varying in kind and far more so in degree can be guided _simultaneously_ by
personal selection; how in strict conformity with its adaptive wants every
part is modified, or preserved unmodified; how a given articulation can
undergo modifications, causing it to disappear on one side, to grow in
volume on another, and to continue unaltered on a third. For every part
that is perfectly adapted, although it can fluctuate slightly, yet can
never undergo a permanent alteration in the ascending or descending
direction because every plus and every minus variation which has attained
selective value would be eliminated by personal selection in the course of
time. Therefore, a definite direction of variation cannot arise in such
cases and we have also reached, as it seems to me, a satisfactory
explanation of the _constancy_ of well-adapted species and characters.
Hitherto I have spoken only of plus and minus variation. But there exist,
as we know, not only variations of size but also variations of _kind_; and
the coloration of the wings of butterflies, which we chose above as our
example, would fall, according to the ordinary usage of speech, under just
this head of variations of quality. The question arises, therefore, Have
the principles just developed any claim to validity in the explanation of
_qualitative_ modifications?
In considering this question it should be carefully borne in mind that by
far the largest part of the qualitative modifications falling under this
head rest on _quantitative_ changes. Of course, chemical transformations,
which usually also involve quantitative {47} alterations, cannot be reduced
to the processes of augmentation described, inasmuch as these, by their
very nature, can be effected only in living elements capable of increase by
propagation; but the interference of selection does not begin originally
with the constitutional predisposition (_Anlagen_) of the germ, i. e. with
the determinants, but with the ultimate units of life, the _biophores_.
A determinant must be composed of heterogeneous biophores, and on their
numerical proportion reposes, according to our hypothesis, their specific
nature. If that proportion is altered, so also is the character of the
determinant. But disturbances of this numerical proportion must result at
once on proof of their usefulness, or as soon as the modifications
determined thereby in the inward character of the determinant turn out to
be of utility. For fluctuations of nutriment and the struggle for
nutriment, with its sequent preference of the strongest, must take place
between the various species of the biophores as well as between the species
of the determinants. But changes in the quantitative ratios of the
biophores appear to us qualitative changes in the corresponding
determinants, somewhat as a simple augmentation of a determinant, for
example, that of a hair, may on its development appear to us as a
qualitative change, a spot on the skin where previously only isolated hairs
stood being now densely crowded with them, and assuming thus the character
of a downy piece of fur. The single hair need not have changed in this
process, and yet the spot has virtually undergone a qualitative
modification. The majority of the changes that appear to us qualitative
rest on invisible _quantitative_ changes, and such can be produced at all
times and _at all stages_ {48} _of the vital units_ by germinal selection.
In a similar manner are induced the most varied qualitative changes of the
corresponding determinants and of the characters conditioned thereby, just
as changes in the numerical proportions of atoms produce essential changes
in the properties of a chemical molecule.
In this way we acquire an approximate conception of the possible mechanical
_modus operandi_ of actual events--namely, of the manner in which the
useful variations required by the conditions of life _can_ always, that is,
very frequently, make their appearance. This possibility is the sole
condition of our being able to understand how different parts of the body,
absolutely undefined in extent, can appear as variational units and vary in
the same or in different directions, according to the special needs of the
case, or as the conditions of life prescribe. Thus, for example, in the
case of the butterfly's wings it rests entirely with utility to decide the
size and the shape of the spots that shall vary simultaneously in the same
direction. At one time the whole under surface of the wing appears as the
variational unit and has the same color; at another the inside half, which
is dark, is contrasted with the outside half which is bright; or the same
contrast will exist between the anterior and posterior halves; or, finally,
narrow stripes or line-shaped streaks will behave as variational units and
form contrasts with manifold kinds of spots or with the broader intervals
between them, with the result that the picture of a leaf or of another
protected species is produced.
I must refrain from entering into the details of such cases and shall
illustrate my views regarding the color-transformations of butterflies'
wings by the simplest {49} conceivable example--viz. that of the uniform
change of color on the entire under surface of the wing.
Suppose, for example, that the ancestral species of a certain
forest-butterfly habitually reposed on branches which hung near the ground
and were covered with dry or rotten leaves; such a species would assume on
its under surface a protective coloring which by its dark, brown, yellow,
or red tints would tend toward similarity with such leaves. If, however,
the descendants of this species should be subsequently compelled, no matter
from what cause, to adopt the habit of resting on the green-leafed branches
higher up, then from that period on the brown coloring would act less
protectively than the shades verging towards green. And a process of
selection will have set in which consisted first in giving preference only
to such persons whose brown and yellow tints showed a tendency to green.
