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[Illustration: NATURE SERIES]
DEGENERATION.
[Illustration]
_NATURE SERIES._
DEGENERATION.
A
CHAPTER IN DARWINISM.
BY
PROFESSOR E. RAY LANKESTER, F.R.S.,
FELLOW OF EXETER COLLEGE, OXFORD.
London:
MACMILLAN AND CO.
1880.
_The Right of Translation and Reproduction is Reserved_
LONDON:
R. CLAY, SONS, AND TAYLOR,
BREAD STREET HILL, E. C.
TO MY FRIEND
ANTON DOHRN
THESE PAGES
Are Dedicated
IN MEMORY OF OUR COMPANIONSHIP
AT JENA AND NAPLES.
E. R. L.
_January, 1880._
DEGENERATION.
DEGENERATION.
It is the misfortune of those who study that branch of science which
our President has done so much to advance—I mean the science of living
things—that they are not able, in the midst of a vast assembly,[1] to
render visible to all eyes the actual phenomena to which their inquiries
are directed. Whilst the physicist and the chemist are able to make
evident to the senses of a great meeting the very things of which they
have to tell, the zoologist cannot hope ever to share with those who
form his audience the keen pleasure of observing a new or beautiful
organism; he cannot demonstrate by means of actual specimens the delicate
arrangements of structure which it is his business to record, and
upon which he bases his conclusions. It is for this reason that he who
would bring to the notice of laymen some matter which at the moment is
occupying the attention of biological students, must appear to be unduly
devoted to speculation—hypothesis—to support which he cannot produce
the facts themselves but merely the imperfect substitutes afforded by
pictures. It is perhaps not altogether a matter for regret that there
should be in one great branch of science, as there is in biology, so very
marked a disproportion between the facilities for demonstrating facts
and the general interest attaching to the theories connected with those
facts. We may be thankful that at the present day we are not likely, in
the domain of biology, to make the mistake (which has been made under
other circumstances) of substituting the mere inspection and cataloguing
of natural objects for that more truly scientific attitude which consists
in assigning the facts which come under our observation to their causes,
or, in other words, to their places in the order of nature. Though we may
rightly object to the attempt which is sometimes made to decry the modern
teachings of biology as not being “exact science,” yet we may boldly
admit the truth of the assertion that we biologists are largely occupied
with speculations, hypotheses, and other products of the imagination. All
true science deals with speculation and hypothesis, and acknowledges as
its most valued servant—its indispensable ally and help-meet—that which
our German friends[2] call “Phantasie” and we “the Imagination.” Our
science—biology—is not less exact; our conclusions are no less accurate
because they are only _probably_ true. They are “probably true” with a
degree of probability of which we are fully aware, and which is only
_somewhat_ less than the probability attaching to the conclusions of
other sciences which are commonly held to be “exact.”
These remarks are addressed to an Association for the advancement
of science—of science which flourishes and progresses by the aid of
suppositions and the working of the imagination. The Association has
been holding its annual sitting in various parts of the British Islands
for more than thirty years, and yet it is still a very common and widely
spread notion that science, that is to say, _true_ science according to
those who hold the notion, does not countenance hypotheses, and sternly
occupies itself with the exact record of fact. On the other hand, there
are many persons who run to an opposite extreme, and call by the name of
science any fanciful attempt to deal with or account for a certain class
of phenomena. The words “science” and “scientific” are used so vaguely
and variously that one might almost come to the conclusion that it would
be well for our Association to plainly state what is that thing for the
advancement of which its meetings are held. I cannot venture to speak
in the name of my colleagues; and no doubt a review of the work done
by the Association would most fitly explain what that body understands
by the word “science.” At the same time it is permissible to take this
opportunity of briefly stating what science is and what it is not, so far
as I am able to judge of the fitting use of the word.
Science is certainly _not_ any and every kind of knowledge. Knowledge of
literature, of the beautiful things which have been written or otherwise
produced by human ingenuity, is not science. Knowledge of the various
manufacturing processes in use by civilised men is not science; nor
knowledge of the names of the stars, or of the joints of a beetle’s leg.
Science cannot be identified with knowledge of any particular _class_ of
objects, however detailed that knowledge may be. It is a common mistake
to consider all knowledge of raw products, of living objects or other
natural objects, as necessarily “science.” The truth is, that a man may
have great knowledge of these things as so many facts, and yet be devoid
of “science.” And, on the other hand, that which is properly called
science embraces not only such subject-matter as that just alluded to,
but also may find its scope in the study of language, of human history,
and of the workings of the human mind.
