marked divergences, but rather exhibit a great harmony of individuals
even in detail, and the conception of species is more readily applied
to these. It would certainly be foolish to give it up, since we should
then lose all possibility of arriving at any sort of orientation
among the enormous wealth of forms in nature. But at the same time we
must not forget that these 'typical' species only appear so to our
short-sighted vision--short-sighted as far as time is concerned--and
that they are connected from long-past times with 'species' which
lived at an earlier date, by just such transition stages as connect
the Celebes species of to-day, which are all living at the same
time. The world of life on the earth only presents at any given time
a 'cross-section of its genealogical tree,' and according as its
branches grow out vertically or horizontally we receive an impression
of typical, sharply defined species or of circles or chains of forms.
In the first case the evolution of new species was associated with the
dying out of the horizontal branches, and the end-twigs of the branch
stand beside each other now apparently isolated and sharply defined;
in the other case only a portion of the ancestral species has been
transmuted, and the other part continues to live alongside of the
species derived from it, and perhaps repeats the process of giving off
a varied race of descendants.
The last thirty years have yielded much palæontological evidence of
the successive stages of species-transformation. In quietly deposited
horizontal strata of the earth's crust, lying one above another, the
whole phyletic history of a group of snail-species has repeatedly
been found in historic order, the oldest in the deepest layer, the
youngest in the uppermost, and the numerous and often very divergent
'species' of a particular deposit are connected by transition forms in
the intermediate strata. From the point of view of time, therefore,
these are not 'typical' species, but circles of forms in a state of
variability.
The most beautiful of such cases are the _Planorbis_ species from the
small lacustrine deposits of Steinheim in Swabia, the _Paludina_ strata
of Slavonia, and various groups of Ammonites.
These cases have been described and discussed so often that I need only
refer to their most essential features.
The _Planorbis_ strata of Steinheim were first investigated, from
the point of view of the theory of descent, by Hilgendorf (1866).
He described nineteen different varieties, which, as they are all
connected in chronological succession with each other, he grouped
together under the name of _Planorbis multiformis_. These little
freshwater snails are found in millions in the strata of the former
lake-basin of Steinheim, and they are arranged in so orderly and
regular a manner that two observers, working independently and at
different times, succeeded in building up the genealogical tree
in almost the same way. According to Alpheus Hyatt, the later
investigator, all the forms are derived from one ancestral form,
_Planorbis lævis_, from which four different series have descended, one
of them splitting up again into three subordinate series.
All the individual members of these series are connected by
intermediate forms in such a manner that a long period of constancy of
forms seems to be succeeded by a shorter period of transformation, from
which again a relatively constant form arises.
We see, therefore, that the idea of species is fully justified in
a certain sense; we find indeed at certain times a breaking up of
the fixed specific type, the species becomes variable, but soon the
medley of forms clears up again and a new constant form arises--_a new
species_, which remains the same for a long series of generations,
until ultimately it too begins to waver, and is transformed once
more. But if we were to place side by side the cross-sections of this
genealogical tree at different levels, we should only see several
well-defined species between which no intermediate forms could be
recognized; these would only be found in the intermediate strata.
The problem we have now to discuss is, how it comes about that
relatively sharply defined species exist which are connected with
ancestral forms further back, but which form among themselves an
exclusive, more or less homogeneous, host of individuals. How does it
happen that we everywhere find a specific type, and not an endless
number of individual forms connected with one another in all directions?
This would require no further explanation if a phyletic evolutionary
force impelled the forms of life to vary in a definite manner, and thus
to become transmuted into new forms in the course of generations. In
that case the whole genealogical tree of the organisms on the earth
must have been potentially contained in the lowest moneron, so that,
given time and the most indispensable general conditions of existence,
the living world just as we know it must have resulted. Nägeli was the
first to express this view, and he followed it out consistently, not
even hesitating to deny the existence of all processes of selection,
and to represent the whole of evolution as a process conditioned by
this phyletic force, which would have given rise to the world of
organisms which has actually arisen, even if the conditions of life at
the different periods of the earth's history had been other than they
were. I have always combated this idea, without however overlooking
that it is based upon facts which--at that time at any rate--gave it a
certain justification. We cannot pass it by without giving some other
interpretation of the facts. Following Nägeli, the botanist Askenasy
championed this view of 'variation in a different direction,' which
gives rise to new forms; and in more recent times Romanes, Henslow,
and Eimer expressed similar views, and--although they did not actually
dispute the existence of processes of selection--they attributed a much
less important rôle to them, and referred the phyletic genealogical
tree of organisms in the main to other and internal causes.
