The prevailing belief that slow and gradual, nearly invisible changes
constitute the process of evolution in the animal and vegetable kingdom,
did not offer a strong stimulus for experimental research. No
appreciable response to any external agency was of course to be
expected. Responses were supposed to be produced, but the corresponding
outward changes would be too small to betray themselves to the
investigator.
The direct observation of the mutations of the evening-primrose has
changed the whole aspect of the problem at once. It is no longer a
matter dealing with purely hypothetical conditions. Instead of the vague
notions, uncertain hopes, and a priori conceptions, that have hitherto
confused the investigator, methods of observation have been formulated,
suitable for the attainment of definite results, the general nature of
which is already known.
To my mind the real value of the discovery [687] of the mutability of
the evening-primrose lies in its usefulness as a guide for further work.
The view that it might be an isolated case, lying outside of the usual
procedure of nature, can hardly be sustained. On such a supposition it
would be far too rare to be disclosed by the investigation of a small
number of plants from a limited area. Its appearance within the limited
field of inquiry of a single man would have been almost a miracle.
The assumption seems justified that analogous cases will be met with,
perhaps even in larger numbers, when similar methods of observation are
used in the investigation of plants of other regions. The mutable
condition may not be predicated of the evening-primroses alone. It must
be a universal phenomenon, although affecting a small proportion of the
inhabitants of any region at one time: perhaps not more than one in a
hundred species, or perhaps not more than one in a thousand, or even
fewer may be expected to exhibit it. The exact proportion is immaterial,
because the number of mutable instances among the many thousands of
species in existence must be far too large for all of them to be
submitted to close scrutiny.
It is evident from the above discussion that next in importance to the
discovery of the prototype of mutation is the formulation of methods
[688] for bringing additional instances to light. These methods may
direct effort toward two different modes of investigation. We may search
for mutable plants in nature, or we may hope to induce species to become
mutable by artificial methods. The first promises to yield results most
quickly, but the scope of the second is much greater and it may yield
results of far more importance. Indeed, if it should once become
possible to bring plants to mutate at our will and perhaps even in
arbitrarily chosen directions, there is no limit to the power we may
finally hope to gain over nature.
What is to guide us in this new line of work? Is it the minute
inspection of the features of the process in the case of the
evening-primroses? Or are we to base our hopes and our methods on
broader conceptions of nature's laws? Is it the systematic study of
species and varieties, and the biologic inquiry into their real
hereditary units? Or is the theory of descent to be our starting-point?
Are we to rest our conceptions on the experience of the breeder, or is
perhaps the geologic pedigree of all organic life to open to us better
prospects of success?
The answer to all such questions is a very simple one. All possibilities
must be considered, and no line of investigation ignored. For myself I
have based my field-researches and my [689] testing of native plants on
the hypothesis of unit-characters as deduced from Darwin's _Pangenesis_.
This conception led to the expectation of two different kinds of
variability, one slow and one sudden. The sudden ones known at the time
were considered as sports, and seemed limited to retrograde changes, or
to cases of minor importance. The idea that sudden steps might be taken
as the principal method of evolution could be derived from the
hypothesis of unit characters, but the evidence might be too remote for
a starting point for experimental investigation.
The success of my test has given proof to the contrary. Hence the
assertion that no evidence is to be considered as inadequate for the
purpose under discussion. Sometime a method of discovering, or of
producing, mutable plants may be found, but until this is done, all
facts of whatever nature or direction must be made use of. A very slight
indication may change forever the whole aspect of the problem.
The probabilities are now greatly in favor of our finding out the causes
of evolution by a close scrutiny of what really happens in nature. A
persistent study of the physiologic factors of this evolution is the
chief condition of success. To this study field-observations may
contribute as well as direct experiments, [690] microscopical
investigations as well as extended pedigree-cultures. The cooperation of
many workers is required to cover the field. Somewhere no doubt the
desired principle lies hidden, but until it is discovered, all methods
must be tried.
