Water-beetle’s larva. All the cells are seen to be in a row continuous
with _h_, the cells of the outermost skin or ectoderm. _p_, pigmented
cells; _r_, retinal cells connected at _o_ with the optic nerve; _g_,
transparent cells (forming a kind of “vitreous body”); _l_, cuticular
lens. (From Gegenbaur’s “Elements of Comparative Anatomy,” after
Grenacher.)]
Before saying anything further on the subject of degeneration, it seems
desirable once more to direct attention to the myelonic or cerebral eye
which the Ascidian tadpole possesses in common with all Vertebrates. All
other animals which have eyes develop the retina or sensitive part of
the eye from their _outer skin_ (See Figs. 21 and 22, and explanation.)
It is easy to understand that an organ which is to be affected by the
light should form on the _surface_ of the body where the light falls.
It has long been known as a very puzzling and unaccountable peculiarity
of Vertebrates, that the retina or sensitive part of the eye grows
out in the embryo as a bud or vesicle of the brain, and thus forms
_deeply_ below the surface and _away from the light_ (see Fig. 23,
and explanation). The Ascidian tadpole helps us to understand this,
for it is perfectly transparent and has its eye actually _inside_
its brain. The light passes through the transparent tissues and acts
on the pigmented eye, lying deep in the brain. We are thus led to the
conclusion—and I believe this inference to be now for the first time
put into so many words—that the original Vertebrate must have been a
transparent animal, and had an eye or pair of eyes _inside_ its brain,
like that of the Ascidian tadpole. As the tissues of this ancestral
Vertebrate grew denser and more opaque, the eye-bearing part of the brain
was forced by natural selection to grow outwards towards the surface,
in order that it might still be in a position to receive the influence
of the sun’s rays. Thus the very peculiar mode of development of the
Vertebrate eye from two parts, a brain-vesicle (Fig. 23, _A a_, and _B p
r_), and a skin-vesicle (Fig. 23, _B e, l_), is accounted for.
[Illustration: FIG. 22.—Section through the eye (“surface-eye”) of a
Marine Worm (Neophanta). _i_, integument spreading over the front of the
eye _c_; _l_, cuticular lens; _h_, cavity occupied by vitreous body; _p_,
retinal cells; _b_, pigment; _o_, optic nerve: _o´_, expansion of optic
nerve.]
[Illustration: FIG. 23.—_A._ Vertical section through the head of a very
young fish, showing in the centre the cavity of the brain _c_. On each
side is a hollow outgrowth (_a_) which will form the retina of the fish’s
eye (“cerebral eye”); _b_, will become the optic nerve connecting the
brain and the retina; _d_, integument.—_B._ Later condition of the hollow
outgrowth (_a_) of _A_. Its outer wall _r_ is pressed against its deeper
wall _p_ by an ingrowth (_l_) from the outer skin (ectoderm) _e_; _r_,
gives rise to the retinal cells, whilst only _l_, the cellular lens, is
derived from the surface of the skin.]
The cases of degeneration which I have up to this point brought forward,
are cases which admit of very little dispute or doubt. They are attested
by either the history of the individual development of the organisms in
question, as in Sacculina, in the Barnacle, and in the Ascidian, or they
are cases where the comparison of the degenerate animal, with others
like it in structure, but not degenerate, renders the hypothesis of
degeneration an unassailable one. Such cases are the Acarus or mite, and
the skin-worm (Demodex).
We have seen that degeneration, or the simplification of the general
structure of an animal, may be due to the ancestors of that animal having
taken to one of two new habits of life, either the parasitic or the
immobile. Other new habits of life appear also to be such as to lead to
degeneration. Let us suppose a race of animals fitted and accustomed to
catch their food, and having a variety of organs to help them in this
chase—suppose such animals suddenly to acquire the power of feeding
on the carbonic acid dissolved in the water around them just as green
plants do. This would lead to a degeneration; they would cease to hunt
their food, and would bask in the sunlight, taking food in by the whole
surface, as plants do by their leaves. Certain small flat worms, by name
Convoluta, of a bright green colour, appear to be in this condition.
Their green colour is known to be the same substance as leaf-green; and
Mr. Patrick Geddes has recently shown that by the aid of this green
substance they feed on carbonic acid, making starch from it as plants
do. As a consequence we find that their stomachs and intestines as well
as their locomotive organs become simplified, since they are but little
wanted. These vegetating animals, as Mr. Geddes calls them, are the exact
complement of the carnivorous plants, and show how a degeneration of
animal forms may be caused by _vegetative nutrition_.
Another possible cause of degeneration appears to be the indirect one of
minute size. It cannot be doubted that natural selection has frequently
acted on a race of animals so as to reduce the size of the individuals.
