SPENCER AS BIOLOGIST: INDUCTIONS OF BIOLOGY
_Growth--Development--Structure and Function--Waste and
Repair--Adaptation--Cell-Life--Genesis--Nutrition and
Reproduction--The Germ-Cells_
_Growth._--Perhaps the widest and most familiar induction of Biology, is
that organisms grow. But there is growth in crystals, in terrestrial
deposits, in celestial bodies; in fact, growth, as being an integration
of matter, is the primary trait of evolution; it is universal, in the
sense that all aggregates display it in some way at some period. "The
essential community of nature between organic growth and inorganic
growth is, however, most clearly seen on observing that they both result
in the same way. The segregation of different kinds of detritus from
each other, as well as from the water carrying them, and their
aggregation into distinct strata, is but an instance of a universal
tendency towards the union of like units and the parting of unlike units
(_First Principles_, Sec. 163). The deposit of a crystal from a solution is
a differentiation of the previously mixed molecules; and an integration
of one class of molecules into a solid body, and the other class into a
liquid solvent. Is not the growth of an organism an essentially similar
process? Around a plant there exist certain elements like the elements
which form its substance; and its increase in size is effected by
continually integrating these surrounding like elements with itself."
And so on.
Passing over the far-fetched statement that the deposit of sediment in
distinct strata illustrates the universal tendency towards the union of
like units and the parting of unlike units, we must point out that
Spencer begins his discussion of organic growth by describing it in such
general terms that its essential characteristic is lost sight of. A
minute crystal of alum is dropped into a saturated solution of alum, and
it grows rapidly under our eyes out of material the same as its own, but
the living creature grows larger at the expense of material _different_
from its own. The grass grows at the expense of air, water, and salts,
and the lamb grows at the expense of the grass. Though the living
creature cannot, of course, transform one element into another, and must
have carbon, hydrogen, oxygen, nitrogen, etc., in its food, it utilises
materials chemically very different from its own complex compounds.
Spencer's inductions as to growth were the following:--
(1) The growth of an organism is dependent on the available supply
of such environing materials as are of like natures with the
matters composing the organism.
(2) Other things being equal, the degree of growth varies according
to the surplus of nutrition over expenditure.
(3) In the same organism the surplus of nutrition over expenditure
differs at different stages, and growth is unlimited or has a
definite limit, according as the surplus does or does not rapidly
decrease. There is almost unceasing growth in organisms that expend
relatively little energy and definitely limited growth in
organisms that expend much energy. [There are many difficulties
here, _e.g._, the apparent absence of a limit of growth in many
very energetic fishes.]
(4) Among organisms which are large expenders of force, the size
ultimately attained is, other things equal, determined by the
initial size. [By initial size Spencer means the bulk of the
organism when it begins to feed for itself.] A calf and a lamb
commence their physiological transactions on widely different
scales; their first increments of growth are similarly contrasted
in their amounts; and the two diminishing series of such increments
end at similarly-contrasted limits.
[But the further we penetrate into details, the more inevitable
seems the conclusion that adult size is _an adaptive phenomenon_;
in other words that growth has been punctuated by natural
selection.]
(5) Where the likeness of other circumstances permits a comparison,
the possible extent of growth depends on the degree of
organization; an inference testified to by the larger forms among
the various divisions and sub-divisions of organisms.
In connection with growth and its limit Spencer made a simple but shrewd
observation, which seems also to have occurred to Prof. Leuckart and to
Dr Alexander James. He pointed out, that in the growth of similarly
shaped bodies the increase of volume continually tends to outrun the
increase of surface. The volume of living matter must grow more than the
surface through which it is kept alive, if the surface remain regular in
contour. In spherical and all other regular units the volume increases
as the cube of the radius, the surface only as the square of the radius.
