Page:The New International Encyclopædia 1st ed. v. 04.djvu/441

* CAVE ANIMALS. 377 CAVE ANIMALS. muscles, tlip retina i-i nioiv rediiceil, no tracp of the lens could he found except in one individual, and no blood-vessels enter the eye, the shape of the eye itself, which lies just beneath the skin, being very variable (Ei<jenmann). The Proteus of Adelsbert; Cave is a salaman- der-like form, allied to our nuul-puppy (Nec- turus). It has external {lills, very weak legs, ending in three toes in the fore and two in the hinder pair. Its body is remarkably slender, white or colorless, and the eyes are minute, just visible beneath the skin. It is noticeable that when this animal is kept in the laboratory, ex- posed to daylight, the stimulus of the light-rays causes the pigment to develop so that the skin turns slightly dark. The lower animals tell the same story of de- generation, blindness and total or partial atrophy of the eye, together with loss of color, and, in a more striking way, the compensation for the loss of vision by a great inerease in length of the antenn;e and other appendages, or the growth of long, slender tactile bristles. Blind Craiiflsh and Insects. — .V notable in- habitant of ilammoth and other caves is the blind crayfish {Orconcctes pellucidus). It dif- fers from its out-of-door allies in being blind, slender-bodied, and colorless. The eyes are pres- ent, but they are much reduced in size and destitute of a cornea and of black pigment, while the colorless body is slender. It is not only blind, but deaf, as A. S. Packard discovered that the auditory sacs are a third smaller, and the auditory hairs in the ears also a third shorter and smaller: hence it is to be inferred that the ears of the blind crayfish are degenerate, and the sense of hearing nearly, if not quite, obso- lete. This creature is also exceedingly timid and cautious in its movements. The eyeless beetles of eaves (Anophthalmus) have no vestige of eye or of optic nerves and ganglia; but in their movements they closely resemble their epig:ean allies. While their bodies and appendages are slender, they grope their way about by means of very long tactile bristles. They act exactly as if enjoying good eyesight. They walk, run, stop to explore the ground, seek their food, and run from the fingers of the in- sect-hunter who tries to seize them with the same agility as beetles provided with eyes. Other beetles, such as Adelops, which has re- tained vestiges of the outer eye; some spiders, comprising an eyeless species, and others with eyes varying in size, some much reduced, spin little webs on the walls of the chambers. Among the harvestmen some have extraordinarily long legs; while the Campodea, a wingless insect of the -Mammoth and other caves both of the United States and Europe, differs from the outdoor form ill its antenn.T and abdominal appendages being greatly exaggerated in length. The cave crickets have eyes, but they do not e.tend into the re- moter parts of the cave, and hence are twilight spciics. and probably cross with other twilight individuals. Besides these, there are many eye- less crur-tacea of different groups — mites, myria- pods, primitive wingless insects, a few flies, worms, and infusoria which go to make up this a.osemblage of sightless troglodytes. Origin and ITistoby. The fauna of caves is evidently composed of the descendants of indi- viduals which have been carried bv various means into the subterranean passages, have be- come adapted to life in perpetual darkness, be- coming isolated, and thus, so long as they are subjected to their peculiar environment, breed true to their species, and show no tendency to relapse to their originally eyed condition. There are, moreover, many blind or eyeless animals, fishes, insects, and Crustacea which live in holes, ant-nests, or in the abysses of the ocean, which, from the same general cause — i.e. absence of the stinuilus of light — have become eyeless and otherwise modified in compensation for the loss of vision. Tile fauna of caves is indeed a most simple and intelligible object lesson in establishing the truth of the evolution theory and the doctrine of use- inheritiince. Lamarck, in 1809, cited the cases of the mole and burrowing sphalax, as well as the Proteus of Austrian caves, as examples of the impoverishment and disappearance of these organs through constant lack of excrci.se. Dar- win candidly admitted that natural selection did not operate in the case of cave animals, but that the loss of eyes was due to disuse. Indeed, the main interest in studies of cave life centres in the obvious bearing of the facts on the theory of descent. The conditions of existence in caverns, subterranean streams, an<l 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. It is obvious that the action of the Lamarckian or primary factors of organic evolution — i.e. change in the environment and use and disuse — are amply .sufficient, when coupled with isola- tion and heredity, and that form of it called use - inheritance, to produce the blind forms. Here, also, we have a case where the transmis- sion by heredity of eyes adapted for vision lapses, I. wing to the profound change of environment, and the animals, after becoming adapted to a life in total darkness, inherit the degenerate eyes a,s well as the specialized tactile organs, elongated appendages, etc., acquired by the modi- fied organisms — a clear example of the trans- mission of acquired characters. The absence of the stimulus of light causes the eye, through disuse, to undergo reduction and atrophy. With this goes, in certain forms, the loss of the optic ganglia and o|)tic nii"es. Packard has found iind stated the following effects of disuse in the invertebrate animals of ifanuuoth and other caves, and it will be realized how profoundly the organisms have been modified : ( 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 (certain Crustacea, harvestmen, Adelops beetle, and the myriapod Pseudotremia). (2) Persistence of the optic lol>es and optic nerves, but total atrophy of the rods and cones, retina (pigment), and facets (blind crayfish). (3) Total atroi)hy of the optie lobes, optic nerves, and all the optic elements, including rods and cones, retina (pigment) and facets (.Anophthalmus beetle, and the myriapod Sco- terpes ) . An interesting fact, confirmatory of the theory of occasional rapid evolution, as opposed to in- variably slow action involved in pure Darwin-