PROOFS FROM VARIATION OF ORGANIC FORMS, ARTIFICIAL AND NATURAL.
As already stated, page 40, the use of the method of experiment in
the field of biology is, unfortunately, very limited. Nevertheless,
it is already beginning to be used more and more in the department
of physiology, and may be used also, to a limited extent, in the
department of morphology. It is true that direct _scientific_
experiments, for the express purpose of producing permanent
modifications of form, and thus testing the theory of evolution, are of
comparatively little value as yet, because the all-important element
of time is wanting. The steps of evolution are so slow, and the time
necessary to produce any sensible effect is usually so great, that, in
comparison, man’s individual lifetime is almost a vanishing quantity.
But, from time immemorial, experiments have been _unconsciously_
made by man on domestic animals and food-plants, which bear directly
on this subject. All domestic animals and food-plants, and many
ornamental flowering plants, have been subjected for ages to a process
of artificial selection acting upon natural variation of offspring.
As wild species are modified, we believe, indefinitely by divergent
variation and _natural_ selection, so domestic species are modifiable
certainly largely, perhaps indefinitely, by divergent variation
and _artificial_ selection by man. We all know the extraordinary
modifications which have thus been gradually brought about in domestic
animals, such as dogs, horses, sheep, pigeons, etc.; in food-plants,
as cereal grains, garden-vegetables, etc., and in ornamental plants,
as roses, dahlias, pinks, etc. We can only give very briefly the
principles of the process by which these extreme modifications are
produced, referring the reader to works specially devoted to this
subject for more complete accounts.
Let it be borne in mind, then (_a_), that inheritance is not only
from the immediate parents, but from the whole line of ancestry. The
inheritance from the immediate parents is, doubtless, usually greater
than from any other _one_ term of the ancestral series--the effect on
the offspring of any previous generation becomes, doubtless, less and
less as the distance from the offspring increases--yet the _sum_ of
the ancestral inheritance is far greater than the immediate parental.
Let it also be borne in mind (_b_) that true breeding from one form
for many generations creates a fund of heredity in that form, and thus
tends to produce fixity, rigidity, or permanence in that form.
[Illustration: FIG. 68.]
Now, the method of producing artificial breeds, sometimes consciously,
sometimes unconsciously, is, briefly, as follows: Suppose it be
desired to obtain a variety of an animal, say a dog, having a
certain character. We start from a common type, _a_ (Fig. 68). If
this type were allowed to breed naturally, the slight divergent
variation of offspring represented by the radiating lines would
neutralize one another by interbreeding, the individual differences
would be “_pooled_” in a common stock, and the species would remain
substantially constant. But if among all these slightly divergent
varieties we select one, _b_, which seems in the right direction, and
ruthlessly destroy all the others (indicated by crossing them out by
the circular line), and breed this variety, _b_, only, we shall get
again a number of divergent varieties. It may be that the larger number
of these will be backward, in the direction of the original type _a_,
on account of the ancestral heredity in that direction, but some will
again be in the desired direction. Let all the varieties other than
the desired one, but especially the backward-going or reverting ones,
be again destroyed, and the one kind only selected which seems to be
in the right direction, viz., _c_. As we push the form thus from
generation to generation in the desired direction, especially if we
attempt to hasten too much the process, the resistance to movement--if
I may use the expression--in that direction becomes greater and greater
(shown by the decreasing distances between the successive points of
divergence, _a_, _b_, _c_, _d_, etc.), and the tendency to reversion
becomes stronger (shown by the greater number and length of the
backward-going lines), until finally it is almost impossible to push
any farther. We will suppose that _x_ is such a limit. But if, now,
we breed true on the point _x_, destroying the reversions or backward
variations for many generations, we will gradually accumulate a fund
of ancestral heredity on this point which increases with every added
generation, until finally the tendency to reversion becomes small. The
variety _breeds true_ without further interference, or with only very
general superintendence. Such a permanent variety is called a _race_.
After a race is firmly established for a sufficient length of time,
and the tendency to reversion is lost, it may itself become a new
point of departure for the formation of new varieties or races, in the
same or other directions. Thus, during even the brief history of man,
have been formed races of the different domestic animals, and useful
and ornamental plants, differing so greatly from each other that, if
found in the wild state, they would unhesitatingly be called different
species, or even in some cases different genera.
Now, if art can vary form so greatly, and in so short time, why may
not Nature in limitless time? If art by artificial selection, why not
Nature by natural selection? Nature is as rigid in selection and as
ruthless in destruction: why may we not expect similar or even much
greater results? The process is similar in the two cases--i. e.,
selection among varieties in offspring, only that the selection
is natural instead of artificial, and the process is so slow that
there is little tendency to reversion in the latter case. Suppose,
then, we have a gradually changing physical environment, or climate.
Among the divergent varieties of any species in each generation,
those would be preserved which are most in accordance with the new
climate, and the others would perish. This is natural selection, or
survival of the fittest. Add to this the effect of the change in
the organic environment. All species are modified by the changing
physical environment; but these modified species again all affect
one another in the competitive struggle for life, and the strongest
or swiftest, or most cunning, survive (natural selection). Add to
this, again, the struggle among the males for possession of the
females--for reproductive opportunities--by which only the strongest
and most courageous, or the most beautiful and attractive, leave
progeny which inherit their peculiarities (sexual selection). Add to
these, finally, _migrations_, voluntary among higher and involuntary
dispersals among lower animals and plants, and the consequent mingling
of faunas and floras--the migrations subjecting them to great change
of environment, both physical and organic, and the mingling producing
fiercer struggle for life--and we have in powerful operation many
causes of modification. Add, I say, all these causes of modification
together, and then make the process slow and continuous through
unlimited time, and where is the limit to the degree of change?
Commencing in any species, from any point of departure, there are
formed first slight modifications which would be called varieties; then
these modifications, continuing in the same direction, form races;
these races by wider separation become species, and species in their
turn become genera, etc. Comparing, again, to a growing tree, varieties
are swelling buds; when they grow into twigs, they are species; when
they branch again into different species, the branching stem becomes a
genus, etc.
We have thus far spoken only of the various forms of one factor, viz.,
the Darwinian factor of selection, whether natural or artificial.
We have dwelt upon this one, because the natural and the artificial
processes are so similar, and the artificial is so controllable. But
there are other factors in operation, in art as well as in nature. We
have already spoken (p. 73) of other factors of natural change. We
have shown how changing physical environment affects _function_, and
function affects _form and structure_, and how these slight changes are
integrated by heredity through many generations. We have also shown how
_use_ or _disuse_ increases or diminishes the size and change the form
of parts, and these changes, also, however slight, are integrated by
heredity.
Now, these factors are operative also in domestication of animals
and cultivation of plants. No environment is so new and peculiar as
domestication and cultivation. The soil and temperature in plants,
food and housing of domesticated animals, tend to change form and
structure of the offspring, although in a way which it is difficult
intelligently to control, and thus are prolific of varieties from
which to select. In fact, they often give rise to great and unexpected
modifications, called sports, which form points of departure for new
varieties and races. Now, in nature, not only are all these causes
and factors of change in constant operation, but they act together in
a peculiarly complex way. All the members of a fauna and flora, and
the physical environment of any locality, constitute together a most
complex and delicately adjusted system of correlated parts. A change in
one part is propagated through the whole system; also, a change in one
factor affects all other factors. When we add to this the large amount
of time, in comparison with individual human life and observation,
necessary to produce visible change of form, we can easily understand
why the process is still imperfectly understood, although the _fact_ is
certain.
But it will be asked, Are there, then, no differences between the
artificially made extreme varieties equivalent, so far as difference
of form is concerned, to species, and real natural species? There
are. If there were not, there would never have been any doubt about
the derivative origin of natural species. But if it be asked, Are not
these differences fundamental, and therefore fatal to the argument for
evolution derived from this source? we answer, we think not. We will
deal frankly and fairly with these differences.
=First Difference, Reversion.=--The strong tendency of artificial
varieties to reversion, even during the process of formation, and
especially their complete reversion to the original type if the hand
of man be withdrawn--i. e., if left to themselves, or become wild--is
supposed to show an essential difference between such varieties,
however extreme, and true species--is supposed, in fact, to prove an
indestructible permanency of specific types. Nature disowns these
artificial forms, and as it were brands them with bastardy. Not only
so, she strives ever to destroy them. The supporting hand of man is
necessary to sustain them. Left to themselves and to Nature, they
quickly revert to the original type. If all the extreme varieties of
dogs, from the greyhound and Newfoundland, on the one hand, to the
terrier and lap-dog on the other, were turned loose on an isolated
island, uninhabited by man but full of other animals, and left there
to shift for themselves--and the island were visited again after a
lapse of a hundred or a thousand years--it is probable that a uniform
species, something like to, though perhaps not identical with, the
wolf, would be found. They would have reverted to the original or
nearly the original wild type from which they were produced by
domestication. All or nearly all that was done by man would have been
undone by Nature. This reversion is one test of species.