Thus a cell, for instance, as it grows, must get into physiological
difficulties, for the nutritive necessities of the increasing volume are
ever less adequately supplied by the less rapidly increasing absorbent
surface. There is less and less opportunity for nutrition, respiration,
and excretion. A nemesis of growth sets in, for waste gains upon,
overtakes, balances, and threatens to exceed repair. Growth may cease at
this limit, and a balance be struck; or the form of the unit may be
altered and surface gained by flattening out, or very frequently by
ramifying processes; or--and this the most frequent solution--the cell
may divide, halving its volume, gaining new surface, and restoring the
balance. In more general terms, growth expresses the preponderance of
constructive processes or anabolism; increase of volume with less rapid
increase of nutritive, respiratory, and excretory surface involves a
relative predominance of katabolism; the limit of growth occurs when
further increase of volume would prejudicially increase the ratio of
katabolism to anabolism; at that point the cell restores the balance by
dividing. And what is true of the unit applies also in a general way to
organs, such as leaves which increase their surface by becoming much
divided, and even to organisms which exhibit many adaptations for
increasing their nutritive, respiratory, and excretory surfaces.
_Development._--Growth is increase in bulk, development is increase in
structure, and Spencer's chief induction in regard to development is
that we see a change from an incoherent, indefinite homogeneity to a
coherent, definite heterogeneity. "The originally like units called
cells become unlike in various ways, and in ways more numerous and
marked as the development goes on. The several tissues which these
several classes of cells form by aggregation, grow little by little
distinct from each other; and little by little put on those structural
complexities that arise from differentiations among their component
units. In the shoot, as in the limb, the external form, originally very
simple, and having much in common with simple forms in general,
gradually acquires an increasing complexity and an increasing unlikeness
to other forms. Meanwhile, the remaining parts of the organism to which
the shoot or limb belongs, having been severally assuming structures
divergent from one another and from that of this particular shoot or
limb, there has arisen a greater heterogeneity in the organism as a
whole." Moreover, "whereas the germs of organisms are extremely similar,
they gradually diverge widely in modes now regular and now irregular,
until in place of a multitude of forms practically alike we finally have
a multitude of forms most of which are extremely unlike." In other
words, there is in individual development (ontogeny) some condensed
recapitulation of the steps in racial evolution (phylogeny).
Furthermore, in the progressing differentiation of each organism there
is a progressing differentiation of it from its environment; it becomes
freer from the environmental grip and more master of its fate. Here
again there is an individual progress parallel to that seen in the
course of historic evolution.
A general criticism must be made, that Spencer thought of the germ-cell
much too simply. It is a microcosm full of intricacy; the nucleus is
often exceedingly definite and coherent; the early cells are often from
the first defined, with prospective values which do not change. The
fertilised ovum has only apparent simplicity; it has a complex
individualised organisation--often visible. No one can doubt that
development is progressive differentiation, but it is rather a
realisation of a complex inheritance of materialised potentialities than
a change from an incoherent, indefinite homogeneity to a coherent,
definite heterogeneity.
_Structure and Function._--To the question, does Life produce
Organisation, or does Organisation produce Life? Spencer answered that
"structure and function must have advanced _pari passu_: some difference
of function, primarily determined by some difference of relation to the
environment, initiating a slight difference of structure, and this again
leading to a more pronounced difference of function; and so on through
continuous actions and reactions." As structure progresses from the
homogeneous, indefinite, and incoherent, so does function, illustrating
progressive division of labour. From an evolutionist point of view,
Spencer argued that life necessarily comes before organisation; "organic
matter in a state of homogeneous aggregation must precede organic matter
in a stage of heterogeneous aggregation. But since the passing from a
structureless state to a structured state is itself a vital process, it
follows that vital activity must have existed while there was yet no
structure: structure could not else arise. That function takes
precedence of structure, seems also implied in the definition of Life.
If Life is shown by inner actions so adjusted as to balance outer
actions--if the implied energy is the _substance_ of Life while the
adjustment of the actions constitutes its _form_; then may we not say
that the actions to be formed must come before that which forms
them--that the continuous change which is the basis of function, must
come before the structure which brings function into shape?"
