or later obtain an influence in determining the viability of their
possessors, either increasing or diminishing it. It is this, in
association with the unlimited multiplication of individuals, which
gives a basis to the principle of transformation, which it is the
immortal merit of Charles Darwin and Alfred Russel Wallace to have
introduced into science: _the principle of selection_. We have seen
that this principle may have a much more comprehensive meaning than
was attributed to it by either of these two naturalists; that there
is not merely a struggle between individuals which brings about their
adaptation to their environment, by preserving those which vary in
the most favourable way and rejecting those which vary unfavourably,
but that there is an analogous struggle between the parts of these
individuals, which, as Wilhelm Roux showed, effects the adaptation
of the parts to their functions, and that this struggle must be
assumed to occur even between the determinants and biophors of the
germ-plasm. There is thus a germinal selection, a competition between
the smaller and larger particles of the germ-plasm for space and
food, and that it is through this struggle that there arise those
definitely and purposefully directed variations of the individual,
which are transmissible because they have their seat in the immortal
germ-plasm, and without which an adaptation of individuals in the sense
and to the extent in which we actually observe it would be altogether
inconceivable. I have endeavoured to show that the whole evolution
of the living world is guided essentially by processes of selection,
in as far as adaptations of the parts to one another, and of the
whole to the conditions of life, cannot be conceived of as possible
except through these, and that all fluctuations of the organism,
from the very lowest up to the highest, are forced into particular
paths by this principle, by 'the survival of the fittest.' This ends
the whole dispute as to whether there are indifferent 'characters'
which have no influence on the existence of the species, for even the
characters most indifferent for the 'person' would not exist unless
the germinal constituents (determinants) which condition them had
been victorious in the struggle for existence over others of their
kind, and even the 'indifferent' characters, which depend solely
upon climatic or other external influences, owe their existence to
processes of germinal selection, for those elements of the determinants
concerned were victorious which throve best under such influences.
But should variations thus produced by external influence increase so
far that they become prejudicial to the survival of their bearers,
then they are either set aside by personal selection or, if that be
no longer possible, they lead to the extinction of the species. Thus
the multitude of small individual variations, which probably occur in
every species, but which strike us most in Man--the differences in the
development of mouth, nose, and eyes, in the hair, in the colour of
skin, &c., as far as they are without significance in the struggle for
existence--depend upon processes of germinal selection, which permitted
the greater development of one group of determinants, or of one kind of
biophor in one case, of another in another. The proportionate strength
of the elements of the germ-plasm is not readily lost at once, but is
handed on to successive generations, and thus even these 'indifferent'
characters are transmitted.
It is obvious that, if the principle of selection operates in nature
at all, it must do so wherever living units struggle together for the
same requirements of life, for space and food, and these units need not
be persons, but may represent every category of vital units, from the
smallest invisible units up to the largest. For in all these cases the
conditions of the selection-process are given: individual variability,
nutrition, and multiplication, transmission of the advantage
attained, and, on the other hand, limitation of the conditions of
existence--especially food and space. The resulting struggle for
existence must, in every category of vital units, be most acute
between the individual members of each category, as Darwin emphasized
in the case of species from the very first, and persistent variations
of a species of living units can only be brought about by this kind
of struggle. Strictly speaking, therefore, we should distinguish as
many kinds of selection-processes as there are categories of living
units, and these could not be sharply separated from one another,
apart from the fact that we have to infer many of them, and cannot
recognize their gradations. Here, as everywhere else, we must break up
the continuity of nature into artificial groups, and it seems best to
assume and distinguish between four main grades of selective processes
corresponding to the most outstanding and significant categories of
vital units, namely: Germinal, Histonal, Personal, and Cormal Selection.
