interesting discoveries in regard to animals living in the depths of
the ocean, all of which exhibit peculiar adaptations to the special
conditions of their life, and especially to the darkness of the great
depths. One of the most striking of these discoveries was that of
the luminous organs which are found not in all but in a great many
animals living on the bottom of the abyssal area, and also among the
animals occurring at various levels above the floor of the abyss.
These are sometimes glands which secrete a luminous substance, but
sometimes complex organs, 'lanterns' which are controlled by the will
of the animal, and suddenly evolve a beam of light and project it in
a particular direction, like an electric searchlight. These organs
have a most complex structure, composed of nerves and lenses, which
focus the light, and on the whole are not unlike eyes. That this sort
of structure should have arisen all at once through a 'mutation' is
inconceivable; it can have originated only from simple beginnings by a
gradual increase of its structure along with continual strict selection
among the variations which cropped up. They all depend upon complicated
'harmonious' adaptation, and cannot possibly have been derived from
mutations, that is, from ready-made structural 'constellations,' unless
we are to call in the aid of the miraculous. But lanterns of this kind
are found in many different kinds of animals--in Schizopod Crustaceans,
in shrimps, in fishes of different genera and families. Many fishes
have long rows of luminous organs on the sides and on the belly, and
these probably serve to light up the sea-floor and facilitate the
finding of food; in others the luminous organs are placed upon the
snout just above the wide voracious mouth, and in that position they
have undoubtedly the significance attributed to them by Chun, namely,
that they attract small animals, just as the electric lamps allure all
sorts of nocturnal animals, and especially insects, in large numbers
to their destruction. But not fishes only, but molluscs, e.g. the
Cephalopods of the great depths, have developed luminous organs, and
one species of Cephalopod has about twenty large luminous organs, like
gleaming jewels, ultramarine, ruby-red, sky-blue and silvery, while
in another the whole surface of the belly is dotted over with little
pearl-like luminous organs. Even if we cannot be quite clear as to the
special use of these lanterns of deep-sea animals, there can be no
doubt that they are adaptations to the darkness of the great depths,
and when we find the _same_ adaptations (in a physiological sense)
in many animals belonging to the most diverse groups, there is no
possibility of referring them to sudden mutations which have arisen
all at once in these groups with no relation to utility, and yet have
not occurred in any animals living in the light. Only 'variations'
progressing and combining in the direction of utility can give us the
key to an explanation of the origin of such structures.
The same is true of the eyes of deep-sea animals. It was believed at
one time that all the inhabitants of dark regions had lost their eyes.
This is the case with many cave animals and the inhabitants of the
lightless depths of our lakes, but in the abyssal zone of the sea it
is only some fishes and Crustaceans whose eyes have degenerated to
the vanishing point. Moreover, the disappearance apparently occurs in
species which are restricted to the ocean-floor in their search for
food, which therefore can make more use of their tactile organs than of
their eyes, for while the ocean-floor undoubtedly contains over wide
areas an abundance of food for these mud-eaters, it is only partly
illuminated, that is, only in places where there are luminous animals
such as polyp-colonies, &c. The fact that so many of the animals of the
great depths are luminous obviously conditions, not that most of the
immigrants into the abyssal zone should lose their eyes as useless,
but that they should adapt them to the light which is very weak in
comparison with that of the superficial layers. The eyes of deep-sea
fishes, for instance, are either enormously large, and therefore suited
for perceiving the faint light of the depths, or they have varied in
another and very characteristic manner: they have become elongated
into a cylinder, which projects far beyond the level of the head. It
looks almost as if the animals were looking through an opera-glass, and
Chun has called these eyes 'telescope-eyes.' A. Brauer has recently
shown what far-reaching variations of the original eye of fishes were
necessary in order to transform it into an organ for seeing in the
dark. These variations, however, have occurred in the eyes of the most
diverse animals in the deep sea, and not only do different families
of deep-sea fishes possess 'telescope-eyes,' but Crustaceans and
Cephalopods as well. Even our owls possess quite a similar structure,
although it does not project beyond the head in the same way. Here
again we have to deal with the phenomenon which Oscar Schmidt in
his time called _convergence_, that is, corresponding adaptations
to similar conditions in animal forms not genealogically connected
with one another. These telescope-eyes are not all descended from
one species which chanced in one of the 'mutation-periods' suddenly
to produce this combination of harmonious adaptations, but they have
risen independently through variation progressing step by step in
the direction of the required end, that is to say, through natural
selection based upon germinal selection. Only thus can their origin be
understood.
