Therefore I do not
doubt that little folds of skin, which originally served as ovigerous
frena, but which, likewise, very slightly aided the act of
respiration, have been gradually converted by natural selection into
branchiae, simply through an increase in their size and the
obliteration of their adhesive glands. If all pedunculated cirripedes
had become extinct, and they have already suffered far more extinction
than have sessile cirripedes, who would ever have imagined that the
branchiae in this latter family had originally existed as organs for
preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, grave cases of difficulty occur, some of
which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs
of fishes offer another case of special difficulty; it is impossible
to conceive by what steps these wondrous organs have been produced;
but, as Owen and others have remarked, their intimate structure
closely resembles that of common muscle; and as it has lately been
shown that Rays have an organ closely analogous to the electric
apparatus, and yet do not, as Matteuchi asserts, discharge any
electricity, we must own that we are far too ignorant to argue that no
transition of any kind is possible.
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