Panmixia can not account for the reduction of the color, since it
returns in some species when they are exposed to the light, and
disappears to a certain extent in others when kept in the dark.
Panmixia, Romanes thinks, may have _helped_ to discharge the color. In
many instances the coloration is a protective adaptation, and therefore
maintained by selection. Panmixia might in such instances lower the
general average to what has been termed the “birth mean.” Proteus is
perhaps such an instance. But in this species the bleached condition
has not yet been hereditarily established, and since each individual is
independently affected “the main cause of change must have been of that
direct order which we understand by the term climatic.”
Since, however, the bleached condition, which in the first instance is
an individual reaction to the absence of light, has become hereditarily
established in Amblyopsis so that the bleaching goes on even when the
young are reared in the light, it is evident that in Amblyopsis we have
the direct effect of the environment on the individual hereditarily
established.
THE EYES OF THE AMBLYOPSIDÆ.--The structure of the eyes has formed the
basis of a separate, fully illustrated paper.[A] The prominent features
in the eyes of the various species must, however, be known before the
question of the origin of these forms and the causes of degeneration
can be seriously considered. The eyes of the species of Chologaster
are normally formed, possessing a lens, pupil, vitreous body, retina,
and optic nerve, and all the eye muscles normal to the fishes. The
eyes are functional. The retina is, however, very much simplified.
The eye of papilliferus is, in this respect, more perfect than the
eye of cornutus. In papilliferus the outer nuclear layer consists of
two series of nuclei, the inner layer of about five series of nuclei,
and the ganglionic layer of a complete single layer of nuclei except
where the optic fibers pass between them, for an optic-fiber layer
is not present. In Chologaster cornutus the outer nuclear layer has
been reduced to one or two series, and the ganglionic layer to cells
widely separated from each other or in rows and little groups, but no
longer forming a complete layer. In Amblyopsis and Typhlichthys the
largest eyes are not more than one twentieth the diameter of those
of Chologaster, or one thousandth of their bulk; the lens is nearly,
if not quite, obliterated; the same is true of the vitreous body and
the optic nerve in the adult. Beyond this the eyes differ much. In
Amblyopsis scleral cartilages are present and prominent, the pigmented
layer is prominent, the outer and inner nuclear layers form one layer
only, two or three cells deep. In T. subterraneus the pigmented layer
is insignificant, and no pigment is ever found in it, while the outer
and inner nuclear layers are still separate. In both these species the
ganglionic layer forms a central core of cells. In Amblyopsis several
or all the eye muscles are present; in Typhlichthys nothing is left of
them.
[A] Archiv f. Entwickelungsmechanik, viii, pp. 545-617, Plates
XI-XV.
Scleral cartilages are not present in Chologaster or Typhlichthys; in
Troglichthys they are very prominent, sometimes _several times as long
as the eye_. While there is no pigment left in Typhlichthys, there
is in Troglichthys. The eye in the former is about 0.168 millimetre
in diameter, while the entire eye of the latter is but about 0.050
millimetre, or less than one third the diameter, and less than one
ninth the bulk.
The entire eye of Troglichthys is smaller than many single cells, and I
shall be pardoned for not going into the details of its structure here.
[Illustration: FIG. 5.--Three views of the head of an _Amblyopsis_,
prepared to show the tactile ridges.]
[Illustration: FIG. 6.--Snout of _Chologaster papilliferus_, to show
the tactile ridges.]
THE TACTILE ORGANS.--The tactile organs are among the most important in
the consideration of the blind forms. Their minute structure will form
the basis of a separate paper. The prominent tactile organs about the
head of Amblyopsis have been mentioned by nearly every writer, and they
have been figured by Putnam-Wyman[B] and Leidig,[C] but the figures
of the distribution of the ridges are worthless. The description of
Professor Forbes[D] of Chologaster papilliferus is the only systematic
enumeration of the ridges that has appeared. The accompanying figures,
drawn by me with the _camera lucida_, and verified and copied by Mr. U.
O. Cox, give the exact extent and position of the ridges in Amblyopsis
and Chologaster papilliferus. It will be seen that in the number and
distribution of the tactile area the two forms agree very closely, the
eyed form having the same number and distribution of ridges or rows
that the blind forms have. In Chologaster papilliferus most of the
ridges are much less prominent than in the blind species, being sunk
into the skin. About the nose and chin, however, the ridges are as
prominent as in the other species. In the small Chologaster cornutus
there are no distinct ridges at all, the tactile organs being arranged
as in other species of fishes. In specimens of the same size the
papillæ are not more prominent in papilliferus than in cornutus. It is
only in the oldest of papilliferus that the papillæ become prominent.
