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Show 172 DIFFICULTIES ON THEORY. [Cn.AP. VI. ovigerous frena, the eggs lying loose at the bottom of the sack, in the ,vell-enclosed shell; but t~ey ~ave large folded branchiro. Now I think no one will dispute that the ovigerous frena in the one family ar~ stri?tly homologous with the hranchiro of the other family ; Indeed, they graduate into each other. Therefore I do not doupt that little folds of skin, which originally serve_d as ovigerous frena, but which, likewise, very slightly aided the act of respiration, have been gradually converte? by nat~.ual s~lection into branchiro: simply t~rough 3:n Increase In their size and the obliteration of thmr adhesive glands. If all pedunculated cirripedes had become extinct, and they have already suffered far more extinction than have sessile cirri pedes, who wo~1ld ever h3:v~ imagine_d that the branchi~ in this latter family had onginally existed as organs foi preventing the ova from being washed out o~ the ~ack? Although we must be extreme!~ cautious In concluding that any organ coul~ !lot possibly ~ave been produced by successive transitional gradations, yet, 1!-ndoubtedly, grave cases of difficulty occur, soine of which will be discussed in my future work. . . One of the gravest is that of neuter 1n~ects, wlnch are often very differently constructed _from mther the ;males or fertile females ; but this case w1ll be treated of 1n the next chapter. The electric organs of fishes offer a~other case of special .difficulty ; it is hnpossible to concmve by what steps these wondrous organs have been yr?d"?-ced; but as Owen and others have remarked, thmr Intimate structure closely resembles that of common muscle; and as it has lately been shown that Rays have an organ closely analogous to the el~ctric apparatus, an~ .yet do not as Matteuchi asserts, discharge any electnmty, we mu~t own that we are far too ignorant to argue that no transition of any kind is possible. . The electric organs offer another and even more serious difficulty; for they occur in only abo'?-t a ~ozen fi~l~es, of which several are widely remote 1n thmr affinities. Generally when the same organ appears in several m~m~ hers of the same elass, especially if in .mem~ers hav1ng very different liabits of life, we may attribute 1ts presence OHAP. VI.] TRANSITIONS OF ORGANS. 173 ~o inheritance from a common ancestor; and its absence 1n some ~f the me~bers to its loss through disuse or natural selection. But If the electric organs had been inherited from one ancient proge:r;it~r thus provided, we might have expected that all electric fishes would have been specially rel~ted to e~ch other. Nor does geology at all lead io tho behef that fonnerly most fishes had electric orO'ans which most of thei~ modified des?endants ~ave lost. 5 Th~ presence of luminous organs In a few Insects belonO'ing to d~ft'erent fan1ilies and orders, offers a p~rallel gase of difficulty. Other c~ses coul~ be given ; for instance in plaJ?-ts, the very curious contnvance of a Inass of pollengrain~, borne on. a foot-~talk with a sticky gland at the end, IS the same 1n Orchis and .Asclepias -O'enera almost as remote as [ossible amongst flowering pYants. In all these cases o · two very distinct species furnished with apparently the same anomalous organ, it should be observed that, although the general appearance and function o~ the organ may be the same, yet smne fundamental difference can generally be detected. I am inclined to believ~ tha~ in nearly the .same way as two men have sometimes Indep.endently _hit on the very same invention, so natui:al selection, working for the good of each being and talnng advantage of analogous variations has some~ imes modi:fi~d in _very ne~rly the same manne~ two parts In two organic bmngs, which owe but little of their structure in common to inheritance from the same ancestor. .Although i~ ;many cases it is most difficult to conjecture by what transitions an organ could have arrived at its present state; yet, considering that the proportion of livIng and known forms to the extinct and unknown is very small, I have been ~stonished h~'Y rarely an organ can be named, towards whwh no transitional grade is known to lead. Th~ truth of t~is remark is indeed shown by that old canon 1n. natural history of "Natura non facit sal tum." We meet With this admission in the writings of almost every experien?ed naturalist ; or, as Milne Ed,vards has :ve~l expre~sed It, nature is prodigal in variety, but niggard 1n Innovation. Why, on the theory of Creation should this be so ? Why should all the parts and organs ~f many |