)RIGIN AND HISTORY. The fauna of eaves is evidently composed of the descendants of which have been carried by various means into the subterranean passages, have be come adopted to life in perpetual darkness, be coming isolated. and thns. so long as they arc 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, inserts. and e•ustacea which live in holes. ;int-nests. or in the abysses of the ocean.
from the same general cause—i.e. of the stimulus of light—have become eyeless and otherwise modified in compensation for the loss of vision.
The 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 inhe•itance. l,amarek, in 'IRO!). 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 exercise. Dar Win candidly admitted that natural selection did not operate in the case of cave animals, but tlmt the loss of eyes was IncI to disuse.
Indeed, the main interest in studies of cave life centres in the obvious hearing of the fact, on 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.
It is obvious that the action of the Lamarckian or primary faeters 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 eves adapted for vision lapses, owing to the profound change of environment, and the animals, after becoming adapted to a life in total darkness, inherit the degenerate eyes as 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 optic nen es. Packard has found and stated the effects of disuse in the invertebrate animals of Mammoth 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 e•ystalline lens (certain crustacea. harvestmen, Adeiops beetle, and the m•riajaal Pseudotremia).
(2) Persistence of the optic lobes and optic nerves, but total atrophy of the rods and cones, retina (pigment arid facets (blind crayfish).
(3) rotal atrophy of the optic lobes. optic nerves, and all the optic elements, including rods and cones, retina (pigment) and facets (Anophthatmus beetle, and the myriapod Seo terpes).
An interesting fact, confirmatory of the theory of occasional rapid evolution, as opposed to in variably slow action involved in pure Darwin ism. is that we never find any vestiges of the optic lobes or optic nerves: if they are wanting at all, they are totally abolished. The atrophy is comparatively rapid. sudden, and wholesale. It was probably so with the loss by disuse of the thumb of the thumbless monkey of South America, which has retained no vestige of the lost member.
The varying degrees of development in the external parts of certain blind animals prove that these forms entered the at different periods. and have been exposed for different ol time to the loss of light. The com pletely eyeless forms are the oldest inhabitants of these regions of Cimmerian darkness. The imperfect lenses and retina., the abolishment of visual nerve: and portions of the brain. are, like all vestigial structures in highly specialized or modified animals of various kinds, like ancient, decayed sign-posts pointing out sonic nearly oblit erated paths now unworn and disused.