Page:The New International Encyclopædia 1st ed. v. 06.djvu/96

* DEGEIfEBATION. T2 forms is from the beginning attended by tliemodi- ticatioQ of organs or of tlie organism, through lack of nutrition or disuse, resulting iu plijsieal degeneracy. We see everywhere throughout the plant and animal kingdoms this jirocess of de- generation, this loss or reduction of parts of organs or I'ntire organs. This is proved by the occurrence of so many vestigial organs or struc- tures in the higher or more specialized plants, and especially animals, as compared with the stem forms, i.e. the ancestral or generalized forms from which they have apparently de- scended. Indeed, there can be no specialization of form and structure without a corresponding reduction of the less useful neighboring parts by partial or total atroi)hy. To illustrate: The foot of the horse, so wonderfully adapted to the animal's mode of life, consists of a single digit, the other four having disapijeared, with the exception of the two splint-bones, which are the relics of two digits. Morphologv- and paleontologj- declare in unmistakable terms that the ancestral form from which the horse family has ilesieiided was a generalized form provided with five functional toes on each foot. See House : DlsisE. In groups of animals with a metamorphosis there are exceptional forms in which development is direct, the stages of development being ab- breviated or suppressed; the organism may then be said to take .1 'short cut' to attain a growth on a level with its allies, who have traveled the long road of development by metamorphosis. The evolution of almost everj- type, as well as the process of development of the embryo of most organisms, particularly of the more specialized animals, is a process of development of one part ht the expense of another, its hypertrophy, due to accelerated development, resulting from use or exercise and consequent increased nutrition; while adjoining parts, or less desirable nr useful parts or organs, are allowed to remain stationary or lapse by atrophy. Adaptation seems not only to involve progressive evolution, but also degen- eration. Thus there a|)|)ears in the growth of the individual as well as in the development or evolution of the race or class to he a competition or struggle for existence, at first tH>tween the cells, and afterwards between the different or- gans, especially those most concerned with the outer world, or with competitive organic forms. The result is that the successful structures re- ceive the most nourishment, while the less useful or favored languish and dwimlle, iHH-oming so many vestiges to tell the tale of defeat and death by atro|)hy. This is exemplified in the case of such an animal as the lobster. In this animal, and throughout the. class of Crustacea, of which it is a tyf>e. the wonderful diversity of form in the segments of which the body and limbs are com- posed, as well as the marvelous degree of special- ization of body and limbs characterizing the thousands of species of the class, are due to the excess of development of one over adjoining parts. Thus the Inigi- carapaci- f)r shield of the lobster is due to t!ie excessive development of the upper or dorsal portion of two head-segments, the cor- responding parts or arches of eleven other seg- mi-nts of the ceplia lot horn x having, during the atrophied by the continuous pressure of the growing carapace. D£G£N£BATION. The wonderful difTerenees between the mouth- parts of the caterpillar and the butterlly are due to the normal or, in some cases, excessive de- velopment of one pair of organs at the ex|KMise of others. Thus the mandibles of the cati'rpillar undergo atrophy in the changes of the chrysalis to the buttenlv; the latter, taking no solid food, has entirely lost its mandibles, while the maxil- la; forming the tongue are, by exercise in taking nectar, greatly developed, as in the butterlly and especially in the liawk-moth. So it is with man. As regsirds his brain and hands he is the most specialized of mammals; but as concerns his feet he is plantigrade, show- ing a tendency to loss of his little toe by atrophy, while already in the higher race he loses his wisdom teeth soon after they appear. Besides this he bears the vestiges of nearly seventy u.se- less structures, among them the vermiform ap pendix, the most useless and positively dangerous of these relics pointing to his origin from som» more generalized mammal. The case is similar with the paddles of the ichthyosaurus, the plesiosaunis, and the whales. They are, at least in the last named, the result of modification by degeneration, resulting in adaptation to the medium inhabited. And so it is with the process of evolution of the birds, pterosaurs, and bats, and the motlitications in the feet of the male, and in the burrowing insects, climbing lizards, mammals, particularly the lemurs, and the thumbless monkeys. There are four chief forms of degeneration: (1) Degeneration during the growth of'the in- dividual, and the phylogeny of the race or class: (2) degi-neration of the individual, more or less total; (3) social and institutional degeneration, often alTecting whole peoples, due to slavery, and to mental, moral, and pathological causes; and (4) individual moral degeneration in the hu- man races, resulting in the production of mental degenerates and criminals. It is worthy of ob- servation that the occurrence of atrophy, of de- generacv in man, is nearly evervwhere foreshad- owed in the rest of the organic worlil. where it is a normal process, and the necessary complement of progressive evolution. The death or elimination of the individual membcT which has In-come useless or out of harmony with its physical and social environ- ment is also in the course of nature. WTien once lapsed by atrophy, an organ or part of an org;in can never, as a rule, be restored ; there are a few exceptions, e.g. that of ihe pigment of a blind Proteus subjected to light, and the organs of certain jilants. The pnx-ess of degeneration may at an early stage be arrested, bvit after a certain stage resumption of the original normal structure becomes impossible. Were it not so, evolution would be vastly less progressive than it has been, and the world would be crowded more densely than now with degenerates. The inunediate causes of degj'neration are lack of nutrition, disuse, and change of habit and instincts, so that an organ or set of organs are thrown out of use. Atrophy, also, as stated by IVmoor, may arise from lack of space, as seen in the reduction in the numl)er of teeth in man compared with lemurs and platyrrliine monkeys, these having six grinding teeth, while man has but five die to a reduction in the size of the jaw. Not only do we observe sporadic cases of degen- eration, but also species and genera, as well as
 * rowth ot the animal, become completely lost or