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form which may accompany the degeneracy of highly organized ani-
mals, is found in the history of the insects collectively known as
Strepsiptera, and of which the genus Stylop>s is the best-known exam-
ple. The male stylops (Fig. 13, a) is an active insect, possessing a




Fig. 13.— Sttlops. (Fi?. c shows the Stylops. in outline, within the body of the Bee ; and Fig.
shows the Stylops removed from the body of its host.)



single pair of wings. These wings are the hinder pair ; the front pair
l)eing represented by a pair of twisted organs {ic), which illustrate
wing-degeneration, possibly through disuse. Both males and females
as they leave the egg are small, active, six-legged beings {d, e), which
crawl about on the bodies of bees. Carried into the hive, the young
stylops behave like the proverbial viper, injuring the community
which gives them shelter by boring their way into the bodies of lar-
val or infant bees. Here the young stylops, casting their skin, become
in the larval interior sluggish, footless grubs. Each possesses a mouth,
small jaws, and a digestive system of simple construction. Mean-
while, bee-development progresses ; and, as the larval bee passes
tlirough its chrysalis state with its styl ops-lodger contained in its inte-
rior, the latter thrusts the front extremity of its body from between
two of the hinder body-segments of the bee. Then the male stylops,
undergoing development in this position, becomes the winged insect {a)
and passes into the world. The female stylops (0), on the other hand,
remain in their places on the bees. They undergo but a slight change
of form, persisting as mere sac-like bodies {c), without legs or diges-
tive system (i), and develop in their interior the eggs from Avhich suc-
ceeding generations of stylops will be produced. Such a case of abso-
lute degeneracy is all the more remarkable in view of the facts that it



384



THE POPULAR SCIENCE MONTHLY.



is limited to one sex alone, and that the free-winged males of stylops
are as highly organized as most of their neighbor insects.

The class of the spiders {Arachnida) offers collective examples of
degeneration and retrogression, which show how large numbers of
animals may acquire lower characters, contrasting with the higher
phases to which other members of their class have attained. The
mites and ticks have unquestionably originated from the same root-
stock as the spiders and scorpions. The development of the two
groups proves this much. But, while the latter animals have advanced
to a high complexity of organization, the mites and ticks have degen-
erated into parasitic forms — or at least exemplify beings which, first
attaining a respectable rank in their own series, have certainly not ad-
vanced upon that rank. Many of the mites, however, exhibit well-
marked degeneration. Only on the hypothesis of sweeping retrogres-
sion can we account for the singular and anomalous condition in which
a certain harmless mite, named Demodex follicidoriim (Fig. 14), spends
its existence. This mite inhabits the sacs or follicles of the human
skin at the sides of the nose. It is a minute, worm-like animal, pos-
sessing eight degenerate rudiments of legs, and a thoroughly rudimen-
tary structure in other respects. Here parasitism has denuded the




Fig. 14.— Demodex (magnified).



Fig. 15.— Linguatulixa.



animal of wellnigh every attribute of its Arachnidan character, and
has left it in a condition analogous in many respects to sacculina itself.
Of the equally curious LinguatiiUna (Fig. 15), inhabiting the "fron-
tal sinuses " or forehead spaces of dogs, wolves, horses, and sheep, the
same remark holds good. The body here is thoroughly worm-like in
shape (J, c), and a digestive and nervous system are to be enumerated
among the possessions of the organism. But not even tlie rudiments
of legs are to be perceived, although the mouth bears certain apologies
for the appendages proper to that region in the mite and spider class.
Yet the young linguatulina (a) exactly resembles the early form of the



