DEGENERATION S A FACTOR IN EV01.1" TION. The progressive development of organic forms is from the beginning attended by the modi fication of organs or of the organism, through lack of nutrition or disuse, resulting in physical degeneracy, \\ e see c w here throughout the plant nod animal kingdoms this process of de generation, this loss or reduction of parts of organs or entire organs. This is proved by the 4,ocurrenee of so many vestigial organs or struc tures in tl e higher or more specialized plants, and especially animals, as compared with the stein forms, i.e. the ancestral or generalized forms from which they have apparently de scended.
Indeed. there can It no specialization of form and structure without a corresponding reduction of the less useful neighboring parts be partial or total atrophy. To illustrate: The foot of the horse. so ttonderfully adapted to the animal's mode on life, consists of a single digit. the other four having disappeared, with the exception of the two splint-bones. which are the relies of two digits. Aloridiology and paleontology declare in unmistakable terms that the ancestral form from which the horse family has lest •ded Nvas a gen•ralized form provided with live functional It.cs on each foot. See IloRSE; DISUSE.
In groups of animals with a metamorphosis I 114.re are exceptional forms in which development is direct. the stages of development being ali t revinted or suppressed; the organism may be said to take .t 'short cut' to attain a growth on a level with its allies, who have traveled the long road of 4.14.veloinnent by metamorphosis.
The evolution of almost every type. as well as the process of development of the embryo of most organisms, particularly of the more specialized anin ads. is a process of development of one part at the expense of another, its hypertrophy. due to accelerated development, resulting front use 44r exervise and consequent increased nutrition: while adjoining parts, or less desirable or useful its or are allowed to remain slat iona ry or lapse by atrophy. Adaptation seems not only
to involve progressive evolution. but also 414.gen ration. Thus there appears in the growth of th4. individual as as in the development or ii tint ion of the race or (lass to he a competition or struggle for existence, at tirst bet \Neel' the calls, and afterwards between the different or gans, especially those most con•erned with the outer world, or With •Onipetitive organie forms, 1 he result is that the suceessful struetures re ceive the 11 ost nourishment. While the less useful 44• favored and dwindle, becoming so many vestiges t4, tell the tale of defeat and death by at ropily.
This is exemplified in the ease of such an mil la 1 as the lobster. I 11 this animal, and throughout the.elass of Crust:leen. of Which it is a t?Ia.. tl, wonderful diversity of form in the scgn 4•11 s of \\ kl1 t he and limbs a re vent po.,d, as well as the marvelous degree of special ization of hod and liinhs cliaraelerizing the I f species of the (lass. are lilt' to the of detelopii ent of oter adjoining parts. Thus the huge e trapoee t r shield of the lobster is 41ife to Ole excessiie 414 t4 'opulent of the tipper or dor-al portion of two head segments, t114. eor iespoo ling part. or •4r.•h•s of eleven other seg ments of the 1.11:(1..th..r.tx having, during the growth of the animal. beemie completely lost or atrophied by the cont imams pressure of the ;,row ing compact'.
The wonderful differences between the mouth parts of the caterpillar and the butterfly are due to the normal 01, in sonic cases, excessive de Nelopm•nt of one pair of organs at the expense 01 others. Thus the mandibles of the caterpillar undergo attophy in the changes of the chrysalis to the butteril:t ; the latter. taking no solid food, has entirely lost its mandibles. while the maxil le forming the tongue arc. by exercise in taking nectar. greatly developed. as in the butterfly and espeeially in the hawk-moth.