With this conception as the best starting point for further
investigation, we may now make a brief survey of the other phase of the
problem. We shall try to connect our observations on the
evening-primroses with the theory of descent at large.
We start with two main facts. One is the mutability of Lamarck's
primrose, and the second is the immutable condition of quite a number of
other species. Among them are some of its near allies, the common and
the small flowered evening-primrose, or _Oenothera biennis_ and _O.
muricata_.
From these facts, a very important question arises in connection with
the theory of descent. Is the mutability of our evening-primroses
temporary, or is it a permanent condition? A discussion of this problem
will give us the means of reaching a definite idea as to the scope of
our inquiries.
Let us consider the present first. If mutability is a permanent
condition, it has of course no beginning, and moreover is not due to the
[691] agency of external circumstances. Should this be granted for the
evening-primrose, it would have to be predicated for other species found
in a mutable state. Then, of course, it would be useless to investigate
the causes of mutability at large, and we should have to limit ourselves
to the testing of large numbers of plants in order to ascertain which
are mutable and which not.
If, on the other hand, mutability is not a permanent feature, it must
once have had a beginning, and this beginning itself must have had an
external cause. The amount of mutability and its possible directions may
be assumed to be due to internal causes. The determination of the moment
at which they will become active can never be the result of internal
causes. It must be assigned to some external factor, and as soon as this
is discovered the way for experimental investigation is open.
In the second place we must consider the past. On the supposition of
permanency all the ancestors of the evening-primrose must have been
mutable. By the alternative view mutability must have been a periodic
phenomenon, producing at times new qualities, and at other times leaving
the plants unchanged during long successions of generations. The present
mutable state must then have been preceded by an immutable [692]
condition, but of course thousands of mutations must have been required
to produce the evening-primroses from their most remote ancestors.
If we take the species into consideration that are not mutable at
present, we may ask how we are to harmonize them with each of the two
theories proposed. If mutability is permanent, it is manifest that the
whole pedigree of the animal and vegetable kingdom is to be considered
as built up of main mutable lines, and that the thousands of constant
species can only be taken to represent lateral branches of the
genealogic tree.
These lateral branches would have lost the capacity of mutating,
possessed by all their ancestors. And as the principle of the hypothesis
under discussion does not allow a resumption of this habit, they would
be doomed to eternal constancy until they finally die out. Loss of
mutability, under this conception, means loss of the capacity for all
further development. Only those lines of the main pedigree which have
retained this capacity would have a future; all others would die out
without any chance of progression.
If, on the other hand, mutability is not permanent, but a periodic
condition, all lines of the genealogic tree must be assumed to show
alternatively [693] mutating and constant species. Some lines may be
mutating at the present moment; others may momentarily be constant. The
mutating lines will probably sooner or later revert to the inactive
state, while the powers of development now dormant may then become
awakened on other branches.
The view of permanency represents life as being surrounded with
unavoidable death, the principle of periodicity, on the contrary,
follows the idea of resurrection, granting the possibility of future
progression for all living beings. At the same time it yields a more
hopeful prospect for experimental inquiry.
Experience must decide between the two main theories. It demonstrates
the existence of polymorphous genera, such as _Draba_ and _Viola_ and
hundreds of others. They clearly indicate a previous state of
mutability. Their systematic relation is exactly what would be expected,
if they were the result of such a period. Perhaps mutability has not
wholly ceased in them, but, might be found to survive in some of their
members. Such very rich genera however, are not the rule, but are
exceptional cases, indicating the rarity of powerful mutative changes.
On the other hand, species may remain in a state of constancy during
long, apparently during indefinite, ages.
[694] Many facts plead in favor of the constancy of species. This
principle has always been recognized by systematists. Temporarily the
current form of the theory of natural selection has assumed species to
be inconstant, ever changing and continuously being improved and adapted
to the requirements of the life-conditions. The followers of the theory
of descent believed that this conclusion was unavoidable, and were
induced to deny the manifest fact that species are constant entities.
