Popular Science Monthly/Volume 36/January 1890/The Effect of Cave Life on Animals, and its Bearing on the Evolution Theory
|THE EFFECT OF CAVE LIFE ON ANIMALS, AND ITS BEARING ON THE EVOLUTION THEORY.|
THE main interest in studies on cave life centers in the obvious bearing of the facts upon the theory of descent. The conditions of existence in caverns, subterranean streams, and deep wells are so marked and unlike those which environ the great majority of organisms, that their effects on the animals which have been able to adapt themselves to such conditions at once arrest the attention of the observer. To such facts as are afforded by cave life, as well as parasitism, the philosophic biologist naturally first turns for the basis of his inductions and deductions as to the use and disuse of organs in inducing their atrophy. It is comparatively easy to trace the effects of absence of light on animals belonging to genera, families, or orders in which eyes are normally almost universally present. As we have seen in non-cavernicolous animals, the eyes are wanting from causes of the same nature as have induced their absence in true cave animals. No animal or series of generations of animals, wholly or in part, lose the organs of vision unless there is a physical appreciable cause for it. "While we may never be able to satisfactorily explain the loss of eyes in certain deep-sea animals from our inability to personally penetrate to the abysses of the sea, we can explore caves at all times of day and night, of winter and summer; we can study the egg-laying habits of the animals, and their embryonic development; we can readily understand how the caves were colonized from the animals living in their vicinity; we can nicely estimate the nature of their food, and its source and amount, as compared with that accessible to out-of-door animals; we can estimate with some approach to exactitude the length of time which has elapsed since the caves were abandoned by the subterranean streams which formed them and became fitted for the abode of animal life. The caves in southern Europe have been explored by more numerous observers than those of this country, and the European cave fauna is richer than the American, but the conditions of European cave life and the effects of absence of light and the geological age of the cave fauna are like those of American caves. Moreover, the cave life of New Zealand and the forms there living in subterranean passages and in wells show that animal life in that region of the earth has been affected in the same manner. The facts seem to point to the origin of the cave forms from the species now constituting a portion of the present Quaternary fauna; hence they are of very recent origin.
The result of cave exploration shows that no plants, even the lowest fungi, with the exception of Oozonium auricomum Link, and perhaps one or two other kinds of fungi common to Europe and America in and out of caves, can so adapt themselves as to live and propagate their species in the total darkness of caverns. They are far more dependent on the influence of light than animals.
"We will now briefly rehearse the facts relating to the changes in structure and color undergone by animals adapted to a life in total darkness in caves, premising that, so far as we know, the Protozoa detected in subterranean waters do not essentially differ from those living in the light. It appears from the following facts that eyeless animals change their color as well as those having eyes:
1. A sponge (Spongilla stygia) found by Dr. Joseph in the waters of Carniolan grottoes, instead of being green, is pellucid and bleached.
2. The Hydra (H. pellucida), also found by Dr. Joseph in the subterranean lakes of Carniola, was, as its name indicates, neither green nor brown, like the two species of the upper world, but pellucid, bleached out, or colorless.
Such was also found by Dr. Joseph to be the case with the smaller crustaceans, such as certain cave species of Cypris, Leptodera, Estheria, and Branchipus (B. pellucidus Jos.).
3. As regards change of color, we do not recall an exception to the general law that all cave animals are either colorless or nearly white, or, as in the case of Arachnida and insects, much paler than their out-of-door relatives.
The worms (planarians and earth-worms) are somewhat paler than their allies living out of caves, but as the normal environment of most planarians and earth-worms is much like those of cave animals, the difference is not so marked, though both of our cave planarian worms are white and eyeless.
All the cave Crustacea, both aquatic and terrestrial, are colorless or whitish, more or less vitreous, and pellucid, the pigment cells being degenerate and functionless. The effects of total darkness seem quite different from the influence to which the eyeless deep-sea Crustacea are exposed, since they, like their fellows with eyes normal or hypertrophied, are said to be of the same flesh and reddish tints common to deep-sea animals.
In the case of the cavernicolous myriapods the bleaching of the body is very marked. In out-of-door myriapods the normal tint of the integument is brown or rarely amber-brown; but the color of the cavernicolous species is white or flesh-white, like a freshly molted myriapod of normal habitat.
The cave species of Arachnida are usually whitish or pale amber-colored, or pale horn, with a reddish tint. Of the mites, some are white, others horn-color, or chitinous. In the family Chernetidæ the cave species are "dull white," or "pale horn with a reddish tint," or "pale yellowish."