Histonal Selection includes all the processes of selection which take
place between the elements of the body (soma), as distinguished from
the germ-plasm, of the Metazoa and Metaphyta, not only between the
'tissues' in the stricter sense, but also between the parts of the
tissues, that is, the lower vital units of which they are composed, and
which Wilhelm Roux, when he published his _Kampf der Teile_ ('Struggle
of the Parts'), called 'molecules.' It occurs between all the parts
of the tissues down to the lowest vital units, the biophors. We must
also reckon under histonal selection the processes of selection
which take place between the elements of the simplest organisms, and
through which these have gradually attained to greater complexity of
structure and increased functional capacity. As long as no special
hereditary substance had been differentiated, variations which arose
in the simplest organisms through selection-processes of this kind
were necessarily transmitted to the descendants, but after this
differentiation had taken place this could no longer occur--'acquired'
modifications of the soma were no longer transmitted, and the
importance of histonal selection was limited to the individual. But
this form of selection must be of the greatest importance in regard to
the adaptations of the parts which develop from the ovum, especially
during the course of development, and it is also indispensable all
through life in maintaining the equilibrium of the parts, and their
adaptation to the varying degree of function required from them (use
and disuse). But its influence does not reach directly beyond the
life of the individual, since it can only give rise to 'transient'
modifications, that is, to changes which cease with the individual life.
In contrast to this is Germinal Selection, which depends upon the
struggle of the parts of the germ-plasm, and thus only occurs in
organisms with differentiation of somatoplasm and germ-plasm,
especially in all Metazoa and Metaphyta--forming in these the basis of
all hereditary variations. But not every individual variation to which
germinal selection gives rise persists and spreads gradually over the
whole species, for, apart from the cases we have already mentioned, in
which indifferent variations favoured by external circumstances gain
the victory, this happens only if the variations in question are of use
to their bearer, the individual. Any variation whatever may arise in a
particular individual purely through germinal selection, but it is only
the higher form of selection--Personal Selection--that decides whether
the variation is to persist and to spread to many descendants so that
it ultimately becomes the common property of the species. Germinal and
personal selection are thus continually interacting, so that germinal
selection continually presents hereditary variations, and personal
selection rejects those that are detrimental and accepts those that
are useful. I will not repeat any exposition of the marvellous way in
which personal selection reacts upon germinal selection, and prevents
it from continuing to offer unfavourable variations, and compels it to
give rise to what is favourable in ever-increasing potency. Although
it apparently selects only the best-adapted _persons_ for breeding,
it really selects the favourable id-combinations of the germ-plasm,
that is, those which contain the greatest number of favourably varying
determinants. We saw that this depends upon the multiplicity of ids
in the germ-plasm, since every primary constituent of the body is
represented in the germ-plasm, not once only, but many times, and it is
always half of the homologous determinants contained in the germ-plasm
of an individual which reach each of its germ-cells, always, moreover,
in a different combination. Thus, with the rejection of an individual
by personal selection, a particular combination of ids, a particular
kind of germ-plasm is in reality removed, and thus prevented from
having any further influence upon the evolution of the species. By
this means germinal selection itself is ultimately influenced, because
only those ids remain unrejected in the germ-plasm whose determinants
are varying in directions useful to the species. Thus there comes about
what until recently was believed to be impossible: the conditions
of life give rise to useful directions of variation, not directly,
certainly, but indirectly.
We may distinguish as a fourth grade of selection Cormal Selection,
that is, the process of selection which effects the adaptation of
animal and plant stocks or corms, and which depends on the struggle of
the colonies among themselves. This differs from personal selection
only in that it decides, not the fitness of the individual person, but
that of the stock as a whole. It is a matter of indifference whether
the stocks concerned are stocks in the actual material sense, or only
in the metaphorical sense of sharing the common life of a large family
separated by division of labour. In both cases, in the polyp-stock as
well as in the termite or ant-colony, the collective germ-plasm, with
all its different personal forms, is what is rejected or accepted. The
distinction between this cormal selection and personal selection is,
therefore, no very deep one, because here too it is in the long run the
two sexual animals which are selected, not indeed only in reference
to their visible features, but also in reference to their invisible
characters, those, namely, which determine in their germ-plasm the
constitution of their neuter progeny or, in the case of polyps, their
asexually reproducing descendants.
