmule derived from each part. (27/54. See some good criticisms on
this head by Delpino and by Mr. G.H. Lewes in the 'Fortnightly Review'
November 1, 1868 page 509.)
Having now endeavoured to show that the several foregoing assumptions are to a
certain extent supported by analogous facts, and having alluded to 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 one another 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 reversions of the same kind; and as,
according to our hypothesis, all the forms of reproduction depend on the
aggregation of gemmules derived from the whole body, we can understand this
remarkable agreement. Parthenogenesis is no longer wonderful, and if we did
not know that great good followed from the union of the sexual elements
derived from two distinct individuals, the wonder would be that
parthenogenesis did not occur much oftener than it does. On any ordinary
theory of reproduction the formation of graft-hybrids, and the action of the
male element on the tissues of the mother-plant, as well as on the future
progeny of female animals, are great anomalies; but they are intelligible on
our hypothesis. The reproductive organs do not actually create the sexual
elements; they merely determine the aggregation and perhaps the multiplication
of the gemmules in a special manner. These organs, however, together with
their accessory parts, have high functions to perform. They adapt one or both
elements for independent temporary existence, and for mutual union. The
stigmatic secretion acts on the pollen of a plant of the same species in a
wholly different manner to what it does on the pollen of one belonging to a
distinct genus or family. The spermatophores of the Cephalopoda are
wonderfully complex structures, which were formerly mistaken for parasitic
worms; and the spermatozoa of some animals possess attributes which, if
observed in an independent animal, would be put down to instinct guided by
sense-organs,--as when the spermatozoa of an insect find their way into the
minute micropyle of the egg.
The antagonism which has long been observed (27/55. Mr. Herbert Spencer
('Principles of Biology' volume 2 page 430) has fully discussed this
antagonism.), with certain exceptions, between growth and the power of sexual
reproduction (27/56. The male salmon is known to breed at a very early age.
The Triton and Siredon, whilst retaining their larval branchiae, according to
Filippi and Dumeril ('Annals and Mag. of Nat. Hist.' 3rd series 1866 page 157)
are capable of reproduction. Ernst Haeckel has recently ('Monatsbericht Akad.
Wiss. Berlin' February 2, 1865) observed the surprising case of a medusa, with
its reproductive organs active, which produces by budding a widely different
form of medusa; and this latter also has the power of sexual reproduction.
Krohn has shown ('Annals and Mag. of Nat. Hist.' 3rd series volume 19 1862
page 6) that certain other medusae, whilst sexually mature, propagate by
gemmae. See also Kolliker 'Morphologie und Entwickelungsgeschichte des
Pennatulidenstammes' 1872 page 12.)--between the repair of injuries and
gemmation--and with plants, between rapid increase by buds, rhizomes, etc.,
and the production of seed, is partly explained by the gemmules not existing
in sufficient numbers for these processes to be carried on simultaneously.
Hardly any fact in physiology is more wonderful than the power of regrowth;
for instance, that a snail should be able to reproduce its head, or a
salamander its eyes, tail, and legs, exactly at the points where they have
been cut off. Such cases are explained by the presence of gemmules derived
from each part, and disseminated throughout the body. I have heard the process
compared with that of the repair of the broken angles of a crystal by re-
crystallisation; 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. But we have
here to encounter two objections which apply not only to the regrowth of a
part, or of a bisected individual, but to fissiparous generation and budding.
The first objection is that the part which is reproduced is in the same stage
of development as that of the being which has been operated on or bisected;
and in the case of buds, that the new beings thus produced are in the same
stage as that of the budding parent. Thus a mature salamander, of which the
tail has been cut off, does not reproduce a larval tail; and a crab does not
reproduce a larval leg. In the case of budding it was shown in the first part
of this chapter that the new being thus produced does not retrograde in
development,--that is, does not pass through those earlier stages, which the
fertilised germ has to pass through. Nevertheless, the organisms operated on
or multiplying themselves by buds must, by our hypothesis, include innumerable
gemmules derived from every part or unit of the earlier stages of development;
and why do not such gemmules reproduce the amputated part or the whole body at
a corresponding early stage of development?
The second objection, which has been insisted on by Delpino, is that the
tissues, for instance, of a mature salamander or crab, of which a limb has
been removed, are already differentiated and have passed through their whole
course of development; and how can such tissues in accordance with our
hypothesis attract and combine with the gemmules of the part which is to be
reproduced? In answer to these two objections we must bear in mind the
evidence which has been advanced, showing that at least in a large number of
cases the power of regrowth is a localised faculty, acquired for the sake of
repairing special injuries to which each particular creature is liable; and in
the case of buds or fissiparous generation, for the sake of quickly
multiplying the organism at a period of life when it can be supported in large
numbers. These considerations lead us to believe that in all such cases a
stock of nascent cells or of partially developed gemmules are retained for
this special purpose either locally or throughout the body, ready to combine
with the gemmules derived from the cells which come next in due succession. If
this be admitted we have a sufficient answer to the above two objections.
