The illustration of the swim-bladder 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 widely different purpose, namely respiration.
Arrainen igeri-maskuriaren adibidea ona da, agerian jartzen digulako garrantzi handiko egitate bat, alegia, hasieran xede baterako, flotaziorako zehazki, eraikiriko organo bat beste helburu guztiz ezberdin baterako organo, arnasketarako alegia, bihur daitekeela.