I) EG EN ERA TION. 385

mites. It possesses two pairs of jointed limbs, and certain style-like
organs pertaining to the mouth. There is thus the clearest evidence
that linguatulina is a degraded animal. It is the degenerate descend-
ant of a free-living and apparently four-legged — or it may be eight-
legged — ancestor ; and its further history seems to afford a clew to the
causes of its retrogression. For the four-legged larvae of linguatulina
escape, while still within the egg, from the nose of the dog or sheep
host which has harbored their parents. Received along with food into
the body of the hare or rabbit, the larval being liberates itself. From
the rabbit's digestive system it bores its way through the tissues to
the liver, thus reminding one strongly of the similar migrations of the
embryo tapeworm. In the liver further changes ensue. Frequent
moltings become the order of the day, and at length they assume a
worm-like aspect and remain thus, still imperfect, until, by transfer-
ence to the body of dog, wolf, or sheep, and by passage to the frontal
sinuses, they acquire perfection of their life-functions. If the history
of these beings teaches us anything concerning their past, it points to
a free and active state as their original condition, and to the probable
acquirement, first, of a lodgment in the digestive system of one ani-
mal as a relatively simple parasite ; and, secondly, of a further modifi-
cation of habit, transferring at once its perfection and completed deg-
radation to the forehead cavities of a second host.

But the conditions which make for the degeneracy of an animal
are, as we have seen in the case of the barnacles, not always associated
with a parasitic habit. Mere fixation, as we have observed, secures
the disappearance of useless organs, such as organs of motion and
sense-organs, which, being possessed by the young form, clearly indi-
cate that the ancestry of the animals in question has at any rate been
capable of leading to better things than the descendants represent in
their existent persons. The sea-squirts, or ascidians, besides serving
as a text for the derivation of vertebrates, and for abnormal ways in
the animal chemistry which imitates the plant's work, have been se-
lected as fruitful objects of discussion by those biologists who find in
the idea of degeneration an explanation of knotty points in natural
history. For the same voice that proclaims the fact that a sea-squirt
— which is a mere rooted bag with a double neck (Fig. 16) — begins life
as a free-swimming, tadpole-like larva (Fig. 17, 5), tells us in the same
breath that there must have been retrogression and degeneration from
an active condition to produce the sac-like adult state. The assertion
that the youthful sea-squirt, moreover, possesses first a rod-like body —
called the notochord (Fig. 17, n) — only found besides in the young of
vertebrate animals, is also to be taken as implying the superiority of
ascidian infancy to sea-squirt maturity. And, when it is added that
the elderly squirt wants the sense-organs and nervous cord which the
larva possesses, it may well be argued that sheer degeneracy of habit
and structure can alone account for the sweeping transformations

VOL. XIX. — 25



386



THE POPULAR SCIENCE MONTHLY.



which mark the phases of ascidian life-history. Thus it is matter of
sober, natural-history fact that a sea-squirt larva, of all invertebrate
animals, is the only being that possesses organs and parts proper to
the young vertebrate or to the adult form of one lower vertebrate in





Fig. 16.— Sea-Sqort.



Fio. 17.— Development op Sea-Squirt.



particular. This adult is the little fish known as the lancelet, which,
in the relative simplicity of its organization, makes a nearer approach
to the poor or sea-squirt relations of the vertebrates than any other
fish.

The fact of vertebrate and sea-squirt relationship is worth dwell-
ing upon, because the topic unquestionably presents one with a com-
mon point of view, whence a survey of the higher development, evolu-
tion, and progress of the vertebrates, and a view of the degeneracy
and retrogression of the sea-squirts, may best be obtained. Reveling
in the freedom of its early life, the larval sea-squirt — presenting, as
already noted, a striking resemblance to the tadpole of the frog, in its
backbone, its nerve-system, and its breathing-sac, or modified throat —
ultimately settles down. Like the youthful barnacle somewhat, the
young sea-squirt attaches itself to a stone or shell by the suckers with
which nature has provided its head. Then succeeds the disappearance
of the tail, with its backbone and its nerve-cord, and the body itself
soon assumes the sac-like shape that betokens the mature ascidian char-
acter. The outer skin becomes tough and leathery, and develops the
cellulose which, by biological right, we should expect to find in plants
alone. Then succeeds the fuller formation of the gill-sac or breathing-
chamber, and of its neighbor compartment, which receives the effete
water of respiration to be ejected by the second mouth of the sac-like



