SPECIAL PROOFS, TAKEN FROM THE GENERAL LAWS OF ANIMAL STRUCTURE, OR
FROM COMPARISON IN THE TAXONOMIC SERIES.
_General Principles._
=Analogy and Homology.=--In biology those organs or parts in different
animals are said to be _analogous_ which, however different their
origin, have a general similarity of form and especially of function;
while those are called _homologous_ which, however different their
general appearance, and however different their function, yet may,
by close examination and extensive comparison, be shown to be
modifications of one another--to be, in fact, originally the same part
modified for different purposes. In the former the parts compared look
and behave as if they were the same, but are not; in the latter they
look and behave entirely differently, but are, in fact, the same part
in disguise.
We can best make this plain by examples. The wing of a bird and
the wing of a butterfly are analogous organs. They have the same
function--i. e., flying; and this function necessitates the same
general form of a flat plane. But they are not at all homologous; they
are not at all the same organ or part. They certainly have never been
formed one out of the other by modification. But the wing of a bird,
the fore-paw of a reptile or mammal, the wing of a bat, and the arm and
hand of a man, though so different in form and function, are homologous
parts. On close examination they are found to have the same general
structure, to be composed of essentially the same pieces, although they
are so greatly modified in order to adapt them to different functions,
that the general or superficial resemblance is now lost. Their
structure is precisely such as it would be if they had all originated
from some archetypal fore-limb by modifications in different directions
of its several parts. By extensive comparison in the taxonomic and
ontogenic series, all the intermediate gradations between these extreme
modifications may be picked up.
[Illustration: FIG. 2.--Lepidosiren.]
Another example. The lungs of a mammal and the gills of a fish are
analogous organs, since they have the same function of aëration of the
blood. But they are not at all homologous: they are not built on the
same plan; by no effort of the mind can we imagine that the former
could have come out of the latter by modification. On the contrary,
we have positive proof that it did not so come. But there is an organ
in the fish which is homologous with the mammalian lung, viz., the
air-bladder, or swim-bladder. We know it--1. Because we can trace in
the taxonomic series all the gradations from the one to the other. In
most fishes the air-bladder is wholly cut off from the gullet, and
only very feebly supplied with blood. It is used and can be used only
for flotation. In others, as the gar-pike, the swim-bladder is quite
vascular and opens by a tube into the throat. Through this opening
air is gulped down from time to time into the bladder, and again
from time to time expelled. In other words, this fish supplements
its gill-breathing by an imperfect lung-breathing. We have here
the beginning of a lung. In still other fishes, viz., the Dipnoi
(_lepidosiren_ and _ceratodus_, Fig. 2), the air-bladder becomes a
more perfect lung--i. e., a very vascular sacculated sac; and there is
not only an opening into the throat, but also from the throat to the
snout. In other words, we have for the first time _nostrils_. These
fishes completely combine gill-breathing with lung-breathing. The step
from these to the lowest amphibian reptiles is so small, that some have
classed the lepidosiren among amphibians instead of fishes. The siredon
or axolotl of New Mexico, the necturus or menobranchus of our Northern
lakes, and the siren of our Southern swamps, have both gills and lungs,
and breathe both air and water; but the lung is very imperfect, being
only a sacculated sac, like the air-bladder of the ceratodus and
lepidosiren. No one doubts that the air-breathing organ of an amphibian
is a true lung; yet we have traced all the gradations between it and
the air-bladder of a fish. We conclude, therefore, that if there be
any such thing as transmutation of organic forms, the lung of higher
animals must have been formed by the process above described.[20]
But we know it still more certainly--2. Because we can trace the change
from the one to the other in the ontogenic series. In the life-history
of the individual we can actually see the one thing change into the
other. The frog, as is well known, when first hatched, is a tadpole. It
has no legs, but locomotes by means of a vertically-expanded tail. It
has no lungs, but breathes water instead of air, by means of gills. It
is in all respects, therefore, a fish, and would be classed as such if
it remained in this condition. But it does not; it gradually loses its
tail and gills, and acquires legs and lungs, and breathes air only. Now
in this change whence came the lungs? From the gills by modification?