The mutation theory gives a clew to the final combination of the two
contending ideas. Reducing the changeability of the species to distinct
and probably short periods, it at once explains how the stability of
species perfectly agrees with the principle of descent through
modification.
On the other hand, the hypothesis of mutative periods is by no means
irreconcilable with the observed facts of constancy. Such casual changes
can be proved by observations such as those upon the evening-primrose,
but it is obvious that a disproof can never be given. The principle
grants the present constancy of the vast majority of living forms, and
only claims the exceptional occurrence of definite changes.
Proofs of the constancy of species have been given in different ways.
The high degree of similarity of the individuals of most of our [695]
species has never been denied. It is observed throughout extended
localities, and during long series of years. Other proofs are afforded
by those plants which have been transported to distant localities some
time since, but do not exhibit any change as a result of this migration.
Widely dispersed plants remain the same throughout their range, provided
that they belong to a single elementary species. Many species have been
introduced from America into Europe and have spread rapidly and widely.
The Canadian horsetail (_Erigeron canadensis_), the evening-primrose and
many other instances could be given. They have not developed any special
European features after their introduction. Though exposed to other
environmental conditions and to competition with other species, they
have not succeeded in developing a new character. Such species as proved
adequate to the new environment have succeeded, while those which did
not have succumbed.
Much farther back is the separation of the species which now live both
in arctic regions and on the summits of our highest mountaintops. If we
compare the alpine flora with the arctic plants, a high degree of
similarity at once strikes us. Some forms are quite identical; others
are slightly different, manifestly representing elementary species of
the same systematic [696] type. Still others are more distant or even
belong to different genera. The latter, and even the diverging, though
nearly allied, elementary species, do not yield adequate evidence in any
direction.
They may as well have lived together in the long ages before the
separation of the now widely distant floras, or have sprung from a
common ancestor living at that time, and subsequently have changed their
habits. After excluding these unreliable instances, a good number of
species remain, which are quite the same in the arctic and alpine
regions and on the summits of distant mountain ranges. As no
transportation over such large distances can have brought them from one
locality to the other, no other explanation is left than that they have
been wholly constant and unchanged ever since the glacial period which
separated them. Obviously they must have been subjected to widely
changing conditions. The fact of their stability through all these
outward changes is the best proof that the ordinary external conditions
do not necessarily have an influence on specific evolution. They may
have such a result in some instances, in others they obviously have not.
Many arctic forms bearing the specific name of _alpinus_ justify this
conclusion. _Astragalus alpinus_, _Phleum alpinum_, _Hieracium alpinum_
and [697] others from the northern parts of Norway may be cited as
examples.
Thus Primula imperialis has been found in the Himalayas, and many other
plants of the high mountains of Java, Ceylon and northern India are
identical forms. Some species from the Cameroons and from Abyssinia have
been found on the mountains of Madagascar. Some peculiar Australian
types are represented on the summit of Kini Balu in Borneo. None of
these species, of course, are found in the intervening lowlands, and the
only possible explanation of their identity is the conception of a
common post-glacial origin, coupled with complete stability. This
stability is all the more remarkable as nearly allied but slightly
divergent forms have also been reported from almost all of these
localities. Other evidence is obtained by the comparison of ancient
plants with their living representatives. The remains in tombs of
ancient Egypt have always afforded strong support of the views of the
adherents of the theory of stability, and to my mind they still do so.
The cereals and fruits and even the flowers and leaves in the funeral
wreaths of Rameses and Amen-Hotep are the same that are still now
cultivated in Egypt. Nearly a hundred or more species have been
identified. Flowers of _Acacia_, leaves of _Mimusops_, [698] petals of
_Nymphaea_ may be cited as instances, and they are as perfectly
preserved as the best herbarium-specimens of the present time. The
petals and stamens retain their original colors, displaying them as
brightly as is consistent with their dry state.