The most frequent and objectionable misuse of the word ‘science’ is
that which consists in confounding science with invention—in applying
the term which should be reserved for a particular kind of knowledge to
the practical applications of that knowledge. Such things as electric
lighting and telegraphs, the steam-engine, gas, and the smoky chimneys
of factories, are by a certain school of public teachers, foremost among
whom is the late Oxford Professor of Fine Art, persistently ascribed
to science, and gravely pointed out as the pestilential products of
a scientific spirit. They are, in fact, nothing of the kind. American
inventors and electric lamps, together with all the factories in
Sheffield, might be obliterated without causing a moment’s concern to
a single student of science. It is of the utmost importance for the
progress and well-being of science that this should be understood; that
the eager practical spirit of the inventor who gains large pecuniary
rewards by the sale of his inventions should not be confounded with
what is totally different and remote from it, namely, the devoted,
searching spirit of science, which, heedless of pecuniary rewards, ever
faces nature with a single purpose—_to ascertain the causes of things_.
It seems to me impossible to emphasise too strongly in such a place
and in such a meeting as this, that Invention is widely separate from,
though dependent on, Science. Invention is worldly-wise, and despises
the pursuit of knowledge for its own sake. She awaits the discoveries
of Science, in order to sell them to civilisation, gathering the golden
fruit which she has neither planted nor tended. Invention follows, it is
true, the footsteps of Science, but at a distance: she is utterly devoid
of that thriftless yearning after knowledge, that passionate desire to
know the _truth_, which causes the unceasing advance of her guide and
benefactress.
We may, it seems to me, say that of all kinds and varieties of knowledge
that only is entitled to the name “science” which can be described as
Knowledge of Causes, or Knowledge of the Order of Nature. It is this
knowledge to which the great founder of European science—Aristotle the
Greek—pointed as _true_ knowledge: τότε ἐπιστάμεθα ὅταν τὴν αἰτίαν
εἴδωμεν. Science is that knowledge which enables us to demonstrate so
far as our limited faculties permit, that the appearances which we
recognise in the world around us are dependent in definite ways on
certain properties of matter: science is that knowledge which enables,
or tends to enable us, to assign its true place in the series of events
constituting the universe, to any and every thing which we can perceive.
The method by which scientific knowledge is gained—knowledge of the
causes of nature—is precisely the same as that by which knowledge of
causes in every-day life is gained. Something—an appearance—has to be
accounted for: the question in both cases is “Through what cause, in
relation to what antecedent is this appearance brought about?” In
scientific inquiry, as in every-day life, a hypothesis—a provisional
answer or guess—is the reply; and the truth of that guess or hypothesis
is then _tested_. This testing is an essential part of the process. “If
my guess be true, then so-and-so as to which I can decide by inspection
or experiment, must be true also,” is the form which the argument takes,
and the inquiry is thus brought to a point where _observation_ can decide
the truth of the hypothesis or first guess. In every-day life we have
often to be content without fully testing the truth of our guesses, and
hurry into action based on such unverified suppositions. Science, on the
other hand, can always wait, and demands again and again the testing and
verification of guesses before they are admitted as established truths
fit to be used in the testing of new guesses and the building up of
scientific doctrine.
The delicately-reared imaginations of great investigators of natural
things have from time to time given birth to hypotheses—guesses at
truth—which have suddenly transformed a whole department of knowledge,
and made the causes of things quite clear which before seemed likely
to remain always concealed. So great is the value of hypothesis,
so essential to scientific discovery, that the most skilled and
highly-trained observer may spend his life in examining and scrutinising
natural objects and yet fail, if he is not guided by hypothesis, to
observe particular facts which are of the uttermost importance for the
explanation of the causes of the things which he is studying. Nature,
it has been said, gives no reply to a general inquiry—she must be
interrogated by questions which already contain the answer she is to
give; in other words, the observer can only observe that which he is led
by hypothesis to look for: the experimenter can only obtain the result
which his experiment is designed to obtain.
For a long time the knowledge of living things, of plants and of animals
could hardly be said to form part of the general body of science, for
the causes of these things were quite unknown. They were kept apart as
a separate region of nature, and were supposed to have been pitched, as
it were, into the midst of an orderly and cause-abiding world without
cause or order: they were strangers to the universal harmony prevailing
around them. Fact upon fact was observed and recorded by students of
plants and animals, but having no hypothesis as to the causes of what
they were studying, the naturalists of twenty years ago, and before that
day, though they collected facts, made slow progress and some strange
blunders. Suddenly one of those great guesses which occasionally appear
in the history of science, was given to the science of biology by the
imaginative insight of that greatest of living naturalists—I would say
that greatest of living men—Charles Darwin.
In the form in which Mr. Darwin presented his view to the world it was no
longer a mere guess. He had already tried it and proved it in an immense
series of observations; it had already been converted by twenty years’
labour on his part into an established doctrine, and the twenty years
which have passed since he published the _Origin of Species_ have only
served to confirm, by thousands of additional tests, the truth of his
original guess.
Space will not allow me to go fully into the history of the Darwinian
theory, but it is necessary for my present purpose that I should state
precisely what that theory is. It involves a number of subordinate
hypotheses which, together with the main hypothesis, furnish us with a
complete “explanation,” as it is called, of the facts which have been
ascertained as to living things; in other words, it assigns living things
to their causes, gives them their place in the Order of Nature.
It is a very general popular belief at the present day that the Darwinian
theory is simply no more than a capricious and anti-theological assertion
that mankind are the modified descendants of ape-like ancestors.
