SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.
Although we must be extremely cautious in concluding that any organ could
not have been produced by successive, small, transitional gradations, yet
undoubtedly serious cases of difficulty occur.
One of the most serious is that of neuter insects, which are often
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; for it is impossible to conceive
by what steps these wondrous organs have been produced. But this is not
surprising, for we do not even know of what use they are. In the gymnotus
and torpedo they no doubt serve as powerful means of defence, and perhaps
for securing prey; yet in the ray, as observed by Matteucci, an analogous
organ in the tail manifests but little electricity, even when the animal is
greatly irritated; so little that it can hardly be of any use for the above
purposes. Moreover, in the ray, besides the organ just referred to, there
is, as Dr. R. McDonnell has shown, another organ near the head, not known
to be electrical, but which appears to be the real homologue of the
electric battery in the torpedo. It is generally admitted that there
exists between these organs and ordinary muscle a close analogy, in
intimate structure, in the distribution of the nerves, and in the manner in
which they are acted on by various reagents.
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