Paleontologic evidence points to the same conclusion. Of course the
remains are incomplete, and rarely adequate for a close comparison. The
range of fluctuating variability should be examined first, but the test
of elementary species given by their constancy from seed cannot, of
course, be applied. Apart from these difficulties, paleontologists agree
in recognizing the very great age of large numbers of species. It would
require a too close survey of geologic facts to go into details on this
point. Suffice it to say that in more recent Tertiary deposits many
species have been identified with living forms. In the Miocene period
especially, the similarity of the types of phanerogamic plants with
their present offspring, becomes so striking that in a large number of
cases specific distinctions rest in greater part on theoretical
conceptions rather than on real facts. For a long time the idea
prevailed that the same species could not have existed through more than
one geologic period. Many distinctions founded on this belief have since
had to be abandoned. [699] Species of algae belonging to the
well-preserved group of the diatoms, are said to have remained unchanged
from the Carboniferous period up to the present time.
Summing up the results of this very hasty survey, we may assert that
species remain unchanged for indefinite periods, while at times they are
in the alternative condition. Then at once they produce new forms often
in large numbers, giving rise to swarms of subspecies. All facts point
to the conclusion that these periods of stability and mutability
alternate more or less regularly with one another. Of course a direct
proof of this view cannot, as yet, be given, but this conclusion is
forced upon us by a consideration of known facts bearing on the
principle of constancy and evolution.
If we are right in this general conception, we may ask further, what is
to be the exact place of our group of new evening-primroses in this
theory? In order to give an adequate answer, we must consider the whole
range of the observations from a broader point of view. First of all it
is evident that the real mutating period must be assumed to be much
longer than the time covered by my observations. Neither the beginning
nor the end have been seen. It is quite obvious that _Oenothera
lamarckiana_ was in a mutating condition when I first [700] saw it,
seventeen years ago. How long had it been so? Had it commenced to mutate
after its introduction into Europe, some time ago, or was it already
previously in this state? It is as yet impossible to decide this point.
Perhaps the mutable state is very old, and dates from the time of the
first importation of the species into Europe.
Apart from all such considerations the period of the direct
observations, and the possible duration of the mutability through even
more than a century, would constitute only a moment, if compared with
the whole geologic time. Starting from this conception the pedigree of
our mutations must be considered as only one small group. Instead of
figuring a fan of mutants for each year, we must condense all the
succeeding swarms into one single fan, as might be done also for _Draba
verna_ and other polymorphous species. In _Oenothera_ the main stem is
prolonged upwards beyond the fan; in the others the main stem is lacking
or at least undiscernable, but this feature manifestly is only of
secondary importance. We might even prefer the image of a fan, adjusted
laterally to a stem, which itself is not interrupted by this branch.
On this principle two further considerations are to be discussed. First
the structure of the [701] fan itself, and secondly the combination of
succeeding fans into a common genealogic tree.
The composition of the fan as a whole includes more than is directly
indicated by the facts concerning the birth of new species. They arise
in considerable quantities, and each of them in large numbers of
individuals, either in the same or in succeeding years. This multiple
origin must obviously have the effect of strengthening the new types,
and of heightening their chances in the struggle for life. Arising in a
single specimen they would have little chance of success, since in the
field among thousands of seeds perhaps one only survives and attains
complete development. Thousands or at least hundreds of mutated seeds
are thus required to produce one mutated individual, and then, how small
are its chances of surviving! The mutations proceed in all directions,
as I have pointed out in a former lecture. Some are useful, others might
become so if the circumstances were accidentally changed in definite
directions, or if a migration from the original locality might take
place. Many others are without any real worth, or even injurious.
Harmless or even slightly useless ones have been seen to maintain
themselves in the field during the seventeen years of my research, as
proved by _Oenothera laevifolia_ and _Oenothera_ [702] _brevistylis_.
Most of the others quickly disappear.
This failure of a large part of the productions of nature deserves to be
considered at some length. It may be elevated to a principle, and may be
made use of to explain many difficult points of the theory of descent.
If, in order to secure one good novelty, nature must produce ten or
twenty or perhaps more bad ones at the same time, the possibility of
improvements coming by pure chance must be granted at once. All
hypotheses concerning the direct causes of adaptation at once become
superfluous, and the great principle enunciated by Darwin once more
reigns supreme.