Though most of my readers, I do not doubt, know how imperfect and
erroneous a conception this is, yet I shall not, I think, be wasting
time in stating what the Darwinian theory really is. In fact, it is
so continuously misrepresented and misunderstood, that no opportunity
should be lost of calling attention to its real character. Bit by bit,
naturalists had succeeded in discovering the order of nature—so far that
all the great facts of the universe, the constitution and movements of
the heavenly bodies, the form of our earth, and all the peculiarities
of its crust, had been successfully assigned to one set of causes—_the
properties of matter_, which are set forth in what we know by the name
of the “laws of physics and chemistry.” Whilst geologists, led by Lyell,
had shown that the strata of the earth’s crust and its mountains,
rivers, and seas were due to the long-continued operation of the very
same general causes—the physico-chemical causes—which at this moment are
in operation and are continuing their work of change, yet the living
matter on the crust of the earth had to be excluded from the grand
uniformity which was elsewhere complete.
The first hypothesis, then, which was present to Mr. Darwin’s mind,
as it had been to that of other earlier naturalists, was this: “Have
not all the varieties or species of living things (man, of course,
included) been produced by the continuous operation of the _same_ set
of physico-chemical causes which alone we can discover, and which alone
have been proved sufficient to produce everything else?” “If this be
so,” Mr. Darwin must have argued (and here it was that he boldly stepped
beyond the speculations of Lamarck and adopted the method by which Lyell
had triumphantly established Geology as a science), “these causes must
still be able to produce new forms, and are doing so wherever they have
opportunity.” He had accordingly to bring the matter to the test of
observation by seeking for some case of the production of new forms
of plants, or of animals, by natural causes at the present day. Such
cases he found in the production of new forms or varieties of plants and
animals, by breeders. Breeders (the persons who make it their business
to produce new varieties of flowers, of pigeons, of sheep, or what not)
make use of two fundamental properties of living things in order to
accomplish their purpose. These two properties are, firstly, that no two
animals or plants, even when born of the same parents, are _exactly_
alike; this is known as Variation: secondly, that an organism, as a rule,
inherits, that is to say, is born with the peculiarities of its parents;
this is known as Transmission, and is simply dependent on the fact that
the offspring of any plant or animal is only a detached portion of the
parent—a chip of the old block, as the saying is. The breeder selects
from a number of specimens of a plant or animal a variety which comes
nearest to the form he wishes to produce. Supposing he wished to produce
a race of oxen with short horns, he would select from his herd bulls and
cows with the shortest horns, and allow these only to breed; they would
_transmit_ their relatively short horns to their offspring, and from
these again the cattle with the shortest horns would be selected by the
breeder for propagation, and so on through several generations. In the
end a very short-horned generation would be obtained, differing greatly
in appearance from the cattle with which the breeder started.
Now we know of no facts which forbid us to suppose that could a breeder
continue his operations indefinitely for any length of time—say for a few
million years—he could convert the short-horned breed into a hornless
breed; that he could go on and thicken the tail, could shorten the
legs, get rid of the hind limbs altogether by a series of insensible
gradations, and convert the race into forms like the Sirenia, or
sea-cows. But if he could do this, you have only to give him a longer
time still and there is no obstacle remaining to the conversion, by the
same kind of process, of a polyp into a worm, or of a worm into a fish,
or even of a monkey into a man.[3]
So far we have supposed the interference of a breeder who selects and
determines the varieties which shall propagate themselves; so far we
have not got a complete explanation, for we must find a substitute in
nature for the _human selection_ exercised by the breeder. The question
arises, then, “Is there any necessary selective process in nature which
could have operated through untold ages, and so have represented the
selective action of the breeder, during an immense period of time?”
Strangely enough, Mr. Darwin was led to the discovery of such a cause
existing necessarily in the mechanical arrangements of nature, by reading
the celebrated book of an English clergyman, the Rev. Mr. Malthus, _On
Population_. On happening to read this book, Mr. Darwin himself tells
us that the idea of “natural selection” flashed upon him. That idea is
as follows. Not only among mankind, but far more largely among other
kinds of animals and of plants, the number of offspring produced by
every pair is immensely in excess of the available amount of the food
appropriate to the particular species in question. Accordingly, there is
necessarily a struggle for existence—a struggle among all those born for
the possession of the small quantum of food. The result of this struggle
is to pick out, or select, a few who survive and propagate the species,
whilst the majority perish before reaching maturity. The fact that no
two members of a species are alike has already been shown to be the
starting-point which enables the breeder to make _his_ selection. So,
too, with _natural_ selection in the struggle for existence; the fact
that all the young born of one species are not exactly alike—but some
larger, some smaller, some lighter, some darker, some short-legged, some
big-eyed, some long-tongued, some sharp-toothed, and so on—furnishes the
opportunity for a selection. Those varieties which are best fitted to
obtain food and to baffle their competitors, gain the food and survive,
the rest perish.