We venture to maintain that everything in the world of organisms that
has permanence and significance depends upon adaptation, and has arisen
through a sifting of the variations which presented themselves, that
is, through selection. Everything is adaptation, from the smallest
and simplest up to the largest and most complex, for if it were not
it could not endure, but would perish. The principle which Empedocles
announced, in his own peculiar and fantastic way, is the dominating
one, and I must insist upon what has so often been objected to as an
exaggeration--that everything depends upon adaptation and is governed
by processes of selection. From the first beginnings of life, up to
its highest point, only what is purposeful has arisen, because the
living units at every grade are continually being sifted according to
their utility, and the ceaseless struggle for existence is continually
producing and favouring the fittest. Upon this depends not only the
infinite diversity of the forms of life, but also, and chiefly, the
closely associated progress of organization.
It cannot be proved in regard to each individual case, but it can be
shown in the main that attaining a higher stage in organization also
implies a predominance in the struggle for existence, because it opens
up new possibilities of life, adaptations to situations not previously
utilizable, sources of food, or places of refuge. Thus a number of the
lower vertebrates ascended from the water to the land, and adapted
themselves to life on dry land or in the air, first as clumsily moving
salamanders, but later as actively leaping frogs; thus, too, other
descendants of the fishes gained a sufficient carrying power of limbs
to raise the lightened body from the ground, and so attained to the
rapid walk of the lizards, the lightning-like leaps of the arboreal
agamas, the brief swooping of the flying-dragons, and ultimately
the continuous flight which we find in the flying Saurians and the
primitive birds of the Jurassic period, and in the birds and bats of
our own day.
It is obvious that each of these groups, as it originated, conquered
a new domain of life, and in many cases this was such a vast one, and
contained so many special possibilities, that numerous subordinate
adaptations took place, and the group broke up into many species and
genera, even into families and orders. All this did not come about
because of some definitely directed principle of evolution of a
mysterious nature, which impelled them to vary in this direction and in
no other, but solely through the rivalry of all the forms of life and
living units, with their enormous and ceaseless multiplication, in the
struggle for existence. They were, and they are still, forced to adapt
themselves to every new possibility of life attainable to them; they
are able to do this because of the power of the lowest vital units of
the germ to develop numerous variations; and they are obliged to do it
because, of the endless number of descendants from every grade of vital
unit, it is only the fittest which survive.
Thus higher types branched off from the lower from time to time,
although the parent type did not necessarily disappear; indeed it
could not have disappeared as long as the conditions of its life
endured; it was only the superfluous members of the parent form that
adapted themselves to new conditions, and as, in many cases, these
required a higher organization, there arose a semblance of general
upward development which simulated a principle of evolution always
upwards. But we know that, at many points on this long road, there
were stations where individual groups stopped short and dropped back
again to lower stages of organization. This kind of retreat was almost
invariably caused by a parasitic habit of life, and in many cases this
degeneration has gone so far that it is difficult to recognize the
relationship of the parasite to the free-living ancestors and nearest
relatives. Many parasitic Crustaceans, such as the Rhizocephalids,
lack almost all the typical characteristics of the crustacean body,
and dispense not only with segmentation, with head and limbs, but also
with stomach and intestine. As we have seen, they feed like the lower
fungi, by sucking up the juices of their hosts, by means of root-like
outgrowths from the place where the mouth used to be. Nevertheless,
their relationship to the Cirrhipedes can be proved from their larval
stages. There are, however, parasites in the kidneys of cuttlefish--the
Dicyemidæ--in regard to which naturalists are even now undecided
whether they ought to form a lowly class by themselves between
unicellular animals and Metazoa, or whether they have degenerated, by
reason of their parasitism, from the flat worms to a simplicity of
structure elsewhere unknown. They consist only of a few external cells,
which enclose a single large internal cell, possess no organs of any
kind, neither mouth nor intestine, neither nervous system nor special
reproductive organs. But although degeneration cannot be proved in this
case, it can be in hundreds of other cases with absolute certainty,
as, for instance, in the Crustacea belonging to the order of Copepods,
which are parasitic upon fishes, in which we find all possible stages
of degeneration, according to the degree of parasitism, that is, to
the greater or less degree of dependence upon the host; for organs
degenerate and disappear in exact proportion to the need for them, and
they thus show us that degeneration also is under the domination of
adaptation.
