Roux[K] attempted chiefly to explain degeneration in the individual. Degeneration is looked upon as the result of a struggle among the parts for (_a_) room and (_b_) food. Without doubting that both these principles are active agents in degeneration, it may be seriously doubted whether they are effective in the degeneration of the eyes in question. Certainly there can be no question of a struggle for room, for the position and room formerly occupied by the eye is now filled with fat, which can not have been operative against the eye. The presence of this large fat mass in the former location of the eye, the large reserve fat mass in the body, the uniformly good condition of the fish and the low vitality, which enables them to live for months without visible food, all argue against the possibility that the struggle for food between parts was an active agent in the degeneration of the eyes. [K] Gesammelte Abhandlungen, 1895. Kohl[L] considers that “_Der Grund und direkter oder indirekter Anlass zum Eintreten der Entwickelungshemmung ist Lichtmangel._” The method of operation of the lack of light he conceived to be as follows: [L] Rudimentäre Wirbelthieraugen, 1893. Other organs were developed to compensate for the disuse of the eye; and as the developmental force was used in the formation of these organs, each succeeding generation developed its eye less. The degeneration is thus explained as the result of a struggle of parts, although this term is nowhere used, acting through the principle of compensation. The same objections may be offered to this explanation of Kohl as to all his theoretical discussions--they are based on the assumption of conditions and processes that have no existence. The high development of ‘compensating’ organs is not primarily the result of the loss of the eye, but the high development of the former organs permitted the disuse and later degeneration of the latter. His whole process is a phylogenetic one, without a preceding ontogenetic one, though on this point he does not seem to be very clear himself, for on one page we are told that degeneration leads to retardation, and on another that degeneration is a consequence of retardation. Ledenfeld[M] endeavors to apply Roux’s _Kampf der Theile_, with reversed selection, to explain the conclusions reached by Kohl on the processes and causes of degeneration. The struggle is represented as taking place between the germ and soma, the former endeavoring to keep the latter at the lowest efficient point as weapon for the germ. If a series of individuals get into the dark the organs of vision are of no advantage and reversed selection will bring about their degeneration. The saving in ontogeny appears first as a retardation and then as a cessation of development. [M] Zoölogischer Centralblatt, 1896.