It is not that the larger
quadrupeds are actually destroyed (except in some rare cases) by the
flies, but they are incessantly harassed and their strength reduced,
so that they are more subject to disease, or not so well enabled in a
coming dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly
perfected at a former period, have been transmitted in nearly the same
state, although now become of very slight use; and any actually
injurious deviations in their structure will always have been checked
by natural selection. Seeing how important an organ of locomotion the
tail is in most aquatic animals, its general presence and use for many
purposes in so many land animals, which in their lungs or modified
swim-bladders betray their aquatic origin, may perhaps be thus
accounted for. A well-developed tail having been formed in an aquatic
animal, it might subsequently come to be worked in for all sorts of
purposes, as a fly-flapper, an organ of prehension, or as an aid in
turning, as with the dog, though the aid must be slight, for the hare,
with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance to
characters which are really of very little importance, and which have
originated from quite secondary causes, independently of natural
selection.
Pages:
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244