Like Nägeli himself, his followers have laid stress upon the fact that
natural selection cannot be the cause of the evolution and succession
of particular species, because the differences which separate species
from species are not of an adaptive nature, and therefore cannot
depend upon selection; but if the step from one species to the next
succeeding one does not depend upon adaptation, then the greater steps
to genera, families, and orders cannot be referred to it either,
since these can only be thought of as depending upon a long-continued
splitting up of species. Genera, families, and all higher groups
must be recognized as conventional categories, not as real divisions
existing in nature itself. Even Treviranus and Lamarck maintained that
the differences between genera depended just as much upon our estimate,
our intellectual convenience, as do the differences between species.
All forms were originally connected, though they may not be so now, and
if the species are really not distinguished by adaptive characters,
then neither are any other grades of our classificatory system,
neither order nor classes, since they all depend originally on the
transmutation of species. It was therefore quite consistent of Nägeli
to seek the mainspring of organic evolution, not in adaptation, but in
an unknown evolutionary force. Thus he refused to recognize adaptation
as a consequence of selection, but regarded it, as Lamarck had done,
as the direct effect of external conditions, and as an entirely
subordinate factor in the transmutation of forms.
Nägeli and his modern successors conceive of phyletic evolution as
depending upon definitely directed variation, resulting from internal
causes and occurring at definite times, which of necessity causes the
existing form to be transformed into a new one. To them the species
appears, so to speak, as a vital crystallization, or to use Herbert
Spencer's phraseology, as an equilibrium of living matter, which
becomes displaced from time to time, and passes over into a new state
of equilibrium, being transmuted into a new species, something like the
pictures in a kaleidoscope. The species is thus something conditioned
from within, which must be as it is and could not be otherwise, just
like a crystal which crystallizes in one particular system and not
in another; it must be just thus or it could not be at all. From the
point of view of this theory it would be easy to understand that the
thousands or millions of individuals composing a species all agree in
essentials--that a specific type exists.
But this conception can hardly be entirely correct, although there is
some truth at its foundation, namely, that germinal variations which
arise independently are the basal roots of all transmutation. But the
species is not simply the result of these internal processes, it is
not even mainly so; it is not the result of an internal, definitely
directed developmental force, even if we attempt to think out such a
force in a purely scientific or mechanical, instead of a mystical,
sense. It seems clear to me that the species is not a life-crystal in
the sense that it must, like a rock-crystal, take form in a particular
way and in no other for purely internal reasons and by virtue of
its physical constitution; the species is essentially a complex of
adaptations, of modern adaptations which have been recently acquired,
and of inherited adaptations handed down from long ago--a complex which
might quite well have been other than it is, and indeed must have been
different if it had originated under the influence of other conditions
of life.
But of course species are not exclusively complicated systems of
adaptations, for they are at the same time 'variation-complexes,'
the individual components of which are not all adaptive, since they
do not all reach the limits of the useful or the injurious. All
transformations arise from a basis of spontaneous chance variations,
just as all forest plants grow from the soil of the forest, but do not
all grow into trees, the adaptive forms which determine the essential
character of the forest; for many species remain small and low, like
the mosses, grasses, and herbs; and these too have a share, though a
subordinate one, in determining the character of the forest, which
depends definitely, though only partially, on the loftier growths.
According to my view all adaptation depends on an alteration in
the equilibrium of the determinant system, such as must arise from
intra-germinal or even general fluctuations in the nutritive supply,
affecting larger or smaller groups of determinants and causing
variation in them to a greater or less degree. And these variations
may be in a quite definite direction, persisted in for internal
reasons, as we have already seen in the section dealing with germinal
selection. These variations are the building-stones out of which,
under the guidance of personal selection, a new specific type, that
is, a new complex of adaptations, can be established. In this type
many indifferent characters are involved, which are just as constant
characters of the species as the adaptations.