DEGENER^iTIOX. 387

body. The eye of the larva likewise disappears, and all that remains
to the adult ascidian is a nerve-mass, called by courtesy the " brain,"
and which serves to regulate the few acts that mark the placid and
rooted existence of the race. Attention has been recently directed in
a special manner to the resemblance which exists between the eye of
the larval sea-squirt and that of vertebrates — a statement to be taken
along with that which conversely declares the unlikeness of the ascid-
ian eye to that of all other invertebrate animals. It is matter of fact
that the chief parts of the eye of a vertebrate animal grow inward as
developments from the skin, and unite with an outgrowth from the
brain. This outgrowth forms the retina, a nervous network of the
eye, whereon the images of things seen are duly received for transmis-
sion to brain and sensorium. Now, in invertebrate animals the retina
is formed from the skin-layer. This latter method of growth, it has
been remarked, is a perfectly natural one. It was to be expected that,
as the retina is to be affected in the discharge of its duty by light-
rays, it should form on the surface of the body where the light-rays
fall. In the vertebrate, and in the sea-quirt larva, the retina, on the
contrary, forms away below the skin-surface, and grows outward from
the brain. Why is this so ? Professor Ray Lankester maintains that
because the ascidian larva is perfectly transparent, the light-rays pass
through to its brain-eye, and thus give rise to sensations of sight.
Hence, if the original and primitive vertebrate animal or root-stock
were like the larval sea-squirt, as we suppose it to have been, its body
would be transparent, and its eye or eyes, situated on its brain, would
receive light-rays through its clear body. But, as the evolution of the
vertebrate race proceeded, the tissues became firmer and denser. By
" natural selection " — or, in other words, by the exercise of accom-
modating power to function — the eyed region of the brain would tend
to grow more and more toward the body's surface, to receive the rays
of light. As development, therefore, proceeded, the mode of growth
of the vertebrate eye would be adapted to the exigencies of its new
surroundings. Thus, to-day, the vertebrate eye grows from without
inward, because light-rays strike naturally on the outer surface of the
body. But it likewise grows from within outward as well, because of
the ancestral and hereditary tendencies which cause it to repeat in the
individual growth the passage to the surface it had to make in the
evolution of the race. If one might add a suggestion to such an ex-
planation, it would consist in an endeavor to account for that affinity
between brain and outer surface of body which we see to exist. Why
the brain should grow outward, as it does in eye, ear, and nose like-
wise, to connect with the body's surface, and so to form organs of
sense, is plain enough. We must bear in mind that the brain itself is
formed from the outer layer or epiblast of the larva, and from the
same layer which develops into the skin. Brain and skin, to begin
with, arise from the same layer. Hence, before even the matter of



3S8 THE POPULAR SCIEXCE MOXTHLY.

ojes falls to be considered, the affinity of the skin-layer and the ner-
Tous system is a fact worth noting. It is this truest of relationships
■which may reasonably enough explain, not merely why the sense-organs
arise from the skin-surface, but also why the brain grows outward to
meet with the structure to which it is so near akin.

Degeneration of a very pronounced kind thus accounts for the
peculiarities of sea-squirt structure to-day. The case of ascidian re-
trogression is likewise the more interesting, seeing that its reverse
side is that of progressive evolution and development of the highest
forms of life the existing world knows. It is, therefore, important to
note in passing that the possibilities of development may include de-
generation of a very marked type, along with progressive evolution of
equally pronounced kind. The category of life's extension includes,
in fact, many possibilities which at first sight might appear of most
unlikely kind ; and, among these possibilities, that of extreme degen-
eration is by no means the least notable as an element in inducing the
material variety of life we behold in the animal and plant worlds of
to-day. The list of causes which lead to the degeneration of living
beings includes, however, other fashions of producing retrogression
than by fixation and parasitic habits, and operates in different ways
upon organisms of varied structure and social or biological rank.
Changes. in food and feeding may thus accomplish degeneration and
induce physiological backsliding of the most typical description. It
is a familiar fact that the animal organism is of relatively higher nat-
ure than the plant, seeing that the animal frame can, as a rule, feed
upon and build up its tissues from organic or living matter only.
Animals, in other words, demand the substance of other animals or of
plants, or of both combined, as a necessity of their commissariat ar-
rangements. Plants, on the other hand, are specially constructive and
elaborative in their feeding. They build up from the non-living mat-
ters aroimd them — carbonic acid, water, ammonia, and minerals — the
tissues of their living bodies. They "transubstantiate" this non-
living matter into living tissue ; and the verdant tints of spring, the
full glory of the summer's blossom, or the mellow ruddiness of au-
tumn's fruits, represents, each in its way, the result at once of the
plant's constructive chemistry and of the elaboration into living matter
of the inorganic materials of air and soil around.