But all such discussions of "structure" and "function" in the abstract
tend to verbal quibbling. We cannot have activity without something to
act, we cannot have metabolism without stuff. No one can tell what the
first thing that lived on the earth was like, what organisation it had,
or what it was able to do, but we may be sure that vital organisation
and vital activity are only static and kinetic aspects of the same
thing. It is quite probable, however, that there is no one thing that
can be called protoplasm, for vital function may depend upon the
inter-relations or inter-actions of several complex substances, none of
which could by itself be called alive; which are, however, held together
in that unity which makes an organism what it is. Just as the secret of
a firm's success may depend upon a particularly fortunate association of
partners, so it may be with vitality.[7]
_Waste and Repair._--Organisms are systems for transforming matter and
energy and the law of conservation holds good. "Each portion of
mechanical or other energy which an organism exerts implies the
transformation of as much organic matter as contained this energy in a
latent state," and the waste must be made good by repair. We thus see
why plants with an enormous income of energy and little expenditure of
energy have no difficulty in sustaining the balance between waste and
repair; we understand the relation between small waste, small activity,
and low temperature in many of the lower animals; we understand
conversely the rapid waste of energetic, hot-blooded animals. The
deductive interpretation of waste is easy, but it is different with
repair, for here the analogy between the organism and an inanimate
engine breaks down. The living creature is a self-stoking,
self-repairing, and also--it may be noted in passing--a self-reproducing
engine. Spencer did not do more than restate the difficulty when he said
that the component units of organisms have the power of moulding fit
materials into other units of the same order.
In passing to consider the ability which an organism often has of
recompleting itself when one of its parts has been cut off, just as an
injured crystal recompletes itself, Spencer was led to the hypothesis
that "the form of each species of organism is determined by a
peculiarity in the constitution of its units--that these have a special
structure in which they tend to arrange themselves; just as have the
simpler units of inorganic matter." "This organic polarity (as we might
figuratively call this proclivity towards a specific structural
arrangement) can be possessed neither by the chemical units nor the
morphological units, we must conceive it as possessed by certain
intermediate units, which we may term _physiological_." But if in each
organism the physiological units which result from the compounding of
highly compound molecules have a more or less distinctive character, the
germ-cell is not so very _indefinite_ after all.
Many of the facts of regeneration are very striking. A crab may regrow
its complex claw, a starfish arm may regrow an entire body. A snail has
been known to regenerate an amputated eye-bearing horn twenty times in
succession, a newt can replace a lost lens, a lizard can regrow its tail
and part of its leg, a stork can regrow the greater part of its bill. In
many cases, the surrender of parts which are afterwards regrown is
exceedingly common, as in some worms and Echinoderms, and is a
life-saving adaptation. Organically, though not consciously, the
brainless starfish has learned that it is better that one member should
perish than that the whole life should be lost. This regenerative
capacity no doubt implies certain properties in the living matter and in
the organism, but we are far from being able to picture how it comes
about. What does seem clear is that the distribution and mode of
occurrence of the regenerative capacity--in external organs often, but
in internal organs very rarely; in most lizard's tails, but not in the
chamaeleon's; in the stork's bill but not in its toes--are _adaptive_,
being related to the normal risks of life, as Reaumur, Lessona, Darwin,
and Weismann have pointed out. According to Lessona's Law, which
Weismann has elaborated, regeneration tends to occur in those organisms
and in those parts of organisms which are in the ordinary course
of nature most liable to injury. To which we must add two
saving-clauses--(_a_) provided that the lost part is of some vital
importance, and (_b_) provided that the wound or breakage is not in
itself very likely to be fatal. In Weismann's words, the theory is, that
"the power of regeneration possessed by an animal or by a part of an
animal is regulated by adaptation to the frequency of loss and to the
extent of the damage done by the loss."
_Adaptation._--Wherever we look in the world of organisms we find
examples of adaptation; we see form suited for different kinds of
motion, organs suited for their uses, constitution suited to
circumstances in such external features as colouring and in such
internal adjustments as the regulation of temperature; we find effective
weapons and effective armour, flowers adapted to insect visitors and
insect visitors adapted to flowers, one sex adapted in relation to the
other, the mother adapted to bearing and rearing offspring, the embryo
adapted to its pre-natal life; everywhere there is adaptation in varying
degrees of perfection. The adaptation is a fact, in regard to which all
naturalists are agreed; difference of opinion arises when we ask how
these adaptations have come to be.
In the chapter "Adaptation" Spencer practically restricted his attention
to a certain kind of adaptation, namely the direct modifications which
result from use or disuse, or from environmental influence. The
blacksmith's arm, the dancer's legs, the jockey's crural adductors,
illustrate direct results of practice; "a force de forger on devient
forgeron." The skin forms protective callosities where it is much
pressed or rubbed, as on the schoolboy's hands or the old man's
toothless gums. The blood-vessels may respond by enlargement to
increased demands made on them; the fingers of the blind become
extraordinarily sensitive.
