In these cases, one of the two organs might with ease be
modified and perfected so as to perform all the work by itself, being
aided during the process of modification by the other organ; and then
this other organ might be modified for some other and quite distinct
purpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into
one for a wholly different purpose, namely respiration. The
swimbladder has, also, been worked in as an accessory to the auditory
organs of certain fish, or, for I do not know which view is now
generally held, a part of the auditory apparatus has been worked in as
a complement to the swimbladder. All physiologists admit that the
swimbladder is homologous, or "ideally similar," in position and
structure with the lungs of the higher vertebrate animals: hence there
seems to me to be no great difficulty in believing that natural
selection has actually converted a swimbladder into a lung, or organ
used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true
lungs have descended by ordinary generation from an ancient prototype,
of which we know nothing, furnished with a floating apparatus or
swimbladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed.
Pages:
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238