between such types as these, than between any others of similar
degree of remove from each other with which we are acquainted. And
inasmuch as almost all groups, as genera, orders, etc., which are
held to be distinct, but adjacent, present certain points of
approximation to each other, the almost daily discovery of
intermediate forms gives us confidence to believe that the pointings
in other cases will also be realized.
γ. Of Transitions.
The preceding statements were necessary to the comprehension of the
supposed mode of metamorphosis or development of the various types
of living beings, or, in other words, of the single structural
features which define them.... As it is evident that the more
comprehensive groups, or those of highest rank, have had their
origin in remote ages, cases of transition from one to the other by
change of character cannot be witnessed at the present day. We
therefore look to the most nearly related divisions, or those of the
lowest rank, for evidence of such change.
It is necessary to premise that embryology teaches that all the
species of a given branch of the animal kingdom (_e.g._, Vertebrate,
Mollusc, etc.) are quite identical in structural character at their
first appearance on the germinal layer of the yolk of the parent
egg. It shows that the character of the respective groups of high
rank appear first, then those of less grade, and last of all those
structures which distinguish them as genera. But among the earliest
characters which appear are those of the species, and some of those
of the individual.
We find the characters of different _genera_ to bear the same
relation to each other that we have already seen in the case of
those definitive of orders, etc. In a natural assemblage of related
genera we discover that some are defined by characters found only in
the embryonic stages of others; while a second will present a
permanent condition of its definitive part, which marks a more
advanced stage of that highest. In this manner many stages of the
highest genus appear to be represented by permanent genera in all
natural groups. Generally, however, this resemblance does not
involve, an entire identity, there being some other immaturities
found in the highest genus at the time it presents the character
preserved in permanency by the lower, which the lower loses. Thus
(to use a very coarse example) a frog at one stage of growth has
four legs and a tail: the salamander always preserves four legs and
a tail, thus resembling the young frog. The latter is, however, not
a salamander at that time, because, among other things, the skeleton
is represented by cartilage only, and the salamander’s is ossified.
This relation is therefore an imitation only, and is called _inexact
parallelism_.
As we compare nearer and nearer relations—_i.e._, the genera which
present fewest points of difference—we find the differences between
undeveloped stages of the higher and permanent conditions of the
lower to grow fewer and fewer, until we find numerous instances
where the lower genus is exactly the same as the undeveloped stage
of the higher. This relation is called that of _exact parallelism_.
It must now be remembered that the permanence of a character is what
gives it its value in defining genus, order, etc., in the eyes of
the systematist. So long as the condition is permanent no transition
can be seen: there is therefore no development. If the condition is
transitional, it defines nothing, and nothing is developed; at
least, so says the anti-developmentalist. It is the old story of the
settler and the Indian: “Will you take owl and I take turkey, or I
take turkey and you owl?”
If we find a relation of _exact parallelism_ to exist between two
sets of species in the condition of a certain organ, and the
difference so expressed the only one which distinguishes them as
sets from each other—if that condition is always the same in each
set—we call them two genera: if in any species the condition is
variable at maturity, or sometimes the undeveloped condition of the
part is persistent and sometimes transitory, the sets characterized
by this difference must be united by the systematist, and the whole
is called a single genus.
We know numerous cases where different individuals of the same
species present this relation of _exact parallelism_ to each other;
and as we ascribe common origin to the individuals of a species, we
are assured that the condition of the inferior individual is, in
this case, simply one of repressed growth, or a failure to fulfill
the course accomplished by the highest. Thus, certain species of the
salamandrine genus amblystoma undergo a metamorphosis involving
several parts of the osseous and circulatory systems, etc., while
half grown; others delay it till fully grown; one or two species
remain indifferently unchanged or changed, and breed in either
condition, while another species breeds unchanged, and has never
been known to complete a metamorphosis.
The nature of the relation of _exact parallelism_ is thus explained
to be that of checked or advanced growth of individuals having a
common origin. The relation of _inexact parallelism_ is readily
explained as follows: With a case of _exact parallelism_ in the
mind, let the repression producing the character of the lower,
parallelize the latter with a stage of the former in which a second
part is not quite mature: we will have a slight want of
correspondence between the two. The lower will be immature in but
one point, the incompleteness of the higher being seen in two
points. If we suppose the immaturity to consist in a repression at a
still earlier point in the history of the higher, the latter will be
undeveloped in other points also: thus, the spike-horned deer of
South America have the horn of the second year of the North American
genus. They would be generically identical with that stage of the
latter, were it not that these still possess their milk dentition at
two years of age. In the same way the nature of the parallelisms
seen in higher groups, as orders, etc., may be accounted for.