But the reason of this tendency to reversion is obvious: First, the
time was too short, the rate of change was too rapid, in the artificial
formation of these varieties. There was not time enough to accumulate
a fund of heredity on each successive stage of the change. Therefore
the form is unstable and the tendency to revert is strong. Compare the
fleeting days and the hurrying impatience of man with the infinite time
and the divine patience of Nature! But mere instability is not the
principal cause of reversion. Secondly, in the case of artificial forms
in a wild state, _natural selection compels reversion_. Every species
in a wild state must of course be in harmony with the environment. But
artificially made forms are in harmony with the artificial environment
of domestication, but not with the environment of nature. In nature
the fittest survive, but artificial breeds are not fit to survive in a
state of nature. They are therefore quickly destroyed in the struggle
for life, or must be modified. Nature immediately begins to select
the fittest, and gradually in the course of time produces one or more
uniform species, similar to that from which they came, or perhaps to
what they would have been by this time if left to the operation of
natural causes under the conditions supposed. But natural species, if
they are formed, as the derivationists suppose, by the operation of
natural causes, can not revert unless the conditions revert; for the
same causes which operated to produce, still continue to operate to
keep, the species. Take an example:
The form, the habits, and the instincts of the pointer have been made
by a slow process of artificial selection of divergent varieties of
offspring, and by training of individuals continued and its effects
accumulated through many generations. But this form and these habits
and instincts, so laboriously produced, would be quickly destroyed by
Nature. The pointer, left to himself, must either change or become
extinct, because not adapted to the wild state. Such instincts and
habits would not only be of no use, but would be incompatible with
success in the struggle for life. But suppose for a moment that these
habits and instincts were useful to the animal in a wild state;
evidently they would be instantly seized upon by natural selection,
and not only perpetuated but intensified until a very distinct species
would be produced. The same is true of all other races of dogs. If the
Newfoundland, the greyhound, and the pug were all turned loose in a
forest, and if each of these kinds were admirably adapted to some place
in the economy of Nature--for some special mode of food-getting without
corresponding disabilities in other directions (as must be the case
if made by natural selection)--there can be no doubt they would each
survive, and their characters intensified; intermediate forms would
disappear (for reasons which we shall see presently), and we would
soon have three distinct species, or perhaps we would even call them
distinct genera.
=Second Difference, Intermediate Forms.=--Natural species are
distinct--marked out with hard and fast lines--while artificially-made
races, even though in their typical forms they differ as much or more
than natural species, shade into one another by insensible gradations.
In answer and explanation of this difference we remark: If species or
modified forms of any kind, whether natural or artificial, are made by
natural causes, and not at once out of hand by supernatural creation,
then of course there must have been gradations in the process of
making. Now, in the artificial case, the whole process as well as the
result lies within the limits of observation, while in the natural case
only the final result. But it will be asked, Why are the gradations
not seen also in the final result? We answer, because the intermediate
forms are eliminated in the struggle for life, and not reproduced by
cross-breeding. If artificial races always bred true--i. e., without
crossing, as natural species do--they would probably soon be as
sharply demarked. Cross-breeding is the great cause of the shadings
between domestic races. This brings me to the third and most important
difference.
=Third Difference, Cross-Fertility.=--Artificially-made races breed
freely and without repugnance with one another, and the offspring
of such cross-breeding is indefinitely fertile. Natural species
will not usually unite with one another, being prevented by sexual
repugnance and other causes. Or, if they do sexually unite, there is
either no offspring, or else the offspring is sterile, and therefore
the intermediate form dies out in the first generation; or else the
offspring is imperfectly fertile, and therefore the intermediate form
is eliminated in a few generations, and the species remain distinct;
or else the offspring is more fertile with the parent stocks, and
therefore revert to the parent stocks, and still the species remain
distinct. Such infertile, or imperfectly fertile, offspring--the result
of crossing of species--are called hybrids.
This is regarded as a most important test of true species, as
contrasted with varieties or races. There are two bases on which
species may be founded. Species may be based on _form_, morphological
species; or they may be based on _reproductive functions_,
physiological species. By the one method a _certain amount of
difference_ of form, structure, and habit, _constitutes species_;
according to the other, if the two kinds breed freely with each other
and the offspring is indefinitely fertile, the kinds are called
varieties, but if they do not they are called species. The two tests,
however, do not always accord. Every now and then we find undoubted
morphological species which may be crossed and produce indefinitely
fertile offspring. Yet it is certainly true that species are usually
cross-sterile, while varieties, whether natural or artificial, are
cross-fertile.
In explanation of this important difference, let it be observed that
there are here two things which must be kept distinct in the mind,
although they are, doubtless, closely allied--viz., sexual repugnance
(psychological element) and cross-sterility (physiological element).
The former is found, of course, only in the higher animals, where
fertilization is _voluntary_. The latter is universal among all
living things. This latter, therefore, is the more fundamental and
essential element, and the former may be regarded as its psychical
sign in the higher animals. It is of this latter, therefore--i. e.,
cross-sterility--that we shall speak mainly.
Suppose, then, we have growing together in the same locality many
species of pines or oaks, or other anemophilous trees. The whole air
is filled with the pollen of many species, and every germ-cell must
receive many kinds of male cells, and yet there are no hybrids, but,
on the contrary, the species remain distinct. So also in case of
hermaphrodite animals, where the fertilization is involuntary; many
aquatic species are found together in the same locality, and the water
is filled with sperm-cells of many different species. Many kinds of
sperm-cells must fall on each germ-cell, and yet there are no hybrids;
the species remain distinct. In all such cases we must suppose that
there is, among the different kinds of male cells, a struggle for the
possession of the germ or female cell, or a sort of sexual selection by
the female cell among the competing male cells, and the fittest--the
most in accord; i. e., those of the same species--prevail. This is
universal. But in the higher animals, in addition to the prepotency
of male cells of the same species, and comparative infertility in
case of union of those of different species, sexual attraction and
sexual repugnance contribute to the same result, and species are thus
doubly separated. Thus sexual selection is of two kinds: selection of
individuals for union (psychical), and selection of sperm-cells for
fertilization (physiological). The one kind is usually the sign of the
other--attraction the sign of fertility, and repugnance of sterility.
But in the domestic state it is all otherwise. Free competition between
individuals or between cells is not allowed. Thus, for example, among
plants, crossings may be forced and hybrids made in gardens which would
never occur in Nature. The florist prevents fertilization in the same
kind and compels fertilization of a different kind. If male cells of
the same kind were allowed to compete, the result would be different.
Doubtless the same method would succeed in many lower animals. So also
in higher animals free competition and sexual selection for union are
often not allowed, and therefore animals of different species, such as
the horse and the ass, unite, which would not do so if they were free
to select as in the wild state. These two are widely distinct species,
sometimes even called genera, and therefore the offspring is infertile;
but two closely allied species, such as two species of wolf, or of the
fox, in a domestic state would probably not only unite but produce
indefinitely fertile offspring. In fact, it is almost certain that the
dog was made by a mixture of several species of wolf, most, perhaps
all, of them now extinct.[30] On the other hand, it is not at all
certain that the extreme varieties of dogs have not passed the limit
of greatest attraction, and therefore of greatest cross-fertility, and
that, if allowed free choice, as in Nature, they would not breed true,
or tend to breed true, with their own kind, and intermediate kinds die
out in the struggle for life.
=Law of Cross-breeding.=--Before going any further in this discussion,
it is necessary to bring out another point of extreme importance in the
formation of varieties, both natural and artificial--a point which I
believe throws light upon the very significance of sex itself--I refer
to the effect of cross-breeding.
It is a curious and most significant fact that different varieties,
both natural and artificial, are, up _to a certain limit_, not only
cross-fertile and cross-attractive, but even more so than individuals
of the same variety. Long experience has shown that very close breeding
of the same variety for a long time fixes the _kind_ but _weakens_ the
_stock_, especially in fertility, while judicious crossing of varieties
strengthens the stock, increasing its fertility, and especially
producing _plasticity_ or _variability_. Therefore breeders, if they
wish to preserve a valuable variety, breed close; but, if they wish to
make new varieties, cross-breed. But we have already seen that species
are usually cross-sterile. Therefore there must be some regular law of
increase to a maximum, and again decrease to zero. It is this law that
I now wish to investigate.