Anyhow, pangenesis seems to throw a considerable amount of light on the
wonderful power of regrowth.
It follows, also, from the view just given, that the sexual elements differ
from buds in not including nascent cells or gemmules in a somewhat advanced
stage of development, so that only the gemmules belonging to the earliest
stages are first developed. As young animals and those which stand low in the
scale generally have a much greater capacity for regrowth than older and
higher animals, it would also appear that they retain cells in a nascent
state, or partially developed gemmules, more readily than do animals which
have already passed through a long series of developmental changes. I may here
add that although ovules can be detected in most or all female animals at an
extremely early age, there is no reason to doubt that gemmules derived from
parts modified during maturity can pass into the ovules.
With respect to hybridism, pangenesis agrees well with most of the ascertained
facts. We must believe, as previously shown, that several gemmules are
requisite for the development of each cell or unit. But from the occurrence of
parthenogenesis, more especially from those cases in which an embryo is only
partially formed, we may infer that the female element generally includes
gemmules in nearly sufficient number for independent development, so that when
united with the male element the gemmules are superabundant. Now, when two
species or races are crossed reciprocally, the offspring do not commonly
differ, and this shows that the sexual elements agree in power, in accordance
with the view that both include the same gemmules. Hybrids and mongrels are
also generally intermediate in character between the two parent-forms, yet
occasionally they closely resemble one parent in one part and the other parent
in another part, or even in their whole structure: nor is this difficult to
understand on the admission that the gemmules in the fertilised germ are
superabundant in number, and that those derived from one parent may have some
advantage in number, affinity, or vigour over those derived from the other
parent. Crossed forms sometimes exhibit the colour or other characters of
either parent in stripes or blotches; and this occurs in the first generation,
or through reversion in succeeding bud and seminal generations, of which fact
several instances were given in the eleventh chapter. In these cases we must
follow Naudin (27/57. See his excellent discussion on this subject in
'Nouvelles Archives du Museum' tome 1 page 151.) and admit that the "essence"
or "element" of the two species,--terms which I should translate into the
gemmules,--have an affinity for their own kind, and thus separate themselves
into distinct stripes or blotches; and reasons were given, when discussing in
the fifteenth chapter the incompatibility of certain characters to unite, for
believing in such mutual affinity. When two forms are crossed, one is not
rarely found to be prepotent in the transmission of its characters over the
other; and this we can explain by again assuming that the one form has some
advantage over the other in the number, vigour, or affinity of its gemmules.
In some cases, however, certain characters are present in the one form and
latent in the other; for instance, there is a latent tendency in all pigeons
to become blue, and, when a blue pigeon is crossed with one of any other
colour, the blue tint is generally prepotent. The explanation of this form of
prepotency will be obvious when we come to the consideration of Reversion.
When two distinct species are crossed, it is notorious that they do not yield
the full or proper number of offspring; and we can only say on this head that,
as the development of each organism depends on such nicely-balanced affinities
between a host of gemmules and nascent cells, we need not feel at all
surprised that the commixture of gemmules derived from two distinct species
should lead to partial or complete failure of development. With respect to the
sterility of hybrids produced from the union of two distinct species, it was
shown in the nineteenth chapter that this depends exclusively on the
reproductive organs being specially affected; but why these organs should be
thus affected we do not know, any more than why unnatural conditions of life,
though compatible with health, should cause sterility; or why continued close
interbreeding, or the illegitimate unions of heterostyled plants, induce the
same result. The conclusion that the reproductive organs alone are affected,
and not the whole organisation, agrees perfectly with the unimpaired or even
increased capacity in hybrid plants for propagation by buds; for this implies,
according to our hypothesis, that the cells of the hybrids throw off
hybridised gemmules, which become aggregated into buds, but fail to become
aggregated within the reproductive organs, so as to form the sexual elements.
In a similar manner many plants, when placed under unnatural conditions, fail
to produce seed, but can readily be propagated by buds. We shall presently see
that pangenesis agrees well with the strong tendency to reversion exhibited by
all crossed animals and plants.