We have, then, to note that the hypothesis that there _must be a
selection_—which was framed or deduced as a “test hypothesis” from the
earlier hypothesis that species have arisen by the action of causes still
competent to produce new forms—led Mr. Darwin to the discovery of this
great cause—the “_natural selection_,” or “survival of the fittest,”
in the struggle for existence. Just as the breeder can slowly change
the proportions of the animals or plants on which he operates, so in
inconceivably long periods of time has this struggling of varieties, and
the consequent natural selection of the fittest, led to the production,
from shapeless primitive living matter, of all the endless varieties of
complicated plants and animals which now people the world. Countless
varieties have died out, leaving only their modified descendants to
puzzle the ingenuity of the biologist.
Of the tens and hundreds of thousands of intermediate forms we know
nothing by direct observation. They have perished as better fitted forms
ousted them in the never-ending conflict. But we feel sure that they once
were in existence, and can infer what was their structure, and what were
their peculiarities, by the study of the structure and attributes of
their now living descendants.
If all the forms of life at present living are the modified offspring of
a smaller number of ancestral forms which have died out, and if these
again were the modified descendants produced by ordinary parentage of a
single original living thing—then the whole series of forms that have
_ever_ lived could, if we had them before us, be arranged in the form of
a great family-tree—the various branches presenting a perfect gradation
of forms arranged one after another, leading down from the terminal twigs
(which would represent the latest forms produced) to larger and larger
branches, until the common trunk representing the original ancestor would
be reached. Our actual means of observing the genealogical affinities of
different kinds of animals and plants may be understood by a further use
of the metaphor of a genealogical tree in shape like an elm or an oak.
Suppose the genealogical tree completely written out—a perfect record—to
be sunk in muddy water so that only its topmost branches and twigs
are here and there visible—then you have a fair notion of the present
condition of the great family of organisms. Only the topmost twigs remain
visible, the rest of the great family-tree of living beings is hidden
from view, submerged beneath the muddy waters of time. Naturalists have,
however, undertaken to reconstruct this great genealogical tree. It is a
main object now in the study both of zoology and of botany to find out
what are the cousinships, what the exact genetic relationships of all
the various species of plants and animals, and so to show, even to the
minutest detail, in what particular ways physico-chemical causes have
brought about and modified the forms of living things.
The task is not quite so difficult as the comparison to a submerged
forest-tree would lead one to expect; at the same time it is more
difficult than those who have boldly attempted it appear to believe. We
have one great help in the carefully worked out systematic classification
of animals and plants according to their structure. We are justified in
assuming as a general law that animals or plants of like structure have
descended from common ancestors—that is to say, that the same kind of
organisation (especially where a number of elaborate details of structure
are involved) has _not been twice_ produced by natural selection. Thus
we are entitled to conclude that _all_ the animals which have a backbone
and pharyngeal gill-slits combined—the Vertebrates, as we call them—have
descended from a common parent; that all the animals with a muscular
foot-like belly and lateral gill filaments, the Molluscs, have also had a
common parent, and so on.
A classification according to structure goes then a long way towards
mapping out the main lines of the family-tree of organisms. We are
further assisted in the task by the fossil remains of extinct organisms
which sometimes give to us the actual ancestors of forms now living. But
the most remarkable aid to the correct building-up of the pedigree of
_animals_ at least (and the remarks which follow are confined to that
division of the organic world), is afforded by the changes—the phases
of development—which every animal exhibits in passing from the small
shapeless egg to the adult condition. The aid which we here obtain
depends on the following facts. Just as we suppose any one animal—say
a dog—to have developed by slow change through an immense series of
ancestors which become simpler and simpler as we recede into the past
until we reach a small shapeless lump of living matter devoid of
structure, so do we find actually as a matter of fact, which any one can
see for himself, that every individual animal begins its individual life
as a structureless particle which is thrown off from its parent, and is
known as the _egg-cell_ (Fig. 1). Gradually passing through a series of
more and more elaborated conditions of structure, that egg grows into the
adult dog. The changes which have taken countless ages in the one case,
are accomplished in a few weeks in the other.
And now we have to note the important fact which makes this process
of development so intensely interesting in relation to the pedigree
of the animal kingdom. There is very strong reason to believe that
it is a general law of transmission or inheritance, that structural
characteristics appear in the growth of a young organism in the order
in which those characteristics have been acquired by its ancestors. At
first the egg of a dog represents (imperfectly, it is true,) in form and
structure the earliest ancestors of the dog; a few days later it has the
form and structure of somewhat later ancestors; later still the embryo
dog resembles less remote ancestors; until at last it reaches the degree
of elaboration proper to its immediate forefathers.
[Illustration: FIG. 1.—An egg: a single corpuscle of protoplasm with
nucleus _b_ _c_, and body _a_.]
Accordingly the phases of development or growth of the young are a brief
recapitulation of the phases of form through which the ancestors of the
young creature have passed. In some animals this recapitulation is more,
in others it is less complete. Sometimes the changes are hurried through
and disguised, but we find here and there in these histories of growth
from the egg most valuable assistance in the attempt to reconstruct the
genealogical tree. The history of the development of the common frog is a
good illustration of the _kind_ of evidence in question.