Only on the assumption that such shades were possible by a displacement in
the quantitative proportions of the different kinds of biophores composing
the determinants of the scales affected, was a further development in the
direction of green possible. Such being the case, however, that development
_had to_ result; because fluctuations in the numerical proportions of the
biophores are always taking place, and consequently the material for
germinal selection is always at hand. At present it is impossible to
determine exactly the magnitude of the initial stages of the deviations
thus brought about and promoted by the sexual blending of characters; but
it may perhaps be ascertained in the future, with exceptionally favorable
material. Pending such special observations, however, it can only be said
_a priori_ that slight changes in the composition of a determinant do not
necessarily {50} condition similar slight deviations of the corresponding
character,--in this case the color,--just as slight changes in the atomic
composition of a molecule may result in bestowing upon the latter widely
different properties. As soon, however, as the beginning has been made and
a definite direction has been imparted to the variation, as the result of
this or that primary variation's being preferred, the selective process
must continue until the highest degree of faithfulness required by the
species in the imitation of fresh leaves has been attained.
That the foregoing process has actually taken place is evidenced not only
by the presence of the beginnings of such transformations, as found for
example in some greenish-tinted specimens of Kallima, but mainly by certain
species of the South American genus Catonephele, all of which are
forest-butterflies, and which, with many species having dark-brown under
surfaces, present some also with bright green under surfaces--a green that
is not like the fresh green of our beech and oak trees, but resembles the
bright under surface of the cherry-laurel leaf, and is the color of the
under surfaces of the thick, leathery leaves, colored dark-green above,
borne by many trees in the tropics.
The difference between this and the old conception of the selection-process
consists not only in the fact that a large number of individuals with the
initial stages of the desired variation is present from the beginning, for
always innumerable plus and minus variations exist, but principally in the
circumstance that the constant uninterrupted progress of the process after
it is once begun is assured, that there can never be a lack of
progressively advantageous variations in a large number of individuals.
Selection, {51} therefore, is now not compelled to wait for accidental
variations but produces such itself, whenever the required elements for the
purpose are present. Now, where it is a question simply of the enlargement
or diminution of a part, or of a part of a part, these variations are
always present, and in modifications of quality they are at least present
in many cases.
This is the only way in which I can see a possibility of explaining
phenomena of _mimicry_--the imitation of one species by another. The useful
variations must be produced in the germ itself by internal
selection-processes if this class of facts is to be rendered intelligible.
I refer to the mimicry of an exempt species by two or three other species,
or, the aping of _different_ exempt patterns by _one_ species in need of
protection. It must be conceded to Darwin and Wallace that some degree of
similarity between the copy and the imitation was present from the start,
at least in very many cases;[17] but in no case would this have been
sufficient had not slight shades of coloring afforded some hold for
personal selection, and in this way furnished a basis for independent
germinal selection acting only in the direction indicated. It would have
been impossible for such a minute similarity in the design, and
particularly in the shades of the coloration, ever to have arisen, if the
process of adaptation rested entirely {52} on personal selection. Were this
so, a complete scale of the most varied shades of color must have been
continually presented as variations in every species, which certainly is
not the case. For example, when the exempt species _Acraea Egina_, whose
coloration is a brick-red, a color common only in the genus Acraea, is
mimicked by two other butterflies, a Papilio and a Pseudacraea, so
deceptively that not only the cut of the wings and the pattern of their
markings, but also that precise shade of brick-red, which is scarcely ever
met with in diurnal butterflies, are produced, assuredly such a result
cannot rest on accidental, but must be the outcome of a _definitely
directed_, variation, produced by utility. We cannot assume that such a
coloration has appeared as an _accidental_ variation in just and in only
these two species, which fly together with the _Acraea_ in the same
localities of the same country and same part of the world--the Gold Coast
of Africa. It is conceivable, indeed, that non-directed variation should
have accidentally produced this brick-red _in a single case_, but that it
should have done so three times and in three species, which live together
but are otherwise not related, is a far more violent and improbable
assumption than that of a causal connexion of this coincidence. Now
hundreds of cases of such mimicry exist in which the color-tints of the
copy are met with again in more or less precise and sometimes in
exceedingly exact imitations, and there are thousands of cases in which the
color-tint of a bark, of a definite leaf, of a definite blossom, is
repeated _exactly_ in the protectively colored insect. In such cases there
can be no question of accident, but _the variations presented to personal
selection must themselves have been produced by the principle of the
survival of the_ {53} _fit!_ And this is effected, as I am inclined to
believe, through such profound processes of selection in the interior of
the germ-plasm as I have endeavored to sketch to you to-day under the title
of germinal selection.
I am perfectly well aware how schematic my presentation of this process is,
and must be at present, owing mainly to our inability to gain exact
knowledge concerning the fundamental germinal constituents here assumed.