Each organism reaches maturity through a longer or shorter course of growth
and development: the former term being confined to mere increase of size, and
development to changed structure. The changes may be small and insensibly
slow, as when a child grows into a man, or many, abrupt, and slight, as in the
metamorphoses of certain ephemerous insects, or, again, few and strongly-
marked, as with most other insects. Each newly formed part may be moulded
within a previously existing and corresponding part, and in this case it will
appear, falsely as I believe, to be developed from the old part; or it may be
formed within a distinct part of the body, as in the extreme cases of
metagenesis. An eye, for instance, may be developed at a spot where no eye
previously existed. We have also seen that allied organic beings in the course
of their metamorphoses sometimes attain nearly the same structure after
passing through widely different forms; or conversely, after passing through
nearly the same early forms, arrive at widely different mature forms. In these
cases it is very difficult to accept the common view that the first-formed
cells or units possess the inherent power, independently of any external
agency, of producing new structures wholly different in form, position, and
function. But all these cases become plain on the hypothesis of pangenesis.
The units, during each stage of development, throw off gemmules, which,
multiplying, are transmitted to the offspring. In the offspring, as soon as
any particular cell or unit becomes partially developed, it unites with (or,
to speak metaphorically, is fertilised by) the gemmule of the next succeeding
cell, and so onwards. But organisms have often been subjected to changed
conditions of life at a certain stage of their development, and in consequence
have been slightly modified; and the gemmules cast off from such modified
parts will tend to reproduce parts modified in the same manner. This process
may be repeated until the structure of the part becomes greatly changed at one
particular stage of development, but this will not necessarily affect other
parts, whether previously or subsequently formed. In this manner we can
understand the remarkable independence of structure in the successive
metamorphoses, and especially in the successive metageneses of many animals.
In the case, however, of diseases which supervene during old age, subsequently
to the ordinary period of procreation, and which, nevertheless, are sometimes
inherited, as occurs with brain and heart complaints, we must suppose that the
organs were affected at an early age and threw off at this period affected
gemmules; but that the affection became visible or injurious only after the
prolonged growth, in the strict sense of the word, of the part. In all the
changes of structure which regularly supervene during old age, we probably see
the effects of deteriorated growth, and not of true development.
The principle of the independent formation of each part, owing to the union of
the proper gemmules with certain nascent cells, together with the
superabundance of the gemmules derived from both parents, and the subsequent
self-multiplication of the gemmules, throws light on a widely different group
of facts, which on any ordinary view of development appears very strange. I
allude to organs which are abnormally transposed or multiplied. For instance,
a curious case has been recorded by Dr. Elliott Coues (27/58. 'Proc. Boston
Soc. of Nat. Hist.' republished in 'Scientific Opinion' November 10, 1869 page
488.) of a monstrous chicken with a perfect additional RIGHT leg articulated
to the LEFT side of the pelvis. Gold-fish often have supernumerary fins placed
on various parts of their bodies. When the tail of a lizard is broken off, a
double tail is sometimes reproduced; and when the foot of the salamander was
divided longitudinally by Bonnet, additional digits were occasionally formed.
Valentin injured the caudal extremity of an embryo, and three days afterwards
it produced rudiments of a double pelvis and of double hind-limbs. (27/59.
Todd 'Cyclop. of Anat. and Phys.' volume 4 1849-52 page 975.) When frogs,
toads, etc., are born with their limbs doubled, as sometimes happens, the
doubling, as Gervais remarks (27/60. 'Compte Rendus' November 14, 1865 page
800.), cannot be due to the complete fusion of two embryos, with the exception
of the limbs, for the larvae are limbless. The same argument is applicable
(27/61. As previously remarked by Quatrefages in his 'Metamorphoses de
l'Homme' etc. 1862 page 129.) to certain insects produced with multiple legs
or antennae, for these are metamorphosed from apodal or antennae-less larvae.
Alphonse Milne-Edwards (27/62. Gunther 'Zoological Record' 1864 page 279.) has
described the curious case of a crustacean in which one eye-peduncle
supported, instead of a complete eye, only an imperfect cornea, and out of the
centre of this a portion of an antenna was developed. A case has been recorded
(27/63. Sedgwick 'Medico-Chirurg. Review' April 1863 page 454.) of a man who
had during both dentitions a double tooth in place of the left second incisor,
and he inherited this peculiarity from his paternal grandfather. Several cases
are known (27/64. Isid. Geoffroy Saint-Hilaire 'Hist. des Anomalies' tome 1
1832 pages 435, 657; and tome 2 page 560.) of additional teeth having been
developed in the orbit of the eye, and, more especially with horses, in the
palate. Hairs occasionally appear in strange situations, as "within the
substance of the brain." (27/65. Virchow 'Cellular Pathology' 1860 page 66.)