Spencer points to the general truth that extra function is followed by
extra growth, but that a limit is soon reached beyond which very little,
if any, further modification can be produced. Moreover, the limited
increase of size produced in any organ by a limited increase of its
function, is not maintained unless the increase of function is
permanent. When the modifying influence is removed, the organism
rebounds or tends to rebound. A lasting change of importance involves a
re-organisation, a new state of equilibrium.
On inductive and deductive grounds, Spencer summed up in four
conclusions:--
(1) An adaptive change of structure will soon reach a point beyond
which further adaptation will be slow.
(2) When the modifying cause has been but for a short time in
action, the modification generated will be evanescent.
(3) A modifying cause acting even for many generations will do
little towards permanently altering the organic equilibrium of a
race.
(4) On the cessation of such cause, its effects will become
unapparent in the course of a few generations.
But two cautions must be emphasised (_a_) that Spencer, in this
discussion, dealt only with those direct adjustments which are referable
to the action of use or disuse, or of surrounding influences; and (_b_)
that we have no security in regarding these as being as such
transmissible.
By adaptations biologists usually mean permanent adjustments, and there
are two theories of the origin of these: (_a_) by the action of natural
selection on inborn variations, or (_b_) by the inheritance of the
directly acquired bodily modifications.
_Cell-Life._--In this chapter, interpolated in the revised edition,
Spencer summed up the main results of the study of the structural units
or cells which build up a body. "Nature everywhere presents us with
complexities within complexities, which go on revealing themselves as we
investigate smaller and smaller objects." Thus protoplasm itself has a
complicated structure; the nucleus of the cell is a little world in
itself; and the cell-firm has other partners, such as the centrosome.
When a cell divides, the readily stainable bodies or chromosomes,
present in definite number within the nucleus, are divided, usually by a
most intricate process, in such a manner that equal amounts are
bequeathed by the mother-cell to each of the two daughter-cells. Spencer
favoured the view that the chromatin, which "consists of an organic acid
(nucleic acid) rich in phosphorus, combined with an albuminous
substance, probably a combination of various proteids" may be peculiarly
unstable and active.
"From the chromatin, units of which are thus ever falling into
stabler states, there are ever being diffused waves of molecular
motion, setting up molecular changes in the cytoplasm. The
chromatin stands towards the other contents of the cell in the same
relation that a nerve-element stands to any element of an organism
which it excites." "We may infer that cell-evolution was, under one
of its aspects, a change from a stage in which the exciting
substance and the substance excited were mingled with approximate
uniformity, to a stage in which the exciting substance was gathered
together into the nucleus and finally into the chromosomes, leaving
behind the substance excited, now distinguished as cytoplasm."
But the suggestion that chromosomes may be stimulating,
change-exciting elements, does not, Spencer goes on to say,
conflict with the conclusion that the chromosomes are the vehicles
conveying hereditary traits. "While the unstable units of
chromatin, ever undergoing changes, diffuse energy around, they may
also be units which, under the conditions furnished by
fertilisation, gravitate towards the organisation of the species.
Possibly it may be that the complex combination of proteids, common
to chromatin and cytoplasm, is that part in which constitutional
characters inhere; while the phosphorised component, falling from
its unstable union and decomposing, evolves the energy which,
ordinarily the cause of changes, now excites the more active
changes following fertilisation."
From this speculation Spencer passes to a brief consideration of
what occurs before and during the fertilisation of the ovum. Before
fertilisation is accomplished the nucleus of the ovum normally
divides twice in rapid succession, and gives off two abortive
cells--known as polar bodies--which come to nothing. The usual
result of this "maturation," as it is called, is that the number of
chromosomes in the ovum is reduced to a half of the normal number
characteristic of the cells of the species to which it belongs. In
the history of the male element or spermatozoon, there is an
analogous reduction, so that when spermatozoon and ovum unite in
fertilisation the normal number is restored. It is now recognised
that the maturation-divisions are useful in obviating the doubling
of the number of chromosomes which fertilisation would otherwise
involve, and it has also been suggested that this continually
recurrent elimination of chromosomes may be one of the causes of
variation.