In the lowest animals and plants multiplication of individuals and
the continuance of the kind are independent of sex, and therefore
in such there may be no sex at all. The sexual elements are not yet
differentiated. An individual divides itself into two; each grows to
the original size and again divides into two, and so on, it may be
indefinitely. In this lowest form of reproduction the individual is
sacrificed to the kind, or else we may regard the kind as an extension
of the individual, and reproduction as a modification of growth. But
there are other sexless modes of reproduction, found in nearly all
plants and many lower animals, in which the individuality is not
sacrificed. The next step in the ascending scale is reproduction by
_budding_. In this case a bud is formed which grows into a perfect
individual, and may remain attached to the parent stalk, forming
together a compound individual, as in most plants and many lower
animals, such as the coral; or it may separate and assume independent
life, as in some plants and many lower animals. In still other animals,
as in many hydrozoa, the budding function is relegated to a special
part, which thus becomes a reproductive _organ_. The next step is the
placing of the budding organ, for greater safety, in an _interior
cavity_. This is the case with aphids. Now, why would not this be an
excellent mode of reproduction for all animals, man included? Why
was sex introduced at all? There are very sufficient reasons, of
many kinds, which may come up later; but the fundamental reason, in
connection with evolution, is _the funding of individual differences
in a common offspring, thereby giving to the offspring a tendency to
divergent variation_.
Now, _non-sexual_ reproduction is _absolute true breeding_. The law
of like producing like is absolute. Heredity is all-powerful, and
tendency to variation is _nil_. These modes of reproduction are in
fact but a modification of growth and an extension of the individual.
Evolution-changes in animals produced in this way only must be very
slow, since the most powerful factor of evolution, viz., natural
selection among divergent varieties of offspring, would be wanting.
In the earliest times, therefore, before sex was yet declared,
we may imagine that physical environment was the great and only
factor of change. Sexual reproduction introduces the new element of
variation of offspring from which Nature makes her selections; and
this element of variation is apparently the result of the union of
_diverse_ individuals, and the funding of these differences in a common
offspring, and thus a double inheritance of individual characteristics
from the parents and a multiple inheritance of the same from the
ancestry. See, then, with this end in view, the pains Nature has
taken to make the difference between the uniting individuals and the
diversity of inheritance by the offspring as great as possible, and
yet the gradual way in which she has accomplished it. As already said,
the lowest form of reproduction is that by _fission_. Next comes
budding in _any_ part indifferently. Next comes the relegation of the
budding function to a particular part. This is the first appearance
of a reproductive _organ_. Next comes the placing of this organ, for
greater safety, within. Thus far all is non-sexual reproduction--all
a modification of growth--an extension of the individual, like the
propagation of plants by cuttings and by buds. Then comes sexual
reproduction in its lowest forms.
It may be well to stop here, to show the entire difference between this
and non-sexual modes. The latter, we have seen, is only a modification
of growth, an extension of the individual. Now, sexual reproduction
is the opposite of all this. Growth is a constant multiplication of
cells. One cell is ever becoming two similar cells--or, if we call them
individuals, one individual is ever becoming two similar individuals.
But in sexual reproduction we have an exactly reverse process. Reduced
to its simplest terms, sexual reproduction is the fusion of _two
diverse_ cells, sperm-cell and the germ-cell, to form _one_ cell,
the ovule--literally, a diverse twain forming one flesh. In its
higher forms it is the union of diverse _individuals_ to bring about
the same result. Instead of one cell becoming two, it is two cells
becoming one; instead of one individual becoming two in the offspring,
it is two individuals becoming one in the offspring. But this great
change was not brought about at once, but only in the most gradual
manner. First, the sexual elements--sperm-cell and germ-cell--are
separated, but _in the same organ_. Then the organs--spermary and
ovary--are separated, but in the _same individual_. This is the
condition of self-fertilizing hermaphroditism so common among plants
and lower animals. Then comes cross-fertilizing hermaphroditism; and
Nature takes much pains and uses many ingenious devices to prevent
self-fertilization and insure cross-fertilization. Now, for the
first time, we have slight individual differences funded in a common
offspring. Then, in order to absolutely forbid self-fertilization, and
at the same time allow greater differences in the crossing individuals
than could be attained in hermaphroditic individuals, the sex organs
are separated in _different individuals_, and fertilization can
only take place by _voluntary union_. Then, to insure the union of
suitable individuals, and forbid the ban between unsuitable, there are
introduced sexual attraction and repulsion. Then, last of all, the
difference between the two sex-individuals becomes greater and greater
as we go up. It is conspicuous only in vertebrates and some insects,
and very conspicuous only in birds and mammals.
We see, then, as we go up the taxonomic, and undoubtedly also the
phylogenic series, that there is a cross-breeding of more and
more diverse individuals, a funding of more and more divergent
characteristics in a common offspring. Why is this? I answer, for the
sake of _better results in the offspring_. This is abundantly shown
by direct experiment. In hermaphroditic plants in which there may
be either self-fertilization or else cross-fertilization with other
individuals of the same species, the latter produces better results in
number and vigor of offspring. But there are other advantages, more
difficult to prove but none the less certain, and of the greatest
importance in evolution: First, as already stated, complexity of
inheritance, like complexity of composition in a chemical substance,
gives instability to the embryo, and thus liability to variation in the
offspring; and this in its turn furnishes the material for selection
of the fittest. Again, it seems to me that there is a direct tendency
to improve the offspring by a sort of struggle in the embryo among the
various qualities inherited from both sides, and a survival of the best
and strongest--a sort of pre-potency of strong qualities.
Can divergence of uniting individuals and the funding of diverse
characteristics go any further? It may. The differences of the uniting
individual may be still further increased, and the resulting offspring
still further improved by the cross-breeding of different varieties
of the same species, for we thus add varietal differences to sexual
differences in the uniting individuals. It is well known that too
close breeding, or consanguineous breeding, or breeding in and in, as
it is variously called, if continued long, has a bad effect on the
offspring, weakening the stock, while judicious crossing of varieties
within certain limits of difference has a good effect, strengthening
the stock and increasing its fertility. It probably does so in two
ways: one direct, by funding many diverse qualities from both sides,
and the survival in the offspring of the strongest and best; the other
indirect, by giving _plasticity_, instability to the embryo, and
variability to the offspring, and therefore abundant material for the
operation of selection, either by man or by Nature. We said, “within
certain limits of difference.” If the difference is extreme, as in
extreme varieties and races, then the effect becomes again bad, and
more and more so as the limit of specific difference is approached;
at which limit at last Nature shuts down and forbids the bans. Thus,
then, there is in cross-breeding a regular law of effect, increasing to
a maximum and again decreasing, which may be graphically represented
by a curve (Fig. 69). In this figure the horizontal line represents
the ordinary level of the type; distances on this line represent
differences, individual, varietal, or specific; ordinates above or
below represent the effect, good or bad, of crossing. Thus s s′
represent two species, and the line between represents their specific
differences; _r r′_ represent different races or permanent varieties;
_v v′_ two strong varieties; _d d′_ ordinary individual differences; _c
c′_ close resembling or consanguineous individuals. The undulating line
represents the effect of crossing these various kinds. It is seen that
“in-and-in breeding,” _c c′_, produces bad effect (negative ordinates);
breeding of ordinary individual differences, _d d′_, keeps the stock at
the ordinary level--in its typical form; crossing two strong varieties,
_v v′_, produces maximum good effect (positive ordinates); crossing
decided races produces again bad effects, which become infinitely bad
as we approach species, S S′.[31]
[Illustration: FIG. 69.]
It is generally admitted that long-continued very close breeding has a
bad effect. Even in plants, Darwin has shown that cross-fertilization
has better effect than self-fertilization, this last being of course
the closest possible breeding. But it is probable that the principal
bad effect is not on the stock but _on the process of evolution_. Very
close breeding weakens the stock, ordinary breeding of individual
differences maintains the stock at the ordinary level and fixes it.
Cross-breeding of varieties strengthens the stock, and also (and this
is its main advantage) produces plasticity in the stock, gives rise
to strong divergent variations, or even sports, and thus becomes a
main agent in evolution. It is probable, moreover, that the higher
the function the more sensitive is it to these effects of breeding.