Certain breeds of sheep bear a whole crowd of horns on their foreheads. As
many as five spurs have been seen on both legs of certain Game-fowls. In the
Polish fowl the male is ornamented with a topknot of hackles like those on his
neck, whilst the female has a top-knot formed of common feathers. In feather-
footed pigeons and fowls, feathers like those on the wing arise from the outer
side of the legs and toes. Even the elemental parts of the same feather may be
transposed; for in the Sebastopol goose, barbules are developed on the divided
filaments of the shaft. Imperfect nails sometimes appear on the stumps of the
amputated fingers of man (27/66. Muller 'Phys.' English Translation volume 1
1833 page 407. A case of this kind has lately been communicated to me.) and it
is an interesting fact that with the snake-like Saurians, which present a
series with more and more imperfect limbs, the terminations of the phalanges
first disappear, "the nails becoming transferred to their proximal remnants,
or even to parts which are not phalanges." (27/67. Dr. Furbringer 'Die Knochen
etc. bei den schlangenahnlichen Sauriern' as reviewed in 'Journal of Anat. and
Phys.' May 1870 page 286.)
Analogous cases are of such frequent occurrence with plants that they do not
strike us with sufficient surprise. Supernumerary petals, stamens, and
pistils, are often produced. I have seen a leaflet low down in the compound
leaf of Vicia sativa replaced by a tendril; and a tendril possesses many
peculiar properties, such as spontaneous movement and irritability. The calyx
sometimes assumes, either wholly or by stripes, the colour and texture of the
corolla. Stamens are so frequently converted into petals, more or less
completely, that such cases are passed over as not deserving notice; but as
petals have special functions to perform, namely, to protect the included
organs, to attract insects, and in not a few cases to guide their entrance by
well-adapted contrivances, we can hardly account for the conversion of stamens
into petals merely by unnatural or superfluous nourishment. Again, the edge of
a petal may occasionally be found including one of the highest products of the
plant, namely, pollen; for instance, I have seen the pollen-mass of an Ophrys,
which is a very complex structure, developed in the edge of an upper petal.
The segments of the calyx of the common pea have been observed partially
converted into carpels, including ovules, and with their tips converted into
stigmas. Mr. Salter and Dr. Maxwell Masters have found pollen within the
ovules of the passion-flower and of the rose. Buds may be developed in the
most unnatural positions, as on the petal of a flower. Numerous analogous
facts could be given. (27/68. Moquin-Tandon 'Teratologie Veg.' 1841 pages 218,
220, 353. For the case of the pea see 'Gardener's Chronicle' 1866 page 897.
With respect to pollen within ovules see Dr. Masters in 'Science Review'
October 1873 page 369. The Rev. J.M. Berkeley describes a bud developed on a
petal of a Clarkia in 'Gardener's Chronicle' April 28, 1866.)
I do not know how physiologists look at such facts as the foregoing. According
to the doctrine of pangenesis, the gemmules of the transposed organs become
developed in the wrong place, from uniting with wrong cells or aggregates of
cells during their nascent state; and this would follow from a slight
modification in their elective affinities. Nor ought we to feel much surprise
at the affinities of cells and gemmules varying, when we remember the many
curious cases given in the seventeenth chapter, of plants which absolutely
refuse to be fertilised by their own pollen, though abundantly fertile with
that of any other individual of the same species, and in some cases only with
that of a distinct species. It is manifest that the sexual elective affinities
of such plants--to use the term employed by Gartner--have been modified. As
the cells of adjoining or homologous parts will have nearly the same nature,
they will be particularly liable to acquire by variation each other's elective
affinities; and we can thus understand to a certain extent such cases as a
crowd of horns on the heads of certain sheep, of several spurs on the legs of
fowls, hackle-like feathers on the heads of the males of other fowls, and with
the pigeon wing-like feathers on their legs and membrane between their toes,
for the leg is the homologue of the wing. As all the organs of plants are
homologous and spring from a common axis, it is natural that they should be
eminently liable to transposition. It ought to be observed that when any
compound part, such as an additional limb or an antenna, springs from a false
position, it is only necessary that the few first gemmules should be wrongly
attached; for these whilst developing would attract other gemmules in due
succession, as in the regrowth of an amputated limb. When parts which are
homologous and similar in structure, as the vertebrae of snakes or the stamens
of polyandrous flowers, etc., are repeated many times in the same organism,
closely allied gemmules must be extremely numerous, as well as the points to
which they ought to become united; and, in accordance with the foregoing
views, we can to a certain extent understand Isid. Geoffroy Saint-Hilaire's
law, that parts, which are already multiple, are extremely liable to vary in
number.
Variability often depends, as I have attempted to show, on the reproductive
organs being injuriously affected by changed conditions; and in this case the
gemmules derived from the various parts of the body are probably aggregated in
an irregular manner, some superfluous and others deficient. Whether a
superabundance of gemmules would lead to the increased size of any part cannot
be told; but we can see that their partial deficiency, without necessarily
leading to the entire abortion of the part, might cause considerable
modifications; for in the same manner as plants, if their own pollen be
excluded, are easily hybridised, so, in the case of cells, if the properly
succeeding gemmules were absent, they would probably combine easily with other
and allied gemmules, as we have just seen with transposed parts.