But I regard its existence as assured, although I by no means underrate the
fact that eminent thinkers, like Herbert Spencer, contest its validity and
believe they are warranted in assuming a germ which is composed of _similar
units_. I strongly doubt whether even so much as a _formal_ explanation of
the phenomena can be arrived at in this manner. So far as direct
observation is concerned, the two theories stand on an equal footing, for
neither my dissimilar, nor Spencer's similar, units of germinal substance
can be _seen_ directly.
The attempt has been recently made to discredit my _Anlagen_, or
constitutional germ-elements, on the ground that they are simply a
subtilised reproduction of Bonnet's old theory of preformation.[18] This
{54} impression is very likely based upon ignorance of the real character
of Bonnet's theory. I will not go into further details here, particularly
as Whitman, in several excellently written and finely conceived essays, has
recently afforded opportunity for every one to inform himself on the
subject. My determinants and groups of determinants have nothing to do with
the preformations of Bonnet; in a sense they are the exact opposites of
them; they are simply _those living parts of the germ whose presence
determines the appearance of a definite organ of a definite character in
{55} the course of normal evolution_. In this form they appear to me to be
an absolutely necessary and unavoidable inference from the facts. There
_must_ be contained in the germ parts that correspond to definite parts of
the complete organism, that is, parts that constitute the reason why such
other parts are formed.
It is conceded even by my opponents that the reason why one egg produces a
chicken and another a duck is not to be sought in external conditions, but
lies in a difference of the germinal substance. Nor can they deny that a
difference of germinal substance must also constitute the reason why a
slight _hereditary_ difference should exist between two filial organisms.
Should there now, in a possible instance, be present between them a second,
a third, a fourth, or a hundredth difference of hereditary character, each
of which could vary from the germ, then, certainly, some second, third,
fourth, or hundredth part of the germ must have been different; for whence,
otherwise, should the heredity of the differences be derived, seeing that
external influences affecting the organism in the course of evolution
induce only non-transmissible and transient deviations? But the fact that
every complex organism is actually composed of a very large number of parts
independently alterable from the germ, follows not only from the comparison
of allied species, but also and principally from the experiments long
conducted by man in artificial selection, and by the consequent and not
infrequent change of only a single part which happens to claim his
interest; for example, the tail-feathers of the cock, the fruit of the
gooseberry, the color of a single feather or group of feathers, and so on.
But a still more cogent proof is furnished by the degeneration of parts
grown {56} useless, for this process can be carried on to almost any extent
without the rest of the body necessarily becoming involved in sympathetic
alteration. Whole members may become rudimentary, like the hind limbs of
the whale, or it may be only single toes or parts of toes; the whole wing
may degenerate in the females of a butterfly species, or only a small
circular group of wing-scales, in the place of which a so-called "window"
arises. A single vein of the wing also may degenerate and disappear, or the
process may affect only a part of it, and this may happen in one sex only
of a species. In such cases the rest of the body may remain absolutely
unaltered; only a stone is taken out of the mosaic.
The assumption, thus, appears to me irresistible, that every such
hereditary and likewise independent and very slight change of the body
rests on some alteration of a _single_ definite particle of the germinal
substance, and not as Spencer and his followers would have it, on a change
of _all_ the units of the germ. If the germinal substance consisted wholly
of like units, then in every change, were it only of a single character,
_each_ of these units would have to undergo exactly the same modification.
Now I do not see how this is possible.
But it may be that Spencer's assumption is the _simpler_ one? Quite the
contrary, its simplicity is merely apparent. Whilst my theory needs for
each modification only a modification of _one_ constitutional element of
the germ, that is, of _one_ particle of the germinal substance, according
to Spencer _every_ particle of that substance must change, for they are all
supposed to be and to remain alike. But seeing that all hereditary
differences, be they of individuals, races, {57} or species, must be
contained in the germ, the obligation rests on these similar units, or
rather the capacity is required of them, to produce in themselves a truly
enormous number of differences. But this is possible only provided their
composition is an exceedingly complex one, or only on the condition that in
every one of them are contained as many alterable particles as according to
my view there are contained determinants in the whole germ. _The
differences that I put into the whole germ, Spencer and his followers are
obliged to put into every single unit of the germinal substance._ My
position on this point appears to me incontrovertible so long as it is
certain that the single characters can vary hereditarily; for, if a thing
can vary independently, that is, _of its own accord_, and _from the germ_,
then that thing must be represented in the germ by some particle of the
substance, _and be represented there in such wise that a change of the
representative particle produces no other change in the organism developing
from the germ than such as are connected with the part which depends on
it_. I conceive that even on the assumption of my constitutional elements
(_Anlagen_) the germ-plasm is complex enough, and that there is no need of
increasing its complexity to a fabulous extent. Be that as it may, the
person who fancies he can produce a complex organism from a _really_ simple
germinal substance is mistaken: he has not yet thoroughly pondered the
problem. The so-called "epigenetic" theory with its _similar_ germinal
units is therefore naught else than an evolution-theory where the primary
constitutional elements are reduced to the molecules and atoms--a view
which in my judgment is inadmissible. A _real_ {58} epigenesis from
absolutely _homogeneous_ and not merely _like_ units is not thinkable.