The smallness of size has been favourable to their survival in the
struggle for existence, and in some cases they have been reduced to even
microscopic proportions. But this reduction of size has, when carried
to an extreme, resulted in the loss or suppression of some of the most
important organs of the body. The needs of a very minute creature are
limited as compared with those of a large one, and thus we may find
heart and blood-vessels, gills and kidneys, besides legs and muscles,
lost by the diminutive degenerate descendants of a larger race. That
this is a possible course of change all will, I think, admit. It is
actually exemplified in Appendicularia—the only adult representative
of the Ascidian tadpole—still tadpole-like in form and structure, but
curiously degenerate and simplified in its internal organs. This kind of
degeneration is also exemplified in the Rotifers, or wheel animalcules,
in the minute Crustacean water-fleas (Ostracoda), and in the Moss-polyps,
or Polyzoa. Roughly then we may sum up the immediate antecedents of
degenerative evolution as, 1, Parasitism; 2, Fixity or immobility;
3, Vegetative nutrition; 4, Excessive reduction of size. This is not
a logical enumeration, for each of these causes involves, or may be
inseparably connected with, one or more of the others. It will serve for
the present as well as a more exhaustive analysis. (See Note C.)
And now we have to note an important fact with regard to the _evidence_
which we can obtain of the occurrence of this process of degeneration. We
have seen that the most conclusive evidence is that of the recapitulative
development of the individual. The Ascidian Phallusia shows itself to
be a degenerate Vertebrate by beginning life as a tadpole. But such
recapitulative development is by no means the rule. Quite arbitrarily,
we find, it is exhibited in one animal and not in a nearly allied kind.
Thus very many animals belonging to the Ascidian group have no tadpole
young—just as some tree-frogs have no tadpoles. It is quite possible,
and often, more often than not, occurs, that _the most important part_
of the recapitulative phases are absent from the developmental history
of an animal. The egg proceeds very rapidly to produce the adult form,
and all the wonderful series of changes showing the animal’s ancestry are
absolutely and completely omitted; that is to say, all those stages which
are of importance for our present purpose. Just as certain bodies pass
from the solid to the liquid state at a bound, omitting all intermediate
phases of consistence, but giving evidence of “internal work” by the
suggestive phenomenon of latent heat—so do these embryos skip long tracts
in the historically continuous phases of form, and present to us only the
intangible correlative “internal work” in place of the tangible series of
embryonic changes of shape.
Now I want to put this case—a supposition—before the reader who has so
far followed me in these pages. Suppose, as might well have happened,
that the Barnacles, one and all, instead of recapitulating in their early
life, were to develop _directly from the egg to the adult form_, as so
many animals do; should we have ever made out that they were degenerate
Crustaceans? Possibly we should: their adult structure still bears
important marks of affinities with crabs and shrimps; but as a matter of
fact before their recapitulative development had been discovered they
were classed by the great Cuvier and other naturalists with the Molluscs,
the mussels and snails.
Suppose again that _all_ the existing Ascidians, as many of them actually
have, had long ago lost their recapitulative history in growth from the
egg: suppose that no such a thing as an Ascidian tadpole existed, but
that the Ascidian’s egg grew as directly as possible into an Ascidian, in
every living species of the group. This might easily be the case. Then
most assuredly we should not have the least notion that the Ascidians
were degenerate Vertebrates. We should still class them where they used
to be classed before the Russian naturalist Kowalewsky discovered the
true history and structure of the Ascidian tadpole. I believe that I
shall have the assent of every naturalist when I say that the vertebrate
character of the Ascidians and the history of their degeneration
would never have been suspected, or even dreamed of, had the Ascidian
tadpoles ceased to appear in the course of the Ascidian development at a
geological period anterior to the present epoch.
This being the case, it must be admitted that it is quite _possible_—I
do not say more than possible—that other groups of animals besides
parasites, Barnacles, and Ascidians, are degenerate. It is quite possible
that animals with considerable complexity of structure, at least as
complex as the Ascidians, may have been produced by degeneration from
still more highly-organized ancestors. Any group of animals to which we
can turn may possibly be the result of degeneration, and yet offer no
evidence of that degeneration in its growth from the egg.
