These are strong words, but we will give a few instances to prove their truth: All physiologists admit that the swim-bladder 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 swim-bladder into a lung, or organ used exclusively for respiration.p.

These are strong words, but we will give a few instances to prove their truth: All physiologists admit that the swim-bladder 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 swim-bladder into a lung, or organ used exclusively for respiration.p.

For instance, a swim-bladder has apparently been converted into an air-breathing lung.p.