Thus retrogressive evolution also is based upon the power of the living
units to respond to changing influences by variation, and upon the
survival of the fittest.
The roots of all the transformations of organisms, then, lie in changes
of external conditions. Let us suppose for a moment that these might
have remained absolutely alike from the epoch of spontaneous generation
onwards, then no variation of any kind and no evolution would have
taken place. But as this is inconceivable, since even the mere growth
of the first living substance must have exposed the different kinds
of biophors composing it to different influences, variation was
inevitable, and so also was its result--the evolution of an animate
world of organisms.
External influences had a twofold effect at every stage upon every
grade of vital unit, namely, that of directly causing variation and
that of selecting or eliminating. Not only the biophors, but every
stage of their combinations, the histological elements, chlorophyll
bodies, muscle-disks, cells, organs, individuals, and colonies, can
not only be caused to vary by the external influences to which they
are subjected, but can be guided by these along particular paths of
variation, so that among the variations which crop up some are better
adapted to the conditions than others, and these thrive better, and
thus alone form the basis of further evolution. In this way definite
tendencies of evolution are produced, which do not move blindly and
rigidly onwards like a locomotive which is bound once for all to the
railroad, but rather in exact response to the external conditions,
like an untrammelled pedestrian who makes his way, over hill and dale,
wherever it suits him best.
The ultimate forces operative in bringing about this many-sided
evolution are the known--and although we do not recognize it as yet,
perhaps the unknown--chemico-physical forces which certainly work only
according to laws; and that they are able to accomplish such marvellous
results is due to the fact that they are associated in peculiar and
often very complex different kinds of combinations, and thus conform
to the same sort of regulated arrangements as those which condition
the operations of any machine made by man. All complex effects depend
upon a co-operation of forces. This is seen, to begin with, in the
chemical combinations whose characters depend entirely upon the number
and arrangement of the elementary substances of which they consist; the
atoms of carbon, hydrogen, and oxygen, which compose sugar, can also
combine to form carbonic acid gas and water, or alcohol and carbonic
acid gas; and the same thing is true if we ascend from the most complex
but still inanimate organic molecules to those chemical combinations
which, in a still higher form, condition the phenomena of life, to the
lowest living units, the biophors. Not only do these last differ in
having life, but they themselves may appear in numerous combinations,
and can combine among themselves to form higher units, whose characters
and effectiveness will depend upon these combinations. Just as man may
adjust various metallic structures, such as wheels, plates, cylinders,
and mainsprings in the combination which we call a watch, and which
measures time for us, so the biophors of different kinds in the living
body may form combinations of a second, third, &c., degree, which
perform the different functions essential to life, and by virtue of
their specific, definite combination of elementary forces.
But if it be asked, what replaces human intelligence in these
purposeful combinations of primary forces, we can only answer that
there is here a self-regulation depending upon the characters of the
primary vital parts, and this means that these last are caused to vary
by external influences and are selected by external influences, that
is, are chosen for survival or excluded from it. Thus combinations of
living units must always result which are appropriate to the situation
at the moment, for no others can survive, although, as we have seen,
they must arise. This is our view of the causes of the evolution of the
world of organisms; the living substance may be compared to a plastic
mass which is poured out over a wide plain, and in its ceaseless
flowing adapts itself to every unevenness, flows into every hole,
covers every stone or post, leaving an exact model of it, and all this
simply by virtue of its constitution, which is at first fluid and then
becomes solid, and of the form of the surface over which it flows.
But it is not merely the surface in our analogy which determines
the form of the organic world; we must take account not only of the
external conditions of existence, but also of the constitution of
the flowing mass, the living substance itself, at every stage of its
evolution. The combination of living units which forms the organism is
different at each stage, and it is upon this that its further evolution
depends; this difference determines what its further evolution _may_
be, but the conditions of life determine what it _must_ be in a
particular case. Thus, in a certain sense, it was with the first
biophors, originating through spontaneous generation, that the whole
of the organic world was determined, for their origin involved not
only the physical constitution by which the variations of the organism
were limited, but also the external conditions, with their changes up
till now, to which organisms had to adapt themselves. There can be
no doubt that on another planet with other conditions of life other
organisms would have arisen, and would have succeeded each other in
diverse series. On the planet Mars, for instance, with its entirely
different conditions as regards the proportions in weight and volume of
the chemical elements and their combinations, living substance, if it
could arise at all, would occur in a different chemical composition,
and thus be equipped with different characters, and without doubt
also with quite different possibilities of further development and
transformation. The highly evolved world of organisms which we may
suppose to exist upon Mars, chiefly on the ground of the presence
of the remarkable straight canals discovered by Schiaparelli, must
therefore be thought of as very different from the terrestrial living
world.