Accordingly, wherever we can note that a group of organisms is
characterized by habits likely to lead to degeneration, such as I have
enumerated, viz., parasitism or immobility, or certain special modes
of nutrition, or again, by minute size of its representatives—there
we are justified in applying the hypothesis of degeneration, even in
the absence of any confirmatory evidence from embryology. When we so
apply this hypothesis we find in not a few cases, in working over the
details of the organization of many different animals by the light
which it affords—that much becomes clear and assignable to cause which,
on the hypothesis either of “balance” or of “elaboration,” is quite
hopelessly obscure. As examples of groups of animals which can thus be
satisfactorily explained I may cite first of all the Sponges: as only
somewhat less degenerate, we have all the Polyps and Coral-animals, also
the Starfishes. Amongst the Mollusca—the group of headless bivalves, the
oysters, mussels and clams, known as the Lamellibranchs, are, when one
once looks at their structure in this light, clearly enough explained
as degenerated from a higher type of head-bearing active creatures like
the Cuttle-fish; whilst the Polyzoa or Moss-polyps stand in precisely
the same kind of relation to the higher Mollusca as do the Ascidians to
the higher Vertebrates: they have greatly degenerated, and become minute
encrusting organisms which, like some of the Ascidians, build up colonies
by plant-like budding growth. The Rotifers, or wheel animalcules, I have
already mentioned as best explained by the supposition that they are the
descendants of far larger and more fully-organized animals provided with
locomotive appendages or limbs: they have dwindled and degenerated to
their present minute size and curiously suggestive structure.
Besides these there are other very numerous cases of animal structure
which can best be explained by the hypothesis of degeneration. A
discussion of these, and a due exposition of the application of the
hypothesis of degeneration to the various groups just cited, would
involve a complete treatise on comparative anatomy and embryology, and
lead far beyond the limitations of this little volume.
All that has been, thus far, here said on the subject of Degeneration is
so much zoological specialism, and may appear but a narrow restriction of
the discussion to those who are not zoologists. Though we may establish
the hypothesis most satisfactorily by the study of animal organization
and development, it is abundantly clear that degenerative evolution
is by no means limited in its application to the field of zoology. It
clearly offers an explanation of many vegetable phenomena, and is already
admitted by botanists as the explanation of the curious facts connected
with the reproductive process in the higher plants. As a further example
of its application in this field, the yeast-plant may be adduced, which
is in all probability a degenerate floating form derived from a species
of mould (Mucor). In other fields, wherever in fact the great principle
of evolution has been recognised, degeneration plays an important part.
In tracing the development of languages, philologists have long made
use of the hypothesis of degeneration. Under certain conditions, in the
mouths and minds of this or that branch of a race, a highly elaborate
language has sometimes degenerated and become no longer fit to express
complex or subtle conceptions, but only such as are simpler and more
obvious. (See Note D.)
The traditional history of mankind furnishes us with notable examples of
degeneration. High states of civilisation have decayed and given place to
low and degenerate states. At one time it was a favourite doctrine that
the savage races of mankind were degenerate descendants of the higher and
civilised races. This general and sweeping application of the doctrine
of degeneration has been proved to be erroneous by careful study of the
habits, arts, and beliefs of savages; at the same time there is no doubt
that many savage races as we at present see them are actually degenerate
and are descended from ancestors possessed of a relatively elaborate
civilisation. As such we may cite some of the Indians of Central America,
the modern Egyptians, and even the heirs of the great oriental monarchies
of præ-Christian times. Whilst the hypothesis of universal degeneration
as an explanation of savage races has been justly discarded, it yet
appears that degeneration has a very large share in the explanation
of the condition of the most barbarous races, such as the Fuegians,
the Bushmen, and even the Australians. They exhibit evidence of being
descended from ancestors more cultivated than themselves.
With regard to ourselves, the white races of Europe, the possibility of
degeneration seems to be worth some consideration. In accordance with a
tacit assumption of universal progress—an unreasoning optimism—we are
accustomed to regard ourselves as necessarily progressing, as necessarily
having arrived at a higher and more elaborated condition than that which
our ancestors reached, and as destined to progress still further. On the
other hand, it is well to remember that we are subject to the general
laws of evolution, and are as likely to degenerate as to progress. As
compared with the immediate forefathers of our civilisation—the ancient
Greeks—we do not appear to have improved so far as our bodily structure
is concerned, nor assuredly so far as some of our mental capacities are
concerned. Our powers of perceiving and expressing beauty of form have
certainly _not_ increased since the days of the Parthenon and Aphrodite
of Melos. In matters of the reason, in the development of intellect, we
may seriously inquire how the case stands. Does the reason of the average
man of civilised Europe stand out clearly as an evidence of progress when
compared with that of the men of bygone ages? Are all the inventions and
figments of human superstition and folly, the self-inflicted torturing of
mind, the reiterated substitution of wrong for right, and of falsehood
for truth, which disfigure our modern civilisation—are these evidences of
progress? In such respects we have at least reason to fear that we may
be degenerate. Possibly we are all drifting, tending to the condition
of intellectual Barnacles or Ascidians. It is possible for us—just as
the Ascidian throws away its tail and its eye and sinks into a quiescent
state of inferiority—to reject the good gift of reason with which every
child is born, and to degenerate into a contented life of material
enjoyment accompanied by ignorance and superstition. The unprejudiced,
all-questioning spirit of childhood may not inaptly be compared to the
tadpole tail and eye of the young Ascidian: we have to fear lest the
prejudices, pre-occupations, and dogmatism of modern civilisation should
in any way lead to the atrophy and loss of the valuable mental qualities
inherited by our young forms from primæval man.