The opponents of the selection theory have often urged against it
this constancy of indifferent characters, but as soon as we cease to
restrict the principle of selection to 'persons,' and extend it also
to the lower categories of vital units, the occurrence of indifferent
characters is easily understood. To illustrate characters of this kind,
Henslow has recently called attention to the species of gentian, whose
flowers have a corona split into five tips in some species and into
six or seven in others, and we cannot possibly ascribe any biological
significance to these specific characters. It is quite possible that
they possess none; but did not even Darwin express his belief that many
peculiarities of form 'are to be attributed to the laws of growth, and
to the mutual influence of parts,' forces which he rightly refrained
from including under 'natural selection' in his sense of the word,
but which we now regard as an expression of intra-selection or of
histonal selection? It is this, in our opinion, which brings about the
co-adaptation of the parts to form a harmonious whole, which admits of
the primary adaptations to the conditions of life being followed or
accompanied by correlative secondary variations, and which plays an
important part in directing the course of every individual development,
and is therefore uninterruptedly active within the organism. We cannot
analyse the factors precisely enough to be able to demonstrate in an
individual case why the corona should be divided into four in one
species of gentian and into five in another, but we can understand
in principle that all adaptations of a species which are not primary
are determined by the compelling influence of intra-selection. And
we need not now rest content even with that, for we know that this
intra-selection--as we have already seen--is active within the
germ-plasm, and it is only a logical consequence of the principle of
germinal selection to suppose that variations of definite determinants
due to personal selection may in the germ-plasm itself give rise
to correlative variations in determinants next to them or related
to them in any way, and that these may possess the same stability
as the primary variation. This seems to me a sufficient reason why
biologically unimportant characters may become constant characters of
the species. Correlation is not effected only in the perfect organism;
it exists at every period of its life, from the germ till death, and
what it brings about is quite as inevitable as what is evoked through
adaptation by means of personal selection.
We can thus also understand that indifferent characters may be
contained not only in individual ids of the germ-plasm, but also
coincidentally in a great majority of them, as soon as we think of
them as dependent upon the characters established through personal
selection, for these must be contained in a majority of the ids.
But there is still another reason why indifferent characters should
become stable, and that is the effect of general variational influences
on all the individuals of a species, as, for instance, in many climatic
varieties, and probably also in many cultivated varieties.
But even when we have fully recognized that, from the arcana of the
germ-plasm, new minimal variations are continually cropping up, which
are biologically indifferent, and nevertheless become variational
tendencies, and may increase even to the extent of causing _visible_
differences, and that therefore varieties of snails or of butterflies,
or of any animal or plant whatever, may originate through germinal
selection alone, it cannot for a moment be supposed that the
transmutation of species depends upon this process exclusively or even
preponderantly. This was Nägeli's mistake, and that of his followers as
well, that he ascribed to his 'principle of perfecting' the essential
rôle in directing the whole movement of evolution, while the general
structure of all species shows us that they are, so to speak, built
up of adaptations. But adaptations could not be--or could only be
fortuitously and exceptionally--the _direct_ result of an internal
power of development, since the very essence of adaptational changes is
that they are variations which bring the organism into harmony with the
conditions of its life. We are therefore forced either to underestimate
greatly the part played by adaptation in every organism--and that
is what Nägeli did--or to leave the standpoint of natural science
altogether, and assume a transcendental force which varied and
adapted the species of organisms _pari passu_ with the changes in the
conditions of life during the geological evolution of our earth. This
would be a sort of pre-established harmony, through which the two
clocks of evolution--that of the earth and that of organisms--kept
exact time, although they had quite different and independent works!
But that the determining significance of adaptations in organic forms
is underestimated even now is evidenced by the continually repeated
statement that species differ, not in their adaptive characters, but in
purely morphological characters, whereas it is obvious that we are far
from being able to estimate the functions of a part with sufficient
precision to be able to say definitely whether the differences between
two nearly allied species are or are not adaptations to different
conditions. The same is true with regard to the other side of the
problem--the conditions of life. These are often to all appearances
identical in two allied species, but even where they are visibly
different it is often difficult to assert that the differences between
the two species can be interpreted with certainty as adaptations to
the specific conditions of life. At an earlier stage we discussed
the protective coloration of butterflies, and we saw that the forest
butterflies of the Tropics frequently mimicked a dry leaf on their
under surfaces. In the various regions of the extensive forest
districts of the Orinoco and the Amazon in South America there are
fifty species of the genus _Anæa_ alone, and in the resting pose all
these bear a most deceptive resemblance to a leaf, yet each of them
differs from the rest in the mingling of its colours, its brilliance,
and usually in markings when these are present. If we wished to be able
to decide whether these specific differences were of an adaptive nature
