HOMOLOGIES OF THE ARTICULATE SKELETON.
We have taken the vertebrate skeleton first, only because this
department is most familiar. But in reality, the most beautiful
illustrations of essential identity of structure in the midst of
infinite diversity of adaptive modification for different functions
and habits of life, and therefore of common origin from a primal form,
are found in the department of articulates. I use the old Cuvierian
department _articulata_, rather than the more modern _arthropods_,
because the former includes worms also. Now, whether worms should
be thus included with arthropods, or deserve a whole department to
themselves it matters not for our purposes. It is generally admitted
that arthropods probably descended from marine worms. They all have
the same general plan of skeletal structure. It will suit my purpose,
therefore, to regard worms as the lowest form of jointed animals.
Here, then, we have an entirely different plan of structure--a
different style of architecture and different mechanical principles
of machinery. Instead of a skeleton within and muscles acting on the
outside, we have the skeleton on the outside, and muscles acting from
within. Instead of two cavities, a neural and visceral, the skeleton
forms but one cavity, in which all organs are inclosed and protected.
Instead of finding the nerve-axis on the dorsal aspect of the body, we
find it on the ventral aspect.
[Illustration:
FIG. 26.--Diagram section across an arthropod, showing the inclosing
skeleton-ring and a pair of jointed appendages, _n_, nervous
center; _v_, viscera; _b_, blood system.
]
Take any articulate animal, for example, a shrimp, a centiped, or
a beetle. Cut it across the body, and look at the end (Fig. 26).
We see a ring of bone (chitin) inclosing all the organs (nervous
system _n_, blood system _b_, and visceral system _v_), and a pair
of jointed appendages, perhaps legs, on each side. Now imagine these
parts repeated in a linear series. The rings repeated make a hollow,
jointed tube or barrel, the appendages repeated make a continuous row
of appendages on each side. Now this is exactly what we actually
find. The whole articulate skeleton is ideally made up of a series
of such repeated rings and appendages, modified according to the
position in the series, and the uses to which they are put. And then
the whole articulate department is made up of such articulate animals
again modified according to place in the scale of articulates. The
modification in the lower forms is slight, and therefore the identity
of the repeated parts is obvious; but as we go up the scale, and
the number and complexity of the functions increase, the adaptive
modification becomes greater and greater, until finally it so obscures
the essential identity, that it requires the most extensive comparison
in the taxonomic series and in the ontogenic series, to pick up the
intermediate links and establish the fact of common origin. In a word,
whether they so originated or not, it is certain that the structure of
articulate animals is exactly such as would be the case if all these
animals were genetically connected, and came originally from a primal
form something like one of the lower crustaceans, or, perhaps, a marine
worm.
[Illustration: FIG. 27.--Shrimp (Palæmonetes vulgaris).]
[Illustration: FIG. 28.--External anatomy of the lobster (after
Kingsley).]
It will be best to take an example from about the middle of the
scale, where the two elements, viz., essential identity and adaptive
modification, are somewhat evenly balanced, and both traceable with
ease and certainty. Take, then, a cray-fish, a lobster, or a shrimp.
This animal (Fig. 27) has twenty or twenty-one rings and pairs of
jointed appendages. The rings are some of them diminished, some
of them increased in size. Sometimes several are consolidated;
sometimes several are partially or wholly aborted. The appendages
are modified in shape and size, according to their position, so as
to make them swimming-appendages (swimmerets), walking-appendages
(legs), eating-appendages (jaws), and sense-appendages (antennæ). For
example, in the abdominal region, or so-called tail, we have seven
segments, all being perfect movable rings, each with its pair of
jointed appendages, except the last, or _telson_. The appendages of the
first ring (Fig. 28, B) are specially modified in the male as organs
of copulation (B′). The next four pairs are modified for swimmerets
(D′) and for use as holders of the eggs in the female. The appendages
of the sixth ring (G) are broad and paddle-shaped, and, together with
the telson or seventh ring (H), form the powerful terminal swimmer.
Going, now, to the cephalo-thorax: in this either a large number of
segments (thirteen or fourteen) are consolidated above to form the
upper shell or carapace; or else, as is more probable, two or three
of the anterior segments have enlarged and grown backward over, and
at the expense of the others, to form this shell. At any rate, it is
certain that the carapace is formed of the dorsal portions of a number
of segments consolidated together. Below, however, the segments are
all distinct, and have each its own pair of appendages. For example,
going forward in this region, the five next pairs of appendages are
greatly enlarged and very strong, and serve the purpose of locomotion.