In variations caused by the direct action of changed conditions, of which
several instances have been given, certain parts of the body are directly
affected by the new conditions, and consequently throw off modified gemmules,
which are transmitted to the offspring. On any ordinary view it is
unintelligible how changed conditions, whether acting on the embryo, the young
or the adult, can cause inherited modifications. It is equally or even more
unintelligible on any ordinary view, how the effects of the long-continued use
or disuse of a part, or of changed habits of body or mind, can be inherited. A
more perplexing problem can hardly be proposed; but on our view we have only
to suppose that certain cells become at last structurally modified; and that
these throw off similarly modified gemmules. This may occur at any period of
development, and the modification will be inherited at a corresponding period;
for the modified gemmules will unite in all ordinary cases with the proper
preceding cells, and will consequently be developed at the same period at
which the modification first arose. With respect to mental habits or
instincts, we are so profoundly ignorant of the relation between the brain and
the power of thought that we do not know positively whether a fixed habit
induces any change in the nervous system, though this seems highly probable;
but when such habit or other mental attribute, or insanity, is inherited, we
must believe that some actual modification is transmitted (27/69. See some
remarks to this effect by Sir H. Holland in his 'Medical Notes' 1839 page
32.); and this implies, according to our hypothesis, that gemmules derived
from modified nerve-cells are transmitted to the offspring.
It is generally necessary that an organism should be exposed during several
generations to changed conditions or habits, in order that any modification
thus acquired should appear in the offspring. This may be partly due to the
changes not being at first marked enough to catch attention, but this
explanation is insufficient; and I can account for the fact only by the
assumption, which we shall see under the head of reversion is strongly
supported, that gemmules derived from each unmodified unit or part are
transmitted in large numbers to successive generations, and that the gemmules
derived from the same unit after it has been modified go on multiplying under
the same favourable conditions which first caused the modification, until at
last they become sufficiently numerous to overpower and supplant the old
gemmules.
A difficulty may be here noticed; we have seen that there is an important
difference in the frequency, though not in the nature, of the variations in
plants propagated by sexual and asexual generation. As far as variability
depends on the imperfect action of the reproductive organs under changed
conditions, we can at once see why plants propagated asexually should be far
less variable than those propagated sexually. With respect to the direct
action of changed conditions, we know that organisms produced from buds do not
pass through the earlier phases of development; they will therefore not be
exposed, at that period of life when structure is most readily modified, to
the various causes inducing variability in the same manner as are embryos and
young larval forms; but whether this is a sufficient explanation I know not.
With respect to variations due to reversion, there is a similar difference
between plants propagated from buds and seeds. Many varieties can be
propagated securely by buds, but generally or invariably revert to their
parent-forms by seed. So, also, hybridised plants can be multiplied to any
extent by buds, but are continually liable to reversion by seed,--that is, to
the loss of their hybrid or intermediate character. I can offer no
satisfactory explanation of these facts. Plants with variegated leaves,
phloxes with striped flowers, barberries with seedless fruit, can all be
securely propagated by buds taken from the stem or branches; but buds from the
roots of these plants almost invariably lose their character and revert to
their former condition. This latter fact is also inexplicable, unless buds
developed from the roots are as distinct from those on the stem, as is one bud
on the stem from another, and we know that these latter behave like
independent organisms.
Finally, we see that on the hypothesis of pangenesis variability depends on at
least two distinct groups of causes. Firstly, the deficiency, superabundance,
and transposition of gemmules, and the redevelopment of those which have long
been dormant; the gemmules themselves not having undergone any modification;
and such changes will amply account for much fluctuating variability.
Secondly, the direct action of changed conditions on the organisation, and of
the increased use or disuse of parts; and in this case the gemmules from the
modified units will be themselves modified, and, when sufficiently multiplied,
will supplant the old gemmules and be developed into new structures.
