SPECIAL PROOFS OF EVOLUTION.
_Introductory._
It will be seen from the preceding chapter that we regard the law of
evolution in its wider sense, viz., the derivative origin of all forms,
organic or other, as axiomatic, and therefore requiring no further
proof. Among scientific men there is no longer any discussion of the
truth of this law, but only of the theories of the causes of the law.
We believe that to the scientific mind there is no other rational mode
of looking at the subject of origin of organic forms. To such a mind,
therefore, all that follows is but the deductive application of that
law in the explanation of the phenomena of organic Nature. But it takes
time for the popular mind to readjust itself to new and revolutionary
truth. Many minds, even among the most intelligent, have not yet
accepted this as the only rational mode of thought. Many men require
further _special proofs_ of the derivative origin of organic forms.
Even to those who accept evolution, these proofs will be interesting
as illustrations of such origin. We will attempt to bring out these
proofs under several heads, the most important of which are: 1. Proofs
from morphology, or the general laws of animal structure; 2. Proofs
from embryology; 3. Proofs from geographical distribution of organic
forms; and, 4. Proofs from artificial breeding. The subject is so vast
that all we can do is to touch lightly only the most salient points
under each of these heads; for, as we have already said, the evidence
is really nothing less than the whole science of biology. Preparatory
to this, however, it is necessary to bring out a little more fully than
before (page 29), though still only in outline, the two antagonistic
views, which may be called the old and the new, or the natural and the
supernatural, of the origin of new organic forms, especially species.
=Origin of New Organic Forms; the Old View briefly stated.=--According
to the old-school naturalists, species are the ultimate elements of
taxonomy: genera, families, orders, etc., may gradually change their
character from age to age, by the introduction of new species; but
species were supposed to be substantially _permanent_. It was necessary
to have some unit for convenience of description and classification,
and this was found to be the best because most stable. As in nearly
all cases of beliefs, this doctrine was held at first somewhat
loosely, as a provisional and convenient view--as a good working
hypothesis--but gradually, under pressure of controversy, became more
strictly formulated, and, as it were, hardened into a scientific
dogma, especially in the hands of Agassiz. According to this view,
the first pair or pairs of each specific kind originated we know not
how, but certainly _at once in its present form_ in full perfection,
and, therefore, presumably by _direct creative_ act of Deity; and then
afterward by the law of generation continued to produce others of the
same pattern indefinitely. Moreover, the first one or more pairs of
each kind multiplied and spread abroad in every direction, _each from
its own center of origin_, as far as physical conditions and struggle
for life with other species would allow. This idea explains tolerably
well the geographical distribution of species as we now find it. For
example, species on different continents are widely different, because
those on each have originated independently where we now find them, and
spread in all directions as far as physical conditions would allow, but
could not reach other continents because of the ocean-barrier. That
this is the only reason they are not there, is shown by the fact that,
if they are carried there, they usually do perfectly well. Even on the
same continent, for the same reason, species may be very different
if separated by impassable barriers such as high mountain-chains or
by climate. But wherever one group of species, originating in one
place, comes in contact on the margin of their range with another
group of species originating in another place, we see no evidence of
_transmutation_ of one form _into_ another, but only _substitution_ of
one fully-formed species _for_ another equally fully formed. Therefore,
we must conclude that physical conditions may limit the range of a
species, but can not transmute it into another. Thus, to say the least,
many of the facts of geographical distribution are well explained
by this idea of creative origin in specific centers and subsequent
permanence of specific form. We say _many_ of the facts; we will show
hereafter that _not all_ can be thus explained.
But the main question is not of geographical but of geological
distribution; not distribution in space, but succession in time.
Species do not continue forever. On the contrary, they have changed
many times in the course of geological history. As conditions become
unfavorable, species die out or become extinct, and others take their
place and carry forward the life and development of the organic
kingdom. Now, how do they change? According to this school of thought,
here also, as in geographical distribution, they are not transmuted
but replaced; here also physical conditions may destroy a species,
but can not transform it into another. As species die out, others are
created at once, out of hand and fully formed in their place; but in
accordance with a preordained plan consistently carried out and working
ever toward higher and higher conditions. Thus, life is continued on
the earth by the alternation of supernatural and natural processes; by
the alternate use of direct and indirect action of Deity: direct in
the introduction of first pairs, indirect through the natural process
of reproduction in the continuance and multiplication of the species.