The number of individual papillæ in each tactile ridge differs
considerably with age (size), so that an exact comparison between
the large Amblyopsis and the much smaller species of Chologaster and
Typhlichthys can not be made. From a number of counts made by Professor
Cox I take the liberty of giving the following: Ridge No. 6 contains,
in Chologaster papilliferus, six organs; in Typhlichthys, eleven; in
two specimens of Amblyopsis, respectively eighty-three and one hundred
and six inches long, twelve and twenty.
[B] American Naturalist, 1872, Plate II, Figs. 1 and 2.
[C] Untersuchungen z. Anatomie und Histologie d. Tiere, Plate
III, Fig. 28.
[D] American Naturalist, 16, 1882, p. 2.
Aside from the tactile organs in ridges, there are many solitary ones
not evident from the surface in Amblyopsis. When the epidermis is
removed by maceration, the dermal papillæ on which these rest give the
whole head a velvety appearance.
In the young, at least of Amblyopsis, each of the tactile organ of the
ridges is provided with a club-shaped filament abruptly pointed near
the end. They wave about with the slightest motion in water, and are so
numerous as to give the whole head a woolly appearance.
To recapitulate the facts ascertained concerning the eye and tactile
organs:
1. The eyes were degenerating and the tactile organs developing beyond
the normal before the permanent underground existence began.
2. The eyes continued to degenerate and the tactile organs to increase
after permanent entrance to underground waters.
3. In the degeneration of the eye the retina leads; the vitreous body
and lens follow; the more passive pigmented layer and sclera remain
longest; the bony orbit is not affected.
BEARING OF THE FACTS GAINED ON THE ORIGIN OF THE CAVE FAUNA.--The
origin of the cave fauna and of the blind fauna are two distinct
questions. This was first recognized by H. Garman. Before, the two
questions were considered as one, and two explanations are prominent
among those suggesting its solution:
1. The explanation of Lankester seems either a pleasantry or the most
unwarranted speculation. He says: “Supposing a number of some species
of Arthropod or fish to be swept into a cavern or to be carried from
less to greater depths in the sea, those individuals with perfect eyes
would follow the glimmer of light, and eventually escape to the outer
air or the shallower depths, leaving behind those with imperfect eyes
to breed in the dark place. A natural selection would thus be effected.
In every succeeding generation this would be the case, and even those
with weak but still seeing eyes would in the course of time escape,
until only a pure race of eyeless or blind animals would be left in the
cavern or deep sea.”
[Illustration: FIG. 7.--Lateral view of _Amblyopsis_, showing the
location of the tactile ridges.]
This process does not, of course, account for the degeneration of the
eye beyond blindness. But, aside from this objection, the humor of his
“glimmer of light” impresses itself very forcibly on one after spending
a day in following the devious windings of a living cave, not to
mention his tendency in cave animals, which are negatively heliotropic,
to follow it. There are other objections. Fishes are annually swept
into the caves, but they are not able to establish themselves in
them. To do this they must have peculiar habits, special methods of
feeding and mating before a successful colonization of caverns can
become successful. Further, if the origin of the cave fauna is due to
accident, the accident must have happened to four species out of six of
the Amblyopsidæ, while none of the numerous other species of fishes
about the caves met with the same accident.
2. The second explanation is that of Herbert Spencer:[E] “The existence
of these blind cave animals can be accounted for only by supposing
their remote ancestors began making excursions into the caves, and,
finding it profitable, extended them, generation after generation,
farther in, undergoing the required adaptations little by little.”
[E] Popular Science Monthly, vol. xliii, pp. 487, 488.
This second view has been modified by H. Garman in so far as he
supposes the adaptations to do without eyes and consequent degeneration
of eyes to occur anywhere where a species has no use for eyes,
enumerating burrowing animals and parasitic animals, concluding that
“the origin of the cave species (nonaquatic especially) of Kentucky
were probably already adjusted to a life in the earth before the caves
were formed.” In this modified sense, Spencer’s view is directly
applicable to the Amblyopsidæ. Hamann goes so far as to suppose that
darkness itself is not the primary cause of degeneration, but unknown
factors in the animal itself.
[Illustration: FIG. 8.--Lateral view of _Chologaster papilliferus_,
showing the location of the tactile ridges.]
The three things to be considered in this connection are (_a_) the
habit of the cave form, (_b_) the modifications to enable the form to
do without the use of light, and (_c_) the structure of the eye as a
result of _a_ and _b_.