[Illustration: FIG. 2.—Tadpoles and young of the Common Frog. 1, Recently
hatched (twice natural size); 2 and 2_a_, same enlarged to show the
external gills; 3 and 4, later stages with gill slits covered by a
membrane leaving only the spiracle (see Fig. 16) as an exit for the
respired water; 5, with hind legs appearing; 6, with both fore and hind
legs; 7, atrophy of the tail; 8, young frog.]
The frog’s egg first gives rise to a little aquatic creature with
external gills and a tail—the tadpole—which gradually loses its gills
and its tail and acquires in their place lungs and four legs (Fig. 2),
so as now to be fitted for life on dry land. From what we otherwise know
of the structure of the frog and the animals to which it is allied, we
are justified in concluding that the tadpole is a recapitulative phase
of development, and represents to us more or less closely an ancestor of
the frog which was provided with gills and tail in the adult state, and
possessed neither legs nor lungs.
[Illustration: FIG. 3.—Adult shrimp of the genus Peneus.]
A less familiar case is that of a certain kind of shrimp, which is
illustrated in the wood-cuts (Fig. 3 and right lower corner of Fig.
4). The little creature which issues from the egg of this shrimp is
known as the “Nauplius form.” Many animals very different in appearance
from this shrimp make their first appearance in the world as Nauplii;
and it appears probable that the Nauplius-phase is the recapitulative
re-presentation of an ancestor common to all this set of animals,
an ancestor which was not _exactly_ like the Nauplius, but not very
different from it.
[Illustration: FIG. 4.—Nauplius larval-form of various Crustacea
(Shrimps, Water-fleas, Barnacles, &c.).]
The Nauplius of our shrimp gradually elongates. At first it has but
three pair of limbs, but it soon acquires additional pairs, and a jointed
body, and thus by gradually adding to its complexity of structure as seen
in Figs. 5 and 6, it approximates more and more to the adult form from
the egg of which it originated.
[Illustration: FIG. 5.—Larva of the Shrimp Peneus.]
[Illustration: FIG. 6.—More advanced larva of the Shrimp Peneus.]
And now we are approaching the main point to which I wish to draw the
reader’s attention. In attempting to reconstruct the pedigree of the
animal kingdom and so to exhibit correctly the genetic relationships of
all existing forms of animals, naturalists have hitherto assumed that the
process of natural selection and survival of the fittest has invariably
acted so as either to improve and elaborate the structure of _all_ the
organisms subject to it, or else has left them unchanged, exactly fitted
to their conditions, maintained as it were in a state of _balance_. It
has been held that there have been some six or seven great lines of
descent—main branches of the pedigree—such as that of the Vertebrates,
that of the Molluscs, that of the Insects, that of the Starfish, and
so on; and that along each of these lines there has been always and
continuously a progress—a change in the direction of greater elaboration.
Each of these great branches of the family-tree is held to be
independent—they all branch off nearly simultaneously from the main
trunk like the leading branches of an oak. The animal forms constituting
the series in each of these branches are supposed to gradually increase
in elaboration of structure as we pass upwards from the main trunk of
origin and climb further and further towards the youngest, most recent
twigs. New organs have, it is supposed, been gradually developed in each
series, giving their possessors greater powers, enabling them to cope
more successfully with others in that struggle for existence in virtue
of which these new organs have been little by little called into being.
At the same time _here and there_ along the line of march, certain forms
have been supposed to have “fallen out,” to have ceased to improve,
and being happily fitted to the conditions of life in which they were
long ago existing, have continued down to the present day to exist in
the same low, imperfect condition. It is in this way that the lowest
forms of animal life at present existing are usually explained, such
as the microscopic animalcules, Amœbæ and Infusoria. It is in this way
that the lower or more simply-made families of higher groups have been
generally regarded. The simpler living Mollusca or shellfish have been
supposed necessarily to represent the original forms of the great race
of Mollusca. The simpler Vertebrates have been supposed necessarily to
represent the original Vertebrates. The simpler Worms have been supposed
necessarily to be the stereotyped representatives of very ancient Worms.
That this is, to a certain extent, a true explanation of the existence
at the present day of _low_ forms of animals is proved by the fact that
we find in very ancient strata fossil remains of animals which differ,
ever so little, from particular animals existing at the present day;
for instance, the Brachiopods (lamp-shells), Lingula and Terebratula,
the King-crabs, and the Pearly Nautilus are found living at the present
day, and are also found with no appreciable difference in very ancient
strata of the earth’s crust; strata deposited so long ago that most of
the forms of life at present inhabiting the earth’s surface had not then
been brought into existence, whilst other most strange and varied forms
occupied their place, and have now for long ages been extinct.