Each species is made according to a pattern in the Divine mind, on a
sort of intellectual die, and then continues to reproduce a succession
of individuals of the same pattern as if struck from the same die
until the die is broken or worn out. Another die is made, of another
pattern, and individuals are struck from this; and so on, throughout
the whole geological history of the organic kingdom. Only, we must
add that the successive dies are made to follow one another according
to a plan which is expressed by the three laws already given on page
11. Thus, the origin of individuals is natural, the origin of species
supernatural; the making of dies is supernatural, the coinage is
natural.
We have stated this view in a too extreme form, in order to make it
clearer. We now, therefore, proceed to qualify somewhat. Specific types
were held, by writers of this school of thought, to be _substantially_
but not absolutely unchangeable. Successive individuals of the same
species were admitted to be not exactly alike. Such slight differences
were called _varieties_. It was admitted, indeed, that species
varied, but it was believed that such variations in any direction
were strictly limited in amount. A species may be compared to a right
cylinder standing on end. As such a cylinder may be tilted slightly
in one direction or another, without overthrowing its equilibrium,
the cylinder tending ever to right itself and return to its original
position, so a species may be varied slightly in one direction or
another without destroying its integrity, the species tending ever to
return to its normal or typical form. But as the cylinder, if pushed
too far from its normal position, is overthrown, so also a species,
if pressed too far in the way of variation from its typical form, is
destroyed, but not changed into another species. As cylinders may be
more or less rigid, depending upon the breadth of their bases, so also
some species are more rigidly set in their typical form, and some are
more plastic to influences causing variations, but in all cases there
is a limit to the amount of oscillation consistent with integrity.
=The New View briefly stated.=--According to Darwin, and all biologists
of the present day, species are variable _without limit_, if only the
causes of change are constant and slow enough in their operation,
and the time long enough. A species must be in harmony with its
environment, for this is the condition of its existence. Now, if the
environment change, the species must _tend_ to change slowly from
generation to generation, so as to readjust its relations in harmony
with the changing environment. If the change of environment be slow,
the readjustment may be successful, and the species will change
gradually into another form, so different that it will be called
a different species, especially if the intermediate gradations be
destroyed. If the change in the environment be too rapid, many species,
especially the more rigid, will be destroyed, while the more plastic
may survive by modification. Thus, at every step in the evolution
of the organic kingdom, some species have died without issue, while
others have saved themselves by changing into new forms in harmony
with the new environment. Comparing to a growing tree, some branches
overshadowed die, while others push on for light, forming new lateral
buds, and dividing as they grow. By continued divergent change species
gradually become genera, genera families, etc. Thus, varieties,
species, genera, families, orders, classes, etc., are only different
degrees of differences formed all in the same way. Varieties are only
commencing species, species commencing genera, and so on. There is
no making and wearing out of dies, and making of new ones; the whole
process is a natural one--the whole series is genetically connected. In
a perfect classification varieties, species, genera, families, orders,
classes, etc., are only different _degrees of blood-kinship_.
So much may be regarded as certain, and out of the field of discussion
among biologists of the present day. It is only in defining this
process more accurately, and especially in the _theory of the causes_
or _factors_ of evolution, that there are still difference and
discussion. The most probable view on this subject we now proceed to
give.