There is only one means of estimating our position, only one means of
so shaping our conduct that we may with certainty avoid degeneration
and keep an onward course. We are as a race more fortunate than our
ruined cousins—the degenerate Ascidians. For us it is possible to
ascertain what will conduce to our higher development, what will favour
our degeneration. To us has been given the power to _know the causes of
things_, and by the use of this power it is possible for us to control
our destinies. It is for us by ceaseless and ever hopeful labour to try
to gain a knowledge of man’s place in the order of nature. When we have
gained this fully and minutely, we shall be able by the light of the past
to guide ourselves in the future. In proportion as the whole of the past
evolution of civilised man, of which we at present perceive the outlines,
is assigned to its causes, we and our successors on the globe may expect
to be able duly to estimate that which makes for, and that which makes
against, the progress of the race. The full and earnest cultivation of
Science—the Knowledge of Causes—is that to which we have to look for the
protection of our race—even of this English branch of it—from relapse and
degeneration.
FOOTNOTES
[1] These pages formed a discourse delivered before the British
Association at Sheffield on the evening of August 22nd, 1879, under the
presidency of Professor Allman, LL.D., F.R.S.
[2] See Note A.
[3] See Note B.
[4] Der Ursprung der Wirbelthiere und das Princip des Functionswechsels.
Leipzig, 1875.
NOTES.
A.
“Die Phantasie ist ein unentbehrliches Gut; denn sie ist es, durch
welche neue Combinationen zur Veranlassung wichtiger Entdeckungen
gemacht werden. Die Kraft der Unterscheidung des isolirenden Verstandes
sowohl, als der erweiternden und zum allgemeinen strebenden Phantasie
sind dem Naturforscher in einem harmonischen Wechselwirken nothwendig.
Durch Störung dieses Gleichgewichts wird der Naturforscher von der
Phantasie zu Träumereien hingerissen, während diese Gabe den talentvollen
Naturforscher von hinreichender Verstandesstärke zu den wichtigsten
Entdeckungen führt.”—JOHANNES MÜLLER, _Archiv für Anatomie_, 1834.
B.
To many persons the conclusion that man is the naturally modified
descendant of ape-like ancestors appears to be destructive of the
belief in an immortal soul, and in the teachings of Christianity; and
accordingly they either reject Darwinism altogether, or claim for man a
special exemption from the mode of origin admitted for other animals.
It seems worth while, in order to secure a calm and unprejudiced
consideration for the teachings of Darwinism, to point out to such
persons that, as a matter of fact, whatever views we may hold with
regard to a soul and the Christian doctrines, they cannot be in the
smallest degree affected by the admission that man has been derived from
ape-like ancestors by a process of natural selection, so long as the
demonstrable fact, not denied by any sane person, is admitted, namely,
that every individual man grows by a process of natural modification
from a homogeneous egg-cell or corpuscle. Assuredly it cannot lower our
conception of man’s dignity if we have to regard him as “the flower of
all the ages,” bursting from the great stream of life which has flowed on
through countless epochs with one increasing purpose, rather than as an
isolated, miraculous being, put together abnormally from elemental clay,
and cut off by such portentous origin from his fellow animals, and from
that gracious Nature to whom he yearns with filial instinct, knowing her,
in spite of fables, to be his dear mother.
A certain number of thoughtful persons admit the development of man’s
body by natural processes from ape-like ancestry, but believe in the
non-natural intervention of a Creator at a certain _definite_ stage in
that development, in order to introduce into the animal which was at
that moment a man-like ape, something termed “a conscious soul,” in
virtue of which he became an ape-like man. It appears to me perfectly
legitimate and harmless for individuals to make such an assumption if
their particular form of philosophy or of religion requires it. Such an
assumption does not in any way traverse the inferences from facts to
which Darwinism leads us; at the same time zoological science does not,
and cannot be expected to, give any support to such an assumption. The
gratuitous and harmless nature of the assumption so far as zoological
science is concerned, and accordingly the baselessness of the hostility
to Darwinism of those who choose to make it, may be seen by the
consideration of a parallel series of facts and assumptions, which puts
the matter clearly enough in its true light.