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Show 196 DIFFICULTIES ON THEORY. CHAP. VI. former period have been transmitted in nearly thedsame ' f slight use· an any state, although now become 0 ve~y '·u actually injurious deviations in theu str~cture; 1 . a1 " ;;ys have been checked by natural selection. . .eei?g ow . f locomotion the ta1l 1s 1n most 1mportant an organ o d .c . . l "ts general presence an use lOr many aquatic a~Ima s, I y land animals which in their lungs purposes 1n so man ' . . . . } 1 or modified swimbladders betray theu aquatic ongin, h be thus accounted for. A well-developed ' , may per aps . . l "t . ht tail having been formed in an aq~atic anima ' 1 mig subsequently come to be worked In for all . sorts of pur-s a fly-flapper an organ of prehension, or as an poses, a ' ·a b aid in turning, as with the dog, though t~e ai must e slight, for the hare, with hardly any tail, can double quickly enough. . In the second place, we may sometimes attr1 ?ute importance to characters which ar~ ~eally of very ht~le importance, and which have originated from q~Ite secondary causes, independently of natural selectiOn. We should remember that climate, food, &c., p~ob~bly have some little direct influence on the org~nisatwn; that characters reappear from the law of revers~on; that correlation of growth will have had a most Important influence in modifying various structures; and fin~lly, that sexual selection will often have largely modified the external characters of animals having a will, to give one male an ad vantage in fighting with anot~~r or in charming the females. Moreover when a modrfication of structure has primarily arisen from the above or other unknown causes, it may at first have been of no advantage to the species, but may subsequently have been taken advantage of by the descendants of the species under new conditions of life and with newly acquired habits. To give a few instances to illustrate these latter CHAP. VI. ORGANS OF LITTLE IMPORTANCE. 197 remarks. If green woodpeckers alone had existed and we did not know that there were many black and ';ied kinds, I dare say that we should have thought that the green colour was a beautiful adaptation to hide this tree-frequenting bird from its enemies; and consequently that it ~as a character of importance and might have been acquired through natural selection; as it is, I. h~ve no doubt that the colour is due to some quite distinct ~ause, probably to sexual selection. A trailing bamboo In the Malay Archipelego climbs the loftiest trees by the aid of exquisitely constructed hooks cluste~ ed around the ends of the branches, and this contnvance, no doubt, is of the highest service to the plant; ~ut as we see nearly similar hooks on many trees whwh are not climbers, the hooks on the bamboo may have arisen from unknown laws of growth, and .have been s~bsequently taken advantage of by the pl~nt undergoing further modification and becoming a climber. The naked skin on the head of a vulture is ?enera~y.looked at as a direct adaptation for wallowing m putridity; and so it may be, or it may possibly be due to the direct action of putrid matter; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the cleanfeeding male turkey is likewise naked. The sutures in the skulls of young mammals have been advanced as a beautifu~ ~daptation for aiding parturition, and no doubt they facilitate, or may be indispensable for this act; but .as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg we may 1· n.1c er that this structure has arisen from the' laws o~ ?rowth, and has been taken advantage of in the partuntwn of the higher animals . . We are profoundly ignorant of the causes producing shght and unimportant variations ; and we are immedi- |