But what is time of eyes adapted to darkness is true in some measure
of all eyes, for the eyes of animals are not mere decorative points
which might be present or absent; they cannot have arisen in any
animal whatever through sudden mutation--they have been laboriously
acquired with difficulty, by the slow increase of gradually perfecting
adaptations; they are parts which bear the most precise internal
correlation with the whole organization of the animal, and which can
only cease to exist when they become superfluous. Thus the origin of
eyes seems to me only conceivable on the basis of germinal selection
controlled towards what is purposeful by natural selection, that is to
say, on a basis of fluctuating variation, and not through chance.
This is the case with all adaptations. Just as the eyes of animals
are adaptations which utilize the light-waves in the interest of the
organism and its survival, the same is true of all the sense-organs,
tactile organs, smelling and tracking organs, organs of hearing, and so
on. The animal cannot do without these; first the lower sense-organs
arose and then the higher; the increasingly high organization of the
animal conditioned this, and a multicellular animal without sensory
structures is inconceivable. The same may be said of the nervous system
as a whole, whose function it is to translate into action the stimuli
received through the sense-organs, whether directly or by means of
intervening nerve-cells, which form central organs of ever-increasing
complexity of composition. As telescope-eyes have evolved in some
groups of deep-sea animals, independently of one another, and certainly
not through the fortuitous occurrence of a mutation, but under the
compulsion of necessity in competition, so all the organs we have just
named, the whole nervous system with all its sense-organs, must have
arisen through the same factors of evolution in numerous independent
genealogical lines. And it must not be supposed that this is all; what
is true of the sense-organs--that they are necessities--is undoubtedly
true also of all parts and organs of the animal body, both as a whole
and in every detail. It cannot be demonstrated in all cases, but it
is nevertheless certain that this applies also to all the organs of
movement, digestion, and reproduction, to all animal groups and also
to the differences between them, even although these may not always be
obvious adaptations to the conditions of life. What part is left for
mutation to play if almost everything is an adaptation? Possibly the
specific differences; and these in point of fact cannot in many cases
be interpreted with certainty as adaptations, though this can hardly
be taken as a proof that they are not. Possibly also the geometrical
skeletons of many unicellulars, in which again we cannot recognize any
definite relation to the mode of life. It is easy enough to conceive
of the wondrously regular and often very complex siliceous skeleton
of the Radiolarians or Diatoms as due to saltatory mutations, and
'leaps' of considerable magnitude must certainly have been necessary
to produce some of the manifold transformations here as everywhere
else. But whether these are or are not without importance for the
life of the organisms, we are in the meantime quite unable to decide.
Here too it is well to be cautious in concluding that these organic
'crystallizations' are without importance, and therefore to infer that
they have arisen suddenly from purely internal causes. One of the
experts on Diatoms, F. Schütt, has shown us that differences in length
in the skeletal process of the Peridineæ have a definite relation to
their power of floating in the sea-water, that the long skeletal arms
or horns which these microscopic vegetable organisms extend into the
surrounding water form a float-apparatus, for their friction against
the particles of the water prevents sinking and enables them to
float for a considerable time at approximately the same level. These
skeletal forms are thus adaptations, and Chun has recently been able to
corroborate the conclusion that this adaptation is exactly regulated,
for the length of these horns varies with the specific gravity of
the different ocean-currents, species with 'monstrously long' horns
occurring, for instance, in the Gulf of Guinea, which is distinguished
by its low salinity and high temperature (Fig. 131, _A_), while in
the equatorial currents with higher salinity and cooler water, and
thus a higher specific gravity, there is a predominance of species
of Peridineæ with 'very short' processes and relatively undeveloped
float-apparatus (Fig. 131, _B_). It could be seen clearly in the course
of the voyage that the long-armed Peridineæ became more abundant as the
ship passed from the North Equatorial current into the Gulf of Guinea,
and that by and by they held the field altogether, but later, when the
'Valdivia' entered into the South Equatorial current, they disappeared
'all at once.' Thus in this case, in which the veil over the relations
between form and function in unicellular organisms has been lifted a
little, we recognize that the smallest parts of the cell-body obey
the laws of adaptation, and consistent thinking must lead us to the
conviction that even in the most lowly organisms the whole structure in
all its essential features depends upon adaptation.
[Illustration: FIG. 131. Peridineæ: species of _Ceratium_. _A_, from
the Gulf of Guinea. _B_, from the South Equatorial currents. After
Chun.]
