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Show CHAP. VI. 190 ON THEORY. DIFFICULTIES y ancient ancestral passed, we should have to loo~r t~ ver forms long since become extinct: s I·n concluding that ' t ely cau 1ou . . We should be ex rem £ m.ed by transitional ld t have been or . an organ cou no . d Numerous cases could be gr:en gradations of some ki~ . 1 f the same organ performing ainongst the l?wer annna ~s~inct functions ; thus the aliat the same time whollyd' t and excretes in the larva 1 · es 1o-es s, mentary cana respH . ' the fish Co bites. In the Hydra, of the dragon-fly and m d . 'de out and the exterior . 1 be turne Insi ' . the anima may . t d the stomach respire. In '11 then diges an · 1' ·r surface WI 1 t' mio-ht easily spec1a 1se, 1 such cases nat u ral seh ec 1on. ed o a part or organ, whw. h ret us gain ' any advantage we fu t' s for one function alone, £ d two nc 1on , . had per orme 't nature by insensible steps. and thus wholly change It~ s .perform simultaneously di · t gans some !IDe Two stinc ot'r in the same I· nd 'IV I'dual .' to give one the same func wn fi h 'th gills or branchire that · t there are s WI · Ins ance, . . 1 d in the water, at the same time breathe the air dissofve . . their swimbladders, this h th breathe ree air In . I t at ey 1 . ductus pneumaticus for Its supp y, latter o.rgand?~:~ ~y highly vascular partitions. In and bemg IVI e mi ht with ease be these cases, one of the two organs :m all the work modified and perfected so as to perfo dification by itself, being aided during the p~oces~ ~f ;;o an might b the other organ ; and then this ot ~ . g b~ modified for some other and quite distinct purpose, or be quite obliterated. . is a ood .The illustration of the swimbladde_r ~ ~:e:rtanlfact one because it shows us clearly the big £y p purpose that' an organ origr. na1 1y cons t ruet e d or o£n e a wholly' namely flotati.o n, may b e conv~r t e.d into Tohnee swori m bladder different purpose, name~y respiration. to the auditory has, also, been ":orked In as ~n a~ce~~or:ot know which organs of certa1n fish, or, or CHAP. VI. TRANSITIONS OF ORGANS. 191 view is now generally held, a part of the auditory apparatus has been worked in as a complement to the swimbladder. All physiologists admit that the swimbladder is homologous, or "ideally similar," in position and structure with the lungs of the higher vertebrate animals: hence there seems to me to be no great difficulty in believing that natural selection has actually converted a swimbladder into a lung, or organ used exclusively for respiration. I can, indeed, hardly doubt that all vertebrate animals having true lungs have descended by ordinary generation from an ancient prototype, of which we know nothing, furnished with a floating apparatus or swimbladder. We can thus, as I infer from Professor Owen's interesting description of these parts, understand the strange fact that every particle of food and drink which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. In the higher Vertebrata the branchire have wholly disappeared- the slits on the sides of the neck and the loop-like course of the arteries still marking in the embryo their former position. But it is conceivable that the now utterly lost branchire might have been gTadually worked in by natural selection for some quite distinct purpose : in the same manner as, on the view entertained by some naturalists that the branchire and dorsal scales of Annelids are homologous with the wings and wingM covers of insects, it is probable that organs which at a very ancient period served for respiration have been actually converted into organs of flight. In considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that I will give one more instance. Pedunculated cirripedes have two minute folds of skin, |