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Show 390 EMBRYOLOGY. [0U.A.P. XIII. As all the organic beings, extinct and recent, which have ever lived on this earth have to be classed toge~her, d 11 have been connected by the finest gradatwns, ~l~e b~~t a or indeed if our collections· were nearly pm:fect, the only possible ~rrangement, would be genealogw~l ; d escen t b m· n0...,. . on n1y vi· ew the hidd. en bond of 1c· onne, ctiofn which naturalists have been. see~Ing under t 1e ter.m o the natural system. On this view we. can undm stand how it is that, in the eyes of most naturahsts, the s~ruct~re of the embryo is even more important ~or classi~cati~n .than that of the adult. For the em br:yo IS the animal In its less modified state ; and in so far It re:eals the structure of Hs progenitor. In two ~roups of annuals, ~owever much they may at present differ fron1 each other In .sti:ucture and habits, if they pass through the same or Sil11Ilar embryonic stages, we may feel assured ~h~t they have both descended from the same or nearly Sil11Ilar parents, and are therefore in that degree closely related. Thus,, community in embryonic structure rev~als comrnuinty of descent. It will reveal this community of descent, however much the structure of the adult ma.y have been 1nodified and obscured· we have seen, for Instance, that cirri pedes can at once' be recognised by their larvoo as belonging to the great c~ass of crustaceans. . As th·e· embryonic state of each speCies a~d group of. species :partially shows us the structure of thmr less mo~Ified ancient :pro-enitors, we can clearly see why anCient and e~trnct forrns of life should resemble the embr~os of. thmr d~scendants,- our existing species. Agassiz behoves th1s to be a law of nature; but I am bound to confess that I only hope to see the law hereafter I?rove~ true. It .can be proved true in those cases alone In whiCh the ancient state now supposed to be represented in many. embry?s, has ~ot been obliterated, either by the successrve variations in a long course of 1nodificatio~ ~aving supervened at a very early age, or by the variations ~av1ng been inherited at an earlier period than t~at a.t which they first appeared. It should also be borne In mind, that the supposed law of resemblance of ancient forms of life to the embryonic stages of. recent forms, may be true, but yet, CHAP. XIII.] RUDIMENTARY ORGANS. 391 ) . owing. to ~he geological .record not extending far enough back In tnne, may remain for a long period or for ever inca~able of .demonstration. ' ' 'Ihus? as I~ semns to .m~, the leading facts in embryolo~ y, which are s~cond In Importance to none in natural his~ory, are explai_ned. on the principle of slight modificatiOns ~ot appean~g, In the many descendants from some o~e ancient progenitor, at a very early period in the life of each, though perhaps caused at the earliest and beinoinheri~ ed at a corr~sp?nding not early period.' Embryof ogy rrses greatly 1n Interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals. Rudi~enta_ry, atrophied, ~r: aborted organs.-Organs or parts 1n this strange condition, bearing the stamp of !nutility, are .extremely common throughout nature. For mstance, rudimentary mammre are very general in the males of mammals : I presume that the "bastard-wino-" in birds may be safely considered as a digit in a ru~H~ enta:y state: in. very many snakes one lobe of the lungs Is rudimentary ; In other snakes there are rudiments of the pelvis and hind limbs. Some of the cases of rudimentary organs are extremely curious ; for instance the presence of teeth in fretal whales, which when grow~ up have not a tooth in their heads; and the presence of teeth which never cut through the gurus, in the upper jaws of our u~born calve~. It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds. Nothing can be plainer !han that wings are formed for flight, yet in how many ~sects do we see wings so reduced in size as to be utterly mcay,able of flight, and not rarely lying under wing-cases, firm y soldered together ! The meaning of rudimentary organs is often quite unmistakeable : for instance there are beetles of the same genus (and even of the sa1ne species) resembling each other. most ?losely in all respects, one of which will have full-sized Wings, and another mere rudiments of membrane; and here it is impossible to doubt, that the rudi- |