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Show 132 LAWS OF VARIATION. (€JHAP. V. this idea · but in the case of the corolla of the U 11?-b~lliferre, it is by no means, as Dr. Hooker. inforr;::d~~t~~ species with the densest heads that the Inn~r th ght flowers most frequently differ. It migh:t have ee:z; ou that the development of the ray-petals by drahidg nou~ ish1nent from certain other parts of the flower .a cda~e thm. r aborti.o n ; but I. n s01ne 0 omp ositre thfel re ItS a 'tIhu er-t ence I. n the seeds of t h e ou t er an d inner ore s WI 1 do"uf any difference in the corolla. Possibly' t!ffise seve~a thferences n1ay be connected with some di erence In e flow of nutriment towards the central and external flowers: we know, at least, that in irregular flower~, those nearest to the axis are oftenest subject to pel? ria, andf becoi~e re ular I mav add as an instance of this, and o a stri.rin~ cas.e of co~relation, that I have recently observed In some garden pelargoniums, that the central flo:ver of the truss often loses the patches of d~rker colour In the two upper petals ; and that when th1s occurs! the adherent nectary is quite aborted; when the colour lS absel?-t from only one of the two upper petals, the nectary IS only much shortened. With respect to the difference in the corolla of the central and exterior flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's idea t~at _the rayflorets serve to attract insects, whose agency IS highly advautao- eous in the fertilisation of plants of these t~o order~ is so far-fetched, as it may at first appear: an~ If it be auvantageous, natural s.election may ~ave co;ne Into play. But in regard to the differences b_oth In the Internal and external structure of the seeds, which ar~ not alw~ys correlated with any differences in the flowers, It seen1s Impossible that they can be. in any way. advantageous to the plant: yet in the Umbelhferre these.diff~rences are of such apparent importance-the seeds b~Ing In so~e cases, according to Tausch, orthospermous In the exterior flowers, and crn1ospennous in the central flowers,-that the elder De Candolle founded his main divisions of the .orde: on analogous differences. Hence ~e see that. modrficatwns of structure, viewed by systematists as of h1gh value, may be wholly due to unknown laws of correlated growth, and CHAP. V] UORRELATION OF GROWTH. 133 without be~ng, as far as we can see, of the slightest service to the species. We may often falsely attribute to correlation of growth structu~es ~hich are co~mon to whole groups of species: and. which In t~uth are simply due to inheritance; for an anCier:t progenitor may have acquired through natural seleotron some one modification in structure and after thou~~nd~ of generations, some other and i~dep~ndent mod1fi~atron; and these two modifications, having been tran.smitted to a whole group of descendants with diverse habits, would naturally be thought to be correlated in some necessary manner: So, again, I do not doubt that some apparen~ correlations, occurring throughout whole orders, are entirely due to the manner alone in which natural selection can ac~. For instance, Alph. De Candolle has. remarked that Winged seeds are never found in fruits which do not open : I should explain the rule by the fact that seeds could not gradually become winged through nat~ral. s~lection, except in f~uits which opened; so that the Individual plants producrng seeds which were a little better fitted to be wafted further, might get an advantage ov.er those producing seeds less fitted for dispersal; and this process could ·not possibly go on in fruit which did not open. . The elder ~eoffro;y and Goethe propounded, at about the same period, thmr law of compensation, or balancement of growth ; or, as Goethe expressed it " in order to spend on one side, nature is forced to eco~omise on the ot?er side." I think this holds true to a certain extent With our domestic productions : if nourishment flows to one part or organ in excess, it rarely flows, at least in e~cess, to an~ther part ; thus it is difficult to get a cow to give much milk and to fatten readily. The same varietyof the cabbage do not yield abundant and nutritious foliage and ~ copious. supply of oil-bearing seeds. When tli~ seeds 1n o"?-r ~ru1ts become atr~phied, the fruit itself gains largely 1n siZe and quality. In our poultry, a large tuf~ o~ ~eathers on the head is generally accompanied by a d1m1nis_hed co~b, ~nd a large beard by diminished watt~ es~ W 1th species In a state of nature it can hardly he |