Whilst we are thus justified by the direct testimony of fossil remains in
accounting for _some_ living forms on the hypothesis that their peculiar
conditions of life have been such as to maintain them for an immense
period of time _in statu quo_ unchanged, _we have no reason for applying
this hypothesis, and this only_, to the explanation of all the more
imperfectly organised forms of animal or plant-life.
It is clearly enough possible for a set of forces such as we sum up under
the head “natural selection” to so act on the structure of an organism
as to produce one of three results, namely these; to keep it _in statu
quo_; to increase the complexity of its structure; or lastly, to diminish
the complexity of its structure. We have as possibilities either BALANCE,
or ELABORATION, or DEGENERATION.
Owing, as it seems, to the predisposing influence of the systems of
classification in ascending series proceeding steadily upwards from the
“lower” or simplest forms to the “higher” or more complex forms,—systems
which were prevalent before the doctrine of transformism had taken firm
root in the minds of naturalists, there has been up to the present day
an endeavour to explain every existing form of life on the hypothesis
that it has been maintained for long ages in a state of Balance; or
else on the hypothesis that it has been Elaborated, and is an advance,
an improvement, upon its ancestors. Only one naturalist—Dr. Dohrn, of
Naples—has put forward the hypothesis of Degeneration as capable of wide
application to the explanation of existing forms of life;[4] and his
arguments in favour of a general application of this hypothesis have
not, I think, met with the consideration which they merit.
The statement that the hypothesis of Degeneration has not been recognised
by naturalists generally as an explanation of animal forms, requires
to be corrected by the exception of certain kinds of animals, namely,
those that are parasitic or quasi-parasitic. With regard to parasites,
naturalists have long recognized what is called _retrogressive
metamorphosis_; and parasitic animals are as a rule admitted to be
instances of Degeneration. It is the more remarkable whilst the
possibility of a degeneration—a loss of organisation making the
descendant far _simpler_ or _lower_ in structure than its ancestor—has
been admitted for a few exceptional animals, that the same hypothesis
should not have been applied to the explanation of other simple forms
of animals. The hypothesis of Degeneration will, I believe, be found to
render most valuable service in pointing out the true relationships of
animals which are a puzzle and a mystery when we use only and exclusively
the hypothesis of Balance, or the hypothesis of Elaboration. It will, as
a true scientific hypothesis, help us to discover causes.
We may now examine a few examples of undeniably degenerate animals, and
first, I may call to mind the very remarkable series of lizard-like
animals which exist in the south of Europe and in other countries, which
exhibit in closely related genera a gradual loss of the limbs—a local or
limited Degeneration. We have the common Lizard (_Lacerta_), with five
toes on each of its well-grown fore and hind limbs; then we have side by
side with this a lizard-like creature, _Seps_, in which both pairs of
limbs have become ridiculously small, and are evidently ceasing to be
useful in the way in which those of _Lacerta_ are useful; and lastly,
we have _Bipes_, in which the anterior pair of limbs has altogether
vanished, and only a pair of stumps, representing the hinder limbs,
remain.
No naturalist doubts that Seps and Bipes represent two stages of
Degeneration, or atrophy of the limbs; that they have, in fact, been
derived from the five-toed four-legged form, and have _lost_ the
locomotor organs once possessed by their ancestors. This very partial
or local atrophy is not however that to which I refer when using the
word Degeneration. Let us imagine this atrophy to extend to a variety
of important organs, so that not only the legs, but the organs of
sense, the nervous system, and even the mouth and digestive organs are
obliterated—then we shall have pictured a thorough-going instance of
Degeneration.
Degeneration may be defined as a gradual change of the structure in
which the organism becomes adapted to _less_ varied and _less_ complex
conditions of life; whilst Elaboration is a gradual change of structure
in which the organism becomes adapted to more and more varied and complex
conditions of existence. In Elaboration there is a new _expression_ of
form, corresponding to new perfection of work in the animal machine.
In Degeneration there is _suppression_ of form, corresponding to the
cessation of work. Elaboration of some one organ _may_ be a necessary
accompaniment of Degeneration in all the others; in fact, this is
very generally the case; and it is only when the total result of the
Elaboration of some organs, and the Degeneration of others, is such as to
leave the whole animal in a _lower_ condition, that is, fitted to less
complex action and reaction in regard to its surroundings, than was the
ancestral form with which we are comparing it (either actually or in
imagination) that we speak of that animal as an instance of Degeneration.
Any new set of conditions occurring to an animal which render its food
and safety very easily attained, seem to lead as a rule to Degeneration;
just as an active healthy man sometimes degenerates when he becomes
suddenly possessed of a fortune; or as Rome degenerated when possessed
of the riches of the ancient world. The habit of parasitism clearly
acts upon animal organisation in this way. Let the parasitic life
once be secured, and away go legs, jaws, eyes, and ears; the active,
highly-gifted crab, insect, or annelid may become a mere sac, absorbing
nourishment and laying eggs.
Reference was made above to the larval stage of a certain shrimp (Figs.
4, 5, 6). Let us now compare these with the young stages of a number
of shrimp-like animals, viz., Sacculina, Lernæocera, Lepas, Cyclops,
Limnetis, (all drawn in Fig. 4), some of which lead a parasitic life.