Turning now to the laws of Inheritance. If we suppose a homogeneous gelatinous
protozoon to vary and assume a reddish colour, a minute separated particle
would naturally, as it grew to full size, retain the same colour; and we
should have the simplest form of inheritance. (27/70. This is the view taken
by Prof. Hackel in his 'Generelle Morphologie' b. 2 s. 171, who says:
"Lediglich die partielle Identitat der specifisch constituirten Materie im
elterlichen und im kindlichen Organismus, die Theilung dieser Materie bei der
Fortpflanzung, ist die Ursache der Erblichkeit.") Precisely the same view may
be extended to the infinitely numerous and diversified units of which the
whole body of one of the higher animals is composed; the separated particles
being our gemmules. We have already sufficiently discussed by implication, the
important principle of inheritance at corresponding ages. Inheritance as
limited by sex and by the season of the year (for instance with animals
becoming white in winter) is intelligible if we may believe that the elective
affinities of the units of the body are slightly different in the two sexes,
especially at maturity, and in one or both sexes at different seasons, so that
they unite with different gemmules. It should be remembered that, in the
discussion on the abnormal transposition of organs, we have seen reason to
believe that such elective affinities are readily modified. But I shall soon
have to recur to sexual and seasonal inheritance. These several laws are
therefore explicable to a large extent through pangenesis, and on no other
hypothesis which has as yet been advanced.
But it appears at first sight a fatal objection to our hypothesis that a part
or organ may be removed during several successive generations, and if the
operation be not followed by disease, the lost part reappears in the
offspring. Dogs and horses formerly had their tails docked during many
generations without any inherited effect; although, as we have seen, there is
some reason to believe that the tailless condition of certain sheep-dogs is
due to such inheritance. Circumcision has been practised by the Jews from a
remote period, and in most cases the effects of the operation are not visible
in the offspring; though some maintain that an inherited effect does
occasionally appear. If inheritance depends on the presence of disseminated
gemmules derived from all the units of the body, why does not the amputation
or mutilation of a part, especially if effected on both sexes, invariably
affect the offspring? The answer in accordance with our hypothesis probably is
that gemmules multiply and are transmitted during a long series of
generations--as we see in the reappearance of zebrine stripes on the horse--in
the reappearance of muscles and other structures in man which are proper to
his lowly organised progenitors, and in many other such cases. Therefore the
long-continued inheritance of a part which has been removed during many
generations is no real anomaly, for gemmules formerly derived from the part
are multiplied and transmitted from generation to generation.
We have as yet spoken only of the removal of parts, when not followed by
morbid action: but when the operation is thus followed, it is certain that the
deficiency is sometimes inherited. In a former chapter instances were given,
as of a cow, the loss of whose horn was followed by suppuration, and her
calves were destitute of a horn on the same side of their heads. But the
evidence which admits of no doubt is that given by Brown-Sequard with respect
to guinea-pigs, which after their sciatic nerves had been divided, gnawed off
their own gangrenous toes, and the toes of their offspring were deficient in
at least thirteen instances on the corresponding feet. The inheritance of the
lost part in several of these cases is all the more remarkable as only one
parent was affected; but we know that a congenital deficiency is often
transmitted from one parent alone--for instance, the offspring of hornless
cattle of either sex, when crossed with perfect animals, are often hornless.
How, then, in accordance with our hypothesis can we account for mutilations
being sometimes strongly inherited, if they are followed by diseased action?
The answer probably is that all the gemmules of the mutilated or amputated
part are gradually attracted to the diseased surface during the reparative
process, and are there destroyed by the morbid action.
A few words must be added on the complete abortion of organs. When a part
becomes diminished by disuse prolonged during many generations, the principle
of economy of growth, together with intercrossing, will tend to reduce it
still further as previously explained, but this will not account for the
complete or almost complete obliteration of, for instance, a minute papilla of
cellular tissue representing a pistil, or of a microscopically minute nodule
of bone representing a tooth. In certain cases of suppression not yet
completed, in which a rudiment occasionally reappears through reversion,
dispersed gemmules derived from this part must, according to our view, still
exist; we must therefore suppose that the cells, in union with which the
rudiment was formerly developed, fail in their affinity for such gemmules,
except in the occasional cases of reversion. But when the abortion is complete
and final, the gemmules themselves no doubt perish; nor is this in any way
improbable, for, though a vast number of active and long-dormant gemmules are
nourished in each living creature, yet there must be some limit to their
number; and it appears natural that gemmules derived from reduced and useless
parts would be more liable to perish than those freshly derived from other
parts which are still in full functional activity.
The last subject that need be discussed, namely, Reversion, rests on the
principle that transmission and development, though generally acting in
conjunction, are distinct powers; and the transmission of gemmules with their
subsequent development shows us how this is possible. We plainly see the
distinction in the many cases in which a grandfather transmits to his
grandson, through his daughter, characters which she does not, or cannot,
possess. But before proceeding, it will be advisable to say a few words about
latent or dormant characters. Most, or perhaps all, of the secondary
characters, which appertain to one sex, lie dormant in the other sex; that is,
gemmules capable of development into the secondary male sexual characters are
included within the female; and conversely female characters in the male: we
have evidence of this in certain masculine characters, both corporeal and
mental, appearing in the female, when her ovaria are diseased or when they
fail to act from old age. In like manner female characters appear in castrated
males, as in the shape of the horns of the ox, and in the absence of horns in
castrated stags. Even a slight change in the conditions of life due to
confinement sometimes suffices to prevent the development of masculine
characters in male animals, although their reproductive organs are not
permanently injured. In the many cases in which masculine characters are
periodically renewed, these are latent at other seasons; inheritance as
limited by sex and season being here combined. Again, masculine characters
generally lie dormant in male animals until they arrive at the proper age for
reproduction. The curious case formerly given of a Hen which assumed the
masculine characters, not of her own breed but of a remote progenitor,
illustrates the close connection between latent sexual characters and ordinary
reversion.
