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Show 134 LAWS OF VARIATION. [OnAP. V. maintained that the law is of universal application ; but many good observei·s, more especially b?tanists, ~elieve in its truth. I "rill not however, here give any Instances, for I see hardly any' way of distinguishing between the effects, on the one hand, of a part being largely ~~veloped through natural selection and another and a~J oining part being reduced by this same process or by dis~se, and, on the other hand the actual withdrawal of nutriment from one part owin;. to the excess of growth in another and adjoining part~ I suspect, also, that some of t~e ca~es of compensation which have been advanced, and likewise some other facts, may be Inerged under a more general principle, namely, that natural selection is continually trying to economise in every part of the organisation. If under changed conditions of life a structure before useful becomes less useful, any diminution, however s1ight in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure. I can thus only understand a fact with which I was much struck when examining cirripedes, and of which many other instances could be given: namely, that when a cirripede is parasitic within another and is thus protected, it loses more or less completely its own ~hell or carapace. This is the case with the male Ibla, and in a truly extraordinary manner with the Proteolepas : for the carripace in all other cirripedes consist of the three highly-important anterior segments of the head enormously developed, and furnished with great nerves and muscles ; but in the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the merest rudiment attached to the bases of the prehensile antennre. Now the saving of a large and complex structure, when rendered superfluous by the parasitic habits of the Proteolepas, though effected by slow steps, would be a decided advantage to each successive individual of the species ; for in the struggle for life to which every animal is exposed, each individ\lal Proteolepas would have a better chance of supporting itself, by less nutriment being wasted in developing a structure now become useles~:;. CHAP. V.] CORREL.A.TION OF GROW'l'II ~ . 135 . Thus, as I believe natural 1 f . In the long run in r~du . s~ cc I~n Will always succeed organisation, as soon as ci~nfs ~~nd~avJng evfly part of the by any Ineans causinO' some t) re super UOUS, Without velopcd in a corresp~ndino- ~e~~~ part to be largely dethat natural selection ma5y ~ ee.l And, conversely, largely developin()' an . pe ~ct Y well succeed in necessary compen~atiof thng~d w:thout requiring as a part. e Ie uction of some adjoining . I~ seems to be a rule, as remarked b I Hilaire, both in varieties and . . Y s. Geoffroy St. part or organ is re eated man Ill .speci~s, that when any the same individuaf (as th y ttibes :n the structure of stamens in pol androus :fl. e ver e rre In snakes, and the whereas the nfmber of thowers) the number is variable; occurs in lesser numbers i~ ~~m~ p~rt Thorgan, when it and some botanists have' furth n.s an . e same author parts are also very liable to v e.r t~em~rked that multiple much a~ this " vegetative repe~fti~~o,~ :~ ~!:upturfe.O Ina~- expression seems to be . ' ro . wen s foregoino- ~emark seem a sign of low organisation ; the • • 5 s connect · h opinion of naturalists that b . ed 1Ith t. every general nature are more variable, th :h.ngs b_'Y In the scale of presume that lowness in th~n ose w Ich are higher. I parts of the or anisation hIS case means ~hat the several for particular fu~ctions. andve reen but httle specialised to perform diversified 'work as ong as the same part has should remain variable th 't w~ can perhaps see why it should have preserved ~r ~ tlsd whh ~atural selection form less carefully than r~ ec ~h eac httle deviation of one special purpose alone w Ienth e part has to serve for which has to cut all sorts .of th· e same way that a knife shape; whilst a tool for some I~~~ ~ay bbe. of almost any be of some particular sha IN- Ic ar o ~ ect had better never be for otten ca P~· atural selection, it should solely throng~ a·nd'~orni.t acd on each part of each beino- R . . ll s a vantage 5' ud1men tary part · t h b · and I believe with t~~~h ~~ een stated bJ: some authors, We shall have to recur to'th e apt tl be ~Ighly variable. tary and aborted oro-ans . a~dgeiner~ll shubJ ect of rudimen- 5 ' WI ere only add that |