All start equally with the recapitulative phase known as the Nauplius;
but whilst the Nauplius of the free-living shrimp grows more and more
elaborate, observe what happens to the parasites—they degenerate
into comparatively simple bodies; and this is true of their internal
structure as well as of their external appearance. The most utterly
reduced of these parasites is the curious Sacculina (Fig. 7) which
infests Hermit-crabs, and is a mere sac filled with eggs, and absorbing
nourishment from the juices of its host by root-like processes.
[Illustration: FIG. 7.—Adult Sacculina (female).]
Lernæocera again, which in the adult condition is found attached to
the gills of fishes, has lost the well-developed legs of its Nauplius
childhood and become an elongated worm-like creature (Fig. 8), fitted
only to suck in nourishment and carry eggs.
[Illustration: FIG. 8.—Adult female Lernæocera.]
Amongst these Nauplii—all belonging to the great group Crustacea, which
includes crabs and shrimps—is one which gives rise to an animal decidedly
degenerate, but not precisely parasitic in its habits. This Nauplius is
the young of the ship’s Barnacle, a curious stalked body, inclosed in a
shell of many pieces (Fig. 9). The egg of the Barnacle gives rise to an
actively swimming Nauplius, the history of which is very astonishing.
After swimming about for a time the Barnacle’s Nauplius fixes its head
against a piece of wood, and takes to a perfectly fixed, immobile state
of life (Fig. 10). The upper figures represent the Nauplius stage of
animals closely resembling the Barnacle: the lower figures show the
transformation of the Nauplius into the young Barnacle. Its organs of
touch and of sight atrophy, its legs lose their locomotor function, and
are simply used for bringing floating particles to the orifice of the
stomach; so that an eminent naturalist has compared one of these animals
to a man standing on his head and kicking his food into his mouth.
[Illustration: FIG. 9.—Adult Barnacle or Lepas (one of the Cirrhipedes).
Natural size.]
[Illustration: FIG. 10.—Development of Cirrhipedes (Barnacle and
Sea-acorn). After Huxley.]
Were it not for the recapitulative phases in the development of the
Barnacle, we may doubt whether naturalists would _ever_ have guessed that
it was a degenerate Crustacean. It was in fact for a long time regarded
as quite remote from them, and placed among the snails and oysters; its
true nature was only admitted when the young form was discovered.
Other parasitic organisms, which exhibit extreme degeneration as
compared with their free-living relatives, might be cited and figured
in profusion, did our limits permit. Very noteworthy are the degenerate
Spiders—the mites, leading to still more degenerate forms, the Linguatulæ.
We have two of these represented in Figs. 11 and 12. The one (Fig. 11),
as compared with a spider is seen still to possess the eight walking
legs, small, it is true, whilst the palps and daggers of the spider have
dwindled to a beak projecting from the front of the globular unjointed
body. In the other the eight legs have become mere stumps, and the body
is elongated like that of a worm.
[Illustration: FIG. 11.—Acarus equi. A degenerate Spider or mite
parasitic on the skin of the horse.]
[Illustration: FIG. 12.—Degenerate Spider (Demodex foliculorum) found in
the skin of the human face.]
The instances of degeneration which we have so far examined are due to
parasitism, except in the example of the Barnacle, where we have an
instance of degeneration due to sessile and immobile habit of life. We
may now proceed to look at some sessile or immobile animals which are not
usually regarded as degenerate, but which, I think, there is every reason
to believe are the degenerate descendants of very much higher and more
elaborate ancestors. These are certain marine animals, the Ascidians, or
sea-squirts. These animals are found encrusting rocks, stones, and weeds
on the sea bottom. Sometimes they are solitary (Fig. 13), but many of
them produce buds, like plants, and so form compound masses or sheets of
individuals all connected and continuous with one another, like the buds
on a creeping plant (Fig. 14).
[Illustration: FIG. 13.—Two adult Ascidians: to the left Phallusia—to
the right Cynthia: the incurrent and excurrent orifices are seen as two
prominences. Half the natural size.]
We will examine one of the simple forms—a tough mass like a leather
bottle with two openings; water is continually passing in at the one
and out at the other of these apertures. If we remove the leathery
outer-case (Fig. 15), we find that there is a soft creature within which
has the following parts:—Leading from the mouth a great throat, followed
by an intestine. The throat is perforated by innumerable slits, through
which the water passes into a chamber—the cloaca: in passing, the water
aërates the blood which circulates in the framework of the slits. The
intestine takes a sharp bend, which causes it to open also into the
cloaca. Between the orifice of the mouth and of the cloaca there is a
nerve-ganglion.
[Illustration: FIG. 14.—A colony of compound Ascidians (Botryllus)
growing on a piece of sea-weed (Fucus). Each star corresponds to eight or
more conjoined Ascidians. Natural size.]