The effects upon the eyes and optic lobes of a life in total darkness are the following:
1. Total atrophy of optic lobes and optic nerves, with or without the persistence in part of the pigment or retina and the crystalline lens (Cæcidotæa, Crangonyx, Chthonius, Adelops, Pseudotremia).
2. Persistence of the optic lobes and optic nerves, but total atrophy of the rods and cones, retina (pigment) and facets (Orconectes).
3. Total atrophy of the optic lobes, optic nerves, and all the optic elements, including rods and cones, retina (pigment) and facets (Anophthalmus, Scoterpes, and? Anthrobia).
An interesting fact confirmatory of the theory of occasional rapid evolution, as opposed to invariably slow action involved in pure Darwinism, is that we never find any rudiments of the optic lobes or optic nerves; if they are wanting at all, they are totally. abolished; there is not a series of individuals with these organs in different degrees of development corresponding to the rudimentary conditions of the eye. The atrophy is comparatively rapid, sudden, and wholesale. On the other hand, we have series, as in Cæcidotæa or Chthonius, where there is but a single, or two or three, or several crystalline lenses, partially enveloped in pigment.
These varying degrees of development in the peripheral parts of the eye prove that the animals entered the caves at different periods, and have been exposed for different lengths of time to the loss of light. For example, those individuals of Chthonius Packardii which live in the Labyrinth of Mammoth Cave are eyeless, or have merely pigment spots; those collected in the Rotunda (which is much nearer the entrance to the cave) have eyes, or at least lenses and a retina. While most individuals of the Cæcidotæa are eyeless, a few have rudimentary eyes. Thus, in the differing conditions of the eyes in different individuals, we have an epitome of the developmental history of the genus Cæcidotæa and its species. Certain Aselli borne into caves or introduced into subterranean streams feeding deep, dark wells, losing the stimulus of the light, begin to lose their eyes and the power of sight. The first step is the decrease in the number of facets and corresponding lenses and retina; after a few generations—perhaps in four or five—the facets become reduced to only four or five; the eye is then useless; then all at once, perhaps after only two or three generations, as a result of disuse, there is a failure in forming images on the retina, and those complicated, elaborate structures, the optic ganglion and optic nerve, suddenly break down and are absorbed, though the external eye still exists in a rudimentary state. These imperfect lenses and retinæ, like all rudimentary organs throughout the animal world, are like ancient, decayed sign-posts, pointing out some nearly obliterated path now unworn and disused. The result of change of environment, with disuse and atrophy of the organs of vision, together with the inheritance of these defects and their establishment as fixed specific and even generic characters, results in the creation of a new natural genus with its assemblage of species, and, if we include all the cave animals thus produced, the creation of a new fauna. It would be a thorough test of the theory of descent if we could keep these creatures in confinement, exposed first to twilight and then to the full light of day, and endeavor to breed a few generations of these blind animals and ascertain whether their descendants would not revert to the original ancestral eyed forms. The Cæcidotæa would perhaps be the best subject for such an experiment; it is so abundant and easy to breed. That the Cæcidotæa has been evolved from some species of Asellus hardly admits of a doubt. Our Asellus communis abounds under sticks and stones, submerged boards and logs, throughout the Northern and central States. Thence it could readily be carried, in cavernous regions like those of southern Illinois, Indiana, and Kentucky, into subterranean streams. The supply must be very great, as the individuals of C. stygia are very abundant; indeed, so far as we know, as much as or even more so than those of Asellus communis.
In the blind crayfish of the caverns of Indiana and Kentucky, and of the similar species (C. hamulatus) inhabiting the Nickajack Cave of Tennessee, we have two aberrant forms belonging to a widely diffused group, whose center of distribution lies in the Mississippi Valley, and which is rich in species and in individuals. All the streams and ditches situated over or near the caves are densely populated with crayfish. I was interested, after finding C. pellucidus in a stream flowing through the Bradford Cave, near New Albany, Indiana, to find the common eyed crayfish of that region in great abundance a few yards from the mouth, outside of the cave, in the shallow brook issuing from the cave itself. That crayfish with eyes can readily enter a cave—probably in time of freshets—is proved by the fact that Cambarus Bartonii is often found in Mammoth Cave, where it finds food; and a small specimen has been found by Mr. Putnam a little paler than usual—i. e., as pale as the darker specimens of C. pellucidus—but the eyes were normal, though it is doubtful if it lives long enough in the cave to breed there.