In this way too, the mutation-period of the evening-primrose is to be
considered as a prototype. Assuming it as such provisionally, it may aid
us in arranging the facts of descent so as to allow of a deeper insight
and a closer scrutiny. All swarms of elementary species are the remains
of far larger initial groups. All species containing only a few
subspecies may be supposed to have thrown off at the outset far more
numerous lateral branches, out of which however, the greater part have
been lost, being unfit for the surrounding conditions. It is the
principle of the struggle for life between elementary species, followed
by the survival of the [703] fittest, the law of the selection of
species, which we have already laid stress upon more than once.
Our second consideration is also based upon the frequent repetition of
the several mutations. Obviously a common cause must prevail. The
faculty of producing _nanella_ or _lata_ remains the same through all
the years. This faculty must be one and the same for all the hundreds of
mutative productions of the same form. When and how did it originate? At
the outset it must have been produced in a latent condition, and even
yet it must be assumed to be continuously present in this state, and
only to become active at distant intervals. But it is manifest that the
original production of the characters of _Oenothera gigas_ was a
phenomenon of far greater importance than the subsequent accidental
transition of this quality into the active state. Hence the conclusion
that at the beginning of each series of analogous mutations there must
have been one greater and more intrinsic mutation, which opened the
possibility to all its successors. This was the origination of the new
character itself, and it is easily seen that this incipient change is to
be considered as the real one. All others are only its visible
expressions.
Considering the mutative period of our evening-primrose [704] as one
unit-stride section in the great genealogic tree, this period includes
two nearly related, but not identical changes. One is the production of
new specific characters in the latent condition, and the other is the
bringing of them to light and putting them into active existence. These
two main factors are consequently to be assumed in all hypothetic
conceptions of previous mutative periods.
Are all mutations to be considered as limited to such periods? Of course
not. Stray mutations may occur as well. Our knowledge concerning this
point is inadequate for any definite statement. Swarms of variable
species are easily recognized, if the remnants are not too few. But if
only one or two new species have survived, how can we tell whether they
have originated-alone or together with others. This difficulty is still
more pronounced in regard to paleontologic facts, as the remains of
geologic swarms are often found, but the absence of numerous mutations
can hardly be proved in any case.
I have more than once found occasion to lay stress on the importance of
a distinction between progressive and retrograde mutations in previous
lectures. All improvement is, of course, by the first of these modes of
evolution, but apparent losses of organs or qualities are [705] perhaps
of still more universal occurrence. Progression and regression are seen
to go hand in hand everywhere. No large group and probably even no genus
or large species has been evolved without the joint agency of these two
great principles. In the mutation-period of the evening-primroses the
observed facts give direct support to this conclusion, since some of the
new species proved, on closer inspection, to be retrograde varieties,
while others manifestly owe their origin to progressive steps. Such
steps may be small and in a wrong direction; notwithstanding this they
may be due to the acquisition of a wholly new character and therefore
belong to the process of progression at large.
Between them however, there is a definite contrast, which possibly is in
intimate connection with the question of periodic and stray mutations.
Obviously each progressive change is dependent upon the production of a
new character, for whenever this is lacking, no such mutation is
possible. Retrograde changes, on the other hand, do not require such
elaborate preliminary work. Each character may be converted into the
latent condition, and for all we know, a special preparation for this
purpose is not at all necessary. It is readily granted that such special
preparation may occur, because the [706] great numbers in which our
dwarf variety of the _Oenothera_ are yearly produced are suggestive of
such a condition. On the other hand, the _laevifolia_ and _brevistylis_
mutations have not been repeated, at least not in a visible way.
From this discussion we may infer that it is quite possible that a large
part of the progressive changes, and a smaller part of the retrograde
mutations, are combined into groups, owing their origin to common
external agencies. The periods in which such groups occur would
constitute the mutative periods. Besides them the majority of the
retrograde changes and some progressive steps might occur separately,
each being due to some special cause. Degressive mutations, or those
which arise by the return of latent qualities to activity, would of
course belong with the latter group.