Spencer suggested another interpretation. He pointed out the
general fact that sexual reproduction (gamogenesis) commonly occurs
when asexual reproduction (agamogenesis) is arrested by
unfavourable conditions, that failing asexual reproduction
initiates sexual reproduction. Now as egg-cells and sperm-cells are
the outcome of often long series of cell divisions (asexual
multiplication), may not the polar bodies, which are aborted cells,
indicate that asexual multiplication can no longer go on, and that
the conditions leading to sexual multiplication have set in? "As
the cells which become spermatozoa are _left_ with half the number
of chromosomes possessed by preceding cells, there is actually that
impoverishment and declining vigour here suggested as the
antecedent of fertilisation." In short, the germ-cells, separately
considered, are cells in which the power of further asexual
multiplication is exhausted, as it is known to become exhausted in
Infusorians and such body-cells as nerve-cells; there arises a
state which initiates a sexual union or amphimixis of the two kinds
of germ-cells, and the decrease in the chromatin is an initial
cause of that state.
We quote this speculation as a good instance of Spencer's continual
endeavour to rationalise puzzling and exceptional facts by showing
that there is a general principle underlying them. But the
objections to his hypothesis are numerous. Mature ova or
spermatozoa will not normally divide if left to themselves, but
that is because they are specialised to secure amphimixis, not
because their powers are in any way declining or impoverished. A
parthenogenetic ovum gives off one polar body--though without
reduction in the number of chromosomes--and then proceeds by
asexual multiplication or ordinary cell division to build up a
body. The spore of a fern or a moss has only half the number of
chromosomes that the cells of its producer have, yet it proceeds by
asexual multiplication or ordinary cell-division to build up the
gametophyte or sexual generation.
_Genesis._--Spencer attempted a classification of the various modes of
reproduction that occur among organisms--asexual reproduction
(agamogenesis) by fission and budding, sexual reproduction (gamogenesis)
by specialised germ-cells usually involving fertilisation or amphimixis,
and all the complications involved in "alternation of generations"
(metagenesis), the development of eggs without fertilisation
(parthenogenesis), and so on. But what gives particular importance to
the chapter on genesis is not the discussion of the modes of
reproduction, but the general conclusion that nutrition and reproduction
are antithetic processes--a very fruitful idea in biology.
Where there is alternation of generation, sexual and asexual, we find
that asexual reproduction continues as long as the forces which result
in growth are greatly in excess of the antagonistic forces. Conversely
the recurrence of sexual reproduction occurs when the conditions are no
longer so favourable to growth. Similarly, where there is no
alternation, "new individuals are usually not formed while the preceding
individuals are still rapidly growing--that is, while the forces
producing growth exceed the opposing forces to a great extent; but the
formation of new individuals begins when nutrition is nearly equalled by
expenditure."
In illustration Spencer points to facts like the following: "Uniaxial
plants begin to produce their lateral, flowering axes, only after the
main axis has developed the great mass of its leaves, and is showing its
diminished nutrition by smaller leaves, or shorter internodes, or both";
"root-pruning" and "ringing," which diminish the nutritive supply,
promote the formation of flower-shoots; high nutrition in plants
prevents or arrests flowering.
Similarly, the aphides or green-flies, hatched from eggs in the spring,
multiply by parthenogenesis throughout the summer; with extraordinary
rapidity one generation follows on another; but when the weather becomes
cold and plants no longer afford abundant sap, males reappear and sexual
reproduction sets in. It has been shown that in the artificial summer of
a green-house, parthenogenesis may continue for four years. In a large
number of cases of ordinary reproduction, _e.g._ in birds, the connexion
between cessation of growth and commencement of reproduction is very
distinct.
It is not difficult to see the advantages in the postponement of sexual
reproduction until the rate of growth begins to decline. "For so long as
the rate of growth continues rapid, there is proof that the organism
gets food with facility--that expenditure does not seriously check
assimilation; and that the size reached is as yet not disadvantageous:
or rather, indeed, that it is advantageous. But when the rate of growth
is much decreased by the increase of expenditure--when the excess of
assimilative power is diminishing so fast as to indicate its approaching
disappearance--it becomes needful, for the maintenance of the species,
that this excess shall be turned to the production of new individuals;
since, did growth continue until there was a complete balancing of
assimilation and expenditure, the production of new individuals would be
either impossible or fatal to the parent. And it is clear that 'natural
selection' will continually tend to determine the period at which
gamogenesis commences, in such a way as most favours the maintenance of
the race."