Therefore, the effect is greater in man than in any other animal. It is
true that many have doubted the bad effect of close breeding in man,
and have brought forward formidable statistics to substantiate their
position; but these doubtless take no account of the most important
function, the psychic, and especially the most important element in
every function, so far as evolution or progress is concerned, viz.,
_plasticity_ or capability of progressive improvement. The tendency
of consanguineous breeding, or even the breeding of persons of like
character and experiences, as in an isolated community, is, if not to
deteriorate the physique, at least to fix, stereotype the character,
and thus to check social progress. Contrarily, the crossing of
varieties of the same race seems not only to strengthen but, by the
diverse inheritance, to produce plasticity of character and capacity
for progress. But the difference between the primary races seems too
great for crossing with advantage. Some degree of sexual repugnance
which undoubtedly exists between the primary races is the psychical
sign of this fact.[32]
If, now, we go back to what we said before taking up this subject of
the effect of cross-breeding, we at once see that there is an apparent
flaw in all our reasonings. If close in-and-in breeding produced
better and more numerous offspring than cross-breeding between slight
varieties, then, indeed, such varieties would be preserved, and
increase in divergence from generation to generation until they became
species. Or, in any case, if, in any way, divergence could reach the
point of extreme varieties or races, or what are called sub-species,
then commencing cross-sterility would complete the separation, and thus
form true species. But how can the process of progressive divergence
begin, when slight varieties are even more fertile by cross-breeding
than by close breeding? Is it not evident that, with every generation,
the slight varieties would cross-breed with one another and with the
parent stock, and thus all varietal differences would be funded into
a common stock, and the type would be preserved unchanged? This, as
already pointed out (p. 76), has always been the chief difficulty
in the way of imagining how varieties can grow into species; and
the difficulty is _only increased_ by our discussion of the law of
cross-breeding. Now, just here, Dr. Romanes’s most important and
prolific idea comes to our help, and, as it seems to us, completely
solves the difficulty.
According to Dr. Romanes, no organ is so subject to variation as
the reproductive, and this in no respect so much as in degrees and
kinds of fertility--we might almost say so subject to freaks of
cross-sterility. Now, suppose we start with any well-defined species
in a state of nature. With every generation there are many slightly
divergent individual varieties, some greater and some less; but these
are all immediately swamped by crossing with one another and with the
parent stock, and the species remains unchanged. But suppose among
these divergent variations there arise, from time to time, some which
affect the reproductive organs in such wise that the variety, though
perfectly fertile with its own kind, is infertile, or imperfectly
fertile, with other varieties, and especially with the parent stock.
The change may be only in the _time_ of flowering in plants, or season
of heat in animals, or it may be actual infertility in sexual union.
Right here we have the beginning of a new species. The variety is
sexually isolated from the parent stock by cross-sterility, and
therefore all its peculiarities, however trivial, are preserved by
true breeding. Cross-breeding is necessary to make species, but true
breeding preserves them. Cross-breeding tends ever to make varieties,
but immediately destroys them again. This constant forming and
swamping, separating and again merging of varieties, like mixing of
dough, makes the whole mass (stock) more and more plastic and subject
to variety. This plasticity finally gives rise to varieties of the kind
which produces species by sexual isolation. By continued merging the
centrifugal forces continually increase, but are continually repressed
by crossing, until finally varieties break away to form species.
Now it is easy to see, from this point of view, why artificial
varieties are cross-fertile. It is because in artificial breeding we
are intent only on making varieties in form, size, color, etc., and not
at all on making any characterized by cross-sterility with the parent
stock. Cross-sterility with the parent stock, or with other varieties,
would be of no advantage, because we control the breeding, and can
breed true if we desire. Sexual isolation is not necessary, because
we can use physical isolation. On the contrary, such cross-sterility
would be a positive disadvantage to the breeder, by limiting the range
of his experiments just where they would be most prolific in making
new varieties. Hence, as might be expected, all domestic varieties are
cross-fertile, unless it be the extreme varieties, which may, in some
instances, have passed the limit of greatest fertility.
If this idea be true, then species which have originated in the same
locality ought to be always cross-sterile, but species which have
grown up apart, in widely separated geographical regions, ought to be
sometimes cross-fertile, because they were isolated by physical not by
sexual barriers. Such, Dr. Romanes thinks, is a fact. It is, however,
a very important point, which ought to be carefully investigated. We
say “_sometimes_.” It is probable that most geographical species also
are cross-sterile; for, although the isolation by cross-sterility of
slight varieties be the main cause of the origin of species, yet a
species formed by isolation of any other kind will gradually become
cross-sterile with other species. Although cross-sterility be the main
cause of divergence, yet divergence beyond a certain limit, however
caused, will bring about cross-sterility, because the reproductive
organs will partake of the general change going on in every part.
=Application.=--Suppose, then, a species breeding naturally in a wild
state. Individual varieties are constantly being formed and again
funded back into the common stock by cross-breeding. If the varieties
thus formed be decided, the cross-breeding will strengthen the stock,
and especially will preserve and increase its plasticity or tendency to
variation. Finally, among the widely divergent varieties there is one
affecting the reproductive organs of several individuals in such wise
that they are infertile, or imperfectly fertile, with the parent stock,
though perfectly fertile among themselves. These form a new species,
which continue to increase indefinitely.
=Objection answered.=--This view completes the answer to an objection
which is often made to evolution: “If natural species are formed by
transmutation, why is it we do not find intermediate links? Why is not
organic nature made up only of individual forms, shading insensibly
into each other in such wise that classification becomes a mere device
to handle more conveniently complex material? Why is it that groups,
especially species, are marked out with hard and fast lines?” We
have heretofore answered this by saying that intermediate forms are
eliminated. So they are, but how? Dr. Romanes’s idea of physiological
selection largely answers this. It is by the funding of _ordinary_
varieties into a common parental stock by crossing, and separating
_specific_ varieties by cross-sterility. Thus the organic field is
broken up into points about which variations oscillate. As every mass
of matter, when closely examined, is found to consist of aggregations
about centers of _cohesive_ attraction as discrete granules or
crystals, and only exceptionally do we find a homogeneous vitreous
structure; even so organic forms aggregate about points of _sexual_
attraction, and the whole mass consists of discrete species, and only
exceptionally--i. e., in domestication--do we find insensible shadings.
Now, species are the smallest aggregate of individuals, as granules
are of molecules. Species are more distinctly marked out by hard and
fast lines than are other taxonomic groups only because they are the
_last_, going downward, that are cross-sterile--because right here is
the change from cross-sterility to cross-fertility.
If this view be true, then in _the same locality_ species ought to be
always distinct and without shadings. If we find shadings at all, it
ought to be in intermediate geographical regions, where isolation is
not sexual but physical. Now, this is exactly what we find to be the
fact. _Innumerable examples of such intermediate forms in intermediate
geographical regions_ are now known, especially among birds and
reptiles, and examples have so increased in modern times, by closer
study, that naturalists, especially ornithologists, have been compelled
to resort to a trinomial nomenclature in order to designate these
geographical sub-species.[33]
If any further explanation is necessary, it will probably be found in
the following suggestions:
1. The number of individual varieties constantly being formed is
almost infinite, but the number of places in nature is very limited.
Now, among the infinite number of slight individual varieties formed
with every generation, the competitive struggle will be severest
between those most nearly alike, because they are competitors for the
_same_ place. Only one kind succeeds, viz., the fittest. Intermediate
forms are, therefore, exactly those which are eliminated in the most
wholesale way. 2. Add to this the fact that, as soon as divergence,
from whatsoever cause, reaches a certain point, sexual repugnance
or cross-sterility, or both, come in to perpetuate and increase the
separation already commenced. 3. Add to this, again, that migrations
in higher animals, and involuntary dispersals in lower animals and
in plants, and the mingling together of different faunas and floras,
produces a still fiercer struggle for life, especially between natives
and invaders, and thus great numbers of forms are destroyed; all but
the fittest are weeded out, and therefore the distinctness of the
remainder is greatly increased. Periods of great changes of physical
geography and of climate, and therefore of wide and general migrations,
are also periods of great weedings-out of unfit forms. Thus it
happens that existing faunas and floras are little else than isolated
_remnants_.
To illustrate, again, by a growing tree: If all the buds of a tree
lived and grew, they would soon become so numerous that they would
together form a solid hemispherical mass, like a coral-head, with no
room between for leaf or light or air. But ninety-nine one-hundredths
of buds die in the struggle for light and air, and therefore the
survivors are distinct growing points, widely separated from each
other. Species are such extreme, but separated, twiglets of the tree of
life.
=Objection.=--But it will be objected, again: The twig-_points_
are, indeed, separate, but the twigs themselves must meet somewhere
lower down, where they began to grow. Intermediate links may be
wanting _now_, but they must, of course, have existed once--i. e., in
previous geological times, and therefore ought to be found fossil. In
distribution in space or geographically, organic kinds may be marked
off by hard-and-fast lines, but, if their derivative origin be true,
in their distribution in _time_ or geologically, there ought to be
many examples of insensible shadings between them. In fact, if we
only had all the extinct forms, the organic kingdom, taken as a whole
and throughout all time, ought to consist not of species at all, but
simply of individual forms, shading insensibly into each other, like
the colors of the spectrum, and our classification ought to be a mere
matter of convenience, having no counterpart in nature. But this is
not the fact. On the contrary, the law of distribution in time is
apparently similar in this respect to the law of distribution in space,
already given (page 169). As in the case of _contiguous_ geographical
faunas, the change is apparently by _substitution_ of one species _for_
another, and not by _transmutation_ of one species _into_ another.