=Factors of Evolution.=--The causes of change or adaptive modification,
or the factors of evolution, are at least _four_ well known, and
probably many more still unknown: 1. The physical environment--heat and
cold, dryness and moisture--affects function of organs, and function
affects structure, and both changed function and changed structure are
inherited by offspring, and so increased from generation to generation,
becoming greater without limit. 2. Increased _use_ or _disuse_ of
organs enforced or permitted by change in the environment, physical
or organic, or both, induces change in form, size, and structure of
the organs; and this change is inherited by the offspring, and so
from generation to generation small differences are integrated until
they become great without limit. These two factors were recognized
by Lamarck. 3. “Natural selection,” or “survival of the fittest,”
among divergent varieties of offspring. This is the distinctive
Darwinian factor. In the two preceding factors the change is during
the _individual lifetime_, and reproduction is supposed to transmit
it unchanged to the offspring. In this factor, on the contrary,
the form and structure are supposed to remain unchanged during the
individual life, but for some unknown cause there are slight variations
in different directions (divergent) in the offspring from the same
parents. Now, when we remember that by reproduction the number of
individuals tends to increase by geometrical progression, and that
in each generation only a very few (on an average only two from all
the offspring of one pair) can survive, it is evident that among
these divergent varieties those will most likely survive which are
most in harmony with the external environment, and which possess the
most efficient organs of defense or of escape, or for food-taking.
The surviving offspring, therefore, will be on the average better in
these respects than their parents. It matters not how little better,
for the integration of even infinitesimal improvements from generation
to generation will eventually produce any required amount of change.
4. To the above Darwin has added also “_sexual selection_.” In
_natural_ selection there is struggle of _all_ for _food_, or _means
of living_. In sexual selection there is a struggle among the _males_
for possession of the _female_, and the _means of procreation_. The
one is connected with the nutritive appetite, the other with the
reproductive appetite. This mode of selection acts in two ways, by the
law of battle and the law of attractiveness. The strongest or the most
attractive males alone, or mainly, leave offspring, which, of course,
inherit their peculiarities; and these are increased indefinitely
by integration through successive generations, thus increasing the
strength or the beauty. Of these two laws, the law of battle is most
conspicuous among mammals, and the law of attractiveness among birds.
It is evident that this factor can not operate among many lower animals
which are hermaphroditic, nor among plants.
Of these acknowledged factors of evolution, the first two were known
to Lamarck and the older evolutionists. The third and fourth are
distinctively Darwinian. According to Darwin, while all these are
operative, the third is the most powerful; but Spencer accords this
distinction to the Lamarckian factors. Many American zoölogists take
the same view.
Such until very recently were all the recognized factors of evolution.
But, within the past year (1886) has taken place, it seems to us,
the most important advance in the theory of evolution since Darwin.
It is the suggestion by Mr. Catchpool,[14] and afterward the more
full elaboration by Dr. Romanes, of another factor, which he calls
“_physiological selection_.”[15]
The great objections to the sufficiency of the theory of evolution,
as left by Darwin, were twofold: 1. While natural selection accounts
completely for the formation of _useful_ structures or adaptive
modifications, and therefore for differences characterizing classes,
orders, families, and even genera--for these are all adaptive--it
can not so completely account for those constituting species; for
these consist mostly of _trivial_ differences in coloration, relative
proportion of parts, which are of _no perceivable use_ in the struggle
for life, and therefore could not be preserved and integrated by
natural selection. Therefore, according to Romanes, natural selection
is a theory of origin of adaptive structures rather than of origin of
species. Comparing to a growing tree, once admit lateral buds started,
and natural selection completely accounts for the growth in different
directions, and therefore for the profuse ramification; but the origin
of the lateral buds is not explained.
2. The second difficulty is as follows: Such commencing differences as
constitute varieties and species not only would not be preserved and
integrated by natural selection unless useful, but would immediately
be _swamped by cross-breeding_ with the parental form. But, as the
whole divergence commences in varieties, evidently it could not
commence at all unless this cross-breeding be in some way prevented.
This may, indeed, be done, without the assumption of any new factor
of evolution, by _migration_; and, hence, migration must be regarded
as an important agent in the creation of new forms, not only by the
effect of a new environment, but also by prevention of the swamping of
commencing species by cross-breeding with the parental form; but in a
crowded locality, without outlet for migration (the very conditions
most favorable for severe competitive struggle, and therefore for
most potent operation of natural selection; and therefore, also,
according to Darwin, for profuse diversification), commencing varieties
could not pass into species, because swamped by cross-breeding.
Once the divergence reaches the point of cross-sterility--i. e., of
species--then, indeed, by true breeding, characters, even though not
useful, may be preserved. But how is it to commence?