They are _walking-appendages_. The next two or three pairs are smaller
and somewhat modified, but not so much as to obscure their essential
similarity to legs. Like legs, they are many-jointed, and like legs,
too, they have gills attached to them. They are called maxillipeds, or
jaw-feet. They are used like hands to gather food and carry it to the
mouth. They are _gathering-appendages_. Then follow three or four pairs
still more modified, and used for mastication. They are called maxillæ
and mandibles. They are _eating-appendages_. Then follow two pairs,
long, many-jointed, with the same kind of curious hinge-joints, which
we have in the legs, undoubtedly homologous with all the others, but
used for an entirely different purpose, and specially modified for that
purpose. They are the antennæ. They are delicate organs of touch and
of hearing, for the ear is situated in the basal joint of the anterior
pair. Last of all, there is still another pair, jointed and movable, on
the ends of which are situated the eyes. These last three, therefore,
are _sense-appendages_. Some writers make this last pair special
organs, not homologous with appendages.
[Illustration: FIG. 29.--Appendages of a prawn (after Cuvier).]
[Illustration: FIG. 30.--Appendages of Nebalia.]
For the sake of greater distinctness, we give the whole series of these
appendages in one of the higher forms, viz., the prawn (Palemon, Fig.
29, and in one of the lower forms, Nebalia, Fig. 30).
[Illustration: FIG. 31.--Vibilia, an amphibod crustacean (after Milne
Edwards).]
That these are really homologous parts is further shown by the fact
that in the case of other crustaceans, such as limulus, the same
appendages, i. e., the appendages of the same body segments, which in
the cases before mentioned are used as feet, become swimmers, while
the appendages corresponding to jaw-feet become walkers; and even what
corresponds to antennæ or sense-appendages, may, as in branchippus,
become powerful claspers. Finally, in all the lowest crustaceans, the
identity is evident, because all the segments and their appendages are
much alike in form and function (Fig. 31).
[Illustration: FIG. 32.--Lithobius forcipatus (after Carpenter).]
We have taken examples from near the middle of the articulate scale,
because, as already stated, both the essential identity and the
adaptive modifications are easily traced. If we go downward in the
scale, the structure becomes more and more generalized, and the rings
and appendages become more and more alike (Fig. 31), until in the most
generalized forms we have only a series of similar rings, with similar
pairs of appendages, except some necessary modifications to form the
head and tail. This is well shown in the centiped (Fig. 32), and still
better in marine worms (Fig. 33). In some marine worms the slight
modification to form the head takes place under our very eyes. These
often multiply by dividing themselves into two. When they do so, they
make a new head and new tail by slight modification of segments and
appendages (Fig. 33).
[Illustration: FIG. 33.--Syllis prolifera.]
If, on the other hand, we go up the scale, we find adaptive
modifications obscuring more and more the simple and obvious identity
of parts, until finally the identity can not be recognized without
extensive comparison in the taxonomic series and study of embryonic
conditions. In crabs--which is a higher form than cray-fish--the tail
or abdomen seems to be wanting, but is only very small and bent under
the body and thus concealed. In all essential respects the structure is
precisely like the cray-fish. In fact, in the embryo, we trace the one
form into the other; for the crab is at first a long-tailed crustacean
(Fig. 34).
[Illustration: FIG. 34.--Development of Carcinus mœnas. A, zoæa stage;
B, megalopa stage; C, final state (after Couch).]
Insects are the highest form of articulates. In these, therefore, we
find the modification is still greater than in crustaceans, though even
here the ring-and-appendage structure is plain enough in most cases.
One of the best evidences of high grade among animals is the gathering
of the segments into distinct groups, and especially the distinctness
of the _head_ as one of these groups. In worms and lower crustaceans
there is no grouping at all, the skeleton being a continuous series
of joints, only slightly modified at the anterior and posterior
extremities. In the higher crustacea, and in spiders and scorpions,
they are grouped into two regions, viz., cephalo-thorax and abdomen.
In insects they are grouped into three very distinct regions--head,
thorax, and abdomen. In insects, therefore, we find for the first
time the head distinctly separated from the rest of the body. This
is an evidence of high grade, because it shows the dominance of
head-functions.