That natural selection punctuates the life to advantage does not
imply that it works directly towards such a remote goal as
species-maintaining; it means that the arrangements which do secure this
end most effectively are those which tend to establish themselves. Those
that do not secure this end are eliminated.
_Nutrition and Reproduction._--Spencer's doctrine of the antithesis
between Nutrition and Reproduction is of great importance in biology,
and we must dwell on it a little longer.
The life of organisms is rhythmic. Plants have their long period of
vegetative growth, and then suddenly burst into flower. Animals in their
young stages grow rapidly, and as the growth ceases reproduction
normally begins; or again, just as perennial plants are strictly
vegetative through a great part of the year or for many successive
years, but have their periodic recurrence of flowers and fruit, so it is
with many animals which after remaining virtually asexual for prolonged
periods, exhibit periodic returns of a reproductive or sexual tide.
Foliage and fruiting, periods of nutrition and crises of reproduction,
hunger and love, must be interpreted as life-tides, punctuated by the
seasons and other circumstances through the agency of Natural Selection,
but none the less as expressions of the fundamental organic rhythm
between rest and work, upbuilding and expenditure, repair and waste,
which on the protoplasmic plane are known as anabolism and
katabolism.[8]
Anabolism and katabolism are the two sides of protoplasmic life, and the
major rhythms of the respective preponderance of these give the
antitheses of growth and multiplication, asexual and sexual
reproduction. The contrasts of metabolism represent the swings of the
organic see-saw; the periodic contrasts correspond to alternate
weightings or lightenings of the two sides.
Spencer's induction that "an approach towards equilibrium between the
forces which cause growth and the forces which oppose growth, is the
chief condition to the recurrence of sexual reproduction," is an
approximate answer to the question--_When_ does sexual reproduction
recur? But there remains, he says, the more difficult question--_Why_
does sexual reproduction recur? _Why_ cannot multiplication be carried
on in all cases, as it is in many cases, by asexual reproduction?
As yet, he says, biology is not advanced enough to give a reply, but a
certain hypothetical answer may be suggested. "Seeing, on the one hand,
that gamogenesis recurs only in individuals which are approaching a
state of organic equilibrium; and seeing, on the other hand, that the
sperm-cells and germ-cells thrown off by such individuals are cells in
which developmental changes have ended in quiescence, but in which,
after their union, there arises a process of active cell-formation; we
may suspect that the approach towards a state of general equilibrium in
such gamogenetic individuals is accompanied by an approach towards
molecular equilibrium in them; and that the need for this union of
sperm-cell and germ-cell is the need for overthrowing this equilibrium,
and re-establishing active molecular change in the detached germ--a
result probably effected by mixing the slightly different physiological
units of slightly different individuals."
Now, while Spencer was probably right in saying that fertilisation
promotes change, we cannot think that he succeeded in finding what he
was seeking, namely a primary physiological reason why sexual
reproduction should occur. It may be pointed out that it is only in a
limited sense that sperm-cells or egg-cells can be spoken of as in a
state of "quiescence," and that it is only in a limited sense that the
organism which has finished growing and is beginning to be sexual can be
spoken of as in a state of general or molecular equilibrium. An egg-cell
is quiescent, as a seed lying in the ground is quiescent, awaiting its
stimulus of warmth and moisture; a sperm-cell is quiescent, as a spore
floating in the air is quiescent, awaiting its appropriate soil. The
egg-cells and sperm-cells cannot be very quiescent since they do so much
when they unite. Moreover, we have simply to recall the facts of natural
parthenogenesis on the one hand or of artificial parthenogenesis on the
other, to see that the quiescence of the egg is a secondary restriction
adapted to secure amphimixis. Moreover, the familiar external and
internal changes which occur in the bodies of organisms when they are
approaching sexual maturity suggest the very opposite of general or
molecular equilibrium.
