Excerpt from Cave Vertebrates of America: A Study in Degenerative Evolution
A cave is a unit of environment so well circumscribed and of such simplicity that we may know its contents, its elements, and its conditions nearly as well as the experimental zoologist knows the contents and conditions of his aquarium. These contents and conditions are of rare uniformity, changing but little from day to night, from season to season, or from decade to decade. The point of chief interest in the cave environment is the total absence of light in all parts except about its mouth. Probably no animals have a more intimate environmental adaptation than those inhabiting caves. This adaptation is largely of color and structure of eye, which modifications are surpassed only by the functional adapta tion of the tactile apparatus of the blind forms.
While no one has followed, and although we may not be able to follow in detail, the steps through which the cave animal has acquired this environmental adaptation, a knowledge of the present condition of their unchanging environment gives us a knowledge of what it has been during their entire period of development.
We know, or can know, what the present stage of their adaptation is. Not in frequently we know what the condition of the animal was at the start of its cave experiences and enough of the steps along its line of evolution (indicated by the degrees of adaptation reached by different members of the group) to enable us to form so clear a picture of its entire route of evolution that we may conjecture what elements of the environment caused the modifications, and by what process they were brought about. We have, in other words, a long experiment conducted by nature unrolled before us.
I propose in this work to give an account of the cave as an environment; to bring together in a revised form the papers on blind and cave vertebrate animals so far published by myself and my students, together with further observations on the species previously considered, to consider the habitat, mode of life, and the origin of the Cuban blind fishes, and to give an account of their eyes.
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