_a._ The prime requisite for a candidate to underground existence is a
negative reaction to light. We found that even the epigæan Chologaster
is negatively heliotropic.
_b._ It must also be evident that a fish depending on its sight to
procure its food can never become a cave form. Sunfishes, which are
annually carried into caves, belong to this class of fishes. They
are always poor when found in the caves, and will never be able to
establish themselves in them. On the other hand, there are no reasons
why fishes detecting their prey either by smell or touch should not
be capable of colonizing caves. The catfishes and Amblyopsidæ belong
to the latter class. It is surprising that more catfishes have not
established themselves in caves. Among the Amblyopsidæ even those
with functional eyes depend on touch and vibrations for their food.
Chologaster has well-developed tactile organs and poor eyes. It is
found chiefly at the mouths of underground streams, but also in the
underground streams themselves. The tactile organs are not different
in kind from those of other fishes, and their high development is not
more marked than their development in the barbels of the catfishes.
The characters which distinguish Chologaster as a fish capable of
securing its food in the dark are emphasized in Typhlichthys, and the
tactile organs are still more highly developed in Amblyopsis. The
eyes of the last two genera are so degenerate that it is needless in
this connection to speak of degrees of degeneration. On account of
the structure of their eyes and their loss of protective pigment they
are incapable of existence in open waters. With the partial and total
adaptation to an underground existence in the Amblyopsidæ and their
negative reaction to light, it is scarcely possible that for this
family the idea of accidental colonization can be entertained for a
moment. Their structure is not as much due to their habitat as their
habitat is to their structure and habit.
Typhlogobius lives in the holes of shrimps under rocks on the coast
of southern California. It is a living example of the origin of blind
forms in dark places remote from caves. Here again the “accidental”
idea is preposterous, since no fish could by accident be carried
into the devious windings of the burrows they inhabit. Moreover, a
number of related species of gobies occur in the neighborhood. They
live ordinarily in the open, but always retreat into the burrows of
crustaceans when disturbed. The origin of the blind species by the
gradual change from an occasional burrow seeker to a permanent dweller
in the dark and the consequent degeneration of the eye is evident here
at once. Among insects the same process and the same results are noted.
We have everywhere the connection of diurnal species with dark-loving
and blind forms, a transition the result of habit entered into with
intent, but no evidence of such a connection as the result of accident.
Also numerous instances of daylight species being swept into caves, but
no instance of one establishing itself there.
This view accounts also for the wide distribution of the blind fishes.
The ancestry of the Amblyopsidæ we may assume to have had a tendency
to seek dark places wherever found, and incipient blind forms would
thus arise over their entire distribution. The structural differences
between Troglichthys and Typhlichthys argue in favor of this, and
certainly the fearless, conspicuous blind fish as at present developed
would have no chance of surviving in the open water. Their wide
distribution after their present characters had been assumed, except
through subterranean waters, would be out of the question entirely.
The same would not be true of the incipient cave forms when they
had reached the stage at present found in Chologaster. It will be
recalled that Chologaster, and even the blind forms, have the habit
of hiding underneath boards and in the darker sides of an aquarium.
These dark-seeking creatures would, on the other hand, be especially
well fitted to become distributed in caves throughout their habitat.
S. Garman’s able argument for the single origin and dispersal of the
blind fishes through epigæan waters was based on the supposition that
the cis-Mississippi and trans-Mississippi forms were identical. The
differences between these species are such as to warrant not only that
they have been independently segregated, but that they are descended
from different genera. The external differences between these species
are trifling, but this was to be expected in an environment where all
the elements that make for external color marking are lacking. The
similarity between Typhlichthys and Amblyopsis is so great that the
former has been considered to be the young of the latter.
Judging from the structure of the eye and the color of the skin,
Troglichthys has been longest established in caves. Amblyopsis came
next, and Typhlichthys is a later addition to the blind cave fauna.
* * * * *
“Those,” said Dr. J. N. Langley, in his sectional address on
Physiology at the British Association, “who have occasion to
enter into the depths of what is oddly, if generously, called the
literature of a scientific subject, alone know the difficulty
of emerging with an unsoured disposition. The multitudinous
facts presented by each corner of Nature form in large part the
scientific man’s burden to-day, and restrict him more and more,
willy nilly, to a narrower and narrower specialism. But that is not
the whole of his burden. Much that he is forced to read consists of
records of defective experiments, confused statement of results,
wearisome description of detail, and unnecessarily protracted
discussion of unnecessary hypotheses. The publication of such
matter is a serious injury to the man of science; it absorbs the
scanty funds of his libraries, and steals away his poor hours of
leisure.”