The theory of homologous groups furnishes important evidence in
favor of derivation. Many orders of animals (probably all, when we
come to know them) are divisible into two or more sections, which I
have called _homologous_. These are series of genera or families,
which differ from each other by some marked character, but whose
contained genera or families differ from each other in the same
points of detail, and in fact correspond exactly. So striking is
this correspondence that were it not for the general and common
character separating the homologous series, they would be regarded
as the same, each to each. Now it is remarkable that where studied
the difference common to all the terms of two homologous groups is
found to be one of _inexact parallelism_, which has been shown above
to be evidence of descent. Homologous groups always occupy different
geographical areas on the earth’s surface, and their relation is
precisely that which holds between successive groups of life in the
periods of geologic time.
In a word, we learn from this source that distinct geologic epochs
coexist at the same time on the earth. I have been forced to this
conclusion[47] by a study of the structure of terrestrial life, and
it has been remarkably confirmed by the results of recent deep-sea
dredgings made by the United States Coast Survey in the Gulf Stream,
and by the British naturalists in the North Atlantic. These have
brought to light types of Tertiary life, and of even the still more
ancient Cretaceous periods, living at the present day. That this
discovery invalidates in any wise the conclusions of geology
respecting lapse of time is an unwarranted assumption that some are
forward to make. If it changes the views of some respecting the
parallelism or coëxistence of faunæ in different regions of the
earth, it is only the anti-developmentalists whose position must be
changed.
Footnote 47:
_Origin of Genera_, pages 70, 77, 79.
For, if we find distinct geologic faunæ, or epochs defined by faunæ,
coëxisting during the present period, and fading or emerging into
one another as they do at their geographical boundaries, it is proof
positive that the geologic epochs and periods of past ages had in
like manner no trenchant boundaries, but also passed the one into
the other. The assumption that the apparent interruptions are the
result of transfer of life rather than destruction, or of want of
opportunities of preservation, is no doubt the true one.
δ. Rationale of Development.
_a. In Characters of Higher Groups._ It is evident in the case of
the species in which there is an irregularity in the time of
completion of metamorphosis that some individuals traverse a longer
developmental line than those who remain more or less incomplete. As
both accomplish growth in the same length of time, it is obvious
that it proceeds with greater rapidity in one sense in that which
accomplishes most: its growth is said to be accelerated. This
phenomenon is especially common among insects, where the females of
perfect males are sometimes larvæ or nearly so, or pupæ, or lack
wings or some character of final development. Quite as frequently,
some males assume characters in advance of others, sometimes in
connection with a peculiar geographical range.
In cases of _exact parallelism_ we reasonably suppose the cause to
be the same, since the conditions are identical, as has been shown;
that is, the higher conditions have been produced by a crowding back
of the earlier characters and an acceleration of growth, so that a
given succession in order of advance has extended over a longer
range of growth than its predecessor in the same allotted time. That
allotted time is the period before maturity and reproduction, and it
is evident that as fast as modifications or characters should be
assumed sufficiently in advance of that period, so certainly would
they be conferred upon the offspring by reproduction. The
_acceleration_ in the assumption of a character, progressing more
rapidly than the same in another character, must soon produce, in a
type whose stages were once the exact parallel of a permanent lower
form, the condition of _inexact parallelism_. As all the more
comprehensive groups present this relation to each other, we are
compelled to believe that _acceleration_ has been the principle of
their successive evolution during the long ages of geologic time.
Each type has, however, its day of supremacy and perfection of
organism, and a retrogression in these respects has succeeded. This
has no doubt followed a law the reverse of acceleration, which has
been called _retardation_. By the increasing slowness of the growth
of the individuals of a genus, and later and later assumption of the
characters of the latter, they would be successively lost.
To what power shall we ascribe this acceleration, by which the first
beginnings of structure have accumulated to themselves through the
long geologic ages complication and power, till from the germ that
was scarcely born into a sand-lance, a human being climbed the
complete scale, and stood easily the chief of the whole?
In the cases of species, where some individuals develop farther than
others, we say the former possess more growth-force, or “vigor,”
than the latter. We may therefore say that higher types of structure
possess more “vigor” than the lower. This, however, we do not know
to be true, nor can we readily find means to demonstrate it.
