128 VABIOUS OBJECTIONS. Cuaf. IV. 



vice, or may formerly have been so, although >vc arc not able 

 to perceive their use ; and these will have been acted on by 

 natural selection, A still larger number of mor}:)hological dif- 

 ferences may certainly be looked at as the necessary result — 

 through pressure, the withdrawal or excess of nutriment, an 

 early-formed jiart affecting a part subsequently developed, cor- 

 relation, etc. — of other adaptive changes, through wliich all 

 species must have j)assed during their long course of descent 

 and modilication. 



No one will maintain that we as yet know the uses of all 

 the pai-ts of any one plant, or the functions of each cell in any 

 one organ. Five or six years ago, endless peculiarities of 

 structure in the flowers of orchids, great ridges and crests, and 

 the relative positions of the various parts AVould have been con 

 sidered as useless moiphological difl'erences ; but now we know 

 that they are of great service, and must have been under the 

 dominion of natural selection. No one at present can explain 

 Avhy the leaves in a spire diverge from each other at certain 

 angles ; but we can see that their arrangement is related to 

 their standing at equal distances from the leaves on all sides ; 

 and we may reasonably expect that the angles will hereafter 

 be shown to follow from some such cause, as the addition of 

 new leaves to the crowded spire in the bud, as inevitably as 

 the angles of a bee"'s cell follow from the manner in which the 

 insects work together. 



In certain Avhole groups of ])lants the ovules stand erect, 

 and in others they are suspended ; and in some few plants 

 within the same ovarium one ovule holds the former and a sec- 

 ond ovule the latter position. These positions seem at first 

 purely moi-jihological and of no physiological signification ; 

 but Dr. Hooker informs me that, of the ovules within the same 

 ovarium, in some cases the upper ones alone and in other cases 

 the lower ones alone are fertilized ; and he suggests that this 

 probably depcn^ls on the direction in which the pollen-tubes 

 enter. If so, the position of the ovules, even when one is erect 

 and the other suspended, would follow from the selection of any 

 slight deviation in position which might favor their fertilization 

 and the production of seed. 



Several plants Ix'longing to distinct orders habitually pro- 

 duce flowers of two kinds — the one open and of the orthnary 

 structure, the other closed and impM'fect. In the latter the 

 l)etals are almost alwaj-s reductxl to the merest rudiments ; the 

 pollen-grains are reduced in diameter ; five of the alternate 



