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Show 46 VARIA'riON UNDER NATURE. [CHAP. II. CHAPTER II. VARIATION UNDER NATURE. . l ecies-Wide ranging, much difl'used, Variability-Individual ddferenceE'-Dou~tfu :fhe la,rger genera in a~y country vary and common species vary most-SpeCles o M ny of the species of the larger more than the species of the s~~iller gene~·a~ abut unequally related to each genera resemble varieties in bemg very c ose y, ' other, and in having restricted ranges. B E applying the principles arrived at in the last ::o~r to orO'anic beings in a state of natu~·e, we must ~ ·lfty discus~ whether these latter are subJect to 1 any Vra~~ation To treat this subject at all propehrly, Ia honlgl cat·:tlogue· of dry facts s1 lO Ul d b e 5O1' ·ven ·' but t de's e s tah res~rve for m future work. Nor shall ~ here lscuss . e . us definftions which have been gi_ven of the te:m ;a~~~es. N 0 one definition has as yet satisfied all natu1 al-ifts. yet every naturalist knows vaguely hhat h~ ~e~ns vvh:n he speaks of a species .. G:enerally tf e ter~ Inc tT~! the unlmown ele-ment of a distinct act o crea Ion. tenn " variety " is almost equally diffic~lt to ~efi~le ; 1 ~~t 1 e community of descent is almost universa . y nnp 10 ' ;l~~u h it can l·arely be proved. We have_ also wl:~t ~re callegd lllOllS t rOSl• t•l eS ., but they graduate Into v·a dn etlebsl . B a monstrosity I presume is nleant ~OJ?e . consl era e a!viation of structure in one part, either IDJ unous dto oS not useful to the species, and n~t ~ener~lly propha~at~ ·. 0~~ authors use the term " vanatlon" In a tee nlca ~ense, im 1 in a modification directly d~e to ~he physical con: d't:? y g f life. and "variations" ln t.hls sense, are su£. ~~~ds n~t to 'be inherited: but who ?an say that th~ Xwarfed condition of shells in the b;racklSh wa~ers o; ~he Baltic or dwarfed plants on Alpme summits, ~d not thicke~ fur of an animal from far northwards, wou CHAP. II.] V .A.RIATION UNDER NATURE. 47 in some cases be inherited for at least s01ne few genm·ations ? and in this case I presume that the form would be called a variety. Again, we have many slight differences 'vhich may be called individual differences, such as are known frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen, from being frequently observed in the individuals of the same species inhabiting the same confined locality. No one supposes that all the individuals of the same species are cast in the very same 1nould. These individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his don1esticated productions. These individual differences generally affect what naturalists consider unimportant parts; but I could show by a long catalogue of facts, that parts which must be called important, whether viewed under a physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am convinced that the most experienced naturalist would be surprised at the number of the cases of variability, oven in important parts of structure, which he could collect on good authority, as I have collected, during a course of years. It should be remembered that systematists are far from pleased at finding variability in important characters, and that there are not many men who 'vill laboriously examine internal and important organs, and compare them in many specimens of the same species. I should never have expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species; I should have expected that changes of this nature could have been effected only by slow degrees : yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Ooecus, which may ahnost be compared to the irregular branching of the stem of a tree. This philosophical naturalist, I may add, has also quite recently shown that the muscles in the larvre of certain insects are very far from uniform. Authors sometimes • |