It may be pointed out that although asexual multiplication persists in
many organisms both large and small, and is sometimes the only method of
multiplication, yet it is apt to be a somewhat expensive process and
would be difficult to arrange for in highly differentiated animals. On
the other hand, asexual multiplication succeeds admirably in many cases;
it does not imply degeneration; it is not inconsistent with the
occurrence of variations; and it is _conceivable_ that it might have
been arranged for even in the highest animals. What other reason can
there be why the circuitous process of sexual reproduction has been
preferred? It may be said that the arrangement by which multiplication
is secured through special germ-cells, more or less apart from the cells
which build up the body, may be justified as an arrangement which
prevents or tends to prevent the transmission of bodily modifications,
many of which are detrimental. But as this cuts both ways, preventing or
tending to prevent the transmission of useful modifications, there must
be some other reason why the circuitous process of sexual reproduction
has been preferred. We believe the answer to be that sexual reproduction
is an adaptive process securing the benefits of amphimixis, for in
amphimixis and in the changes preparatory to it, there is an important
_source of variation_. In one of his essays Weismann wrote as follows:--
"Sexual reproduction is well known to consist in the fusion of two
contrasted reproductive cells, or perhaps even in the fusion of
their nuclei alone. These reproductive cells contain the germinal
material or germ-plasm, and this again, in its specific molecular
structure, is the bearer of the hereditary tendencies of the
organisms from which the reproductive cells originate. Thus in
sexual reproduction two hereditary tendencies are in a sense
intermingled. In this mingling, I see the cause of the hereditary
individual characteristics; and in the production of these
characters, the task of sexual reproduction. It has to supply the
material for the individual differences from which selection
produces new species."
When we inquire into the reasons for the occurrence of a process
such as sexual reproduction, there are four different questions
which may be put: (1) We may inquire into the historical evolution
of the process, so far as that can be legitimately imagined or
inferred from still persistent grades. (2) We may try to discover
what factors may have operated in the course of evolution in
raising the process from one step of differentiation to another.
(3) We may also try to show how the process is justified by its
advantages either self-regarding or species-maintaining. (4) We may
inquire into the physiological sequences in the internal economy of
the individual organism which lead up to the process in question.
There is no doubt always an immediate necessity for the occurrence
of an organic process, but we are in many cases quite unable at
present to do more than describe the series of events without
understanding their causal nexus. The reason for this is apparent,
since the organism is much more than a detached inanimate engine;
it is a system which has summed up in it the long results of time,
the history of ages. Its rhythms and periodicities and crises
puzzle us because they originated under conditions which obtained
untold millennia ago. Thus some processes in higher animals may
have had originally a reference to tides from the reach of which
their present possessors are far withdrawn.
We have entered on this digression partly for clearness sake, and
partly to explain why Spencer had, as we think, very limited
success in his answer to the question: Why does sexual reproduction
occur? The curious reader may be referred to the discussion of
these problems in _The Evolution of Sex_, Contemporary Science
Series, Revised Edition, 1901.
_The Germ-Cells._--But we cannot leave the interesting chapter on
genesis without referring to another of Spencer's conclusions, which
does not seem to us to be quite consistent with facts.
"The marvellous phenomena initiated by the meeting of sperm-cell and
germ-cell, or rather of their nuclei, naturally suggest the conception
of some quite special and peculiar properties possessed by these cells.
It seems obvious that this mysterious power which they display of
originating a new and complex organism, distinguishes them in the
broadest way from portions of organic substance in general.
Nevertheless, the more we study the evidence the more are we led towards
the conclusion that these cells are not fundamentally different from
other cells." The evidence he gives is: (1) that small fragments of
tissue in many plants and inferior animals may develop into entire
organisms; (2) that the reproductive organs producing eggs and sperms
are organs of low organisation, with no specialities of structure "which
might be looked for, did sperm-cells and germ-cells need endowing with
properties unlike those of all other organic agents." "Thus, there is no
warrant for the assumption that sperm-cells and germ-cells possess
powers fundamentally unlike those of other cells."
To this it must be answered: (1) though sperm-cells and egg-cells, being
living units, cannot be "_fundamentally_ unlike" other living units,
such as ordinary body-cells, yet they may be very unlike them; (2) that
the germ-cells are very unlike ordinary body-cells is shown by the fact
that they can do what no single body-cell can do, build up a whole
organism; (3) so specific are germ-cells that in certain cases and in
favourable conditions a small fraction of an egg, bereft of its own
nucleus, may, if fertilised, develop into an entire and normal larva;
(4) it is quite consistent with the idea of evolution that in lower
organisms the contrast between body-cells and germ-cells should be less
pronounced than in higher forms. But the fundamental answer is found
when we inquire into the history of the germ-cells. In many cases, and
the list is being added to, the future reproductive cells are segregated
off at an early stage in embryonic development. Even before
differentiation sets in, the future reproductive cells may be set apart
from the body-forming cells. The latter develop in manifold variety into
skin and nerve, muscle and blood, gut and gland; they differentiate, and
may lose almost all protoplasmic likeness to the mother ovum. But the
reproductive cells are set apart; they take no share in the
differentiation, but remain virtually unchanged, continuing unaltered
the protoplasmic tradition of the original fertilised ovum. After a
while their division-products will be liberated as functional
reproductive cells or germ-cells, handing on the tradition intact to the
next generation.