The nearest out-of-door ally of Cambarus pellucidus is Cambarus affinis. On the other hand, the nearest lucicolous ally of C. hamulatus is perhaps C. latimanus.
It is instructive to find that, in regard to the development of the eyes, and the slenderness, size, and color of the body, these two cave crayfish closely resemble each other, though obviously originating, as Prof. Faxon states, from species belonging to quite different sections of the genus Cambarus, and to a different, more southern, river valley. These facts appear to prove beyond question that the cave species of crayfish in the United States have descended from quite different species of Cambarus, belonging to different zoögeographical areas. Had the two species of blind crayfish been produced instantaneously by special creation, as popularly supposed and advocated in the past by some naturalists, why should the accessory genital organs (gonopoda) differ so much that on this account they belong to different sections of the genus Cambarus?
The cave Phalangidæ, or harvest-men, whose habits and distribution in Europe as well as the United States, both as regards lucicolous and cavernicolous forms, have been given in much detail elsewhere, illustrate clearly the theory that certain subterranean forms, living deep in the soil, under stones in the cave regions of both hemispheres, especially in France and Austria, have been carried into caves, have survived the loss of out-of-door conditions, becoming adapted to the new and strange environment, losing their eyes totally or in part from disuse of those organs, and have bred true to the new specific characters thus established, and are now as unchangeable as the physical conditions in which they live.
The cave spiders in all important respects exemplify the same rule. They belong to, or are closely allied to, genera rich in species in the cavernous regions they inhabit, and which live in dark places. Although scarcely necessary in its changed environment, where there are no hydrographic changes, no winter and summer, and few enemies to contend with, the most aberrant form, the completely eyeless Anthrobia of Mammoth Cave, still spins a silk cocoon around its eggs; while in Weyer's Cave Nesticus pellidus Emerton spins a cocoon for its eggs; and either this species or its fellow troglodyte, Linyphia incerta Emerton, or both species, spin a weak, irregular web, consisting of a few threads. Is not this a useless habit, a simple survival of ancestral traits?
It was noticed that the number of individuals of different species was greater in the smaller shallower caves, such as the Weyer and Carter Caverns; each of these groups of caves has three species, while in Mammoth Cave there is but one, and the individuals are less common. Moreover, all are darker than Anthrobia, all have eyes, and the number of eyes is variable. These facts show that Anthrobia and the eyed forms have originated from species living in partial darkness at or near the mouths of the caverns. In Mr. Emerton's description of Linyphia incerta it will be seen how variable are the number of eyes. From this it may be inferred that the specific characters of this form, as regards the eyes at least, have not been firmly established, and hence it has only recently become a true troglodyte.
In the foregoing examples we have as yet not discovered in this country any connecting links between the eyed and blind or eyeless species of cave animals. But in a series of specimens of a cave myriapod, Pseudotremia cavernarum, which is abundant in the Wyandotte and Carter Caves, we have what we regard as good, if not complete, evidence that this cave form has directly originated from a common and widely distributed out-of-door form. The cave Pseudotremia has black eyes, composed of from twelve to fifteen facets arranged in a triangular area; of one hundred and fifty specimens examined none were found to be eyeless. In a large cave like Wyandotte there is little variation in this species as regards size, proportion, or color (being white with a slight flesh tint). But in Bradford Cave, a grotto in Indiana, only three hundred to four hundred yards deep, where the conditions are naturally more variable, the species likewise varied more in proportion of parts, and in respect to the eyes, which were more rudimentary, while the individuals were. whiter.
We have attempted to show that the only known species of the myriapod genus Pseudotremia has been derived from the widely diffused Lysiopetalum lactarium (Say); it differs in having only about half as many segments as in its out-of-door parent form (this diminution in the number of segments being, due to arrest of development); in the smaller, rudimentary eyes, while the antennse are slenderer and longer. Now, in the Carter Caves of eastern Kentucky we found specimens which prove to us that the cave form is only a modified L. lactarium. In those caves Pseudotremia cavernarum is only partly bleached, being brownish; the eyes are larger, having from twenty-three to twenty-five facets; and the general appearance of the specimens is such, especially the prominent ridges on the latero-dorsal tubercles, that the specimens might be mistaken for pale, partly bleached L. lactarium; yet the variety (Carterensis) is true to its generic character, having half as many segments as in Lysiopetalum. Why the number of body segments should be so greatly diminished in the cave form is only explicable on the ground that it is due to an arrest of development, or that the cave form has descended from some unknown species of Lysiopetalum, with half the number of segments as L. lactarium.