With those animals and plants which habitually produce several forms, as with
certain butterflies described by Mr. Wallace, in which three female forms and
one male form co-exist, or, as with the trimorphic species of Lythrum and
Oxalis, gemmules capable of reproducing these different forms must be latent
in each individual.
Insects are occasionally produced with one side or one quarter of their bodies
like that of the male, with the other half or three-quarters like that of the
female. In such cases the two sides are sometimes wonderfully different in
structure, and are separated from each other by a sharp line. As gemmules
derived from every part are present in each individual of both sexes, it must
be the elective affinities of the nascent cells which in these cases differ
abnormally on the two sides of the body. Almost the same principle comes into
play with those animals, for instance, certain gasteropods and Verruca amongst
cirripedes, which normally have the two sides of the body constructed on a
very different plan; and yet a nearly equal number of individuals have either
side modified in the same remarkable manner.
Reversion, in the ordinary sense of the word, acts so incessantly, that it
evidently forms an essential part of the general law of inheritance. It occurs
with beings, however propagated, whether by buds or seminal generation, and
sometimes may be observed with advancing age even in the same individual. The
tendency to reversion is often induced by a change of conditions, and in the
plainest manner by crossing. Crossed forms of the first generation are
generally nearly intermediate in character between their two parents; but in
the next generation the offspring commonly revert to one or both of their
grandparents, and occasionally to more remote ancestors. How can we account
for these facts? Each unit in a hybrid must throw off, according to the
doctrine of pangenesis, an abundance of hybridised gemmules, for crossed
plants can be readily and largely propagated by buds; but by the same
hypothesis dormant gemmules derived from both pure parent-forms are likewise
present; and as these gemmules retain their normal condition, they would, it
is probable, be enabled to multiply largely during the lifetime of each
hybrid. Consequently the sexual elements of a hybrid will include both pure
and hybridised gemmules; and when two hybrids pair, the combination of pure
gemmules derived from the one hybrid with the pure gemmules of the same parts
derived from the other, would necessarily lead to complete reversion of
character; and it is, perhaps, not too bold a supposition that unmodified and
undeteriorated gemmules of the same nature would be especially apt to combine.
Pure gemmules in combination with hybridised gemmules would lead to partial
reversion. And lastly, hybridised gemmules derived from both parent-hybrids
would simply reproduce the original hybrid form. (27/71. In these remarks I,
in fact, follow Naudin, who speaks of the elements or essences of the two
species which are crossed. See his excellent memoir in the 'Nouvelles Archives
du Museum' tome 1 page 151.) All these cases and degrees of reversion
incessantly occur.
It was shown in the fifteenth chapter that certain characters are antagonistic
to each other or do not readily blend; hence, when two animals with
antagonistic characters are crossed, it might well happen that a sufficiency
of gemmules in the male alone for the reproduction of his peculiar characters,
and in the female alone for the reproduction of her peculiar characters, would
not be present; and in this case dormant gemmules derived from the same part
in some remote progenitor might easily gain the ascendancy, and cause the
reappearance of the long-lost character. For instance, when black and white
pigeons, or black and white fowls, are crossed,--colours which do not readily
blend,--blue plumage in the one case, evidently derived from the rock-pigeon,
and red plumage in the other case, derived from the wild jungle-cock,
occasionally reappear. With uncrossed breeds the same result follows, under
conditions which favour the multiplication and development of certain dormant
gemmules, as when animals become feral and revert to their pristine character.
A certain number of gemmules being requisite for the development of each
character, as is known to be the case from several spermatozoa or pollen-
grains being necessary for fertilisation, and time favouring their
multiplication, will perhaps account for the curious cases, insisted on by Mr.
Sedgwick, of certain diseases which regularly appear in alternate generations.