An early isolation of the reproductive cells has been observed in the
harlequin fly (_Chironomus_) and in some other insects, in the aberrant
worm-type _Sagitta_, in leeches, in thread-worms, in some Polyzoa, in
some small Crustaceans known as Cladocera, in the water-flea _Moina_, in
some Arachnoids (Phalangidae), in the bony fish _Micrometrus aggregatus_,
and in other cases. In the development of the threadworm of the horse
according to Boveri, the very first cleavage of the ovum establishes a
distinction between somatic and reproductive cells. One of the first two
cells is the ancestor of all the cells of the body; the other is the
ancestor of all the germ-cells. "Moreover, from the outset the
progenitor of the germ-cells _differs from the somatic cells not only in
the greater size and richness of the chromatin of its nucleus, but also
in its mode of mitosis_ (_division_), for in all those blastomeres
(segmentation-cells) destined to produce somatic cells a portion of the
chromatin is cast out into the cytoplasm, where it degenerates, and
_only in the germ-cells is the sum-total of the chromatin retained_" (E.
B. Wilson, _The Cell in Development and Inheritance_, 1896, p. 111).
In the majority of cases, we admit, the reproductive cells _are not to
be seen_ in early segregation, and the continuous lineage from the
fertilised ovum cannot be traced. In the majority of cases, the
germ-cells are seen as such after considerable differentiation has gone
on, and although they are linear descendants of the ovum, their special
lineage cannot be traced. But it seems legitimate to argue from the
clear cases to the obscure cases, and to say that the germ-cells are
those cells which retain the complete complement of heritable qualities.
Adopting the conception of the germ-plasm as the material within the
nucleus which bears all the properties transmitted in inheritance, we
may still say, in Weismann's words, "In every development a portion of
this specific germ-plasm, which the parental ovum contains, is unused in
the upbuilding of the offspring's body, and is reserved unchanged to
form the germ-cells of the next generation.... The germ-cells no longer
appear as products of the body, at least not in their more essential
part--the specific germ-plasm; they appear rather as something opposed
to the sum-total of body-cells; and the germ-cells of successive
generations are related to one another like generations of Protozoa." In
terms of this conception, which fits many facts, we may say that in
plants and lower animals the distinction between germ-plasm and
somato-plasm has not been much accentuated, and that in some organisms
the body-cells retain enough undifferentiated germ-plasm to enable them
in small or large companies to regrow an entire organism.
It may be said that Spencer must also have regarded the germ-cells as
containing the whole complement of hereditary qualities. _It must be
so._ The point is that he rejected the theory which gives a rational
account of how the germ-cells have this content and their power of
developing into an organism, like from like. The sentence in which he
points out that the reproductive organs have "none of the specialities
of structure which might be looked for, did the sperm-cells and
germ-cells need _endowing with properties_ unlike those of all other
organic agents," shows how far he deliberately stood from the conception
we have outlined.
Here we may note that the "Inductions" regarding Heredity are
discussed in our eleventh chapter, and those regarding Variation in
our twelfth chapter. We have not dealt with the suggestive concrete
sections which deal with structural and functional evolution,
partly because they are too concrete to be dealt with briefly, and
partly because they are saturated with the hypothesis of the
transmission of acquired characters. Spencer's most important
conclusion in regard to the Laws of Multiplication is referred to
under the heading Population.
FOOTNOTES:
[7] See J. Arthur Thomson's _Progress of Science in the Nineteenth
Century_, 1903, p. 317, and E. B. Wilson's _The Cell in Development and
Inheritance_, 1900.
[8] P. Geddes and J. Arthur Thomson, _The Evolution of Sex_, revised
edition, 1901, p. 238.