In like manner the Mammoth Cave hairy myriapod, Scoterpes Copei, was evidently derived from some species of the hairy genus Trichopetalum. Scoterpes has no trace of eyes, and differs from Trichopetalum in the longer legs and slightly longer and slenderer antennæ. There is no reasonable doubt but that Scoterpes is a bleached Trichopetalum which has lost its eyes, and consequently has longer legs. Some systematists may yet refer it to Trichopetalum, to which it has the same relations as Anophthalmus to Trechus. It should be observed that several myriapods found in twilight within the mouths of caves, such as species of Polydesmus and Cambala, are more or less bleached, showing the change wrought by a life in partial darkness after a limited number of generations.
The Podurans afford instances of the modification of color especially. Whether living in caves in the central States or in Utah, the common cosmopolitan Tomocerus plumbeus is bleached, retaining its eyes, though they are of diminished size. This is, however, rather a twilight than a true cave species.
The beetles of the genera Anophthalmus and Adelops are the best-known examples of cave animals. The Adelops of Mammoth Cave and a few adjoining caves—the only species in this country of the genus—is blind, but possesses rudiments of the outer eye, several corneal lenses surviving. On the other hand, the species of this or the closely allied representative genus Bathyscia, to which they are now referred by Dr. Horn, are very numerous in Europe, and are scavengers in habit. Bedel, in his list of the cave insects of Europe (1875), states that sixty-five species are known, and that several others were known but not described, and that probably further explorations in the region of the Pyrenees, both in France and Spain, will lead to further discovery of species. It appears that not all the species live in caves, but occur in the open air under large stones, moss, vegetable detritus, or at the entrances to caves. It is apparent, then, that the cave animals are emigrants from out of doors, and that the cave species, by isolation from the light and from interbreeding with out-of-door forms, as well as by adaptation to total darkness, have become fixed species with separate generic characters.
Equally instructive and explanatory of the origin of cave animals in general is the genus Ariophthalmus. In the caverns of the central United States there are only eight species, and none occur elsewhere in America, though we have two or three species of Trechus, one at least not infrequent, and Trechus micans is common to both hemispheres. Not alone loss of sight and eyes, but other modifications of the body, legs, and antennae, evidently the result of loss of sight, occur, so universal is the modification of the organism. It is evident that southern Europe is the zoögeographical center of this subgenus, for sixty-four species of completely eyeless beetles referred to this genus have already been discovered in the caves of Austria, Italy, France, and Spain. Lately, however, owing to the studies of Putzeys, and especially of De Perrin, the genus Anophthalmus has been united to Trechus, since there is a series of forms with more or less rudimentary eyes connected with the eyed species of Trechus. Bedel also tells us that in all the species of Trechus there is a natural tendency to penetrate into grottoes, even when ordinarily they live in the open air buried in the earth under stones.
It seems reasonable to conclude that the cave species, which are without optic ganglia, optic nerves, and any traces of eyes, had originally, by adaptation to total darkness, become isolated, and that their characteristics after being fixed by heredity have been transmitted for generations, becoming as unchanging in their way as the physical conditions of darkness and uniform temperature surrounding them. Those living in the open air in the soil under stones, or at or just within the entrances to caves, vary most as regards the eyes, as we have found to be the case with the other forms previously mentioned.
This intimate dependence on the physical conditions of life is so plainly shown in these animals, that we can well understand how potent have been the factors (i. e., change from light to total darkness and an even cave temperature) which have operated on out-of-door forms to induce variation. Given great changes in the physical surroundings, inducing loss of eyes from disuse, the abolition in some cases of the optic ganglia and optic nerves, the elongation of the appendages, isolation from out-of-door allies, and the transmission by heredity owing to close in-and-in breeding within the narrow fixed limits of the cave, and are not these collectively veræ causæ; do they not fully account for the original variations and their fixation; in short, can we not clearly understand the mode of origin of cave species and genera? What room is there in a case like this or in that of parasitic animals for the operation of natural selection? The latter principle only plays, it has seemed to us, a very subordinate and final part in the set of causes inducing the origin of these forms.