All value has been denied my doctrine of determinants[19] on the ground
that it only shifts the riddles of evolution to an invisible terrain where
it is impossible for research to gain a foothold.
Now I have indeed to admit that no information can be gained concerning my
determinants, either with the aided or with the unaided eye. But
fortunately there exists in man another organ which may be of use in
fathoming the riddles of nature and this organ which is called the brain
has in times past often borne him out in the assumption of invisible
entities--entities that have not always proved unfruitful for science by
reason of that defect, in proof whereof we may instance the familiar
assumptions of atoms and molecules. Probably the biophores also will be
included under that head if the determinants should be adjudged utterly
unproductive. But so far I have always held that assumptions of this kind
_are_ really productive, if they are only capable of being used, so to
speak, as a _formula_, whereby to perform our computations, unconcerned for
the time being as to what shall be its subsequent fate. Now, as I take it,
the determinants have had fruitful results, as their application to various
biological problems shows. Is it no advance that we are able to reduce the
scission of a form of life into two or several forms subject to separately
continued but recurrent changes,--I refer to dimorphism and
polymorphism,--that we are able to reduce such phenomena to the formula of
male, female, and worker determinants? It has been, I think, {59} rendered
conceivable how these diverse and extremely minute adaptations could have
developed side by side in the same germ-plasm, under the guidance of
selection; how sterile forms could be _hereditarily_ established and
transformed in just that manner which best suits with their special duties;
and how they themselves under the right circumstances could subsequently
split up into two or even into three new forms. Surely at least the unclear
conception of an _adaptively_ transformative influence of food must be
discarded. It is true, we cannot penetrate by this hypothesis to the last
root of the phenomena. The hotspurs of biology, who clamor to know
forthwith how the molecules behave, will scarcely repress their
dissatisfaction[20] with such provisional knowledge--forgetful that _all
our knowledge is and remains throughout provisional_.
But I shall not enter more minutely into the question whether epigenesis or
evolution is the right foundation of the theory of development, but shall
content myself with having shown, first, that it is illusory to imagine
that epigenesis admits of a simpler structure of the germ, (the precise
opposite is true,) and secondly, that there are phenomena that can be
understood only by an evolution-theory. Such a phenomenon is {60} the
_guidance of variation by utility_, which we have considered to-day. For
without primary constituents of the germ, whether they are called as I call
them, determinants, or something else, _germinal selection_, or guidance of
variation by personal selection, is impossible; for where all units are
alike there can be no struggle, no preference of the best. And yet such a
guidance of variation exists and demands its explanation, and the early
assumptions of a "definitely directed variation" such as Naegeli and
Askenasy made are insufficient, for the reason that they posit only
_internal_ forces as the foundations thereof, and because, as I have
attempted to show, the harmony of the direction of variation with the
requirements of the conditions of life subsists and represents the riddle
to be solved. _The degree of adaptiveness which a part possesses itself
evokes the direction of variation of that part._
This proposition seems to me to round off the whole theory of selection and
to give to it that degree of inner perfection and completeness which is
necessary to protect it against the many doubts which have gathered around
it on all sides like so many lowering thunder-clouds. The moment variation
is determined substantially though not exclusively by the adaptiveness
itself, all these doubts fall to the ground, with _one_ exception, that of
the utility of the initial steps. But just this objection is the least
weighty. Without doubt the theory requires that the initial steps of a
variation should also have selective value; otherwise personal selection
and hence germinal selection could not set in. Since, however, as I have
before pointed out, _in no case can we pretend to a judgment regarding the
selective value of a modification, or have any_ {61} _experience thereof_,
therefore the assumption that in a given case where a character is
transformed the original initial steps of the variation did have selective
value, is not only as probable as the opposed assumption that they had
none, but is _infinitely more probable_, for with this we can give an
intelligible explanation of the mysterious fact of adaptation, while with
that we cannot. Consequently, unless we are resolved to give up all
attempts whatsoever at explanation, we are forced to the assumption that
the initial steps of all actually affected adaptations possessed selective
value.