If the horns of the Peridineæ grow to twelve times the usual length
in adaptation to life in sea-water with a salinity increased to the
extent of .002 per cent., then undoubtedly not only the protoplasmic
particles of the body which form the horns, but all the rest as well,
may be capable of adaptation; and if the _Peridinium_ protoplasm has
this power of adapting itself to the external conditions, then the
capacity for adaptation must be a general character of all unicellular
organisms, or rather of all living substance. As will be seen later
on, we shall be brought to the same conclusion by different lines of
evidence. But a recognition of this must greatly restrict the sphere of
operation which we can attribute to saltatory mutations in the sense
in which the term is used by De Vries, for adaptations from their very
nature cannot arise suddenly, but must originate gradually and step by
step, from 'variations' which combine with one another in a definite
direction under the influence of the indirect, that is, selective
influence of the conditions.
According to the theory of De Vries it seems as if 'variations,'
augmented by selection, could never become constant, and that even the
degree to which they can be augmented is very limited. As far as this
last point is concerned, De Vries seems to me to overlook the fact
that every increase in a character must have limits set by the harmony
of the parts, which cannot be exceeded unless other parts are being
varied at the same time. Artificial selection, in fact, in many cases
reaches a limit which it cannot pass, because it has no control over
the unknown other parts which ought to be varied, in order that the
character desired may be increased still further. Natural selection
would in many cases be able to accomplish this, provided that the
variation is useful. But of what use is it to the beetroot when its
sugar-content is doubled, or to the Anderbeck oats to be highly prized
by man? And yet many individual characters have been very considerably
increased in domesticated animals by selection: of these we need only
call to mind the Japanese cock with tail-feathers twelve feet long.
But undoubtedly these artificial variations do not usually 'breed true'
in the sense that De Vries's mutations of _Œnothera lamarckiana_ did,
that is to say, they only transmit their characters in purity with the
continual co-operation of artificial selection. This at least appears
to be the case, according to De Vries, in the ennobled cereal races,
which, if cultivated in quantities, rapidly degenerate. In many animal
breeds, however, this is not the case to the same degree; many, indeed
the majority, of the most distinct races of pigeon breed true, and only
degenerate when they are crossed with others.
De Vries regards it as a mistake to believe that artificial selection,
persevered in for a long time, will succeed in producing a breed
which will--as he expresses it--be independent of further selection
and will maintain itself in purity. Experience cannot decide this, as
we have not command over the unlimited time necessary for selection,
but theoretically it is quite intelligible that a variation which had
arisen through selection would be more apt to breed true the longer
selection was practised, and there is nothing to prevent it becoming
ultimately quite as constant as a natural species. For, at the
beginning of breeding, we must assume that the variation is contained
in only a small number of ids; as the number of generations mounts up,
more and more numerous ids with this variation will go to make up the
germ-plasm, and the more the breed-ids preponderate the less likelihood
will there be that a reversion to the parent-form will be brought about
by the chances of reducing-division and amphimixis. That most if not
all breeds of pigeon still contain ids of the ancestral form in the
germ-plasm, although probably only a small number of them, we see from
the occasional reversion to the rock-dove which occurs when species
are repeatedly crossed, but that ancestral ids may also be contained
in the germ-plasm of long-established natural species is shown by the
occurrence of zebra-striping in horse-hybrids. We can understand why
these ancestral ids should not have been removed long ago from the
germ-plasm by natural selection, since they are not injurious and may
remain, so to speak, undetected. It is only when they have an injurious
effect by endangering the purity of the new species-type that they can
and must be eliminated by natural selection, and this does not cease
to operate, as the human breeder does, but continues without pause or
break.
I therefore regard it as a mistake on the part of De Vries to exclude
fluctuating variation from a share in the transformation of organisms.
Indeed, I believe that it plays the largest part, because adaptations
cannot arise from mutations, or can only do so exceptionally, and
because whole families, orders, and even classes are based on
adaptations, especially as regards their chief characters. I need only
recall the various families of parasitic Crustaceans, the Cetaceans,
the birds, and the bats. None of these groups can have arisen through
saltatory, perhaps even retrogressive, 'mutation': they can only have
arisen through variation in a definite direction, which we can think of
only as due to the selection of the fluctuations of the determinants of
the germ-plasm which are continually presenting themselves.
The difference between 'fluctuating' variability and 'mutation' seems
to me to lie in this: that the former has always its basis only in
a small majority of the ids of the germ-plasm, while the mutation
must be present in most of the ids if it is to be stably transmitted
from the very first. How that comes about we cannot tell, but we
may suppose that similar influences causing variation within the
germ-plasm may bring about variation of many ids in the same direction.