So also in _successive_ geological faunas, the change seems rather
by substitution than by transmutation. In both cases species seem to
come in suddenly, with all their specific characters perfect, remain
substantially unchanged as long as they last, and then die out and
are replaced by others. Certainly this looks much like immutability
of specific forms, and supernaturalism of specific origin. We have,
we believe, satisfactorily explained this in the case of geographical
distribution (page 201), but how can we explain it in the case of
geological distribution?
=Answer.=--1. The reason for this, given by Darwin and other
evolutionists, is the extremely fragmentary character of the geological
record. If the existing faunas and floras are but isolated remnants,
the rest having been _destroyed_ by migrations and conflicts, how
much more are fossil faunas and floras but fragmentary remnants, the
rest having been _lost_, partly because never preserved, and partly
by destruction of the record! If from this cause existing species are
widely separated, how much more ought we to expect to find fossil
species distinct and widely separated!
This is undoubtedly in most cases a true and sufficient answer, yet we
think the fragmentariness of the geological record has been overstated.
While it is true that there are many and wide gaps in the record; while
it is true, also, that even where the record is continuous many forms
may not have been preserved, yet there are some cases, especially
in the Tertiary fresh-water deposits, where the record is not only
continuous for hundreds of feet in thickness, but the abundance of
life was very great, and the conditions necessary for preservation
exceptionally good. In such cases the number of fossil species found
on each horizon seems to be as great as in existing faunas over equal
space. The record in these cases seems to be continuous and without
break, and crowded with fossil forms; and yet, although the species
change greatly, and perhaps many times, in passing from the lowest to
the highest strata, we do not usually, it must be acknowledged, find
the gradual transitions we would naturally expect, if the change were
effected by gradual transformations. The incompleteness of the record,
therefore, although a true and important cause, is not the whole cause.
In further and completer answer to this greatest of all objections, we
will throw out the following suggestions:
2. We must remember that considerable latitude is allowed by the
anti-derivationists to _variation of species;_ so much so, indeed, that
it is often difficult to draw the line between well-marked varieties
and closely-allied species. Now, according to the derivationist,
these strong varieties, breeding usually true, are naught else than
commencing species.
3. On every side and everywhere, both in existing faunas and in fossil
forms, but especially in the latter, we find innumerable examples of
transitions, or intermediate forms, between all the _higher groups_,
such as genera, families, orders, and classes. It is, in fact, by means
of these that the great law of differentiation from generalized types
has been established. It is, therefore, only between _species_ that
such intermediate forms are rare.
4. But even between species such intermediate forms, though rare, have
been pointed out, both in existing and in extinct faunas. But the
opposition contend that, in all such cases, the previously supposed
species are only varieties. We have already (page 61) spoken of the
obvious fallacy involved in this position. Species are first defined
as forms distinct and without intermediate links, and then we are
challenged to find such links; and when, with much labor, we find them,
they say the supposed species are not species, but only varieties.
But there are some cases in which this subterfuge will not do. There
are cases in which the transitions are between forms so extreme that
they can not, by any stretch of the term, be called varieties. We will
select and dwell upon but one striking example, viz., the fossil forms
of the Tertiary fresh-water deposits of Steinheim.
In Würtemberg, near the little village of Steinheim, are found certain
strata of sand and lime, which are evidently deposits from a small
lake of Tertiary times. The deposits are extremely rich in fossil
shells, especially of the different species of the genus _Planorbis_.
As the deposits seem to have been continuous for ages, and the fossil
shells very abundant, this seemed to be an excellent opportunity to
test the theory of derivation. With this end in view, they have been
made the subject of exhaustive study by Hilgendorf in 1866,[34] and by
Hyatt in 1880.[35] In passing from the lowest to the highest strata
the species change greatly and many times, the extreme forms being so
different that were it not for the intermediate forms they would be
called not only different species but different genera. And yet the
gradations are so insensible that the whole series is nothing less
than a demonstration, in this case at least, of origin of species by
derivation with modifications. The accompanying plate of successive
forms (Fig. 70), which we take from Prof. Hyatt’s admirable memoir,
will show this better than any mere verbal explanation. It will be
observed that, commencing with four slight varieties--probably sexually
isolated varieties--of one species, each series shows a gradual
transformation as we go upward in the strata--i. e., onward in time.
Series I branches into three sub-series, in two of which the change of
form is extreme. Series IV is remarkable for great increase in size
as well as change in form. In the plate we give only selected stages,
but in the fuller plates of the memoir, and still more in the shells
themselves, the subtilest gradations are found.
[Illustration: FIG. 70.--Transformations of Planorbis (after Hyatt).
_Series IV._ 1, Pl. levis: Undorf. 2, Pl. Steinheimensis;
3, tenuis-Steinheimensis; 4, tenuis; 5, discoideus; 6,
trochiformis-discoideus; 7, trochiformis: Steinheim.
_Series III._ 8, Pl. levis: Undorf. 9, Pl. oxystomus; 10, supremus;
11, supremus var. turrita: Steinheim.
_Series II._ 12, Pl. levis: Undorf. 13, Pl. crescens-parvus; 14, 15,
crescens: Steinheim.
_Series I._ _Sub-series 3._ 16, Pl. levis: Undorf. 17, Pl.
minutus-levis; 18, minutus; 19, 20, triquetrus: Steinheim.
_Sub-series 2._ 21, Pl. minutus; 22, 23, denudatus-minutus; 24,
denudatus var. distortus: Steinheim. _Sub-series 1._ 25, Pl.
costatus-minutus; 26, costatus; 27, 28, costatus var--: Steinheim.
The specimens from Undorf all belong to an older Tertiary period than
that at Steinheim.
]
This case is striking, partly because it is a very favorable one,
but mainly because it has been so carefully studied. There can be no
doubt that equally careful study would reveal the same transition
in many other cases. Nor are such transitions confined to the lower
forms of life, though they are probably more abundant there. According
to Cope, the nicest gradations may be traced between some of the
extinct mammalian species so abundant in the Tertiary deposits of the
West--especially between the species of the extinct generalized
family of _Oredontidæ_[36] The same is probably true of the many
extinct species of the horse family.
It is interesting to observe that the details of the process of change
in the forms of _Planorbis_ are in accord with Dr. Romanes’s views.
The change does not seem to have been uniform but somewhat paroxysmal.
The forms seem to remain stable for a long time, and then a few break
into several different forms, while the more rigid die out. It is as if
cross-breeding had kept the type true, but at the same time increased
its tendency to variation, until finally one or more varieties became
sexually isolated and thus formed new species.
5. But still the question remains: Why are transitional forms _rare_ in
all cases, especially between species--so rare that they are eagerly
sought and highly prized? I believe that the true reason of this is
that _the steps of evolution are not always uniform_.
Nearly all evolutionists have assumed and even insisted on uniformity,
as the opposite of catastrophism and of supernaturalism, and therefore
as essential to the idea of evolution. They say that the constancy of
the action of the forces of change necessitates the uniformity of the
rate of change. But, in fact, this is not always nor even usually true.
Causes or forces are constant, but phenomena everywhere and in every
department of Nature are _paroxysmal_. The forces producing storms
and lightning, and volcanoes and earthquakes, are or may be constant;
yet the phenomena are in the highest degree paroxysmal. Wherever in
nature we have a constant force and a strong resistance, we find more
or less paroxysmal action. For this reason the wind blows in puffs,
the friction of wind on water produces waves, water running in small
pipes issues in pulses. The reason is obvious, as may be seen by
the following examples: Suppose lifting forces within the earth are
resisted by crust-rigidity. The forces accumulate uniformly until the
resistance gives way, and suddenly we have an earthquake. Water running
with great resistance in small pipes is checked, but soon accumulates
additional force, which overcomes the resistance, only to be again
checked, and so on, and therefore runs in pulses. Now, the course of
evolution of the whole earth may be likened to such a current; there
are forces of movement and forces of resistance--progressive forces
and conservative forces. The progressive force is accumulative, the
resisting force is constant. Thus, in all evolution or history,
whether of the earth or of society, there are periods of comparative
quiet, during which the forces of change are gathering strength, and
periods of revolution or rapid change, during which these forces show
themselves in conspicuous effects.