This assumption of a stray and isolated production of varieties is to a
large degree supported by experience in horticulture. Here there are no
real swarms of mutations. Sudden leaps in variability are not rare, but
then they are due to hybridization. Apart from this mixture of
characters, varieties as a rule appear separately, often with intervals
of dozens of years, and without the least suggestion of a common cause.
It is quite superfluous to go into details, as we have dealt with the
horticultural [707] mutations at sufficient length on a previous
occasion. Only the instance of the peloric toadflax might be recalled
here, because the historic and geographic evidence, combined with the
results of our pedigree-experiment, plainly show that peloric mutations
are quite independent of any periodic condition. They may occur anywhere
in the wide range of the toad-flax, and the capacity of repeatedly
producing them has lasted some centuries at least, and is perhaps even
as old as the species itself.
Leaving aside such stray mutations, we may now consider the probable
constitution of the great lines of the genealogic tree of the evening
primroses, and of the whole vegetable and animal kingdom at large. The
idea of drawing up a pedigree for the chief groups of living organisms
is originally due to Haeckel, who used this graphic method to support
the Darwinian theory of descent. Of course, Haeckel's genealogic trees
are of a purely hypothetic nature, and have no other purpose than to
convey a clear conception of the notion of descent, and of the great
lines of evolution at large. Obviously all details are subject to doubt,
and many have accordingly been changed by his successors. These changes
may be considered as partial improvements, and the somewhat picturesque
form of Haeckel's pedigree might well be replaced by [708] more simple
plans. But the changes have by no means removed the doubts, nor have
they been able to supplant the general impression of distinct groups,
united by broad lines. This feature is very essential, and it is easily
seen to correspond with the conception of swarms, as we have deduced it
from the study of the lesser groups.
Genealogic trees are the result of comparative studies; they are far
removed from the results of experimental inquiry concerning the origin
of species. What are the links which bind them together? Obviously they
must be sought in the mutative periods, which have immediately preceded
the present one. In the case of the evening-primrose the systematic
arrangement of the allied species readily guides us in the delimitations
of such periods. For manifestly the species of the large genus of
_Oenothera_ are grouped in swarms, the youngest or most recent of which
we have under observation. Its immediate predecessor must have been the
subgenus _Onagra_, which is considered by some authors as consisting of
a single systematic species, _Oenothera biennis_. Its multifarious forms
point to a common origin, not only morphologically but also
historically. Following this line backward or downward we reach another
apparent mutation-period, which includes the origin of [709] the group
called _Oenothera_, with a large number of species of the same general
type as the _Onagra-forms, Still farther downward comes the old genus
_Oenothera_ itself, with numerous subgenera diverging in sundry
characters and directions.
Proceeding still farther we might easily construct a main stem with
numerous succeeding fans of lateral branches, and thus reach, from our
new empirical point of view, the theoretical conclusion already
formulated.
Paleontologic facts readily agree with this conception. The swarms of
species and varieties are found to succeed one another like so many
stories. The same images are repeated, and the single stories seem to be
connected by the main stems, which in each tier produce the whole number
of allied forms. Only a few prevailing lines are prolonged through
numerous geologic periods; the vast majority of the lateral branches are
limited each to its own storey. It is simply the extension of the
pedigree of the evening-primroses backward through ages, with the same
construction and the same leading features. There can be no doubt that
we are quite justified in assuming that evolution has followed the same
general laws through the whole duration of life on earth. Only a moment
of their lifetime is disclosed to us, but it [710] is quite sufficient
to enable us to discern the laws and to conjecture the outlines of the
whole scheme of evolution.
A grave objection which has, often, and from the very outset, been urged
against Darwin's conception of very slow and nearly imperceptible
changes, is the enormously long time required. If evolution does not
proceed any faster than what we can see at present, and if the process
must be assumed to have gone on in the same slow manner always,
thousands of millions of years would have been needed to develop the
higher types of animals and plants from their earliest ancestors.