This likewise holds good, more or less strictly, with other weakly inherited
modifications. Hence, as I have heard it remarked, certain diseases appear to
gain strength by the intermission of a generation. The transmission of dormant
gemmules during many successive generations is hardly in itself more
improbable, as previously remarked, than the retention during many ages of
rudimentary organs, or even only of a tendency to the production of a
rudiment; but there is no reason to suppose that dormant gemmules can be
transmitted and propagated for ever. Excessively minute and numerous as they
are believed to be, an infinite number derived, during a long course of
modification and descent, from each unit of each progenitor, could not be
supported or nourished by the organism. But it does not seem improbable that
certain gemmules, under favourable conditions, should be retained and go on
multiplying for a much longer period than others. Finally, on the view here
given, we certainly gain some insight into the wonderful fact that the child
may depart from the type of both its parents, and resemble its grandparents,
or ancestors removed by many hundreds of generations.
CONCLUSION.
The hypothesis of Pangenesis, as applied to the several great classes of facts
just discussed, no doubt is extremely complex, but so are the facts. The chief
assumption is that all the units of the body, besides having the universally
admitted power of growing by self-division, throw off minute gemmules which
are dispersed through the system. Nor can this assumption be considered as too
bold, for we know from the cases of graft-hybridisation that formative matter
of some kind is present in the tissues of plants, which is capable of
combining with that included in another individual, and of reproducing every
unit of the whole organism. But we have further to assume that the gemmules
grow, multiply, and aggregate themselves into buds and the sexual elements;
their development depending on their union with other nascent cells or units.
They are also believed to be capable of transmission in a dormant state, like
seeds in the ground, to successive generations.
In a highly-organised animal, the gemmules thrown off from each different unit
throughout the body must be inconceivably numerous and minute. Each unit of
each part, as it changes during development, and we know that some insects
undergo at least twenty metamorphoses, must throw off its gemmules. But the
same cells may long continue to increase by self-division, and even become
modified by absorbing peculiar nutriment, without necessarily throwing off
modified gemmules. All organic beings, moreover, include many dormant gemmules
derived from their grandparents and more remote progenitors, but not from all
their progenitors. These almost infinitely numerous and minute gemmules are
contained within each bud, ovule, spermatozoon, and pollen-grain. Such an
admission will be declared impossible; but number and size are only relative
difficulties. Independent organisms exist which are barely visible under the
highest powers of the microscope, and their germs must be excessively minute.
Particles of infectious matter, so small as to be wafted by the wind or to
adhere to smooth paper, will multiply so rapidly as to infect within a short
time the whole body of a large animal. We should also reflect on the admitted
number and minuteness of the molecules composing a particle of ordinary
matter. The difficulty, therefore, which at first appears insurmountable, of
believing in the existence of gemmules so numerous and small as they must be
according to our hypothesis, has no great weight.
The units of the body are generally admitted by physiologists to be
autonomous. I go one step further and assume that they throw off reproductive
gemmules. Thus an organism does not generate its kind as a whole, but each
separate unit generates its kind. It has often been said by naturalists that
each cell of a plant has the potential capacity of reproducing the whole
plant; but it has this power only in virtue of containing gemmules derived
from every part. When a cell or unit is from some cause modified, the gemmules
derived from it will be in like manner modified. If our hypothesis be
provisionally accepted, we must look at all the forms of asexual reproduction,
whether occurring at maturity or during youth, as fundamentally the same, and
dependent on the mutual aggregation and multiplication of the gemmules. The
regrowth of an amputated limb and the healing of a wound is the same process
partially carried out. Buds apparently include nascent cells, belonging to
that stage of development at which the budding occurs, and these cells are
ready to unite with the gemmules derived from the next succeeding cells. The
sexual elements, on the other hand, do not include such nascent cells; and the
male and female elements taken separately do not contain a sufficient number
of gemmules for independent development, except in the cases of
parthenogenesis. The development of each being, including all the forms of
metamorphosis and metagenesis, depends on the presence of gemmules thrown off
at each period of life, and on their development, at a corresponding period,
in union with preceding cells. Such cells may be said to be fertilised by the
gemmules which come next in due order of development. Thus the act of ordinary
impregnation and the development of each part in each being are closely
analogous processes. The child, strictly speaking, does not grow into the man,
but includes germs which slowly and successively become developed and form the
man. In the child, as well as in the adult, each part generates the same part.
Inheritance must be looked at as merely a form of growth, like the self-
division of a lowly-organised unicellular organism. Reversion depends on the
transmission from the forefather to his descendants of dormant gemmules, which
occasionally become developed under certain known or unknown conditions. Each
animal and plant may be compared with a bed of soil full of seeds, some of
which soon germinate, some lie dormant for a period, whilst others perish.
When we hear it said that a man carries in his constitution the seeds of an
inherited disease, there is much truth in the expression. No other attempt, as
far as I am aware, has been made, imperfect as this confessedly is, to connect
under one point of view these several grand classes of facts. An organic being
is a microcosm--a little universe, formed of a host of self-propagating
organisms, inconceivably minute and numerous as the stars in heaven.