Now, that there have been such periods of rapid revolutionary change
in the history of the earth, there can be no doubt. The history of the
earth is marked by periods of comparative quiet, during which life was
exceptionally abundant and prosperous, and change of organic forms
slow and uniform--separated by periods of disturbance, revolution,
rapid changes of physical geography and climate, and consequently of
comparatively rapid and sweeping changes in organic forms. These form
the division-lines between great eras of the earth’s history, and
are always marked by extensive unconformity of the strata, showing
the changes of physical geography above spoken of, and by apparently
sudden and sweeping change in life-forms, showing the great changes of
climate and other physical conditions. Unfortunately, in all cases of
unconformity of strata, there is, of course, a break in the continuity
of the record; and when the unconformity is very general a portion of
the record may be irrecoverably lost. The consequence is, that there is
an apparent break also in the continuity of life-forms. It looks, at
first sight, like wholesale extermination of old and recreation of new
forms. But undoubtedly the break in the continuity of life is apparent
only, as is shown by the loss in the record. If we could recover the
whole record, as indeed we sometimes do, we should find in all cases
that there is no break in the continuity of evolution, but only more
rapid rate of change at these times. But to this cause of rapid rate
of progress--i. e., change of physical environment--we must add change
of organic environment induced by the physical. We have already seen
(p. 179) that extensive changes in physical geography and climate are
always accompanied by wide migrations and dispersals of species, the
mingling of faunas and floras, and the severer struggle for life,
and the sweeping weeding-out of all but the fittest, and the change
of these latter, making them still fitter. These two causes of rapid
change, viz., change of climate and migrations, together with the loss
of record, we believe completely account for those sweeping changes,
not only of species but even of genera, families, and orders which
characterize the passage from one great era to another.
But this does not yet explain the apparent discontinuity between
_consecutive_ species in the same locality in continuous, conformable
strata, or the rarity of transitional forms when one species takes the
place of another in an apparently continuous record. In such continuous
deposits the successive faunas do indeed gradate insensibly into one
another, but apparently as in contiguous geographical regions (p. 200)
by substitution, not by transmutation. How shall we explain this?
On this point I throw out some suggestions: 1. In the modification of
species, too, as well as in other progressive changes, we may imagine
two forces operating, one progressive, the other conservative--the
one external, the other internal. The external progressive force
consists of all the factors of change already mentioned, the internal
conservative is the law of heredity, of like producing like. A changing
environment tends continually and increasingly to change of organisms,
but change is resisted by heredity, which tends to adhere, within
narrow limits, to the same form. But since the external force or
tendency to change increases constantly--since the discord between the
environment and the organism becomes ever greater, there must come a
time when either the species is destroyed, or else the resistance of
heredity gives way, and rapid change takes place. The alternative is
presented to the species to transform or perish; and in one or perhaps
in two or three generations we have an amount of change which, under
other circumstances, might take a hundred generations to accomplish.
These rapid changes are in fact exactly what in artificial varieties
we call sports. We do not know all the conditions which determine
sports in domestication, and still less what determines large and
widely-divergent variations, and therefore rapid origin of many
divergent species, in geological history. But one thing seems probable,
viz., that, when a species begins to change, it continues to change
easily and in many directions. When resistance gives way it takes some
time, many generations, for heredity to gather force again. Hence,
young species are plastic, fluent, because heredity, on any one point,
has not yet accumulated. But as soon as a stable form is again reached,
then, by accumulating a fund of heredity, the form tends to become more
and more rigid, until often it becomes too rigid to yield to modifying
influences, and therefore becomes extinct. By far the greater number
of species do thus become extinct and leave no progeny, while the few
more plastic forms are modified in several directions, and the number
of forms may, after a little time, be undiminished or even increased.
2. As to the _cause_ of rapid changes of form during revolutionary
or critical periods in the earth’s history, Brooks has introduced
an idea which is very suggestive, and deserves serious attention.
We have above spoken of the progressive element as external. Brooks
regards both elements as internal, and represented by the two sexes.
The male represents the progressive, the female the conservative
element. The one tends to divergent variation, the other to fixity
of type by heredity. I think we will all admit that, as a general
rule, in man (and probably all the higher animals) the male is more
highly differentiated into many divergent forms--the female is more
like the type-form of the species. In man, the male is certainly more
diversified in form, in expression, and in character. If they have the
keenest ear for musical pitch, they are also most often music-deaf;
if they have the sharpest perception of color, they are also most
often color-blind; if among them we find the brightest intellects, we
also find the dullest and most stupid; if there are among them more
geniuses, so, also, there are more cranks. The same is also, probably,
true of other animals, in proportion to their grade of organization.
The operation of these two equally necessary elements is well shown
in every advancing society. The initiative of every movement, in all
directions, good or bad, is determined by the male; the conservation
of whatever balance of good there may be, seems to be mainly by the
female. The male tries all things, the female holds fast that which
is good. By the one society gains a little in each generation; by the
other the gain is conserved and made a new point of departure. The one
is ever building hastily a scaffolding and platform; the other ever
consolidating into a permanent structure. Now, according to Brooks,
what is true in the plane of social progress is true also in the lower
plane of organic evolution. In sexual union, and in the resulting
offspring, the sperm-cell is the element which tends to divergent
variation, and the germ-cell to fixity of type, through heredity. In
artificial breeding, then, we ought to make new varieties by proper use
of the sire; we ought to preserve them true by proper management of the
dam.
But, again, it is believed that in many lower animals, especially
insects, the high-feeding of the mother, and consequent good condition
of the ovum, tends to the production of female offspring. It seems
almost certain that, in butterflies, the sex is not yet declared
in the caterpillar stage. According to the careful experiments of
Mrs. Treat,[37] if the caterpillars be well fed, they become female
butterflies; but, if poorly fed, they make males. One purpose of this
provision of Nature is, doubtless, to provide for the greater draught
on the vitality of the female in reproduction.
Now for the application. In good times in the history of a species,
when everything is prosperous, external conditions are favorable, and
food is abundant, females are in excess, and individuals are greatly
multiplied. Under these conditions, evolution would be slow and
uniform. But in _bad_ times in the history of a species, when external
conditions were unfavorable, not only would there be excess of males,
but these, through the influence of the changing environment, as well
as through the dominance of the male element, would be more than
usually varied in character. Among the strongly divergent varieties
thus formed, the fittest--i. e., those most in accord with the changing
environment--would survive and leave offspring partaking of their
character. We have already repeatedly said that the severer pressure of
a rapidly-changing environment determines correspondingly rapid changes
in organic forms. It may do so in many ways; but, according to Brooks,
one of the most important ways is by determining an excess of the male
element.
In brief, then, the causes of rarity of transitional forms among
fossils are--1. The change being, for the reasons given, comparatively
rapid, the _number of generations_ between consecutive species are
few, perhaps only one. 2. Times of rapid change are also times of
unfavorable conditions, and therefore the number of _individuals_
in each generation is small, and all the smaller--in Brooks’s
view--because of the fewness of females. When we remember that fossils
are but a small fraction of the actual faunas and floras of the
time, surely these two causes go far toward explaining the rarity of
links between _species_. 3. Add to these the existence of periods of
wide-spread changes in physical geography and climate, and consequent
wide migrations and dispersals of species, and we sufficiently account
for those sweeping changes in species, genera, families, and orders,
which mark the limits of the great eras, and which are made still more
abrupt, and apparently supernatural, by the loss of record at these
times.[38]
=Objection.=--There is still one more objection which will be made. We
have drawings of plants, animals, and men, by Egyptian artists, who
lived at least three thousand years ago, and the species of the one and
the races of the other are still the same. Still better, we have among
the wrappings of Egyptian mummies the very plants themselves, leaves
and flowers perfectly preserved, and even colors almost perfect. Yet
the species are exactly the same as grow in Egypt to-day. If species
are made by gradual transmutation, surely there ought to have been some
change in three thousand years.
=Answer=.--It may be well to note that this apparent permanence is true
of _races_ of men as well as of _species_ of animals and plants. But
the very men who insist on permanence of species are equally insistent
on the variability of varieties and races. The objection, therefore,
proves too much. We shall not insist on this, however, because as
derivationists we regard races as naught else than commencing species,
and therefore subject to the same laws. We are not striving for triumph
in debate, but only for truth. The true answer will, we believe, be
found among the following suggestions:
1. Three thousand years seems a long time in human history, but in
geological history it is but a day. This, the usual answer, is no doubt
a true one, but hardly, we think, sufficient. When we remember the
enormous change which has taken place in faunas and floras since the
end of the Tertiary, if change still continues at the same rate, surely
it ought to be distinctly perceptible in three thousand years.
2. But we must remember that such changes are usually more or less
paroxysmal; not, indeed, so sudden as to break the continuity of life,
but far more rapid at some times than at others. The last critical
or revolutionary period of rapid change was the Glacial epoch. Since
that time--i. e., during the human period--a new equilibrium has been
established, and the changes in organic forms have been very slow.