I need only recall what I have already said as to the origin of
saltatory variations, such as the copper-beech and similar cases. The
experiments made by De Vries seem to me to give a weighty support to
my interpretation of these phenomena. De Vries himself distinguishes a
'pre-mutation period,' just as I have assumed that the variations which
spring suddenly into expression have been in course of preparation
within the germ-plasm by means of germinal selection for a long time
beforehand. At first perhaps only in a few ids, but afterwards in
many, a new state of equilibrium of the determinant-system would be
established, which would remain invisible until the chance of reducing
division and amphimixis gave predominance to a decided majority of the
'mutation-ids.' In the experiments made by De Vries the same seven new
'species' were produced repeatedly and independently of one another in
different generations of _Œnothera lamarckiana_, and we thus see that
the same constellations (states of equilibrium) had developed in many
specimens of the parent plant, and that it depended on the proportion
in which the ids containing these were represented in the seed whether
one or another of the new 'species' was produced.
My interpretation, according to which a larger or smaller number of
ids were the bearers of the new forms, receives further support from
the experiments, for the new species did not always breed true. Thus
De Vries found one species, _Œnothera scintillans_, which only yielded
35-40 per cent. of heirs, or in another group about 70 per cent.; the
other descendants belonged to the forms _lamarckiana_ or _oblonga_, but
the number of pure heirs could be increased by selection!
I cannot devote sufficient space to go fully into these very
interesting experiments; but one point must still be referred to:
the parent form, _Œnothera lamarckiana_, was very variable from
the beginning, that is, it exhibited a high degree of fluctuating
variability. This tells in favour, on the one hand, of a deep-rooted
connexion between 'variation' and 'mutation,' and, on the other hand,
it indicates that saltatory variation may be excited by transference
to changed conditions of life--as Darwin in his day supposed, and as
I have endeavoured to show in the foregoing discussion. De Vries
assumes mutation-periods, I believe rightly; but they are not periods
prescribed, so to speak, from within, as those who believe in a
'phyletic force' must suppose; they are caused by the influences of the
environment which affect the nutritive stream within the germ-plasm,
and which, increasing latently, bring about in part mere variability,
in part mutations, just as I have indicated in the section on Isolation
(vol. ii. p. 280), and indeed, in one of my earliest contributions to
the theory of descent[30]. In that essay I suggested the conclusion
that periods of constancy alternate with periods of variability,
basing my opinion on general considerations, and on Hilgendorf's study
of the Steinheim snail-shells. According to De Vries's _Œnothera_
experiments we may assume that periods of increased variability may
lead to the marked variations sometimes affecting several characters
simultaneously, and occurring in many ids, which have hitherto been
called 'saltatory' variations, and which we should perhaps do well to
call in future, with De Vries, _mutations_.
[30] _Ueber den Einfluss der Isolirung auf die Artbildung_, Leipzig,
1872, p. 51.
We cannot yet determine how far the influence of such mutations
reaches. I think it is plain that De Vries himself overestimated it,
but how many of the species-types which we find to-day depend upon
mere mutation can only be decided with any certainty after further
investigations. For the present it is well to be clear as to the
validity of the general conclusion, that all 'complex,' and especially
all 'harmonious,' adaptation, must depend, not upon 'mutation,' but
only upon 'variation' guided by selection. As species are essentially
complexes of adaptation, originating from a basis of previous complexes
of adaptation, there remains, as far as I can see, only the small field
of indifferent characters to be determined by mutations, unless indeed
we are to include under the term 'mutation' all variations which gain
stability, but this would be merely a play upon words. In my opinion
_there is no definable boundary line between variation and mutation,
and the difference between these two phenomena depends solely on
the number--larger or smaller--of ids which have varied in the same
direction_.
LECTURE XXXIV
ORIGIN OF THE SPECIFIC TYPE (_continued_)
Illustration of phyletic evolution by an analogy--Reconciliation
of Nägeli and Darwin--Unity of the specific type furthered by
climatic variation--By natural selection: illustration from aquatic
animals--Direct path of evolution--Natural selection works in
association with amphigony--Influence of isolation in defining the
specific type--Duration of the periods of constancy--The Siberian
pine-jays--Species are, so to speak, 'variable crystals'--Gradual
increase of constancy and decrease of reversions--Physiological
segregation of species through mutual sterility--Romanes's
physiological selection--Breeds of domestic animals mutually fertile,
presumably therefore 'amiktic' species also--Mutual fertility
in plant species--Mutual sterility certainly not a condition of
the splitting up of species--Splitting up of species without
amphigony--Lichens--Splitting up of species apart from isolation and
mutual sterility, _Lepus variabilis_.