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Show 148 LAWS OF VARIATION. CHAP. v. . . f 1' £ structure before useful be-changed conditiOns o d~ e. auti'on however slight, in its 1 f 1 any Imin ' . .t' comes ess use u' . d by natural selectiOn, 10r t ]1 be smze on . developmen ' Wl • d. . dual not to have its nutriment it will profit. t~e In IV~n useless structure. I can thus wasted in building up 'th hich I was much struck d t d a fact WI w only un ers . an. g c.u n.p ed es, a nd of which man.y o. thde r when examinlidn be m. ven : name ly' that when a cunpe e i.n stance·sr cou 'thi. n abn-o the r and is thus protected, it lose.s IS parasi IC WI 1 t 1 'ts own shell or carapace. This more or less ?omp e e ~ 1 Ibl and in a truly extraordi-is the case Withit~~h~;,~oteo~pas: for the c~rapace in nary man~e~ w . t f the three highly-Important all o~er Cirnpedes c~ns~es ~ead enormously develope?, an tenor segments o t es and muscles ; but m · h d 'th grea nerv and furn~s. e WI rotected Proteolepas, the whole antethe parasitic and p . d d to the merest rudiment . rt of the head Is re uce N rJor pa h b of the prehensile antennoo. ow attache~ tot e ases om lex structure, when ren-the savmg of a large and c ~t. habi'ts of the Proteo-dered superf l uous by the paras1 1c ld be a deci.d ed h ffi t d by slow steps, wou lepas, thoug e ec e . . d. vidual of the species; advantage to each successive In .I animal is ex· for in the str~g~le_ for ~e !~o7:1~: ~::~have a better Posed each rndividual ro P t · t being ' . . lf b less nu nmen chance of supporting Itse ' y become useless. · d 1 · a structure now wasted In eve oping l 1 t' will always sue· Thus as I believe, natura se ec Ion . art ceed in,the long run in reduci~g. and ~:::gs:;:~!us, of the organisation, as soon a~ It IS ren other part to be without by any means causing s.omed And con· largely developed in a correspondmg e~re;iy well sue· versely, that natural selection may per.:~out requiring ceed in largely developing any organd, wt~ of some ad· as a necessary compensat I.o n the re uc 10n joining part. CHAP. V. CORHELATION OF GROWTH. 149 It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in varieties and in species, that when any part or organ is repeated many times in the structure of the same individual (as the vertebrre in snakes, and the stamens in polyandrous flowers) the number is variable; whereas the number of the same part or organ, when it occurs in lesser numbers, is constant. ~rhe same author and some botanists have further remarked that multiple parts are also very liable to variation in structure. Inasmuch as this "vegetative repetition," to use Prof. Owen's expression, seems to be a sign of low organisation; the foregoing remark seems connected with the very general opinion of naturalists, that beings low in the scale of nature are more variable than those which are higher. I presume that lowness in this case means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why • natural selection should have preserved or rejected each little deviation of form less carefully than when the part has to serve for one special purpose alone. In the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular object had better be of some particular shape. Natural selection, it should never be forgotten, can act on each part of each being, solely through and for its advantage. Rudimentary parts, it has been stated by some authors, and I believe with truth, are apt to be highly variable. We shall have to recur to the general subject of rudimentary and aborted organs; and I will here only add that their variability seems to be owing to their uselessness, and therefore to natural selection having no power to check deviations in their structure. Thus |