[Illustration:
FIG. 35.--External anatomy of Caloptenus spretus, the head and thorax
disjointed; up, uropatagium; _f_, furcula; _c_, cercus (drawn by
J. T. Kingsley).
]
The insect, such, for example, as a grasshopper, consists of seventeen
or eighteen segments (Fig. 35). Of these, four belong to the head,
three to the thorax, and about ten to the abdomen. Those of the abdomen
are all separated and movable; those of the thorax and head are more or
less consolidated. The appendages of the head-segments become antennæ
and jaw-parts, i. e., mandibles--maxillæ and labium; the appendages
of the thorac-segments become legs (the wings are not homologous with
appendages), while those of the abdomen are aborted. The steps of the
gradual consolidation on the one hand, and the abortion on the other,
may be traced in the embryo or larva--i. e., in the caterpillar or the
grub of a bee or a beetle. In the caterpillar, for example, there is
no grouping into three regions, there is no consolidation, and all the
segments have appendages. Again, the almost infinite variety in the
mouth-parts among insects, brought about by adaptive modifications for
biting, for piercing, and for sucking, and yet the essential identity
of all to the more simple and generalized structure of the grasshopper,
is an admirable illustration of the same principle. But to dwell upon
these minor points would carry us too far.
=Illustration of the Law of Differentiation.=--We have here, in the
modifications of segments and appendages of articulates, an admirable
illustration of the most fundamental law of evolution, viz., the law of
differentiation. As we have already seen (page 21), perhaps the most
beautiful and certainly the most fundamental illustration of this law
is found in the development of cell-structure. Commencing in the lowest
animals, and in the earliest embryonic stages of the higher animals,
from a condition in which all are alike, the _cells_ as we go upward
quickly diverge into different forms to produce different tissues and
perform different functions. Here, then, we have a perfect example of
essential identity and adaptive modification. It is the very best type
of differentiation. So also skeletal _segments_, commencing, in the
lowest articulates and in earliest embryonic stages of the higher, all
alike, as we go upward in either series, begin immediately to diverge
in various directions (divergent variation), taking different forms to
subserve different uses. Here, again, therefore, is an illustration
of the law of differentiation. Lastly, in the articulate department,
commencing with the lowest forms and earliest embryonic conditions,
and we may add earliest geological times, and going up either series
from generalized forms very much alike, the _individuals_ are gradually
differentiated into many special forms, in order to adapt them to
the diversified modes of life actually found in nature. Thus cells,
segments, individuals, are all alike affected by this most fundamental
law.
We have taken our illustrations from only the two departments of
vertebrata and articulata, because these are the most familiar to the
reader, and also have been most carefully studied. We have shown that
the general structure of all vertebrates is precisely what it would
be if they all had come from one primal vertebrate form, and that
of all articulates what it would be if all had come from one primal
articulate form. The only _natural_ explanation, and, therefore, the
only scientific explanation of this, is that _they were really thus
derived_. The same kind of evidence may be drawn from the study of
other departments, but to pursue the subject any further in this
direction would carry us beyond the limits which we have assigned. We
desire only to explain the nature, not to give all, of the evidence.
The examples given will be sufficient for the purposes of illustration.
The whole proof is nothing less than the whole science of comparative
anatomy.
Vertebrates, then, were derived from a primal vertebrate, articulates
from a primal articulate, and so for other departments. But whence were
these _primals_ derived? Are there any intermediate links between, any
deeply concealed common plan of structure underlying these primary
groups, showing a common origin? It must be confessed that, in their
_mature_ condition, there seems to be but little evidence of such.
These primary groups seem to be built on different plans, to be
fundamentally of different styles of architecture. Therefore Darwin,
in the true spirit of inductive caution--that true scientific spirit
which keeps strictly within the limits of evidence--commences with four
or five distinct primal kinds, from which by divergent variation all
animals were descended. Nevertheless, the truly scientific biologist
must ever strongly incline to believe that these also came from some
_primal animal_, and even that both animals and plants were derived
from some primal form of _living thing_; that as, in the taxonomic
series, the animal and vegetal kingdoms in their lowest forms merge
undistinguishably into one another; as in the ontogenic series the
animal and plant germ are one, so also in the phylogenic series the
earliest organisms were simply living things, but not distinctively
animal nor vegetal. Science, therefore, whose mission is to trace
origins as far back as possible, must ever strive to find connecting
links between the primary groups. Some such have been supposed to
have been discovered. Some find the origin of vertebrates among the
molluscoids (ascidians); some find the origins of both vertebrates and
articulates among marine worms (annelids). This point is still too
doubtful to be dwelt upon here. It may be that we seek in vain for such
connecting links among existing forms. It may well be that the point of
separation of these great primary groups (unless we except vertebrates)
was far lower even than these low forms. Both phylogeny and ontogeny
seem to indicate this. In the earliest fauna known, the primordial (for
if there was life in the archæan it was not yet differentiated into a
fauna), all the great departments, except the vertebrates, seem to
have been represented. In embryonic development, too, the point of
connection or even of similarity, between the great departments, is
found, as we shall see hereafter, only in the earliest stages--i. e.,
lower down than any but the lowest existing forms, viz., the protozoa.