3. Remember, again, that in evolution _all_ species do not change. On
the contrary, most become rigid, and either remain unchanged, almost
indefinitely, or else die out and leave no descendants. Only the more
plastic forms change into other species, but usually into several
other species, and thus the number of forms may be undiminished, even
though the larger number of old forms leave no descendants. It is true,
therefore, of this as well as of other epochs, that the greater number
of species are permanent.
4. It is not impossible--indeed, it is in exact accordance with the
laws of evolution--that organic forms are more permanent now than
ever before. Evolution is a growth; the forces of growth must exhaust
themselves. Evolution proceeds by constant differentiation and
specialization, but extreme specialization always arrests evolution.
In ontogenic evolution, for example, cell-structure becomes more
and more specialized, but also thereby more and more rigid, and,
when specialization is complete, evolution stops, and cell-forms are
permanent. It is this which limits the cycle of every evolution. So is
it precisely with evolution of the organic kingdom, except that the
cycle is much longer. Here, also, every step is by specialization, and
yet specialization fixes the form, and finally arrests the advance on
that line. Thus, throughout the whole geological history of the earth,
the larger number of forms, by specialization, become rigid and perish,
while the fewer, more generalized, and more plastic forms take up the
march and carry it forward a step, only to be themselves specialized
and fixed. If we compare, again, to a tree: each twig finishes its
growth, flowers, fruits, and dies; other buds take up the growth and
carry it forward. By specialization the highest condition of a certain
form of life is attained, but other possibilities are shut off. Extreme
specialization is the flowering and fruiting--the end and completion
of twig-life. Now, obviously, this specialization and respecialization
can not go on forever. When it is complete in every direction it must
cease, and forms become permanent, or else perish. When it flowers it
must die.
Now, is not the advent of man in many ways a sign of the completeness
of organic evolution? Certain it is that with man there begins an
entirely new form of evolution. Certain it is that with man evolution
is transferred from the organic to the social plane, from the material
to the psychical. Certain it is that the forces, the conditions
and results of this evolution, are wholly different from those of
the other. In organic evolution the organism must conform to the
environment; in human evolution the environment is made to conform to
the wants of the organism. The one is unconscious and involuntary,
passive under the dominating laws of Nature; the other is conscious,
voluntary progress toward an ideal, _by the use_, among other means,
of the laws of Nature. The one is by change of external form--i. e.,
change of species--the other by change of brain-structure. Now, does
not the commencing of the cycle of this new evolution imply the
closing of that of the old? The two may overlap somewhat now, but it
is evident that, when the cycle of human evolution culminates, when
highly civilized man shall have taken possession of the whole earth,
the whole organic kingdom must be readjusted to his wants. All organic
forms must be either domesticated or destroyed. Organic forms will no
longer be modified by natural but wholly by artificial selection.
* * * * *
There are many other supposed objections which have been urged, but
these are mostly not objections to evolution, but only to some _special
theory_ of evolution--Darwinian, Spencerian, Lamarckian, or other.
=Origin of Beauty=.--For example, it has been urged that natural
selection can only account for _useful_ structures; but _beauty_ is
as universal and as conspicuous in nature as _use_. In many cases
Darwin has shown that beauty is useful, and in such cases it is, of
course, seized upon by selection and intensified. Thus, the gorgeous
coloring of birds and insects is largely due to sexual selection.
Beauty is attractive, and therefore the most beautiful prevail in
securing reproductive opportunities. This character is, therefore,
perpetuated in the offspring, and intensified from generation to
generation. But, of course, this can apply only to higher animals,
in which the sexes are separate and sexual union voluntary. It can
not apply to self-fertilizing hermaphrodites; and yet in these, also,
we often find the most gorgeous coloring. Again, Darwin has very
ingeniously and successfully explained the case of the beauty and
fragrance of flowers of hermaphroditic plants by another principle,
viz., that of _insect-selection_. Insects are attracted by the most
showy and fragrant flowers, and thus become the means of carrying
pollen from flower to flower, insuring fertilization, and especially
cross-fertilization. The most beautiful and fragrant flowers are most
certain to be fertilized, and thus beauty and fragrance become useful
to the plant, and therefore are selected and intensified.
These and many other cases of beauty may doubtless be explained by
showing that it is useful; but beauty which is without any use can
not be explained by natural selection. Now, as already said, the
most gorgeous beauty is lavishly distributed even among the lowest
animals, such as marine shells and polyps, where no such explanation
is possible. The process by which such beauty is originated and
intensified is wholly unknown to us.
=Incipient Organs=.--Again, Mivart has drawn attention to another
difficulty in the way of natural selection as an explanation even of
useful organs. Darwin does not, of course, attempt to account for the
_origin_ of varieties. As we have already seen, he assumes divergent
variation of offspring as the necessary material on which natural
selection operates. He who shall explain the origin of varieties will
have made another great step in completing the theory of evolution. But
not only does not natural selection explain the _origin_ of varieties,
but neither can it explain the _first steps_ of advance toward
usefulness. An organ must be already useful before natural selection
can take hold of it to improve it. It can not make it useful, but only
_more useful_. For example, if fins commenced as buds from the trunk,
it is difficult to see how they could be of any use, and therefore
how they could be improved by natural selection until they were of
considerable size, and especially until muscles were developed to move
them. Until that time they would seem to be a hindrance to be removed
by natural selection, instead of a use to be preserved and improved. It
would seem that many organs must have passed through this _incipient
stage_, in which their use was prospective.
Much that is very interesting might be said on these and similar points
of difficulty, but all this lies entirely aside from the scope of
this work. As already said, these are not objections to evolution or
derivation, but only to _Darwinism_, or any other special theory, as a
_sufficient explanation_ of the process of evolution. They only show
that we do not yet fully understand this process; that there are still
other and perhaps greater factors of evolution than is yet dreamed of
in our philosophy.
In the foregoing chapters on special evidences, and especially in the
last two, the reader will observe many points of doubt, discussion, and
difference of opinion. Let it not be concluded on that account that the
_law of evolution_ is still in the region of uncertainty. It can not be
too strongly insisted on that the fact of evolution as a universal law
must be kept distinct from the causes, the factors, the conditions, the
processes, of evolution. The former is certain, the latter are still
imperfectly understood.
PART III.
_THE RELATION OF EVOLUTION TO RELIGIOUS THOUGHT._
CHAPTER I.
INTRODUCTORY.
From what has preceded, the reader will perceive that we regard the
law of evolution as thoroughly established. In its most general
sense, i. e., as a law of continuity, it is a necessary condition of
rational thought. In this sense it is naught else than the universal
law of necessary causation applied to forms instead of phenomena. It
is not only as certain as--it is far more certain than--the law of
gravitation, for it is not a contingent, but a necessary truth like the
axioms of geometry. It is only necessary to conceive it clearly, to
accept it unhesitatingly. The consensus of scientific and philosophical
opinion is already well-nigh, if not wholly, complete. If there are
still lingering cases of dissent among thinking men, it is only because
such do not yet conceive it clearly--they confound it with some special
form of explanation of evolution which they, perhaps justly, think not
yet fully established. We have sometimes in the preceding pages used
the words evolutionist or derivationist; they ought not to be used any
longer. The day is past when evolution might be regarded as a school
of thought. We might as well talk of gravitationist as of evolutionist.
If, then, evolution as a law be certain, if, moreover, it is a law
affecting not only one part of Nature--the organic kingdom--and one
department of science--biology--but the whole realm of Nature and every
department of science, yea, every department of thought, changing our
whole view of Nature and modifying our whole philosophy, the question
presses upon us, “What will be its effect on religious belief, and
therefore on moral conduct?” This is a question of gravest import.
To answer it, however imperfectly, is the chief object of this work.
Except for this, it would probably never have been undertaken. All that
goes before is subsidiary to this.
But I will doubtless be met at the very threshold by an objection
from the scientific side. Some will say--because it is the fashion
now to say--that as simple, honest truth-seekers, we have nothing to
do with its effect on religion and on life. They say we must follow
Truth wherever she leads, utterly regardless of what may seem to us
moral consequences. This I believe is a grave mistake, the result of a
reaction, and on the whole a wholesome and noble reaction, against the
far more common mistake of sacrificing truth to a supposed good. But
the reaction, as in most other cases, has gone much too far. There is
a true _philosophic_ ground of justification for the reluctance with
which even honest truth-seekers accept a doctrine which seems harmful
to society. Effect on life is, and ought to be, an important element
in _our estimate of the truth of any doctrine_. It is necessary for me
to show this, in order to justify this part of my work.