The food which is taken by an adult animal is either assimilated, to
be consumed in immediate activity of some kind, or stored for future
use, and the excess is rejected from the body. We have no reason to
suppose that the same kind of material could be made to subserve the
production of life-force by any other means than that furnished by a
living animal organism. The material from which this organism is
constructed is derived first from the parent, and afterward from the
food, etc., assimilated by the individual itself so long as growth
continues. As it is the activity of assimilation directed to a
special end during this latter period which we suppose to be
increased in accelerated development, the acceleration is evidently
not brought about by increased facilities for obtaining the means of
life which the same individual possesses as an adult. That it is not
in consequence of such increased facilities possessed by its parents
over those of the type preceding it, seems equally improbable when
we consider that the characters in which the parent’s advance has
appeared are rarely of a nature to increase those facilities.
The nearest approach to an explanation that can be offered appears
to be somewhat in the following direction:
There is every reason to believe that the character of the
atmosphere has gradually changed during geologic time, and that
various constituents of the mixture have been successively removed
from it, and been stored in the solid material of the earth’s crust
in a state of combination. Geological chemistry has shown that the
cooling of the earth has been accompanied by the precipitation of
many substances only gaseous at high temperatures. Hydrochloric and
sulphuric acids have been transferred to mineral deposits or aqueous
solutions. The removal of carbonic acid gas and the vapor of water
has been a process of much slower progress, and after the expiration
of all the ages a proportion of both yet remains. Evidence of the
abundance of the former in the earliest periods is seen in the vast
deposits of limestone rock; later, in the prodigious quantities of
shells which have been elaborated from the same in solution. Proof
of its abundance in the atmosphere in later periods is seen in the
extensive deposits of coal of the Carboniferous, Triassic and
Jurassic periods. If the most luxuriant vegetation of the present
day takes but fifty tons of carbon from the atmosphere in a century,
per acre, thus producing a layer over that extent of less than a
third of an inch in thickness, what amount of carbon must be
abstracted in order to produce strata of thirty-five feet in depth?
No doubt it occupied a long period, but the atmosphere, thus
deprived of a large proportion of carbonic acid, would in subsequent
periods undoubtedly possess an improved capacity for the support of
animal life.
The successively higher degree of oxidization of the blood in the
organs designed for that function, whether performing it in water or
air, would certainly accelerate the performances of all the vital
functions, and among others that of growth. Thus it may be that
_acceleration_ can be accounted for, and the process of the
development of the orders and sundry lesser groups of the Vertebrate
kingdom indicated; for, as already pointed out, the definitions of
such are radically placed in the different structures of the organs
which aerate the blood and distribute it to its various
destinations.
But the great question, What determined the direction of this
acceleration? remains unanswered. One cannot understand why more
highly-oxidized blood should hasten the growth of partition of the
ventricle of the heart in the serpent, the more perfectly to
separate the aerated from the impure fluid; nor can we see why a
more perfectly-constructed circulatory system, sending purer blood
to the brain, should direct accelerated growth to the cerebellum or
cerebral hemispheres in the crocodile.
_b. In Characters of the Specific Kind._ Some of the characters
usually placed in the specific category have been shown to be the
same in kind as those of higher categories. The majority are,
however, of a different kind, and have been discussed several pages
back.
The cause of the origin of these characters is shrouded in as much
mystery as that of those which have occupied the pages immediately
preceding. As in that case, we have to assume, as Darwin has done, a
tendency in Nature to their production. This is what he terms “the
principle of variation.” Against an unlimited variation the great
law of heredity or atavism has ever been opposed, as a conservator
and multiplier of type. This principle is exemplified in the fact
that like produces like—that children are like their parents,
frequently even in minutiæ. It may be compared to habit in
metaphysical matters, or to that singular love of time or rhythm
seen in man and lower animals, in both of which the tendency is to
repeat in continual cycles a motion or state of the mind or sense.
Further, but a proportion of the lines of variation is supposed to
have been perpetuated, and the extinction of intermediate forms, as
already stated, has left isolated groups or species.
The effective cause of these extinctions is stated by Darwin to have
been a “natural selection”—a proposition which distinguishes his
theory from other development hypotheses, and which is stated in
brief by the expression, “the preservation of the fittest.” Its
meaning is this: that those characters appearing as results of this
spontaneous variation which are little adapted to the conflict for
subsistence, with the nature of the supply, or with rivals in its
pursuit, dwindle and are sooner or later extirpated; while those
which are adapted to their surroundings, and favored in the struggle
for means of life and increase, predominate, and ultimately become
the centers of new variation. “I am convinced,” says Darwin, “that
natural selection has been the main, but not exclusive, means of
modification.”
