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Show 48 VARIATION UNDER NATURE. [CHAP. II. argue in a circle when they state that important organs never vary; for these same authors practically rank that character as important (as some few naturalists have honestly confessed) which does not vary ; and, under this point of view, no instance of an important part varying will ever be found: but under any other point of view man~ instances assuredly can be given. There is one point connected with individual differ-ences, which seems to me extremely perplexing: I refer to those genera which have soinetimes been called "protean" or ~'polymorphic," in which the species present an inordinate an1ount of variation ; and hardly two naturalists can agree which forms to rank as species, and which as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera df insects, and several genera of Brachiopod shells. In most polyn1orphic genera some of the species have fixed and definite characters. Genera which are polymorphic in one country seem to be, with some few exceptions, polymorphic in other countries, and likewise, judging from Brachiopod shells, at former periods of time. These facts seem to be very perplexing, for they seem to show that this kind of variability is independent of the conditions of life. I a1n inclined to suspect that we see in these polyn1orphic genera variations in points of structure which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter will be explained. Those forms which possess in some considerable degree the character of species, but which are so closely similar to some other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species, are in several respects the most important for us. We have every reason to believe that many of these doubtful and closely-allied forms have permanently retained their characters in their o .. wn country for a long time ; for as long, as far as we know, as have good and true species. Practically, when a naturalist can unite two forms together by others having intermediate characters, he treats the one as a variety Cn.A,P. II.] DOUBTFUL SPECIES. 49 of the other, ranking the most the one first described as th co~mon, but sometimes the variety. But case' f e spe~Ies, and the other as not here enumerate so!e~i~reat di~cudlty,_ ~hich I will or not to rank on~ form as es occu; In em ding whether when. they are closely connec~eda~Ie~y t of and~ther, .even nor will the commonl Y In erme 1ate hnks . termediate links alwa~~a~e:~~ hJ;_b~ffiature of the in: many cases, however one for . e I culty. ~n very another, not because the intm:.e~i~~:f.e1 asha vanety of been found but because 1 111 rs ave actually suppose either that they d~n:o ogy leads the o?server to formerly bave existed. and h somew~~re exist, or may entr;y: of doubt and conJecture i:~e a Wdl e door for the lienee · d t · · pene · ed as a sp~~~s ~re~~~~l~f whether. a. form should be rank-ing sound judgment and y,.~he opini.on of naturalists havguide to follow. W Wti he experier:ce seems the only 'd b e mus, owever 1n m d CI e y a majority of natural' t £ £ any cases, e-well- known varieties can be :a~e~r h:V hhll-marked and ranked as species by at least w IC a':e not been That varieties of this dou~~fuf ~~:_~etentf~udfges. common cannot be dis ut d Ie are ar rom un-of Great. Britain, of Fr~nc: ~r o1~hprre· the several :floras up by different botanists and see ;ha~Ited Sta;t:-s, drawn ber of forins have been ;anked b ab sutrp!-'lSing numspecies and b h Y one v anist as good W . ' Y anot. er as mere varieties. Mr. H. 0 atson, to whom I he und rl bl' . . . ance of all kinds has 1 ~r £ cep o Igation for assist-which are ' mar ~e or me 182 British plants 1 ll b generally considered as varieties but wh' h' lave a een ranked b b t · ' Ic making this list he has rmit~~~nists as s:pe.cies; and in b'?-t which nevertheless have been~~~1:e~I~Ing vari~ties, ;~l~r!s sph!lies, and he has entirely omitted s~v~~~hio-'h\; polym~~hi~ ~~=:· M~ n~r b f;en~ra, i~cluding the :1ost whereas Mr Bentham . . a Inf, on gives 251 species, 139 doubtfui for I gives on y .112,-a ~ifference of each birth and ms h' hAmon~~t hanimals which unite for forms, ranked by:n~czo ~re ~ tig ly loc?motive, doubtful 3 . o ogis as a species and by another |