The animal frame, therefore, presents us— amid exceptions to the
above rule in both animal and plant series — with relatively greater
complexity of organs and tissues than the plant-body presents. This
statement simply reechoes what commonplace observation daily de-
monstrates. Hence, it may be a natural enough inference that what-
ever causes tend to bring the animal feeding nearer in type to that of
the plant will tend to simplify animal structure, and so to produce re-
trogression and degeneration of the animal type. Many animals are
thus known to develop chlorophyly or the green color we see charac-



DEGENERA TIOX.



389



teristically in every leaf. Through the combined operation of this
green color — either singly or aided by the leaf -protoplasm — and the
action of light, plants decompose the carbonic acid of the air, as every
schoolboy knows, and, retaining the carbon to aid in the formation of




Fig. 18.— nTDR.E. (In both figures young hydra? are represented budding from the side of the

parent).

Starch, set free the oxygen, which thus returns to the atmosphere, and
is welcomed by the animal hosts. The hydra, or common fresh-water
polyp (Fig. 18), many animalcules, and certain worms of a low type,
possess this chlorophyl. Like dishonest manufacturers, they seem to
have infringed the patent-rights of the plant to
elaborate this green color. And it is no longer
matter of theory, but ascertained fact, that these
green animals are capable, like the plants, of ab-
sorbing carbonic acid — usually a fatal gas to the
animal constitution — and of elaborating starch
therefrom like their plant neighbors. Thus
simpler mode of feeding, ob-
viating the necessities of animal
existence in the way of diges-
tive apparatus, has apparently
led to the simplification of
structure. Degeneration has
followed in the worms just
mentioned, as the result of
their imitation and acquirement
of vegetative powers of feed-
ing ; and it is probable that
other alterations in the way of
dietary, of less sweeping char-
acter than that just mentioned,
will affect, in like retrogressive
fashion, the animal constitution.

Some of the most curious cases of degeneration known to us illus-
trate the total disappearance of digestive apparatus even in some
beings, in which, as in the stylops already mentioned, one sex becomes




FlO. 19.— KOTIPKRA.



390 THE POPULAR SCIEXCE MONTHLY.

retrogressive while the other sex remains structurally fully developed.
Such a case is illustrated by the males of those remai-kable organisms,
the Botifera, or " wheel-animalcules " (Fig. 19). These minute creat-
ures, inhabiting our fresh waters, may be desiccated and dried, and
revived, on the application of moisture, many times in succession.
But in their ordinary existence, and in the details of their structure,
the " wheel-animalcules " present details equally interesting with their
exhibition of " potential vitality." The female animalcules possess a
complete digestive system, a set of water vessels, a nervous ganglion,
and other belongings ; but their partners are decidedly inferior creat-
ures, since their digestive system becomes totally abortive, while in
size the males are likewise far excelled by the lady rotifers. IIow this
degeneration and disappearance of digestive apparatus and the inferi-
ority of size have been produced in the male rotifers may be a matter
regarding which difference of opinion will certainly exist in biological
minds. The fact that retrogression is here illustrated, however, can
not be questioned. It may also be added that, in all jjrobability, the
extreme development of the function of perpetuating the species and
the extraordinary fertility of production witnessed in these animal-
cules, may satisfactorily account for the abrogation of digestion in
favor of reproduction. Thus, to the other causes of degeneration in
animal life and structure, we may append that which takes origin from
the extreme or excessive development of one function over another.
Physiological development in one direction, overstepping the natural
and ordinary limits, runs concurrently with destruction of life's equi-
librium, and naturally tends to produce degeneration and simplification
of other organs and other duties of life.

