economy, can not have affected the eye for reasons already stated. The mere loss of the force of heredity, unless this was caused by disuse, or the process of germinal selection, can not have brought about the conditions, because some parts have been affected more than others.
Considering the parts most affected and the parts least affected, the degree of use is the only cause capable of explaining the conditions. Those parts most active during use are the ones reduced most—viz., the muscles, the retina, optic nerve and dioptric appliances, the lens and vitreous parts. Those organs occupying a more passive position, e. g., the scleral cartilages, have been much less affected. The lens is one of the latest organs affected, not at all during use, possibly because during use it would continuously be in use. It disappears most rapidly
after the beginning of absolute disuse both ontogenetically and phylogenetically. All indications point to use and disuse as the effective agents in molding the eye. The process, however, does not give results with mathematical precision. In Typhlichthys subterraneus the pigmented layer is affected differently from that of Amblyopsis. The variable development of the eye muscles in different species would offer another objection if we did not know of the variable condition of these structures in different individuals. Chilton has objected to the application of the Lamarckian factor to explain degeneration, on account of the variable effects of degeneration in various invertebrates. But such differences in the reaction are still less explainable by any of the other theories.