My object in the next place is to show that the structure and
life-history of these Ascidians may be best explained on the hypothesis
that they are instances of degeneration; that they are the modified
descendants of animals of higher, that is more elaborate structure,
and in fact are degenerate Vertebrata, standing in the same relation
to fishes, frogs, and men, as do the barnacles to shrimps, crabs, and
lobsters.
[Illustration: FIG. 15.—Anatomy of an Ascidian (Phallusia). At the top
is the mouth, to the right the orifice of the cloaca. In the cloaca
lies an egg, and above it the oblong nerve ganglion. The perforated
pharynx follows the mouth and leads to the bent intestine which is seen
to open into the cloaca. The space around the curved intestine is the
body-cavity; in it are seen oval bodies, the eggs, and quite at the lower
end the curved heart. The root-like processes at the base serve to fix
the Ascidian to stones, shells, or weed.]
The young of some, but by no means of all these Ascidians, have a form
totally different from that of their parents. The egg of Phallusia
gives rise to a tadpole, a drawing of which placed side by side with
the somewhat larger tadpole of the common frog is seen in the adjoining
figure (Fig. 16). The young Ascidian has the same general shape as the
young frog, but not only this; the resemblance extends into details,
the internal organs agreeing closely in the two cases. Further still as
shown by the beautiful researches of the Russian naturalist, Kowalewsky,
the resemblance reaches absolute identity when we examine the way in
which the various organs arise from the primitive egg-cell. Tail, body,
spiracle, eye, and mouth, agree in the two tadpoles, the only important
difference being in the position of the two mouths and in the fact that
the Ascidian has one eye while the frog has two.
[Illustration: FIG. 16.—Tadpole of Frog and of Ascidian. Surface view.]
[Illustration: FIG. 17.—Tadpole of Frog and of Ascidian. Diagrams
representing the chief internal organs.]
Now let us look at the internal organs (Fig. 17). There are four
structures, which are _all four_ possessed at some time of their lives
by all those animals which we call the Vertebrata, the great branch of
the pedigree to which fishes, reptiles, birds, beasts, and men belong.
And the combination of these marks or structural peculiarities is an
overwhelming piece of evidence in favour of the supposition that the
creatures which possess this combination are derived from one common
ancestor. Just as one would conclude that a man whom one might meet,
say on Salisbury Plain, _must_ belong to the New Zealand race, if it
were found not only that he had the colour, and the hair, and the shape
of head of a New Zealander, but also that he was tattooed like a New
Zealander, carried the weapons of a New Zealander, and, over and above
in addition to these proofs, that he talked the Maori language and none
other; so here, in the case of the vertebrate race, there are certain
qualities and possessions, the accumulation of which cannot be conceived
of as occurring in any animal but one belonging to that race. These four
great structural features are—first, the primitive backbone or notochord;
second, the throat perforated by gill-slits; third, the tubular
nerve-centre or spinal cord and brain placed along the back; and lastly,
and perhaps most distinctive and clinching as an evidence of affinity,
the myelonic or cerebral eye.
Now let us convince ourselves that these four features exist not only
in the frog’s tadpole, as they do in all fishes, reptiles, birds, and
beasts, but that they also exist in the Ascidian tadpole, and, it may be
added, co-exist in no other animals at all.
The corresponding parts are named in Figs. 16 and 17, in such a way as to
render their agreement tolerably clear, whilst in Fig. 18 a more detailed
representation of the head of an Ascidian tadpole is given.
[Illustration: FIG. 18.—Ascidian Tadpole with a part only of the tail
_C_. _N_, nervous system with the enlarged brain in front and the narrow
spinal cord behind _n_; _N´_, is placed in the cavity of the brain:
_O_, the single cerebral eye lying in the brain; _a_, similarly placed
auditory organ; _K_, pharynx; _d_, intestine; _o_, rudiment of the mouth;
_ch_, notochord or primitive backbone. (From Gegenbaur’s “Elements of
Comparative Anatomy.”)]
It is clear then that the Ascidians must be admitted to be Vertebrates,
and must be classified in that great sub-kingdom or branch of the animal
pedigree. The Ascidian tadpole is very unlike its parent the Ascidian,
and has to go through a process of _degeneration_ in order to arrive
at the adult structure. The diagrams which are reproduced in Figs. 19
and 20, show how this degeneration proceeds. It will be observed, that
in somewhat the same manner as the young barnacle, the young Ascidian
fixes itself to a stone by its head: then the tail with its notochord
and nerve-chord atrophies. The body grows and gradually changes its
shape, whilst the cloacal chamber forms. The brain remains quite small
and undeveloped, and the remarkable myelonic eye (the eye in the brain)
disappears. The number of gill-slits increases as the animal grows in
size and its outer skin becomes tough and leather-like.
[Illustration: FIG. 19.—Degeneration of Ascidian Tadpole to form the
adult. The black pieces represent the rock or stone to which the Tadpole
has fixed its head.]
[Illustration: FIG. 20.—Very young Ascidian with only two gill-slits.
Compare with Fig 15; which is, however, seen from the other side, so that
left there corresponds to right here.]