No; but from an organ similar in character and position to the
air-bladder of a ceratodus, or a lepidosiren. This organ has gradually
developed into a lung. The steps of the change are briefly as follow:
First, the breathing is wholly water-breathing by gills. Next, by the
development of this other organ, it is partly water-breathing by
gills, and partly air-breathing by lungs. Lastly, the gills gradually
dry up, and the lungs develop more and more, until the breathing is
wholly by lungs.
We have dwelt somewhat upon this example, because it is an excellent
example of what we mean by homology, and also because we will have
occasion to use it again. But so important, for all that follows in
this part, is a clear idea on the subject of homology, that it will be
best to familiarize the mind of the reader with it by means of a few
examples drawn from plants.
A potato is analogous to a root--a tuberous root like that of a
dahlia or a sweet-potato--but is not at all homologous with these.
On the contrary, it is homologous with a stem. It is essentially an
underground, leafless branch, which has thickened enormously at the
point by accumulation of starch. The evidence of this is found in
the fact that it has rudimentary leaves (scales) arranged in regular
spiral order of phylotaxis, each with its axillary bud (eyes). It is
still more clearly shown by the fact that buds above-ground which, if
let alone, would form leafy branches, may be made to become tubers by
covering them with earth or dead leaves, and thus excluding the light;
and, conversely, underground buds which, if let alone, would form
tubers, may be made to grow into leafy branches by exposing them to the
light.
Take another example: The broad, flat, elliptical, green masses so
characteristic of the cactus family, and usually called their leaves,
are indeed _analogous_ to leaves in color, form, and function; for
they are green and flat, and assimilate carbonic acid and water (CO2
and H2O) like leaves. But they are not, in truth, leaves, but modified
stems, for they have the essential structure of stems, with their
pith, wood, medullary rays, and bark, and may be traced through all
gradations into the ordinary cylindrical form of stems. Where are
their leaves, then? Their spines are their abortive leaves. These
are arranged spirally like leaves, and bear buds in their axils like
leaves. They are, in truth, leaves, modified to perform the function of
defensive armor; while their function has been delegated to the stem
flattened for this purpose.
[Illustration:
FIG. 3.--A branch of young acacia, showing change from one form of
leaf to the other; _a_, _b_, _c_, _d_, successive stages of
change; _l_, _s_, leaf stalk which gradually changes into the
blade in _c_, _d_, and _e_.
]
One more example: The acacias, of which there are fifteen to twenty
species in California, introduced from Australia, form two groups
having extremely different styles of leaves. We will call them the
feather-leaved and the simple-leaved acacias. In the former, the leaves
are very finely bipinnate, and the general aspect of the foliage is
extremely feathery and graceful. In the latter the leaves are simple,
ovate, and, curiously enough, set on edge; and the general aspect
of the tree is therefore rather stiff. It seems at first incredible
that leaves so different and aspects so diverse should belong to
plants of the same genus. But a little close examination shows that,
as usual, the botanists are right and the popular judgment wrong.
The plumose-leaf is the normal leaf-form for this genus. The simple
leaf is not only abnormal, but in a homological sense is not a
leaf at all--i. e., it does not correspond to the part called the
_blade_ in ordinary simple leaves of other trees. In the seedling of
the simple-leaved acacias, and sometimes for a considerable time in
the young tree, the leaves are all plumose. As the tree matures it
gradually changes its dress and puts on its _toga virilis_. The gradual
change from the one form to the other may easily be traced in the same
tree, and even often in the same branch (Fig. 3). The steps of the
change (_a_, _b_, _c_, and _d_) are shown in the following figure,
drawn from nature. It is seen, by bare inspection of the figure, that
the so-called leaf, _d_, of the simple-leaved acacias, is really the
vertically-expanded leaf-stalk, _l_, _s_, the true leaf or blade being
wholly aborted. The whole structure of this so-called leaf is different
from that of a true blade. For example, its style of ribbing is
parallel, its position is edgewise to the sky, its palisade cells are
on both sides alike, etc. To emphasize this difference, botanists call
such an apparent leaf a _phyllodium_, or phyllode.
After these illustrations we now repeat the definitions in different
words. Analogy has reference to _general resemblance_ of form
determined by _similarity of function_, however different the origins
of the parts compared may be. Homology has reference to _community
of origin_, however obscured to the superficial observer such common
origin may be by modifications necessary to adapt to different
functions. Observe, then, there are two ideas here which must be kept
distinct. One is common origin, always shown by deep-lying, essential
identity of structure; the other is adaptive modification for function.