How far the theory of degeneration we have thus briefly discussed
may be applied in explanation of the peculiarities of animal structure,
remains as a task for the future of biology to satisfactorily determine.
Possibly the corrections which the future of every hypothesis carries
with it may be many and sweeping. The deductions and inferences
we extract from a study of degeneration to-day may perchance be
falsified by the higher and newer views of the to-morrow of biological
science. But enough has been said to show that, even in a cursory
review of the doctrine of degeneration and retrogression, many jihases
of living histories become theoretically plain ; and it argues hopefully
for the correctness and value of the doctrine before us that it has, so
far as it has been logically pursued, fitted compactly and harmoniously
enough with ascertained facts and with received views of the origin
of animals and plants. That higher forms of life than the sea-squirt
and insect race are by no means exempt from the influence of retro-
gressive change is an observation worth noting at the close of our
researches. We know, for instance, of lowly structures in shell-fish
life appearing in the midst of highly organized frames. A mussel,
a cockle, or an oyster, whose early development runs in parallel lines



DEGENERATION. 3^1 .

to that of the snail and whelk class, is nevertheless esteemed less
highly organized than the latter. The mussel or oyster tribe possess
no head ; the snails and their allies, as every one knows, not merely
exhibit a well-developed head, but have that extremity provided with
eyes, tentacles or feelers, and other addenda of the front region of
the animal body. Hence it is more than probable that the mussel,
headless and inclosed in its shell and possessing relatively little inter-
est in the affairs of the outer world, is an example of a degenerated
type of mollusks. The mussels and their relations stand, in fact, at
the opposite extreme of development in this respect from those well-
known mollusks, the cuttle-fishes. In these creatures, the tendency to
head-development — or what Professor Dana calls " cephalization " —
reaches its maximum, as any one may readily enough suppose on look-
ing at an octopus or squid, with its great head, its enormous eyes, and
its nerves massed together to form a brain inclosed in a kind of skull.
Even as compared with the earlier cuttle-fishes — whose shells, under
the name of ammonites and the like, we find fossilized in large num-
bers — the squids and cuttles of to-day present, in the extreme devel-
opment of head, a noteworthy advance. Thus, while the one molluscan
tribe of mussels and their neighbors has degenerated and gone to its
own lowly place in the series, other groups, starting on an equal foot-
ing, have advanced, and, through progressive evolution, have produced
those higher manifestations of molluscan life that teem in the seas of
to-day. Even among the vertebrate animals we meet with examples
of degenerative tendency which are not so easily explicable as the
foregoing illustrations. In most snakes only one lung is fully devel-
oped, as a rule, the companion organ being rudimentary and degener-
ate. In birds, the egg-producing organs are similarly developed on
one side only. How degeneration should be thus partial and affect
one half of an animal's frame, so to speak, is very hard to discover.
External conditions of life and the influences of surroundings could
apparently possess little effect in inducing such an unsymmetrical re-
trogression of parts. Most probably we shall find the solution of such
conditions to exist within the operation of some deep-seated law of the
living constitution, and in the effects of that law in molding, or even
contorting, the animal frame.

It constitutes one of the chief glories of biological science, as
pursued among us to-day, that its studies are of far-reaching order,
and lead, as the results of their natural extension, to the considera-
tion of fields of thought often widely removed from the original topic
which interests the reader. The present subject of degenerative
changes, regarded as part and parcel of the living constitution, can
readily be shown to possess applications far removed from zoology
and botany, and extending into the most intimate spheres and phases
of human history itself. Degenerative change in human tissues is
medically symptomatic of very many of the ills to which flesh is heir.



392 THE POPULAR SCIENCE MONTHLY.

Tissues and organs degenerate in individual animals, as animal frames
retrogress in their entirety. Cells retrograde and fibers degenerate
in our bodies, just as the sea-squirt's frame exhibits, as a whole, a
universal, physiological backsliding. Nor may many of our diseases
alone be esteemed mere examples of degeneration affecting our tis-
sues. The termination and decline of life itself and the age that
really " melts in unperceived decay " are, in reality, examples of natural
degeneration also. The decline of existence is largely a retrogression
of structure. There can be no such thing as a really " green old age,"
any more than we can speak of " the sere and yellow " of the autum-
nal leaf as imitating the verdant nature of the spring blossom. Nay,
stranger still is it to discern that the full flush of life's vigor is ac-



Online LibraryD. S. (David Samuel) MargoliouthThe Popular science monthly (Volume 19) → online text (page 47 of 110)