But upon the earth things could not have been very different from
what they actually are, even if we allow a good deal to chance and
assume that the form of seas and continents might have been quite
different, the folding of the surface into mountains and valleys, and
the formation of rents and fissures, with the volcanoes that burst
from them, need not have turned out exactly as it has done. In that
case many species would never have arisen, but others would have taken
their place; on the whole, the same types of species-groups would
have succeeded each other in the history of the earth. Let us suppose
that the Sandwich Islands, like many other submarine volcanoes, had
never risen above the surface of the sea, then the endemic species of
snails, birds, and plants which now live there could not have arisen,
and if the volcanic group of the Galapagos Islands had arisen from
the sea not in their actual situation, but forty degrees further
south or north, or 1,000 kilometres further west, then it would have
received other colonists, and probably fewer of them, and a different
company of endemic species would be found there now. But there would
be terrestrial snails and land-birds none the less, and on the whole
we may say that both the extinct and the living groups of organisms
would have arisen even with different formations of land and sea, of
heights and depths, of climatic changes, of elevations and depressions
of the earth's crust, at least in so far as they are adaptations to
the more general conditions of life and not to specialized ones. The
great adaptation to swimming in the sea, for instance, must have taken
place in any case; swimming worms, swimming polyps (Medusæ), swimming
vertebrates, would have arisen; terrestrial animals would have evolved
also, on the one hand from an ancestry of worms in the form of jointed
animals and land or freshwater worms, and again from an ancestry of
fishes. Aerial animals would also undoubtedly have evolved even if the
lands had been quite differently formed and bounded, and I know of no
reason why the adaptation to flight should not have been attempted in
as many different ways as it has actually been by so many different
groups--the insects, the reptiles (the flying Saurians of the Jurassic
period), the extinct _Archæopteryx_, the birds, and the bats among
mammals.
We can trace plainly in every group the attempt not only to spread
itself out as far as possible over as much of the surface of the earth
as is accessible to it, but also to adapt itself to all possible
conditions of life, as far as the capacity for adaptation suffices.
This is very obvious from the fact that such varied groups have striven
to rise from life on the earth to life in the air, and have succeeded
more or less perfectly, and we can see the same thing in all manner of
groups. Almost everywhere we find species and groups of species which
emancipate themselves from the general conditions of life in their
class, and adapt themselves to very different conditions, to which
the structure of the class as a whole does not seem in the least
suited. Thus the mammals are lung-breathers, and their extremities
are obviously adapted for locomotion on the solid earth, yet several
groups have returned to aquatic life, as, for instance, the family of
otters and the orders of seals and whales. Thus among insects which
are adapted for direct air-breathing, certain families and stages
of development have returned to aquatic life, and have developed
breathing-tubes by means of which they can suck in air from the surface
of the water into their tracheal system, or so-called tracheal gills,
into which the air from the water diffuses. But the most convincing
proof of the organism's power of adaptation is to be found in the
fact that the possibility of living parasitically within other
animals is taken advantage of in the fullest manner, and by the most
diverse groups, and that their bodies exhibit the most marvellous and
far-reaching adaptations to the special conditions prevailing within
the bodies of other animals. We have already referred to the high
degree reached by these adaptive changes, how the parasite may depart
entirely from the type of its family or order, so that its relationship
is difficult to recognize. Not only have numerous species of flat worms
and round worms done this, but we find numerous parasites among the
great class of Crustaceans; there are some among spiders, insects,
medusoids, and snails, and there are even isolated cases among fishes.