This difficulty has been severely felt by all Darwinists. It seems to
us that it is largely met by Dr. Romanes. According to Romanes, no
organ is so subject to varietal changes as the _reproductive_, and
these in no respect so much as in degrees of fertility. Unfortunately,
these changes are not visible, and must be judged of only by the
results. It is not uncommon, for example, to find sterility between
individuals (sexual incompatibility) who are both of them perfectly
fertile with other individuals. Similarly, cross-sterility, partial
or complete, is not uncommon between varieties or races, as Mr.
Darwin has long ago noticed. It very generally, as we know, occurs
between, and, in fact, is constantly used as a test of, species. Now,
this cross-sterility with parent stock, which we find so constant a
character of species, and which, therefore, must _have commenced as a
partial cross-sterility_ in varieties, is it _antecedent or consequent
to other variations_? It has been usual to suppose it consequent
to a certain amount of divergence, viz., that which constitutes,
or at least approaches, species. But, according to Romanes, it is
_antecedent_. Among many other variations, this is that one which
originates species, because it prevents reversion by cross-breeding
with the parent stock, and insures true breeding with its own kind.
In a word, it sexually isolates the species. Suppose, then, a species
multiplying indefinitely in one locality: trivial variations of many
kinds, and in many directions, occur among the offspring. These are
merged by cross-breeding into the original type, which, therefore,
remains unchanged. But, from time to time, among these variations
there occur some affecting the reproductive organs in such wise as to
produce partial or complete cross-sterility with the parent form. This
is the beginning of a new species. It breeds true with its own kind,
and therefore all the associated variations external and visible, and
therefore constituting species, although trivial and of no use in the
struggle for life, are preserved.
This view completely accounts for the cross-fertility of artificial
breeds equivalent in other respects to species; for cross-sterility
is not an end aimed at by the breeder, it being easy to prevent
cross-breeding, if desired, by artificial isolation. But, if this view
be true, species from widely-different geographical regions ought also
to be often cross-fertile, because, having been formed by geographical
isolation, sexual isolation was not a necessary factor in their
formation. This point deserves testing by careful observation.
It may be, and has been, objected to Dr. Romanes’s claims, that this is
no new factor; that physiological selection is only a form of natural
selection. This objection, it seems to us, is little more than a play
upon words. It certainly is selection, and by a _natural_ process,
and therefore in some sense a natural selection, but not in the sense
of Darwin. It is not a selection of individuals _fittest to survive_;
for cross-fertile individuals are as fit to survive as individuals,
though not as species, as are cross-sterile. Natural selection is
intent only on preserving the best individuals; physiological selection
on preserving the kind. Natural selection continues the direction of
progress unchanged; physiological makes new directions.
In addition to all these factors of _organic_ evolution, there is still
another far higher factor characteristic of man alone. This is the
_conscious, voluntary co-operation of the thing evolving--the spirit
of man--in the work of its own evolution_. This may be called the
_rational factor_. This, the most important factor of human evolution,
is usually ignored by writers on evolution--either as non-existent,
or else as lying beyond the domain of science. We will emphasize its
importance by taking it up more fully in the next chapter.
It will be observed that Darwin and his followers take divergent
variations of offspring simply as a known fact, upon which natural
selection operates to produce progressive modification; and, as the
cause of variation in offspring is wholly unknown, such variations are
often spoken of as fortuitous. But, of course, it is well understood
that nothing in Nature is really fortuitous. They may, however, for
all purposes of natural selection be thus regarded until we know
their cause. It is evident, then, that if we, with Darwin, take
natural selection, as the most important known factor, the really most
important cause of evolution is the _cause_ of varieties. This is the
_unknown_ fundamental factor. As Darwin reduced Agassiz’s three formal
laws of succession to more general laws of life, and thus made one
important step in the advance of biological science, so he who shall
explain the _cause_ of divergent variation will make another important
step by reducing the phenomena to still more general and fundamental
laws of life.
In conclusion, let me again impress upon the reader that all the doubt
and discussion, above described, as to the factors of evolution, is
entirely aside from the truth of evolution itself, concerning which
there is no difference of opinion among thinkers.