Page:Popular Science Monthly Volume 57.djvu/407

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T may now be profitable to take up the causes leading to the small degree of degeneration found in Chologaster, the degenerations of the eye in Amblyopsis, Typhlichthys and Troglichthys to a mere vestige, together with the total disappearance of some of the accessory structures of the eye, as the muscles.

In the outset of this consideration we must guard against the almost universal supposition that animals depending on their eyes for food are or have been colonizing caves, or that the blind forms are the results of catastrophes that have happened to eyed forms depending on their eyesight for their existence. This idea, so prevalent, vitiates nearly everything that has been written on the degeneration of the eyes of cave animals.

Another word of warning ought perhaps to be added. The process of degeneration found in the Amblyopsidæ need not necessarily be expected to be identical with the degeneration of the same organs in another group of animals, and, however much the conditions in one group may illuminate the conditions in another, cross-country conclusions must be guarded against.

The degeneration of organs ontogenetically and phylogenetically has received a variety of explanations:

1. The organ diminishes with disuse (ontogenetic degeneration)—Lamarck, Roux, Packard), and the effect of this disuse appears to some extent in the next generation (phylogenetic degeneration—Lamarck, Roux, Packard, Kohl).

2. Through a condition of panmixia the general average maintained by selection is reduced to the birth mean in one generation (ontogenetic—Romanes, Lankester, Lloyd Morgan, Weismann) to the greatest possible degeneration in succeeding generations (phylogenetic—Weismann), or but little below the birth average of the first generation (Weismann's later view, Romanes, Morgan, Lankester).

3. Through natural selection (reversed), the struggle of persons, the organ may be caused to degenerate either (A) by the migration of persons with highly developed eyes from the colony living in the dark (Lankester), or (B) through economy of weight and nutriment or liability to injury (phylogenetic purely—Darwin, Romanes).

4. Through the struggle of parts for room or for food an unused