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Show 130 NATURAL SELECTION. CHAP. IV. . to us only from having been found in w~ICh.larte :uow_:s we here and there see a thin straga 10SSI sa e. . . f k 1 down in a tree, gling br.anhchb springincghaf:= ~asorbee:w favoured and is nd whic y some . 11 :till alive on its summit, so we occas~ona. y see .an . ll'ke the Ornithorhynchus or Lepidosnen, whiCh a. nima I all degree connects by I· ts a ffi ni't I' es t wo 1a rge 1n some sm 1 b d b h f li·~e and which has apparent y een save ranc es o 1' ' . . · d t t d from fatal competition ~y havmg Inhabite a pro ec e station. As buds give rise by growth to fresh buds, ~nd these, if vigorous, branch out and ove~op on a~ sid~s many a feebler branch, so by gen~ration. I believe. It has been with the great Tree of Life, which fills with its dead and broken branches the crust of th~ earth, and covers the surface with its ever branchi?g and beautiful ramifications. CHAP, v. LAWS OF VARIATION. 131 CHAPTER V. LAWS OF VARIATION. Effects of external conditions- Use and disuse, combined with natural selection; organs of flight and of vision-Acclimatisation - Correlation of growth - Compensation and economy of growth-False correlations-Multiple, rudimentary, and lowly organised structures variable-Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable-Species of the same genus vary in an analogous manner-Reversions to long lost characters-Summary. I HAVE hitherto sometimes spoken as if the variations -so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature-had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. Some authors believe it to be as much the function of the reproductive system to produce individual differences, or very slight deviations of structure, as to make the child like its parents. But the much greater variability, as well as the greater frequency of monstrosities, under domestication or cultivation, than under nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations. I have remarked in the first chapter-but a long catalogue of facts which cannot be here given would be necessary to show the truth of the remark-that the reproductive system is eminently susceptible to changes in the conditions of life; and to |