=Relation of the True and the Good.=--There is a necessary and
indissoluble connection between truth and usefulness. We all at once
admit this connection in one direction. We all admit that a truth must
eventually have its useful application. It may not be _now_, nor in ten
years, nor in a century, nor even in a millennium, but some time in the
future it will vindicate its usefulness. No truth is trivial or useless
in its relation to human life, for man is a part of Nature, and his
life must be in accordance with the laws of Nature. Every one admits
this, but not every one admits the converse proposition, viz., that
whatever doctrine or belief, in the long run and throughout the history
of human advancement, has tended to the betterment of our race, must
have in it an element of truth by virtue of which it has been useful;
for man’s good can not be in conflict with the laws of Nature. Also,
whatever in the long run and in the final outcome tends to the bad in
human conduct, ought to be received, even by the honest truth-seeker,
with distrust as containing essential error. The reason of this will
now be further explained.
=Relation of Philosophy to Life.=--There are three primary divisions
of our psychical nature, viz., sensuous, intellectual, and volitional
or moral. There are three corresponding primary processes necessary
to make a complete rational and satisfactory philosophy: (1) There
is first the _instreaming_ of the external world through the senses,
as impressions. These we call facts or phenomena. (2) The elaboration
of these facts within, by the _intellect_, into a compact, consistent
structure. This we call knowledge. (3) The outgoing of this knowledge
by the _will_ into the world as right or wise conduct. Now these three
are all equally necessary. All these three portions of our complex
nature are equally urgent to be satisfied.[39] But, unfortunately,
scientific workers are too apt to think only 1 and 2 necessary--that
true facts elaborated into consistent theory are all we need care for.
Theologians and metaphysicians, on the other hand, seem to think only 2
and 3 necessary. They elaborate a theory consistent in all its parts,
exquisitely woven in beautiful and delicate pattern, and apparently
satisfactory in its application to the right conduct of life, but are
less careful to inquire whether it is in harmony with facts derived
from the senses. But, we repeat, all three are equally necessary. The
first gathers the materials, the second constructs the edifice, the
third, by _use_, by practical application, _tests_ whether it be a fit
building to live in, whether it is constructed on sound architectural
principles. The tendency of the olden time was to neglect the first,
the tendency of the present time is to neglect the third. But we repeat
with stronger emphasis that this third element is equally necessary.
All admit that successful application in art is the surest test of
the truth of science. Now, social conduct is the art corresponding to
our philosophy of life, and therefore is the sure test of its truth.
It follows, therefore, that unless all these three primary divisions
of our nature are satisfied by any doctrine, there must result an
ineradicable confusion and discord in our psychical nature, and
cordial acceptance is not only impossible but irrational. We insist
upon this the more because it has become the fashion in these latter
days of dominance of science, to say that to inquire into effects on
society is inconsistent with the scientific spirit, and unworthy of the
honest truth-seeker. But, observe, I am speaking of effects on society
only as a _test of truth_. I would not swerve a hair’s breadth from
absolute devotion to truth. It is necessary, indeed, to inquire into
effects on society, but we must inquire only in the patient spirit
characteristic of the truth-seeker. Whatever is really true will surely
vindicate itself by its beneficence, if we will only wait patiently for
final results. Evolution is no exception to this universal truth. It
will surely vindicate its beneficence, but we must wait yet a little
while--not very long.
So much it was necessary to say in justification of the inquiry which
constitutes this third part of our work. But, after this justification,
the question returns with additional emphasis, “What will be the effect
of the universal acceptance of the law of evolution on religious
thought, and through this on the right conduct of life?”
There can be no doubt that evolution, as a law affecting all science
and every department of Nature, must fundamentally affect the whole
realm of thought, and profoundly modify our traditional views of
Nature, of God, and of man. There can be no doubt that we are now on
the eve of a great revolution. But, as in all great revolutions, so
in this, the first fears as to its effects are greatly exaggerated.
To many, both friends and foes of Christianity, evolution seems to
sweep away the whole foundation, not only of Christianity, but of all
religion and morals, by demonstrating a universal materialism. Many are
ready to cry out in anguish, “Ye have taken away our gods, what have we
more? Ye have destroyed our dearest hopes and noblest aspirations, what
more is left worth living for?” But I think all who are at all familiar
with the history of the so-called conflict between religion and science
will admit this is not the first time this cry has been raised against
science. They have heard this danger-cry so often that they begin
to regard it as little more than a wolf-cry--scientific wolf in the
religious fold. It may not be amiss, then, to stop a moment to trace
rapidly the main points of this conflict--to discuss the various forms
of this scientific wolf.
First, then, it came in the form of the _heliocentric theory of
the planetary system_. We once thought the earth the center of the
universe, and so firm that it can not be moved. But science shows that
it moves about the sun, and spins unceasingly on its axis. Every one
has heard of the terror of the sheep produced by this discovery, and
the nearly tragic results to the bold scientist. But now we look back
with wonder that there should have been any trouble at all. Would any
Christian now consent to give up the grand conceptions of Nature and
of God thus opened to the human mind--the idea of infinite space full
of worlds, of which our earth is one, moving in silent harmony as in a
mystic dance? Verily, this wolf has proved itself a harmless, nay, a
very noble beast, and lies down in peace with the lambs.
Next, it came in the shape of the _law of gravitation_, as sustentation
of the cosmos by law and resident forces. The effect of this on
religious thought was even more profound, though less visible on the
surface, because only perceived by the most intelligent. It seemed
at that time to remove God from the course of Nature. This was the
real ground of the skepticism of the last century, and also the real
motive of Voltaire’s ardent advocacy of Newton’s views before these
were generally accepted in France. But now, who would give up this
grand idea--this conception of law pervading infinite space--the same
law which controls the falling of a stone guiding also the planetary
orbs in their fiery courses? This is indeed the divine spheral music,
inaudible but to the ear of science, accompanying the celestial dance.
Next, it came in the form of the _antiquity of the earth_ and of the
cosmos. The earth which we had fondly thought made specially for us
about six thousand years ago; sun, moon, and stars, which we had vainly
imagined shone only for our behoof--these, science tells us, existed
and each performed its due course inconceivable ages before there was a
man to till the ground or contemplate the heavens. Some of my readers
may still remember the horror, the angry dispute which followed the
promulgation of these facts. But now, who would consent to give up the
noble conception of infinite time thus opened to the human mind and
become forever the heritage of man?
Next, it came in the form of the _antiquity of man_. It is probable,
nay, certain, that man has inhabited the earth far longer than we had
previously supposed we had warrant for believing. The controversy on
this question and the dread of its result has indeed not yet entirely
subsided. Some timid people still look askance at this wolf, but I
think all intelligent people accept it and find it harmless.
Next, and last, it comes now in the form of _evolution_--of the origin
of all things, even of organic forms, by _derivation_--of _creation
by law_. We are even now in the midst of the terror created by this
doctrine. But what is evolution but law throughout infinite time? The
same law which now controls the development of an egg has presided
over the creation of worlds. Infinite space and the universal law
of gravitation; infinite time and the universal law of evolution.
These two are the grandest ideas in the realm of thought. The one is
universal sustentation, the other universal creation, by law. There is
one law and one energy pervading all space and stretching through all
time. Our religious philosophy has long ago accepted the one, but has
not yet had time to readjust itself completely to the other. A few more
years, and Christians will not only accept, but love and cherish this
also for the noble conceptions it gives of Nature and of God.
But some will exclaim, “Noble conceptions of God, say you! Why, it
utterly obliterates the idea of God from the mind. All other conflicts
were for outworks--this strikes at the citadel. All others required
only readjustment of claims, rectification of boundaries betwixt
science and religion--this requires nothing less than unconditional
surrender. Evolution is absolute materialism, and materialism is
incompatible with belief in God, and therefore with religion of any
kind whatsoever!” Before proceeding any further, it becomes necessary
to remove this difficulty out of the way.
CHAPTER II.
THE RELATION OF EVOLUTION TO MATERIALISM.
It is seen in the sketch given in the previous chapter that, after
every struggle between theology and science, there has been a
readjustment of some beliefs, a giving up of some notions which really
had nothing to do with religion in a proper sense, but which had
become so _associated_ with religious belief as be to confounded with
the latter--a giving up of some line of defense which ought never to
have been held because not within the rightful domain of theology at
all. Until the present the whole difficulty has been the result of
misconception, and Christianity has emerged from every struggle only
strengthened and purified, by casting off an obstructing shell which
hindered its growth. But the present struggle seems to many an entirely
different and far more serious matter. To many it seems no longer
a struggle of theology, but of essential religion itself--a deadly
life-and-death struggle between religion and materialism. To many,
both skeptics and Christians, evolution seems to be synonymous with
blank materialism, and therefore cuts up by the roots every form of
religion by denying the existence of God and the fact of immortality.
That the enemies of religion, if there be any such, should assume and
insist on this identity, and thus carry over the whole accumulated
evidence of evolution as a demonstration of materialism, although
wholly unwarranted, is not so surprising; but what shall we say of the
incredible folly of her friends in admitting the same identity!