Organs of the most diverse origin may resemble by adaptive modification
for the same function. This is analogy. Organs of the same origin
may assume very different appearance by adaptive modifications for
different functions. This is homology. In the latter case, which is
the one that concerns us, a profound study of essential structure
and structural relations to other parts, and especially extensive
comparison in the taxonomic and ontogenic series, will usually
detect the homology, or common origin, in spite of the obscurations
produced by adaptive modifications. It is seen, also, that analogy is
a superficial resemblance, easily detected by the popular eye, and
therefore embodied in popular language; while homology is a deep-seated
and essential resemblance, detected often only by profound study and
extensive comparison. Now, one of the strongest proofs of the truth
of evolution is taken from the homologies of animal structure. Common
origin completely explains homology. Every other explanation is
transcendental, and therefore unscientific.
=Primary Divisions of the Animal Kingdom.=--Now, the animal kingdom
consists of several primary divisions, called sub-kingdoms or
departments. The animals in these groups differ so essentially from
one another in their _plan of structure_, that it is difficult, if
not impossible, to trace any structural relation between them--to
imagine how the members of one could have been derived from those of
another--or conceive the common stem from which they all separated.
In other words, it is impossible, in the present state of knowledge,
to trace homology with any certainty from one group to another. But
within the limits of each primary group the homology is easy. Some
naturalists--Agassiz and Cuvier--have made four or five of these
primary groups. Some--Huxley--have made eight. Some make nine or
ten.[21] We will not trouble ourselves to settle this question; for
all agree to make _vertebrata_ and _articulata_ or _arthropoda_ two of
them, and all our illustrations will be drawn from these. Other groups
are too unfamiliar to the general reader to serve our purpose.
Now, as already stated, homology can not be traced with any certainty
between the primary groups, but within the limits of each group it may
be traced with ease and beauty. Analogy, however, being connected with
function, and function being universal, can be traced throughout the
animal kingdom. While, therefore, it is probable, nay, almost certain,
that all animals have had a common origin, we can not yet trace these
great departments by homology to that common origin. But the common
origin of each department is quite clear. For example, the structure of
all vertebrate animals is precisely such as would be the case if all
came from one primal vertebrate, variously modified to adapt to various
modes of life. Also, the structure of all arthropods is precisely
such as would be if all came from one primal arthropod, which, from
generation to generation, became gradually modified in different
directions, in order to adapt itself to various modes of life. But
between arthropods and vertebrates we can not yet clearly see a common
origin, although there doubtless was such.
These great departments may, therefore, be compared to _natural
styles of animal architecture_. As there are various styles of human
architecture--Oriental, Egyptian, Greek, Gothic--each of which may be
variously modified to adapt it to all the different purposes for which
buildings are made, without destroying, though perhaps obscuring, the
integrity of the style; so the different primary groups or departments
may be regarded as different styles of animal structure, each of which
may be and has been modified in many ways to adapt it to various habits
and modes of life, obscuring but not destroying the general style.
Or they may be compared to natural _machines_. As a steam-engine, by
modification, may be adapted to many kinds of purposes, obscuring,
perhaps, but not destroying the essential identity of structure;
even so the vertebrate machine by modification may be, and has been,
adapted to many kinds of purposes, and thus become a swimming-machine,
a crawling-machine, a flying-machine, a running-and leaping-machine,
without destroying, although obscuring, the essential identity of
structure. As in architecture, æsthetic principles of form may be
traced through each style, but not from style to style, while the
mechanical principles of construction run through all alike; so also
in animal architecture, the laws of form and styles of structure are
traceable with ease only within the limits of each primary group,
while the laws of function are traceable through all groups alike.
Or, again, and finally: Each of these departments may be compared
to a _tree_, with branches, twigs, and spray, all obviously coming
from one common stem, but each stem seems separate. They are, indeed,
probably, themselves only great branches of one common trunk, but their
connection is too remote and obscure to be made out clearly by means
of homology. Other evidences, however, drawn from other sources, as we
shall see hereafter, are not wholly wanting.