That it has been to a large extent the means of preservation of
those structures known as specific, must, I think, be admitted. They
are related to their peculiar surroundings very closely, and are
therefore more likely to exist under their influence. Thus, if a
given genus extends its range over a continent, it is usually found
to be represented by peculiar species—one in a maritime division,
another in the desert, others in the forest, in the swamp or the
elevated areas of the region. The wonderful interdependence shown by
Darwin to exist between insects and plants in the fertilization of
the latter, or between animals and their food-plants, would almost
induce one to believe that it were the true expression of the whole
law of development.
But the following are serious objections to its universal
application:
First: The characters of the higher groups, from genera up, are
rarely of a character to fit their possessors especially for
surrounding circumstances; that is, the differences which separate
genus from genus, order from order, etc., in the ascending scale of
each, do not seem to present a superior adaptation to surrounding
circumstances in the higher genus to that seen in the lower genus,
etc. Hence, superior adaptation could scarcely have caused their
selection above other forms not existing. Or, in other words, the
different structures which indicate successional relation, or which
measure the steps of progress, seem to be equally well fitted for
the same surroundings.
Second: The higher groups, as orders, classes, etc., have been in
each geologic period alike distributed over the whole earth, under
all the varied circumstances offered by climate and food. Their
characters do not seem to have been modified in reference to these.
Species, and often genera, are, on the other hand, eminently
restricted according to climate, and consequently vegetable and
animal food.
The law of development which we seek is indeed not that which
preserves the higher forms and rejects the lower after their
creation, but that which explains why higher forms were created at
all. Why in the results of a creation we see any relation of higher
and lower, and not rather a world of distinct types, each perfectly
adapted to its situation, but none properly higher than another in
an ascending scale, is the primary question. Given the principle of
advance, then natural selection has no doubt modified the details;
but in the successive advances we can scarcely believe such a
principle to be influential. _We look rather upon a progress as the
result of the expenditure of some force fore-arranged for that end._
It may become, then, a question whether in characters of high grade
the habit or use is not rather the result of the acquisition of the
structure than the structure the result of the encouragement offered
to its assumed beginnings by use, or by liberal nutrition derived
from the increasingly superior advantages it offers.
ε. The Physical Origin of Man.
If the hypothesis here maintained be true, man is the descendant of
some preëxistent generic type, the which, if it were now living, we
would probably call an ape.
Man and the chimpanzee were in Linnæus’ system only two species of
the same genus, but a truer anatomy places them in separate genera
and distinct families. There is no doubt, however, that Cuvier went
much too far when he proposed to consider Homo as the representative
of an order distinct from the quadrumana, under the name of bimana.
The structural differences will not bear any such interpretation,
and have not the same value as those distinguishing the orders of
mammalia; as, for instance, between carnivora and bats, or the
cloven-footed animals and the rodents, or rodents and edentates. The
differences between man and the chimpanzee are, as Huxley well puts
it, much less than those between the chimpanzee and lower
quadrumana, as lemurs, etc. In fact, man is the type of a family,
Hominidæ, of the order Quadrumana, as indicated by the characters of
the dentition, extremities, brain, etc. The reader who may have any
doubts on this score may read the dissections of Geoffroy St.
Hilaire, made in 1856, before the issue of Darwin’s _Origin of
Species_. He informs us that the brain of man is nearer in structure
to that of the orang than the orang’s is to that of the South
American howler, and that the orang and howler are more nearly
related in this regard than are the howler and the marmoset.
The modifications presented by man have, then, resulted from an
acceleration in development in some respects, and retardation
perhaps in others. But until the _combination_ now characteristic of
the genus Homo was attained the being could not properly be called
man.
And here it must be observed that as an organic type is
characterized by the coëxistence of a number of peculiarities which
have been developed independently of each other, its distinctive
features and striking functions are not exhibited until that
coëxistence is attained which is necessary for these ends.
Hence, the characters of the human genus were probably developed
successively; but few of the indications of human superiority
appeared until the combination was accomplished. Let the opposable
thumb be first perfected, but of what use would it be in human
affairs without a mind to direct? And of what use a mind without
speech to unlock it? And speech could not be possible though all the
muscles of the larynx but one were developed, or but a slight
abnormal convexity in one pair of cartilages remained.
It would be an objection of little weight could it be truly urged
that there have as yet no remains of apelike men been discovered,
for we have frequently been called upon in the course of
paleontological discovery to bridge greater gaps than this, and
greater remain, which we expect to fill. But we _have_ apelike
characters exhibited by more than one race of men yet existing.
But the remains of that being which is supposed to have been the
progenitor of man may have been discovered a short time since in the
cave of Naulette, Belgium, with the bones of the extinct rhinoceros
and elephant.