or not, we should first of all require to know in what kind of forest
two neighbouring species lived, and in what places, among what sort
of leaves, they were in the habit of settling. Even then we should at
best only know whether the species _A_ was better protected, as far as
our own eyes were concerned, among the leaves of the forest _A´_ than
the species _B_, and conversely; but we could not tell whether they
_required_ this protection, or whether the species _A_, if transferred
to the forest _B´_, would be more frequently discovered and destroyed
by its enemies than in its own forest-home, and that alone could
prove the difference to be biologically important, that is, to have
selection-value. The difficulty, indeed the impossibility, of arriving
at such decisions can perhaps be better illustrated by an example from
our indigenous fauna. No one doubts that the upper surface of the
anterior wing in the so-called banner-moth (_Catocala_) possesses a
very effective protective colouring; by day the moths rest with wings
spread out flat upon tree trunks, wooden fences, walls, &c., and they
are so excellently suited to their environment that they are usually
overlooked both by man and animals. But each of the twelve German
species of _Catocala_ has a special protective colouring; in _Catocala
fraxini_ it is a light grey, in _Catocala nupta_, a dark ash-grey, in
_Catocala elocata_ rather a yellowish-brown grey, in _Catocala sponsa_
an olive brown, in _Catocala promissa_ a mingling of whitish-grey
and olive brown, and so on. All these colourings are protective; but
could any even of our most experienced and sharp-sighted entomologists
prove that each of these different shades of colour depends upon
adaptation to the usual resting-place of the particular species to
which it belongs? And yet it is on _a priori_ grounds highly probable
that this is the case. But even this would by no means dispose of the
whole problem, for each of these protective colour schemes is composed
of several, often many, tints; it must be so if they are to fulfil
their end at all, for a uniformly coloured wing would contrast with
the bark of every tree and with every wooden fence. The wing-surface
must therefore bear on a lighter background a number of lines and
streaks varying from brown to black, and usually running zigzag across
the wing; beside these are spots of lighter colour, which complete
the deceptive picture. This 'marking' of the wing is similar in all
twelve species, and yet in each it is different in detail. It is
constant in each, and thus is a specific character. But who would
venture to undertake the task of proving that each of these streaks,
spots, zigzag lines, &c., is or is not adaptive--that the details are
necessary adaptations to the resting-place which had become habitual
to the species, or, on the other hand, simply expressions of the
variational tendencies of the elements of marking, depending upon
germinal selection? This would be an impossible task, and yet we are
here dealing with a character which, _as a whole_, is undoubtedly
adaptive; in many of the differences between other species even that is
not certain.
It seems to me, therefore, hardly reasonable to talk of the
'insufficiency of natural selection' because we are not able to
demonstrate that the minutiæ of specific characters are adaptational
results. Personal selection intervenes whenever the variations produced
by germinal selection attain to selection-value; and whether we can
determine the exact point at which this takes place in individual cases
is, as I have said before, theoretically quite indifferent.
Moreover, there are cases in which we _can_ prove that specific
differences are of an adaptive nature. When, of two nearly related
species of frog, the spermatozoon of one possesses a thick head and
that of the other a thin head, and when at the same time the micropyle
through which alone the spermatozoon can make its way into the ovum is
wide in the first species and narrow in the second, we have before us a
specific character which is obviously adaptive.
In order to gain clearness as to the significance of natural selection
in the restricted sense, that is of personal selection, it seems to
me much more important to study the different groups of animals and
plants with special reference to what they undoubtedly exhibit in the
way of adaptation. For that reason I discussed different groups of
adaptations in detail in some of the preceding lectures, although, or
rather because, they all teach us that every part of every species,
whether animal or plant, even every secretion, and indeed every habit,
every inherited instinct, is subject to adaptation to the conditions of
life. It seems difficult to refuse to admit that this is the natural
impression which this study conveys, and it is strengthened as our
knowledge increases; _that every essential part of a species is not
merely regulated by natural selection, but is originally produced
by it_, if not in the species under consideration at the time, then
in some ancestral species; and, further, that every part can adjust
itself in a high degree to the need for adaptation. It was not without
a purpose that I discussed the phenomenon of mimicry so fully, for
it, above all others, teaches us how great a power of adaptation the
organism possesses, and what insignificant and small parts may be
transformed, in a remarkable degree, in accordance with some actual
need. We saw that a butterfly might assume a colouring which diverged
entirely from that of its nearest relatives, but which caused it to
resemble an immune species of a different family, and thereby protected
it more effectively from persecution. Such a case can no more be due to
a dominating phyletic force than to a chance and sudden displacement
of the state of equilibrium of the determinant system; it can depend
only on natural selection, that is, on a sifting out of the diverse
variations offered by germinal selection, and the unhampered expression
and augmentation of those favoured.
But it is not only these minute variations, insignificant in relation
to the whole structure of the animal, which can be determined by
natural selection. The same applies to the phyletic evolution as a
whole; even that is not directed by the assumed internal principle of
development.