Now it is not at all probable that the duration of life on earth
includes such an incredibly long time. Quite on the contrary the
lifetime of the earth seems to be limited to a few millions of years.
The researches of Lord Kelvin and other eminent physicists seem to leave
no doubt on this point. Of course all estimates of this kind are only
vague and approximate, but for our present purposes they may be
considered as sufficiently exact.
In a paper published in 1862 Sir William Thomson (now Lord Kelvin) first
endeavored to show that great limitation had to be put upon the enormous
demand for time made by Lyell, Darwin and other biologists. From a
consideration [711] of the secular cooling of the earth, as deduced from
the increasing temperature in deep mines, he concluded that the entire
age of the earth must have been more than twenty and less than forty
millions of years, and probably much nearer twenty than forty. His views
have been much criticised by other physicists, but in the main they have
gained an ever-increasing support in the way of evidence. New mines of
greater depth have been bored, and their temperatures have proved that
the figures of Lord Kelvin are strikingly near the truth. George Darwin
has calculated that the separation of the moon from the earth must have
taken place some fifty-six millions of years ago. Geikie has estimated
the existence of the solid crust of the earth at the most as a hundred
million years. The first appearance of the crust must soon have been
succeeded by the formation of the seas, and a long time does not seem to
have been required to cool the seas to such a degree that life became
possible. It is very probable that life originally commenced in the
great seas, and that the forms which are now usually included in the
plankton or floating-life included the very first living beings.
According to Brooks, life must have existed in this floating condition
during long primeval epochs, and evolved nearly all the main branches of
the animal and vegetable kingdom [712] before sinking to the bottom of
the sea, and later producing the vast number of diverse forms which now
adorn the sea and land.
All these evolutions, however, must have been very rapid, especially at
the beginning, and together cannot have taken more time than the figures
given above.
The agency of the larger streams, and the deposits which they bring into
the seas, afford further evidence. The amount of dissolved salts,
especially of sodium chloride, has been made the subject of a
calculation by Joly, and the amount of lime has been estimated by Eugene
Dubois. Joly found fifty-five and Dubois thirty-six millions of years as
the probable duration of the age of the rivers, and both figures
correspond to the above dates as closely as might be expected from the
discussion of evidence so very incomplete and limited.
All in all it seems evident that the duration of life does not comply
with the demands of the conception of very slow and continuous
evolution. Now it is easily seen, that the idea of successive mutations
is quite independent of this difficulty. Even assuming that some
thousands of characters must have been acquired in order to produce the
higher animals and plants of the present time, no valid objection is
raised. The demands of the biologists and the results of [713] the
physicists are harmonized on the ground of the theory of mutation.
The steps may be surmised to have never been essentially larger than in
the mutations now going on under our eyes, and some thousands of them
may be estimated as sufficient to account for the entire organization of
the higher forms. Granting between twenty and forty millions of years
since the beginning of life, the intervals between two successive
mutations may have been centuries and even thousands of years. As yet
there has been no objection cited against this assumption, and hence we
see that the lack of harmony between the demands of biologists and the
results of the physicists disappears in the light of the theory of
mutation.
Summing up the results of this discussion, we may justifiably assert
that the conclusions derived from the observations and experiments made
with evening-primroses and other plants in the main agree satisfactorily
with the inferences drawn from paleontologic, geologic and systematic
evidence. Obviously these experiments are wonderfully supported by the
whole of our knowledge concerning evolution. For this reason the laws
discovered in the experimental garden may be considered of great
importance, and they may guide us in our further inquiries. Without
doubt many minor [714] points are in need of correction and elaboration,
but such improvements of our knowledge will gradually increase our means
of discovering new instances and, new proofs.
The conception of mutation periods producing swarms of species from time
to time, among which only a few have a chance of survival, promises to
become the basis for speculative pedigree-diagrams, as well as for
experimental investigations.
[715]