DEGENERATION AS A FACTOR IN EVOLUTION. Degeneration is a term used in biology to those not infrequent cases where an entire organism falls below the structural level of its young stages (ontogenetic degeneration) or its ancestors (phylogenetic de generation) ; or where an organ in the same way loses its fullness of function and becomes more or less atrophied, abortive and simplified. Thus many parasitic worms, crustaceans, etc., are emphatically simpler than their free-swim ming larva, and the sessile adult ascidian shows only traces of the vertebrate characters of the embryo. Examples of the degeneration of indi vidual organs are the tail of a frog; the digest ive system of the mature mayfly; the pineal gland of man (primitively a medium eye) ; the human wisdom-tooth; the spint-bones of the horse (metacarpals and metatarsals of lost ' digits) ; the lungs of certain salamanders, etc. As in the case of the feet of the horse the per fection of the entire structure is often depend ent on the atrophy or loss of certain of their parts. The loss of the hind legs in the whale unquestionably permit's it to offer less resistance to its motions in the water. On the other hand, the degeneration of an organ may be accom panied by no very great advantage to its owner. Thus a crustacean which starts with a well developed eye may exhibit the gradual loss of this on assuming a dark habitat. Degeneration must be distinguished (1) from occasional abortion; (2) from reversion to an ancestral type; and (3) from the occurrence .of rudimen tary and undeveloped organs where a character possessed by ancestral types remains more or less undeveloped, or shows itself only in embry onic life. Degeneration may be due to the en vironment. Absence of food, heat, light, etc., may mean the absence of the necessary stimulus for the growth and maintenance of the organs; or superfluity of food may cause one system to preponderate over others. Nor can it be doubted that cassation of function checks the food-sup ply to a given organ, and in other ways helps to bring about its degeneration. Again, the mere pressure of a hypertrophied organ on its neighbors, particularly in the embryonic stages of growth, is capable of causing their atrophy But on the other hand, some less obvious cause — the fatigue of early life, a constitutional slug gishness, etc.— may share in conditioning de generation, as in the case of the Tunicata. Weismann and others, however, would explain degeneration by what they call the non-opera tion' of natural selection. On this view, organs are not only developed but maintained by nat ural selection; and if it happens that an organ is no longer an advantage in this struggle for ex istence (for example eyes in, dark caves), then natural selection no longer maintains that organ and it disappears in the course of generation.
In other words retrogressive variations have an equal chance of survival to that of progressive variations, and as the ways in which a compli cated organ like the eye can deteriorate are vastly greater in number than those in which it can improve, the interbreeding of the surviving cave animals is sure to create sooner or later a race with defective eyes. Weismann applies this ultra-Darwinian conception especially to cases of purely phylogenetic degeneration—for example, to the slightly developed wings of the Apteryx. Many cases of degeneration properly so called appear hardly to require his subtle explanation, but find a sufficient one in the nature of the environment, in the effects of stopped function and in the constitution of the organism. The problem of ontogenetic degeneration is im mersed in the obscurity that shrouds all the phenomena grouped under the law of recapitu lation (q.v.). Degeneration is curiously dis tributed in the animal kingdom. There are groups, such as the vertebrates, in which with all the degeneration that may occur, a certain high level of structure is everywhere maintained. There are groups, such as the ascidians, which are distinguished by the universality of a de generation at once phylogenetic and ontogenetic. Again, there are groups like the arthropods, where progressive and retrogressive lines of evolution are found in the closest proximity and interconnection. The Crustacea contain highly developed free-swimming forms, like the lobster or crab; hopeless vegetative parasites, like Sac culina, which nevertheless has a free-swimming larva; slightly modified ectoparasites, degener ate non-parasitic forms, such as the barnacles; free-swimming forms which pass through a vegetative parasitic stage in their existence; and forms in even more involved degrees of abor tion. These forms are related to one another in a manner but distantly suggesting the extent of their degeneracy; neighboring genera may be at very different levels of organization. The sects are also characterized by a mingling of degenerate and non-degenerate forms. (See DEGENERACY; ENVIRONMENT; EVOLUTION, etc.). Consult Demoor, Massart and Vanderville, 'Evolution by Atrophy' (New York 1899) • Lankester, E.Ft., (Degeneration) (London 1880) ; Morel, des deginerescences) (Paris 1857).